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Effects of water availability on carbon and water exchange in a young ponderosa pine forest: Above- and belowground responses
Abstract
Changes in the hydrological cycle, as predicted and currently observed, are expected to significantly impact the water and carbon balance of water-limited forest ecosystems. However, differences in the water-sensitivity of component processes make carbon balance predictions challenging. To examine responses of ecosystem components to water limitations, we conducted a study of tree, soil and ecosystem-level processes in a young ponderosa pine stand under natural summer drought (control) and increased soil water conditions (watered). Weekly-averaged tree transpiration (Ttree), gross ecosystem photosynthesis (GPP) and soil CO2 efflux (Rstree; nearby trees) were related with soil water content (SWC; polynomial form: TtreeR2 = 0.98 and RstreeR2 = 0.91, logarithmic form: GPP R2 = 0.86) and declined rapidly when relative extractable soil water (REW) was <50%. The sensitivity of daily variations in canopy conductance (Gs) to vapor pressure deficit was affected by SWC (R2 = 0.97; logarithmic function), decreasing at REW <50%. Watering maintained REW at about 70% in July and August but positively affected tree carbon and water dynamics only at the end of summer when fluxes in the control treatment were strongly water-limited. A tight coupling of above- and belowground fluxes became apparent. In the control treatment, root-rhizosphere respiration (Rr) decreased along with GPP and Ttree (R2 = 0.58) as drought progressed, while watering maintained Rr, Ttree and Gs at a significantly higher level than those of the unwatered trees in late summer. In contrast, microbial respiration responded instantaneously and strongly to the watering compared to the control treatment. The net effect was that increased soil water availability during the typical dry growing season has a negative effect on the short-term seasonal ecosystem C balance due to a larger increase in decomposition than photosynthesis. However, longer-term effects remain uncertain. In summary, our study highlights that understanding the dissimilar response of tree dynamics and soil decomposition to water availability is a key component in predicting future C sequestration in water-limited forest ecosystems.
... Although the FvCB model is standard for leaf-scale photosynthesis and its environmental response on timescales of minutes to hours, DVMs and ESMs using FvCB produce divergent results for ecosystem-level fluxes and their response to the environment at longer timescales (Rogers et al., 2017). This is due to assumptions that have to be made about photosynthetic parameters that are not predicted by the FvCB model: stomatal conductance (g s ) and the maximum rates of RuBisCO carboxylation (V cmax ) and electron transport (J max ) for ribulose-1,5-bisphosphate (RuBP) regeneration, which together determine the relationship between c i and A. Common approaches for determining the values of V cmax and J max in DVMs and ESMs are to prescribe fixed values per plant functional type (PFT) and attempt to simulate the distribution of PFTs in space, or to use empirical relationships between leaf N and V cmax and simulate leaf N internally or prescribe it per PFT (Smith and Dukes, 2013;Rogers, 2014). ...
... In other vegetation models, a L is commonly ascribed a value of 0.72-0.88 (Rogers et al., 2017). Values of ϕ 0 used here are accordingly lower than values for the intrinsic quantum yield reported from experimental studies (Long et al., 1993;Singsaas et al., 2001). ...
... We compare the simulated spatial distribution of GPP from global-scale simulations against seven different remotesensing-data-driven GPP estimates with global coverage and two Sun-induced fluorescence (SiF) data products. The global GPP estimates are from the following models: MTE (Jung et al., 2011), FLUXCOM ("RS+METEO" setup) (Tramontana et al., 2016), MODIS GPP (MOD17A2H Collections 55 and 6) (Running et al., 2004;Zhao et al., 2005;Running et al., 2015), BESS (Jiang and Ryu, 2016), BEPS (He et al., 2018;Chen et al., 2016), and VPM (Zhang et al., 2017). A more detailed description of these models and aggregation to a common grid of 0.5 • and monthly resolution can be found in Luo et al. (2018). ...
Terrestrial photosynthesis is the basis for vegetation growth and drives the land carbon cycle. Accurately simulating gross primary production (GPP, ecosystem-level apparent photosynthesis) is key for satellite monitoring and Earth system model predictions under climate change. While robust models exist for describing leaf-level photosynthesis, predictions diverge due to uncertain photosynthetic traits and parameters which vary on multiple spatial and temporal scales. Here, we describe and evaluate a GPP (photosynthesis per unit ground area) model, the P-model, that combines the Farquhar–von Caemmerer–Berry model for C3 photosynthesis with an optimality principle for the carbon assimilation–transpiration trade-off, and predicts a multi-day average light use efficiency (LUE) for any climate and C3 vegetation type. The model builds on the theory developed in Prentice et al. (2014) and Wang et al. (2017a) and is extended to include low temperature effects on the intrinsic quantum yield and an empirical soil moisture stress factor. The model is forced with site-level data of the fraction of absorbed photosynthetically active radiation (fAPAR) and meteorological data and is evaluated against GPP estimates from a globally distributed network of ecosystem flux measurements. Although the P-model requires relatively few inputs, the R2 for predicted versus observed GPP based on the full model setup is 0.75 (8 d mean, 126 sites) – similar to comparable satellite-data-driven GPP models but without predefined vegetation-type-specific parameters. The R2 is reduced to 0.70 when not accounting for the reduction in quantum yield at low temperatures and effects of low soil moisture on LUE. The R2 for the P-model-predicted LUE is 0.32 (means by site) and 0.48 (means by vegetation type). Applying this model for global-scale simulations yields a total global GPP of 106–122 Pg C yr−1 (mean of 2001–2011), depending on the fAPAR forcing data. The P-model provides a simple but powerful method for predicting – rather than prescribing – light use efficiency and simulating terrestrial photosynthesis across a wide range of conditions. The model is available as an R package (rpmodel).
... Although the FvCB model is standard for leaf-scale photosynthesis and its environmental response on timescales of minutes to hours, DVMs and ESMs using FvCB produce divergent results for ecosystem-level fluxes and their response to the environment at longer timescales (Rogers et al., 2017). This is due to assumptions that have to be made about photosynthetic parameters that are not predicted by the FvCB model: stomatal conductance (g s ) and the maximum rates of RuBisCO carboxylation (V cmax ) and electron transport (J max ) for ribulose-1,5-bisphosphate (RuBP) regeneration, which together determine the relationship between c i and A. Common approaches for determining the values of V cmax and J max in DVMs and ESMs are to prescribe fixed values per plant functional type (PFT) and attempt to simulate the distribution of PFTs in space, or to use empirical relationships between leaf N and V cmax and simulate leaf N internally or prescribe it per PFT (Smith and Dukes, 2013;Rogers, 2014). ...
... In other vegetation models, a L is commonly ascribed a value of 0.72-0.88 (Rogers et al., 2017). Values of ϕ 0 used here are accordingly lower than values for the intrinsic quantum yield reported from experimental studies (Long et al., 1993;Singsaas et al., 2001). ...
... We compare the simulated spatial distribution of GPP from global-scale simulations against seven different remotesensing-data-driven GPP estimates with global coverage and two Sun-induced fluorescence (SiF) data products. The global GPP estimates are from the following models: MTE (Jung et al., 2011), FLUXCOM ("RS+METEO" setup) (Tramontana et al., 2016), MODIS GPP (MOD17A2H Collections 55 and 6) (Running et al., 2004;Zhao et al., 2005;Running et al., 2015), BESS (Jiang and Ryu, 2016), BEPS (He et al., 2018;Chen et al., 2016), and VPM (Zhang et al., 2017). A more detailed description of these models and aggregation to a common grid of 0.5 • and monthly resolution can be found in Luo et al. (2018). ...
Terrestrial photosynthesis is the basis for vegetation growth and drives the land carbon cycle. Accurately simulating gross primary production (GPP, ecosystem-level apparent photosynthesis) is key for satellite monitoring and Earth System Model predictions under climate change. While robust models exist for describing leaf-level photosynthesis, predictions diverge due to uncertain photosynthetic traits and parameters which vary on multiple spatial and temporal scales. Here, we describe and evaluate a gross primary production (GPP, photosynthesis per unit ground area) model, the P-model, that combines the Farquhar-von Caemmerer-Berry model for C3 photosynthesis with an optimality principle for the carbon assimilation-transpiration trade-off, and predicts a multi-day average light use efficiency (LUE) for any climate and C3 vegetation type. The model is forced here with satellite data for the fraction of absorbed photosynthetically active radiation and site-specific meteorological data and is evaluated against GPP estimates from a globally distributed network of ecosystem flux measurements. Although the P-model requires relatively few inputs and prescribed parameters, the R² for predicted versus observed GPP based on the full model setup is 0.75 (8-day mean, 131 sites) – better than some state-of-the-art satellite data-driven light use efficiency models. The R² is reduced to 0.69 when not accounting for the reduction in quantum yield at low temperatures and effects of low soil moisture on LUE. The R² for the P-model-predicted LUE is 0.37 (means by site) and 0.53 (means by vegetation type). The P-model provides a simple but powerful method for predicting – rather than prescribing – light use efficiency and simulating terrestrial photosythesis across a wide range of conditions. The model is available as an R package (rpmodel).
... There is a pressing need to scrutinize the response of WUE in water-limited environments where WUE is directly influenced by interannual variations in drought stress. In separate prior studies in the semi-arid forests of Oregon, we examined interannual variability in WUE or WUE i in young and mature ponderosa pine forests and found that they generally increased as summer soil water deficit and VPD increased and were higher in more extreme drought years Ruehr et al., 2012;Thomas et al., 2009;Vickers et al., 2012). We found higher WUE in young than mature and old ponderosa pine (5, 3, and 4 g C m −2 mm −1 , respectively) when adjusted for leaf area index (LAI) differences . ...
... Both sites were re-forested in the early 1990s and had similarly aged ponderosa pines. At US-Me6, sap flux measurements were conducted for four trees from May through October in 2010 and 2011, using the methods described above (Ruehr et al., 2012). Errors associated with temperature gradients were 7% overestimation at YP and were corrected accordingly. ...
... In 2005, prolonged dryness led to a decrease in the ratio of T to ET primarily due to an extremely low T (< 0.2 mm) which resulted in a substantial increase in WUE i_T beginning in mid-August. As found in an experiment by Ruehr et al. (2012), the increase in WUE i after August was because g s declined more than photosynthesis. The VPD-induced response of g s was strongly affected by soil water availability, which affected T more than ET. ...
The Pacific Northwest (PNW) region of the United States has some of the most productive forests in the world. As precipitation regimes may shift with changing climate in this area, droughts are predicted to increase in both frequency and degree of severity, which will have a significant impact on already drought-prone ecosystems. When modeling ecosystem responses to drought, it is important to consider the physiology of individual tree species since the variations in drought sensitivity among species is easily overlooked when plants are characterized using broad plant functional types. Here we explore the use of inherent water-use efficiency as an index of drought sensitivity in semi-arid young and mature ponderosa pine forests and a mesic mature Douglas-fir forest in the PNW. Summer maximum of an evapotranspiration-based WUE (WUEi) was 2.5 times higher in young and mature pines in semi-arid climate than Douglas-fir in mesic climate (12.2 and 11.3 versus 4.7gCkPa per kg H2O, respectively). In contrast, annually averaged WUEi was similar among the sites (2.8gCkPa per kg H2O for pines and 2.4gCkPa per kg H2O for Douglas-fir). The effect of drought stress on WUEi was most pronounced in young pine, followed by mature pine and Douglas-fir (32, 11, and 6% increase in WUEi per % decline in soil water content, respectively) which reflect differences in age-related ecosystem structure (root system, stem capacitance, and soil water holding capacity). Among sites, the responses of WUEi to climate variability were largely driven by changes in evapotranspiration (ET) compared to gross primary productivity (GPP). However, in areas where evaporation is the primary component of ET, such as the open canopy ponderosa forests of the PNW, the contribution of soil processes to ET can overshadow the reaction of vegetation transpiration (T) to changes in water availability. In these cases, utilizing a transpiration-based WUE (WUEi_T) in vegetation models will yield a more accurate representation of plant activity during drought. These results highlight the importance of incorporating differences in species- and age-related WUEi in models in diverse forest types at regional and global scales to improve predictions in ecosystem responses to climate change.
... To understand the influence of changes in forest age structure on the land-surface exchanges of carbon and water with the atmosphere, two even-aged young ponderosa pine forests were also included for comparison with the multi-aged ponderosa stands. The US-Me3 site is a 3-m-tall stand with seasonal grass understory which was clear-cut and replanted by the US Forest Service in 1987 (Vickers et al., 2012), while the US-Me6 stand is dominated by an~23-year old ponderosa pine plantation with an average height of 5.2 m after a severe fire during the late 1990s (Ruehr et al., 2012). The climate of this region is Mediterranean with hot dry summers, and cool wet springs and winters, with precipitation mainly falling as rain and occasionally as snow. ...
... A wide range of different software programs (EdiRe developed by the University of Edinburgh, UK) are used for real-time and post-processing analysis (Goulden et al., 2006;Peichl et al., 2010a). Data collection and regular calibrations of tower-based flux measurements followed the description by Ruehr et al. (2012). The data are qualitychecked, and data gaps due to system failure or data rejection are gapfilled using standardized methods (Falge et al., 2001;Papale et al., 2006) to obtain complete and standardized data sets. ...
... In water-limited ecosystems, increases in SWC may also increase GPP (Guo et al., 2015) and R e (Riveros-Iregui et al., 2012). However, even though photosynthetic and respiratory CO 2 fluxes respond similarly to temperature and water availability, the magnitudes and sensitivities can differ (Ruehr et al., 2012;Biederman et al., 2016). Larger NEP in the year with less annual precipitation (Fig. 6) may be correlated with more frequent precipitation events hindering photosynthetic GPP in the Mediterranean climate. ...
Disturbances and climatic changes significantly affect forest ecosystem productivity, water use efficiency (WUE) and carbon (C) flux dynamics. A deep understanding of terrestrial feedbacks to such effects and recovery mechanisms in forests across contrasting climatic regimes is essential to predict future regional/global C and water budgets, which are also closely related to the potential forest management decisions. However, the resilience of multi-aged and even-aged forests to disturbances has been debated for > 60 years because of technical measurement constraints. Here we evaluated 62 site-years of eddy covariance measurements of net ecosystem production (NEP), evapotranspiration (ET), the estimates of gross primary productivity (GPP), ecosystem respiration (Re) and ecosystem-level WUE, as well as the relationships with environmental controls in three chronosequences of multi- and even-aged coniferous forests covering the Mediterranean, temperate and boreal regions. Age-specific dynamics in multi-year mean annual NEP and WUE revealed that forest age is a key variable that determines the sign and magnitude of recovering forest C source-sink strength from disturbances. However, the trends of annual NEP and WUE across succession stages between two stand structures differed substantially. The successional patterns of NEP exhibited an inverted-U trend with age at the two even-aged chronosequences, whereas NEP of the multi-aged chronosequence increased steadily through time. Meanwhile, site-level WUE of even-aged forests decreased gradually from young to mature, whereas an apparent increase occurred for the same forest age in multi-aged stands. Compared with even-aged forests, multi-aged forests sequestered more CO2 with forest age and maintained a relatively higher WUE in the later succession periods. With regard to the available flux measurements in this study, these behaviors are independent of tree species, stand ages and climate conditions. We also found that distinctly different environmental factors controlled forest C and water fluxes under three climatic regimes. Typical weather events such as temperature anomalies or drying-wetting cycles severely affected forest functions. Particularly, a summer drought in the boreal forest resulted in an increased NEP owing to a considerable decrease in Re, but at the cost of greater water loss from deeper groundwater resources. These findings will provide important implications for forest management strategies to mitigate global climate change.
... ability to predict future effects on forest ecosystems is limited by uncertainty regarding the relative roles of evaporative demand/temperature and precipitation in triggering drought stress , and by understanding their corresponding effects on component processes like photosynthesis and heterotrophic respiration (Ruehr et al., 2012). For example, gross primary productivity is generally found to decrease more than respiration during drought conditions (Schwalm et al., 2009;Ruehr et al., 2012), because photosynthesis is limited by both soil drought and high temperatures, while soil moisture constraints on heterotrophic respiration may be partially compensated by temperature (Irvine et al., 2008;Ruehr et al., 2012). ...
... ability to predict future effects on forest ecosystems is limited by uncertainty regarding the relative roles of evaporative demand/temperature and precipitation in triggering drought stress , and by understanding their corresponding effects on component processes like photosynthesis and heterotrophic respiration (Ruehr et al., 2012). For example, gross primary productivity is generally found to decrease more than respiration during drought conditions (Schwalm et al., 2009;Ruehr et al., 2012), because photosynthesis is limited by both soil drought and high temperatures, while soil moisture constraints on heterotrophic respiration may be partially compensated by temperature (Irvine et al., 2008;Ruehr et al., 2012). ...
... ability to predict future effects on forest ecosystems is limited by uncertainty regarding the relative roles of evaporative demand/temperature and precipitation in triggering drought stress , and by understanding their corresponding effects on component processes like photosynthesis and heterotrophic respiration (Ruehr et al., 2012). For example, gross primary productivity is generally found to decrease more than respiration during drought conditions (Schwalm et al., 2009;Ruehr et al., 2012), because photosynthesis is limited by both soil drought and high temperatures, while soil moisture constraints on heterotrophic respiration may be partially compensated by temperature (Irvine et al., 2008;Ruehr et al., 2012). ...
Predicting the net effects on the carbon and water balance of semi-arid
forests under future conditions depends on ecosystem processes responding to
changes in soil and atmospheric drought. Here we apply a combination of field
observations and soil–plant–atmosphere modeling (SPA) to study carbon and
water dynamics in a regenerating ponderosa pine forest. The effects of soil
and atmospheric drought were quantified based on a field irrigation
experiment combined with model simulations. To assess future effects of
intensifying drought on ecosystem processes, the SPA model was run using
temperature and precipitation scenarios for 2040 and 2080.
Experimentally increased summer water availability clearly affected tree
hydraulics and enhanced C uptake in both the observations and the model.
Simulation results showed that irrigation was sufficient to eliminate soil
water limitation and maintaining transpiration rates, but gross primary
productivity (GPP) continued to decrease. Observations of stomatal
conductance indicated a dominant role of vapor pressure deficit (VPD) in
limiting C uptake. This was confirmed by running the simulation under reduced
atmospheric drought (VPD of 1 kPa), which largely maintained GPP rates at
pre-drought conditions.
The importance of VPD as a dominant driver was underlined by simulations of
extreme summer conditions. We found GPP to be affected more by summer
temperatures and VPD as predicted for 2080 (−17%) than by reductions in
summer precipitation (−9%). Because heterotrophic respiration responded
less to heat (−1%) than to reductions in precipitation (−10%),
net ecosystem C uptake declined strongest under hotter (−38%) compared
to drier summer conditions (−8%).
Considering warming trends across all seasons (September–May: +3 °C
and June–August: +4.5 °C), the negative drought effects were
largely compensated by an earlier initiation of favorable growing conditions
and bud break, enhancing early season GPP and needle biomass. An adverse
effect, triggered by changes in early season allocation patterns, was the
decline of wood and root biomass. This imbalance may increase water stress
over the long term to a threshold at which ponderosa pine may not survive,
and highlights the need for an integrated process understanding of the
combined effects of trends and extremes.
... And, according to the burst detection over the years, keywords, such as "carbon dioxide", "model", "climate", "growth", "temperature", "biomass", "global warming", "CO2", "global change", "dynamics", "adjustments", "tallgrass prairie", and "atmospheric CO2" have been emerged more than 10 years, which reflects that the CEI field is ongoing and expanding. Climate change (evaluated CO2, drought) [55], hydrological cycle changes (water availability), and additional carbon input (nutrients) [56][57][58] change the soil carbon cycle process of the ecosystem by affecting soil respiration. Through meta-analysis, these experimental results can quantitatively detect the impact and synergistic impact of global environmental and human factors on soil respiration, provide information for regional and global ecology and climate models, and predict the state of future ecosystems and climate systems [59]. ...
... And, according to the burst detection over the years, keywords, such as "carbon dioxide", "model", "climate", "growth", "temperature", "biomass", "global warming", "CO 2 ", "global change", "dynamics", "adjustments", "tallgrass prairie", and "atmospheric CO 2 " have been emerged more than 10 years, which reflects that the CEI field is ongoing and expanding. Climate change (evaluated CO 2 , drought) [55], hydrological cycle changes (water availability), and additional carbon input (nutrients) [56][57][58] change the soil carbon cycle process of the ecosystem by affecting soil respiration. Through meta-analysis, these experimental results can quantitatively detect the impact and synergistic impact of global environmental and human factors on soil respiration, provide information for regional and global ecology and climate models, and predict the state of future ecosystems and climate systems [59]. ...
Irrigation, as one of the most impactful human interventions in the terrestrial water cycle, has been arousing great attention due to research on the impacts of its interaction with climate. In this paper, we used a scientometric analysis method to explore the overall publication output of the climatic effects of irrigation (CEI) field from the Web of Science Core Collection (WSCC) database, covering the time period from 1993 to 2022. And, through a visual scientific citation analysis tool, CiteSpace, we studied the knowledge structure, disciplinary trajectory, frontier hotspots, and academic impacts in the field of CEI. Using topic screening, 2919 publications related to irrigation climate were searched. CEI research has gone through the knowledge germination stage (1993–2005), knowledge accretion stage (2006–2012), and the knowledge prosperity stage (2013–2022), respectively. Ecology, earth, and marine are the most influential disciplines of research in this field, and they are influenced by earth, geology, geophysics and plant, ecology, zoology. AWM and SOTTE are the most popular journals currently. The academic impacts of scientific stakeholders are uneven. European and American countries have profound influence in the research field. The keyword of “Climate change” is the turning point in the co-word analysis network, and research hotspots focus on “carbon dioxide”, “model”, “climate”, “growth”, “temperature”, “biomass”, “global warming”, “CO2”, “global change”, “dynamics”, “adjustments”, and “atmospheric CO2”. The knowledge base of the CEI field can be divided into 14 clusters, such as cotton production, semi-arid condition, and irrigation water supply, and these three clusters are the three largest among them. This paper offers a comprehensive scientometric review of CEI, and, to some degree, provides some reference for the relevant research on the climate effects of irrigation, which will be beneficial to understand the current research situation and development trend in this field, as well as provide state-of-the-art and future perspectives.
... Second, while control experiments in the laboratory or growth chamber (e.g., Grossiord et al., 2017) can help resolve the contribution to water stress, this is seldom achieved under field conditions in mature trees, which is critical for validating control experiments and making realistic predictions. Furthermore, previous reports of stomatal response to variations in SM and vapour pressure deficit (VPD) tended to focus on temperate environments with relatively high SM and low VPD and a limited range of variability, even in dry conditions Hochberg et al., 2017;Domec et al., 2009;Grossiord et al., 2017;Maseyk et al., 2008;Ruehr et al., 2012). Therefore, the environmental factors and use of mature trees under field conditions extend the range of habitats and conditions and improve the understanding of how trees will respond to drier conditions that may apply to the Mediterranean and possibly other mesic environments undergoing climate change. ...
... Thus, studies intending to disentangle the effects of SM and VPD on transpiration have revealed that VPD seemingly exerts more influence than SM on stomatal closure (Damour et al., 2010;Novick et al., 2016), indicating a direct link between increased VPD and reduced g s . However, studies on pine trees revealed that under semiarid conditions (mean annual rainfall of 415 mm), the main limiting factor of gas exchange was SM and not VPD (Grossiord et al., 2017) and that stomatal conductance sensitivity to VPD was governed by SM (Ruehr et al., 2012). Domec et al. (2009 found that in moist sites with apparently sufficient water supply (annual rainfall >1200 mm), the main limiting factor for tree transpiration shifted from VPD to SM as the soils became drier. ...
Climate change is often associated with increasing vapour pressure deficit (VPD) and changes in soil moisture (SM). While atmospheric and soil drying often co-occur, their differential effects on plant functioning and productivity remain uncertain. We investigated the divergent effects and underlying mechanisms of soil and atmospheric drought based on continuous, in situ measurements of branch gas exchange with automated chambers in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil-atmospheric drought (low SM, high VPD) during the rainless Mediterranean summer and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (gbr ), transpiration rate (E) and net photosynthesis (Anet ) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and gbr to the massive rise in VPD (to 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing the importance of plant water potential in the gbr and E response to VPD. These results demonstrate that avoiding drought on the supply side (SM) and relying on plant hydraulic regulation constrains the effects of atmospheric drought (VPD) as a stressor on canopy gas exchange in mature pine trees under field conditions.
... Evidence from empirical (Table 2) and theory-based models (Table S2) and binned-averaging analysis (Figures 3-6) indicates that the negative effects of high VPD on surface conductance and ecosystem photosynthesis (Anthoni et al., 1999;Noormets et al., 2008;Sulman et al., 2016;Wagle et al., 2015) decrease the carbon uptake of pulse events in semiarid ecosystems. These findings were validated experimentally by Ruehr et al. (2012), who used irrigation to alleviate soil water stress (analogous to a pulse event) in a semiarid pine forest and found that high VPD was associated with declines in canopy conductance that decreased GEP. ...
... reductions in substrate exudation by water-stressed plants (Anthoni et al., 1999;Baldocchi et al., 2006;Chen et al., 2009;Kuzyakov & Gavrichkova, 2010;Roby et al., 2019;Ruehr et al., 2012). Because autotrophic respiration generally scales with GEP (Chapin et al., 2011), autotrophic respiration likely decreases during pulses as high VPD reduces GEP. ...
Intermittent rain events drive dynamic pulses of carbon and water exchange in many arid and semiarid ecosystems. Although soil moisture is known to control these pulses, the effect of atmospheric dryness on pulses is not well documented. Here we hypothesized that vapor pressure deficit (VPD) modulates net ecosystem production (NEP) and ecosystem‐scale water use efficiency (WUE) during pulse events due to its effects on canopy stomatal conductance and evapotranspiration. We quantified relationships between VPD and carbon and water exchange during growing season rain events and tested their generality across four semiarid flux sites with varied vegetation in the southwest United States. Across grassland, shrubland, and savanna sites, we found that high VPD during pulses suppressed ecosystem photosynthesis and surface conductance to a greater degree than respiration or evapotranspiration, particularly when soil moisture was high. Thus, periods of high VPD were associated with a 13–64% reduction in NEP and an 11–25% decrease in WUE, relative to moderate VPD conditions. Sites dominated by shrubs with the C3 photosynthetic pathway were more sensitive to VPD than sites dominated by C4 grasses. We found that a 1 kPa increase in VPD reduced the average NEP of pulse events by 13–56%, which illustrates the potential for projected increases in atmospheric demand to reduce the net productivity of semiarid ecosystems.
... In the literature, many studies have focused on carbon exchanges in differentaged or rotations (the period between planting and harvest) of temperate forests (Peichl et al. 2010a;Law et al. 2003;Clark et al. 2004;Humphreys et al. 2006;Krishnan et al. 2009;Amiro et al. 2010;Goulden et al. 2011;Baldocchi 2019;Xu et al. 2020). However, there is a need for studies that can improve our understanding of how different-aged managed forests respond to multiple environmental drivers, such as concurrent heat and drought events (Williams et al. 2013;Ruehr et al. 2012;Allen et al. 2010;Reichstein et al. 2013;Frank et al. 2015). ...
Background
Most North American temperate forests are plantation or regrowth forests, which are actively managed. These forests are in different stages of their growth cycles and their ability to sequester atmospheric carbon is affected by extreme weather events. In this study, the impact of heat and drought events on carbon sequestration in an age-sequence (80, 45, and 17 years as of 2019) of eastern white pine ( Pinus strobus L.) forests in southern Ontario, Canada was examined using eddy covariance flux measurements from 2003 to 2019.
Results
Over the 17-year study period, the mean annual values of net ecosystem productivity (NEP) were 180 ± 96, 538 ± 177 and 64 ± 165 g C m –2 yr –1 in the 80-, 45- and 17-year-old stands, respectively, with the highest annual carbon sequestration rate observed in the 45-year-old stand. We found that air temperature (Ta) was the dominant control on NEP in all three different-aged stands and drought, which was a limiting factor for both gross ecosystem productivity (GEP) and ecosystems respiration (RE), had a smaller impact on NEP. However, the simultaneous occurrence of heat and drought events during the early growing seasons or over the consecutive years had a significant negative impact on annual NEP in all three forests. We observed a similar trend of NEP decline in all three stands over three consecutive years that experienced extreme weather events, with 2016 being a hot and dry, 2017 being a dry, and 2018 being a hot year. The youngest stand became a net source of carbon for all three of these years and the oldest stand became a small source of carbon for the first time in 2018 since observations started in 2003. However, in 2019, all three stands reverted to annual net carbon sinks.
Conclusions
Our study results indicate that the timing, frequency and concurrent or consecutive occurrence of extreme weather events may have significant implications for carbon sequestration in temperate conifer forests in Eastern North America. This study is one of few globally available to provide long-term observational data on carbon exchanges in different-aged temperate plantation forests. It highlights interannual variability in carbon fluxes and enhances our understanding of the responses of these forest ecosystems to extreme weather events. Study results will help in developing climate resilient and sustainable forestry practices to offset atmospheric greenhouse gas emissions and improving simulation of carbon exchange processes in terrestrial ecosystem models.
... The greatest impact of heatwaves on the Rs rate and its variability occurs during the morning (07:00-11:00) compared to non-heatwave conditions. We extend the above analysis to the additional ten sites across the CONUS for which in addition to Rs we had access to hourly temperature data for heatwave analysis 18,37,38,[44][45][46][47][48]56 (see Table 1 and Fig. 1), we find similar Rs responses to heatwaves across all sites. Each panel in Fig. 4 displays the probability of exceeding the Rs mean values [P ex (%)]. ...
In the wake of climate change, extreme events such as heatwaves are considered to be key players in the terrestrial biosphere. In the past decades, the frequency and severity of heatwaves have risen substantially, and they are projected to continue to intensify in the future. One key question is therefore: how do changes in extreme heatwaves affect the carbon cycle? Although soil respiration (Rs) is the second largest contributor to the carbon cycle, the impacts of heatwaves on Rs have not been fully understood. Using a unique set of continuous high frequency in-situ measurements from our field site, we characterize the relationship between Rs and heatwaves. We further compare the Rs response to heatwaves across ten additional sites spanning the contiguous United States (CONUS). Applying a probabilistic framework, we conclude that during heatwaves Rs rates increase significantly, on average, by ~ 26% relative to that of non-heatwave conditions over the CONUS. Since previous in-situ observations have not measured the Rs response to heatwaves (e.g., rate, amount) at the high frequency that we present here, the terrestrial feedback to the carbon cycle may be underestimated without capturing these high frequency extreme heatwave events.
... Root respiration is a major component of RE and is tightly correlated with photosynthesis (Peichl, Brodeur, Khomik, & Arain, 2010). Research from other ENF forests emphasizes the coupling of above-and belowground processes to their sensitivity to drought stress Ruehr, Martin, & Law, 2012). Similar results reported at site level studies (Allard, Ourcival, Rambal, Joffre, & Rocheteau, 2008) and global synthesis of ENF (Schwalm et al., 2010) also showed that the synchronous response of GEP and RE to summer drought result in a neutral impact on net carbon balance in the ENF. ...
Climate extremes such as heatwaves and droughts are projected to occur more frequently with increasing temperature and an intensified hydrological cycle. It is important to understand and quantify how forest carbon fluxes respond to heat and drought stress. In this study, we developed a series of daily indices of sensitivity to heat and drought stress as indicated by air temperature (Ta) and Evaporative Fraction (EF). Using normalized daily carbon fluxes from the FLUXNET Network for 34 forest sites in North America, the seasonal pattern of sensitivities of net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (RE) in response to Ta and EF anomalies were compared for different forest types. The results showed that warm temperatures in spring had a positive effect on NEP in conifer forests but a negative impact in deciduous forests. GEP in conifer forests increased with higher temperature anomalies in spring but decreased in summer. The drought‐induced decrease in NEP, which mostly occurred in the deciduous forests, was mostly driven by the reduction in GEP. In conifer forests, drought had a similar dampening effect on both GEP and RE, therefore leading to a neutral NEP response. The NEP sensitivity to Ta anomalies increased with increasing mean annual temperature. Drier sites were less sensitive to drought stress in summer. Natural forests with older stand age tended to be more resilient to the climate stresses compared to managed younger forests. The results of the Classification and Regression Tree analysis showed that seasons and ecosystem productivity were the most powerful variables in explaining the variation of forest sensitivity to heat and drought stress. Our results implied that the magnitude and direction of carbon flux changes in response to climate extremes are highly dependent on the seasonal dynamics of forests and the timing of the climate extremes.
... Transpiration is a major component of evapotranspiration in terrestrial ecosystems (Jasechko et al., 2013;Schlesinger & Jasechko, 2014) and is strongly linked to carbon dynamics such as gross primary production (Law et al., 2002), soil and root respiration (Irvine, Law, Martin, & Vickers, 2008;Ruehr, Martin, & Law, 2012), and tree growth (Smith & Sperry, 2014) in forest ecosystems. Transpiration potentially changes with forest management practices (e.g., planting, ageing, thinning, and clearcutting) due to changes in tree and forest structure (e.g., stem diameter at breast height (DBH) and stem density; Previous studies reported that whole-tree transpiration (Q t ) is related to various tree attributes, such as DBH (Čermák, Kučera, & Nadezhdina, 2004;von Allmen et al., 2012), sapwood area (A s_tree ; ...
Previous studies reported relationships between stem diameter at breast height (DBH) and whole‐tree transpiration (Qt) across a variety of species and locations. It might be possible to develop a relationship between DBH and Qt with smaller variations when we focused on a single species. We attempted to develop such a relationship for Japanese cypress (Chamaecyparis obtusa), which is one of the major plantation species in Japan. We collated Qt for 51 Japanese cypress trees from nine different‐sized and ‐aged stands using the sap flux method. We found a strong linear correlation between DBH and the reference value of Qt at a vapor pressure deficit of 1 kPa (R = 0.883). This was a consequence of a strong correlation between DBH and sapwood area (AS_tree) (R = 0.973) and the absence of a correlation between DBH and sap flux density (R = –0.043). We confirmed that using the relationship between DBH and AS_tree, while assuming typical responses of sap flux density to meteorological factors, provides reasonable Qt estimates. This study also demonstrated how the DBH–Qt relationship can be applied to estimate changes in EC with changing forest management.
... Experimental irrigation has been implemented most extensively in forest plantations, where water supplementation is often an important management tool (e.g., Jokela, Dougherty & Martin 2004). More recently, water addition PMEs have been conducted to assess climate change impacts in water-limited woodlands or dry forests (Ruehr, Martin & Law 2012;Plaut et al. 2013). In contrast, despite predictions that wetter sites will likely experience more extreme wet spells (IPCC 2013), the response of mesic forests to precipitation additions has been poorly investigated, with previous studies utilizing relatively small plots (e.g., Yavitt, Wright & Wieder 2004;McCulley, Boutton & Archer 2007;Gao et al. 2017; but see Hanson, Todd & Amthor 2001). ...
Precipitation regimes are changing in response to climate change, yet understanding of how forest ecosystems respond to extreme droughts and pluvials remains incomplete. As future precipitation extremes will likely fall outside the range of historical variability, precipitation manipulation experiments ( PME s) are critical to advancing knowledge about potential ecosystem responses. However, few PME s have been conducted in forests compared to short‐statured ecosystems, and forest PME s have unique design requirements and constraints. Moreover, past forest PME s have lacked coordination, limiting cross‐site comparisons. Here, we review and synthesize approaches, challenges, and opportunities for conducting PME s in forests, with the goal of guiding design decisions, while maximizing the potential for coordination.
We reviewed 63 forest PME s at 70 sites world‐wide. Workshops, meetings, and communications with experimentalists were used to generate and build consensus around approaches for addressing the key challenges and enhancing coordination.
Past forest PME s employed a variety of study designs related to treatment level, replication, plot and infrastructure characteristics, and measurement approaches. Important considerations for establishing new forest PME s include: selecting appropriate treatment levels to reach ecological thresholds; balancing cost, logistical complexity, and effectiveness in infrastructure design; and preventing unintended water subsidies. Response variables in forest PME s were organized into three broad tiers reflecting increasing complexity and resource intensiveness, with the first tier representing a recommended core set of common measurements.
Differences in site conditions combined with unique research questions of experimentalists necessitate careful adaptation of guidelines for forest PME s to balance local objectives with coordination among experiments. We advocate adoption of a common framework for coordinating forest PME design to enhance cross‐site comparability and advance fundamental knowledge about the response and sensitivity of diverse forest ecosystems to precipitation extremes.
... Forest sites with observation lengths no less than 5 years (six sites) were also included for an additional response analysis to further validate our conclusions (Table S1). More detailed information for each site can be found in the supporting information (Acosta et al., 2013;Baldocchi et al., 2005;Berbigier et al., 2001;Bergeron et al., 2007;Cook et al., 2004;Delpierre et al., 2016;Gough et al., 2013;Goulden et al., 2006;Grünwald & Bernhofer, 2007;Holzinger et al., 2006;Jarvis et al., 2004;Jung et al., 2009;Knohl et al., 2003;Kurbatova et al., 2008;Lindauer et al., 2014;Marcolla et al., 2003;Monson et al., 2002;Montagnani et al., 2009;Pilegaard et al., 2011;Rey et al., 2002;Ruehr et al., 2012;Stoy et al., 2014;Tchebakova et al., 2002;Tedeschi et al., 2006;Thum et al., 2007;Urbanski et al., 2007;Valentini et al., 1996;Vickers et al., 2009;Zielis et al., 2014). ...
Changing climate and increasing atmospheric CO2 significantly regulate forest water use efficiency (WUE). However, magnitudes of the WUE trends and underlying processes driving these patterns in two major forest types, deciduous broadleaf forests (DBFs) and evergreen needleleaf forests (ENFs), across the Northern Hemisphere remain poorly understood. We investigated the WUE trends over the past two decades using eddy covariance observations from 26 forest sites from the FLUXNET2015 data set. Our analyses revealed a greater increase in WUE in DBFs than that in ENFs. The decreased stomatal conductance (Gs) mostly contributed to the increase in WUE in the DBFs, whereas the increased gross ecosystem productivity acted as the main trigger for the increase in WUE in the ENFs. The vapor pressure deficit substantially increased in the DBFs, triggering the decrease in Gs. In contrast, the slight CO2 fertilization and the limited stomatal constraint contributed to the increased gross ecosystem productivity in the ENFs.
... A series of software programs were used for post-processing analysis. Data collection and regular calibrations of the tower-based flux measurements were completed as described by Ruehr et al. (2012). ...
Changing climate, especially extreme weather event, is exerting considerable impacts on the hydrological and biogeochemical processes in forests worldwide. A deep understanding of climate change–terrestrial feedbacks is essential to predict future regional/global carbon and water budgets, which can be used to develop potential strategies for forest management. In this study, totally 11 years of eddy covariance tower measurements of CO2 and H2O fluxes, as well as the relevant environmental variables were analyzed to reveal the effects of climate anomalies on vegetation phenology, carbon sequestration and ecosystem water-use efficiency (WUE) dynamics in a humid temperate deciduous forest. Warmer spring temperatures altered the phenological phases with the green-up date advanced approximately 3.5 days per °C, and the extended growing season of about 3 days per °C, reaching the peak in 2012. Because of spring temperature anomaly, the shift from carbon source to sink occurred nearly 40 days earlier than usual. But the abnormal carbon dynamics happened during the peak growth period. Correlation analyses indicated that the amount of precipitation dominantly controlled the capacity of forest carbon sequestration (NEP) in this area. Therefore, the subsequent water scarcity owing to the extremely dry summer together with heatwave severely decreased the forest NEP by about 64.1%. Further analyses implied that the sharp reduction in gross primary production (GPP) rather than ecosystem respiration (Re) resulted to the decrease in NEP. Both GPP and evatranspiration (ET) were larger during the springtime in 2012 than those at adjacent years. But severe summer drought reduced the ecosystem WUE and yielded the lowest GPP and ET. Further work must focus on improving the recognition of forest feedback to climate systems.
... All these differences are the result of the modified stress functions, but -more importantly -of the new (highresolution) land cover fractions used in GLEAM v3 which report an overall larger fraction of bare soils over the continents. The higher contribution of bare soil evaporation and the lower volumes of transpiration, especially in semi-arid regions like the Sahel, result in closer agreement with the partitioning obtained from other data sets (Wang et al., 2014;Schlesinger and Jasechko, 2014;Miralles et al., 2016a;Good et al., 2015). Nonetheless, Miralles et al. (2016a) recently raised awareness about the use of satellite-based evaporation algorithms to assess the contribution from different evaporation components, and suggested avoiding the use of any single model in isolation due to the large differences found in inter-model comparisons. ...
The Global Land Evaporation Amsterdam Model (GLEAM) is a set of
algorithms dedicated to the estimation of terrestrial evaporation and
root-zone soil moisture from satellite data. Ever since its development in
2011, the model has been regularly revised, aiming at the optimal
incorporation of new satellite-observed geophysical variables, and improving
the representation of physical processes. In this study, the next version of
this model (v3) is presented. Key changes relative to the previous version
include (1) a revised formulation of the evaporative stress, (2) an optimized
drainage algorithm, and (3) a new soil moisture data assimilation system.
GLEAM v3 is used to produce three new data sets of terrestrial evaporation
and root-zone soil moisture, including a 36-year data set spanning
1980–2015, referred to as v3a (based on satellite-observed soil moisture,
vegetation optical depth and snow-water equivalent, reanalysis air
temperature and radiation, and a multi-source precipitation product), and two
satellite-based data sets. The latter share most of their forcing, except for
the vegetation optical depth and soil moisture, which are based on
observations from different passive and active C- and L-band microwave
sensors (European Space Agency Climate Change Initiative, ESA CCI) for the
v3b data set (spanning 2003–2015) and observations from the Soil Moisture
and Ocean Salinity (SMOS) satellite in the v3c data set (spanning
2011–2015). Here, these three data sets are described in detail, compared
against analogous data sets generated using the previous version of GLEAM
(v2), and validated against measurements from 91 eddy-covariance towers and
2325 soil moisture sensors across a broad range of ecosystems. Results
indicate that the quality of the v3 soil moisture is consistently better than
the one from v2: average correlations against in situ surface soil moisture
measurements increase from 0.61 to 0.64 in the case of the v3a data set and
the representation of soil moisture in the second layer improves as well,
with correlations increasing from 0.47 to 0.53. Similar improvements are
observed for the v3b and c data sets. Despite regional differences, the
quality of the evaporation fluxes remains overall similar to the one obtained
using the previous version of GLEAM, with average correlations against
eddy-covariance measurements ranging between 0.78 and 0.81 for the different
data sets. These global data sets of terrestrial evaporation and root-zone
soil moisture are now openly available at www.GLEAM.eu and may be used
for large-scale hydrological applications, climate studies, or research on
land–atmosphere feedbacks.
... Therefore rewetting of soil following dry periods, might give rise to flushes of highly concentrated exudates (Mencuccini et al., 2013). As alternate states of moisture availability in soil are common and could increase in the future, it is important to understand how exudation of plant metabolites is affected by soil moisture availability (Ruehr et al., 2012). Furthermore, whether root exudation is regulated by below-ground processes at the root level or by above-ground parts of the plant, would inform on the degree of connection between different organs of the plant. ...
Numerous compounds are exuded by roots that play a central role in microbial decomposition and stabilization of soil carbon. The release of root exudates is sensitive to drought, but it is unclear how compound-specific exudates are related to drought-induced changes in plant metabolism. We investigated drought effects on root exudate quality and quantity for sunflower (Helianthus annuus) and soybean (Glycine max). We analyzed metabolites in phloem and root biomass extracts, to investigate whether root exudation is controlled by above- or below-ground processes. Sunflower and soybean showed different drought responses. Sunflower increased rates of exudation after rewetting (+330% in C) but the composition of metabolites remained unchanged compared to the control (constant moisture). Soybean did not change rates but the composition of metabolites changed with increased concentrations of osmolites (proline and pinitol). For specific groups, positive relationships were observed between exudates and phloem (amino-acids and organic acids) and between exudates and root biomass (sugars). Our results indicate that drought can induce different responses in plant metabolism causing changes in the quantity or composition of root exudates following rewetting. Furthermore, our results suggest that metabolism in shoots can influence exudation of organic acids and amino acids, while roots have a stronger control over exudation of sugars.
... At each windthrow, seven vegetation-free plots were created by repeatedly removing above-ground biomass (1 m 9 1 m) and at distances of 7 AE 1 m from the closest tree, thus making R a negligible (e.g. see Ruehr, Martin & Law (2012)). At HC a trenching approach was used to separate R h and R a (Subke, Inglima & Cotrufo 2006); three trenches were dug down to either bedrock or a maximum depth of 80 cm, each encompassing an area of 1Á5 m 9 2 m. ...
1. Intensifying forest disturbance regimes are likely to impact heavily on future carbon (C) budgets of forest ecosystems. Our understanding of how forest disturbance affects the sources of soil CO2 efflux (Fs) is however poor. This may lead to uncertainties over future C sink estimates of forest ecosystems and associated feedbacks to the atmosphere.
2. We investigated the impact of forest windthrow on the heterotrophic and autotrophic sources of Fs, underlying biotic and abiotic drivers (i.e. plant community composition, soil organic matter (SOM) properties and soil microclimate), and consequences for soil organic carbon (SOC) stocks in situ along a disturbance chronosequence in the European Alps. This chronosequence facilitated the study of temporal changes in the above parameters between the third and sixth years after windthrow.
3. Along the chronosequence, structural equation modelling revealed that soil temperature, soil moisture, SOM properties, and plant community composition explained 90% of the variation in Fs. While no direct interactions among plants and SOM properties could be determined, plants significantly affected soil microclimate. Windthrow had no obvious effect on Fs because reduced autotrophic soil respiration (Ra) was offset by a ~60% increase in heterotrophic soil respiration (Rh), principally due to increased soil temperatures. Ra after windthrow was dominated by grasses and herbs rather than trees; however, a high abundance of ectomycorrhizal fungi suggests an important indirect tree contribution to post-windthrow Ra. SOC stocks significantly declined over the post-windthrow period.
4. Our results show that Rh was by far the dominant source of Fs after forest windthrow. Since C losses from Rh and SOC stocks were in the same order of magnitude, this study demonstrates that post-windthrow declines in SOC stocks were mainly driven by a temperature-related increase in Rh.
http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12805/abstract
... For instance, Figure 3 shows a root exclusion plot, where a trench was dug around the plot and lined with a nonpermeable barrier to prevent root ingrowth, and plants growing within the plot were clipped . A similar effect can be achieved at a smaller spatial scale by pounding a section of large-diameter nonpermeable pipe into the ground to prevent root ingrowth (Heinemeyer et al., 2011;Ruehr et al., 2012). Typically, trenched plots extend 30-100 cm into the ground and therefore do not exclude deep roots, but they exclude the majority of roots, which are typically concentrated near the soil surface. ...
Soil respiration, which is the CO2 produced by the biological activity of soil organisms, is a major flux within the global carbon cycle, emitting about 10 times more CO2 to the atmosphere annually than fossil fuel combustion (0080, 0390 and 0590). Soil respiration is typically measured over relatively small areas (< 1 m2) using surface chambers or buried CO2 sensors. Owing to the limited diffusivity of soil, CO2 is generally much more concentrated in soil than in the atmosphere. CO2 is transported to the soil surface primarily via molecular diffusion, following concentration gradients. Transient atmospheric events such as wind gusts and precipitation can impact CO2 transport in soil, causing deviations between CO2 flux rates and rates of CO2 production by soil organisms. Over long timescales (seasonal to interannual) and through space, variability in soil fluxes generally correlates with factors that influence plant growth and microbial activity, including soil moisture, temperature, and the availability of nutrients and carbon substrates. The large magnitude of soil respiration, and the fact that it is often observed to increase exponentially with temperature, has led to predictions that warming-related increases in soil CO2 emissions could cause an acceleration of climate change (0145 and 9010). The majority of soil respiration research therefore seeks to understand how soil respiration responds to climate and land-use, and to incorporate these feedbacks into global climate models.
... For example, soil moisture limitations tended to be higher in shortstatured ecosystems than in forests (Fig. 3a), which may highlight the importance of plant reliance on stored water or deep rooting systems in taller ecosystems 6 . Plant water use strategy (for example, isohydric or anisohydric) has also been identified as a factor determining the sensitivity of stomatal conductance to VPD 1,27,28 . Accounting for all these sources of variability was outside the scope of this particular study, but should motivate future research. ...
Soil moisture supply and atmospheric demand for water independently limit - and profoundly affect - vegetation productivity and water use during periods of hydrologic stress. Disentangling the impact of these two drivers on ecosystem carbon and water cycling is difficult because they are often correlated, and experimental tools for manipulating atmospheric demand in the field are lacking. Consequently, the role of atmospheric demand is often not adequately factored into experiments or represented in models. Here we show that atmospheric demand limits surface conductance and evapotranspiration to a greater extent than soil moisture in many biomes, including mesic forests that are of particular importance to the terrestrial carbon sink. Further, using projections from ten general circulation models, we show that climate change will increase the importance of atmospheric constraints to carbon and water fluxes in all ecosystems. Consequently, atmospheric demand will become increasingly important for vegetation function, accounting for >70% of growing season limitation to surface conductance in mesic temperate forests. Our results suggest that failure to consider the limiting role of atmospheric demand in experimental designs, simulation models and land management strategies will lead to incorrect projections of ecosystem responses to future climate conditions.
... On the global scale, GPP changes by 4.7 % 0.01 −1 and NPP by 5.5 % 0.01 −1 in response to soil wetness. Although experiments also show rapid reductions in carbon assimilation due to drought stress (e.g., Ruehr et al., 2012;Xia et al., 2014), the magnitude of such influence is difficult to evaluate because different metrics and depths of soil water are used in measurements. Isoprene emissions from PS_BVOC show similar soil-wetness responses to that of GPP (Fig. S10c), indicating that drought reduces BVOC emissions. ...
The terrestrial biosphere has experienced dramatic changes in recent decades.
Estimates of historical trends in land carbon fluxes remain uncertain because
long-term observations are limited on the global scale. Here, we use the Yale
Interactive terrestrial Biosphere (YIBs) model to estimate decadal trends in
land carbon fluxes and emissions of biogenic volatile organic compounds
(BVOCs) and to identify the key drivers for these changes during 1982–2011.
Driven by hourly meteorology from WFDEI (WATCH forcing data methodology
applied to ERA-Interim data), the model simulates an increasing trend of
297 Tg C a−2 in gross primary productivity (GPP) and 185 Tg
C a−2 in the net primary productivity (NPP). CO2 fertilization is
the main driver for the flux changes in forest ecosystems, while meteorology
dominates the changes in grasslands and shrublands. Warming boosts summer GPP
and NPP at high latitudes, while drought dampens carbon uptake in tropical
regions. North of 30° N, increasing temperatures induce a
substantial extension of 0.22 day a−1 for the growing season; however,
this phenological change alone does not promote regional carbon uptake and
BVOC emissions. Nevertheless, increases of leaf area index at peak season
accounts for ~ 25 % of the trends in GPP and isoprene emissions at
the northern lands. The net land sink shows statistically insignificant
increases of only 3 Tg C a−2 globally because of simultaneous
increases in soil respiration. Global BVOC emissions are calculated using two
schemes. With the photosynthesis-dependent scheme, the model predicts
increases of 0.4 Tg C a−2 in isoprene emissions, which are mainly
attributed to warming trends because CO2 fertilization and inhibition
effects offset each other. Using the MEGAN (Model of Emissions of Gases and
Aerosols from Nature) scheme, the YIBs model simulates global reductions of
1.1 Tg C a−2 in isoprene and 0.04 Tg C a−2 in monoterpene
emissions in response to the CO2 inhibition effects. Land use change
shows limited impacts on global carbon fluxes and BVOC emissions, but there
are regional contrasting impacts over Europe (afforestation) and China
(deforestation).
Atmospheric CO2 flux inversions require as input an estimate of spatial and temporal correlations of errors in their estimate of the prior mean. Some previous studies have used the differences in CO2 daily average flux estimates produced by terrestrial carbon cycle models and eddy covariance measurements to constrain the flux error correlations. Since inversions are starting to resolve the daily cycle, we set out to examine the correlations at sub‐daily time scales, as well as the correlations across years. To this end, we examine the autocorrelations in the difference between net ecosystem‐atmosphere exchange measurements from 75 AmeriFlux towers and temporally downscaled high‐spatial‐resolution flux estimates from the Carnegie‐Ames‐Stanford Approach (CASA) terrestrial carbon cycle model. We find that the daily cycle is prominent in these hourly autocorrelations and that these autocorrelations persist across years. We propose a family of functions to model these temporal correlations in atmospheric inversions, and use cross validation to determine which of the correlation functions best fits autocorrelation data from towers not in the training set. Correlation functions with a component that attempts to model the daily cycle in the differences match correlations from other towers better than those without. Those models that reproduce the same correlation structures at 1‐year intervals while modulating the amplitudes of the correlations between those intervals improve the fit still further.
Vegetation in mountainous areas contributes about 36% to the global gross primary productivity (GPP). However, the influences of topography on radiation and water redistributions in mountain ecosystems are so far ignored in existing global GPP data sets. Here, an eco‐hydrological model was adopted to simulate 30 m resolution mountain and flat GPP over 16 watersheds. Then, a topographical correction index (TCI) was developed based on simulated soil water redistribution (TCIwater), radiation redistribution (TCIrad), and redistribution of climate factors (TCIclim). Finally, the proposed TCI was applied to four GPP data sets. The mean‐bias‐error (MBE), determination coefficient (R²), and Root‐Mean‐Square‐Error (RMSE) between mountain GPP and flat GPP (or GPP data sets) were used for evaluation. Results showed that the MBE of flat GPP before correction (194 g C m⁻² yr⁻¹) was reduced to 126, 94, and 2 g C m⁻² yr⁻¹ after the corrections of TCIwater, TCIrad, and TCIclim, highlighting the effectiveness of integrated redistribution information in correcting the topographical effect on GPP estimation. The relationship between mountain and flat GPP after the TCI correction was improved at the 30 m resolution (increasing R² by 0.09 and reducing RMSE by 90 g C m⁻² yr⁻¹) and 480 m resolution (increasing R² by 0.13 and reducing RMSE by 178 g C m⁻² yr⁻¹). Regarding the four GPP data sets after the TCI correction, the MBE of 183 g C m⁻² yr⁻¹ was averagely reduced to 17 g C m⁻² yr⁻¹, and RMSE was reduced by 118 g C m⁻² yr⁻¹ at 480 m resolution. This study suggests that integrating topography‐induced interactions into current GPP data sets is a feasible way to understand the carbon budget in mountain ecosystems.
Solar-induced chlorophyll fluorescence (SIF) is a rapidly advancing front in modeling global terrestrial gross primary production (GPP). Canopy total SIF emissions (SIF total) are mechanistically linked to the plant photo-synthesis, and can be estimated from satellite observed SIF (SIF obs) through radiative transfer modeling. However , the current satellite SIF obs and thus SIF total are available only at coarse spatial resolutions from several kilometers to tens of kilometers, inhibiting the application at fine spatial scales. Here, we proposed an algorithm to generate both global high-resolution SIF total (HSIF total) and high-resolution SIF obs (HSIF obs) at 1 km from low-resolution SIF obs (LSIF obs) from the TROPOspheric Monitoring Instrument (TROPOMI), which has a spatial resolution at nadir of 3.5 km by 5.6-7 km. Our statistical method is based on the law of energy conservation and uses satellite derived fraction of absorbed photosynthetically active radiation, fluorescence efficiency, and the escape probability of fluorescence. We evaluated the accuracy of our HSIF total using the Orbiting Carbon Observatory-2 SIF (R 2 = 0.78). We found that the spatial resolution had clear effects on the relationship between HSIF total and GPP. We also compared HSIF total to 8-day averaged tower GPP from 135 flux sites and found that they were better correlated when HSIF total was averaged over a 1-km radius around the tower than when averaged over a larger radius. Our study provided a unique high-resolution HSIF total product, which will advance the estimation of GPP by extrapolating site-level relationships to the global scale.
Global warming intensifies the hydrological cycle, which results in changes in precipitation regime (frequency and amount), will likely have significant impacts on soil respiration (Rs). Although the responses of Rs to changes in precipitation amount has been extensively studied, there is little consensus on how Rs will be affected by changes in precipitation frequency (PF) across the globe. Here, we synthesized the field observations from 296 published papers to quantify the effects of PF on Rs and its components using meta‐analysis. Our results indicated that the effects of PF on Rs decreased with an increase in background mean annual precipitation. When the data were grouped by climate conditions, increased PF showed positive effects on Rs under the arid condition but not under the semi‐humid or humid conditions, whereas decreased PF suppressed Rs across all the climate conditions. The positive effects of increased PF mainly resulted from the positive response of heterotrophic respiration under the arid condition while the negative effects of decreased PF were mainly attributed to the reductions in root biomass and respiration. Overall, our global synthesis provided for the first time a comprehensive analysis of the divergent effects of PF on Rs and its components across climate regions. This study also provided a framework for understanding and modelling responses of ecosystem carbon cycling to global precipitation change.
Accurate estimation of gross primary productivity (GPP) is essential for understanding the terrestrial carbon budget. Current large‐scale GPP estimates are often obtained at coarse resolutions without considering the subpixel heterogeneity, leading to scaling errors in results. Here, to further characterize (a) the critical sub‐upscaling process causing the largest error and (b) the contributions of various heterogeneity factors in causing the scaling errors, a hydrology‐vegetation model was used to estimate GPP at the 30 m resolution (assumed as reality), and other coarser resolutions (60, 120, 240, 480, and 960 m, assumed as approximations) for 16 mountainous watersheds. Then, GPP scaling errors in the upscaling process of surface heterogeneity were investigated by the root mean squared error between the reality and approximations. Results showed that any surface heterogeneity aggregation from fine to coarse resolutions (e.g., 30–960 m) could cause GPP scaling errors (133 ± 40 gCm⁻²yr⁻¹), and the aggregation from medium to coarse resolutions (e.g., 240–960 m) may be the largest source. More specifically, GPP scaling errors caused by the vegetation heterogeneity aggregation from fine to medium resolutions were relatively small, and the GPP errors caused by the surface topography aggregation from fine to coarse resolutions were all non‐negligible. Elevation aggregation caused larger GPP scaling error than the aggregations of land cover, leaf area index, slope, and aspect. This work highlights the need to consider surface heterogeneity (especially the elevation information) when modeling mountain vegetation GPP at coarse resolutions.
Accurate estimation of evapotranspiration (ET) and the partitioning of ET into transpiration (T r), soil evaporation (E s) and interception (E i) is critical to understand water cycle and land-atmosphere feedback. In this study, we evaluated the performances of two remote sensing-based ET models at multiple scales, and analyzed the uncertainties in ET partitioning due to the model structures. These two models were Simple Terrestrial Hydros-phere Model (SiTH) developed by our team and the Global Land Evaporation Amsterdam Model (GLEAM). As far as ET were concerned, the two models exhibited relatively good performances at different scales. However, it was found that GLEAM performed relatively poor at evergreen broadleaf forest (R 2 = 0.34; RMSE = 0.87 mm day −1 ; NSE = −0.28). In addition, the seasonal pattern of simulated ET by GLEAM at the tropical rainforest was not consistent with the observations. Furthermore, great discrepancies in ET partitioning were observed between the two models. Generally, GLEAM tended to underestimate E s (slope = 0.02; R 2 = 0.004), and overestimate T r (slope = 1.51; R 2 = 0.78) compared to the observations. The underestima-tions of E s by GLEAM may partly be due to the ignorance of soil evaporation under vegetation canopy. On the contrary, SiTH displayed relatively good performances in estimations of E s (slope = 0.76; R 2 = 0.62) and T r (slope = 0.98; R 2 = 0.51). However, both of the two models failed to properly simulate E i , although GLEAM (slope = 0.55; R 2 = 0.83) performed slightly better than SiTH (slope = 0.40; R 2 = 0.95). Global multi-year average ratios of T r , E s , and E i to ET for GLEAM and SiTH were 0.76, 0.09, 0.15 and 0.67, 0.25, 0.08 respectively. In future studies, it is important to investigate direct observations on different components of ET, especially on the interception, to improve our understanding on the ET processes.
Understanding the dependencies of the terrestrial carbon and water cycle with meteorological conditions is a prerequisite to anticipate their behaviour under climate change conditions. However, terrestrial ecosystems and the atmosphere interact via a multitude of variables across temporal and spatial scales. Additionally these interactions might differ among vegetation types or climatic regions. Today, novel algorithms aim to disentangle the causal structure behind such interactions from empirical data. The estimated causal structures can be interpreted as networks, where nodes represent relevant meteorological variables or land-surface fluxes and the links represent the dependencies among them (possibly including time lags and link strength). Here we derived causal networks for different seasons at 119 eddy covariance flux tower observations in the FLUXNET network. We show that the networks of biosphere–atmosphere interactions are strongly shaped by meteorological conditions. For example, we find that temperate and high-latitude ecosystems during peak productivity exhibit biosphere–atmosphere interaction networks very similar to tropical forests. In times of anomalous conditions like droughts though, both ecosystems behave more like typical Mediterranean ecosystems during their dry season. Our results demonstrate that ecosystems from different climate zones or vegetation types have similar biosphere–atmosphere interactions if their meteorological conditions are similar. We anticipate our analysis to foster the use of network approaches, as they allow for a more comprehensive understanding of the state of ecosystem functioning. Long-term or even irreversible changes in network structure are rare and thus can be indicators of fundamental functional ecosystem shifts.
Land surface models intended for large‐scale applications are often executed at coarse resolutions, and the sub‐grid heterogeneity is usually ignored. Here, a spatial scaling algorithm that integrates the information of vegetation heterogeneity (land cover type and leaf area index) and surface topography (elevation, slope, relative azimuth (Raz) between the sun and the slope background, sky‐view factor, and topographic wetness index), was proposed to correct errors in gross primary productivity (GPP) estimates at a coarse spatial resolution. An eco‐hydrological model named BEPS‐TerrainLab was used to simulate GPP at 30 and 480 m resolutions for 16 mountainous watersheds selected globally. Results showed that an obvious improvement on GPP estimates at 480 m resolution was achieved after the correction in comparison with GPP modeled at 30 m resolution, with the determination coefficient increased by 0.38 and mean bias error reduced by 203gCm⁻² yr⁻¹. The combination of all the seven factors made the largest improvement for GPP estimation at 480 m resolution, suggesting that a larger improvement would be achieved when more factors of surface heterogeneity are considered. More specifically, our results indicated that five factors, including land cover type and leaf area index regarded as integrated outcomes of all the environmental conditions, Raz and sky‐view factor associated with radiation redistribution, and slope related to soil water redistribution, were especially important in the spatial scaling procedure. This study suggests that incorporating the information of surface heterogeneity into the spatial scaling algorithm is useful for improving coarse resolution GPP estimates over mountainous areas.
Practically all records of eddy‐covariance flux measurements are affected by gaps, caused by several reasons. In this work, we propose analog period (AP) methods for gap‐filling, and test them for filling gaps of latent heat flux at AmeriFlux sites. The essence of the methods is to look for periods in the record that bear a strong resemblance, in the variable to be filled, to the periods immediately before and after the gap. Similarity between periods is gauged by the coefficient of determination, and the search for similar periods and their ranking is straight forward. The methods are developed in a univariate version (that uses only the latent heat flux data series itself) and several multivariate ones, that incorporate sensible heat flux, ground heat flux, and net radiation data. For each set of independent variables used for gap‐filling, the methods are tested against an existing gap‐filling procedure with similar data requirements. Thus, the univariate version is tested against the mean diurnal variation method, and the multivariate versions are tested against corresponding simple and multiple linear regression techniques that use energy‐budget data, and in one case the evaporative fraction as well. In our tests, the proposed univariate version performs better than the mean diurnal variation method, and the multivariate versions perform better than simple/multiple linear regression methods. An often used available computer package, REddyProc, was also tested as a basis of comparison. In general, the proposed methods (in univariate and multivariate versions) and simple/multiple linear regressions performed better than REddyProc. For the datasets analysed, gap filling via the evaporative fraction method performed poorly.
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Understanding the dependencies of the terrestrial carbon and water cycle is a prerequisite to anticipate their be- haviour under climate change conditions. However, terrestrial ecosystems and the atmosphere interact via a multitude of vari- ables, time- and space scales. Additionally the interactions might differ among vegetation types or climatic regions. Today, novel algorithms aim to disentangle the causal structure behind such interaction from empirical data. Visualising the estimated structure in networks, the nodes represent relevant meteorological determinants and land-surface fluxes, and the links dependencies among them possibly including their lag and strength. Here we show that biosphere-atmosphere interactions are strongly shaped by meteorological conditions. For example, we find that temperate and high latitude ecosystems during peak productivity exhibit very similar biosphere-atmosphere interaction networks as tropical forests. In times of anomalous conditions like drought though, both ecosystems behave more like Mediterranean ecosystems during their dry season. Our results demonstrate that ecosystems from different climate or vegetation types have similar biosphere-atmosphere interactions if their meteorological conditions are similar. We anticipate our analysis to foster the use of network approaches as they allow a more comprehensive understanding of the state of ecosystem functioning. Long term or even irreversible changes in network structure are rare and thus can be indicators of fundamental functional ecosystem shifts.
Rain pulses followed by interstorm drying periods are the fundamental units of water input into ecosystems on subweekly time scales. It is essential to understand landscape-scale vegetation responses on these unit time scales as they may describe sensitivity of landscape water, carbon, and energy cycles to shifts in rainfall intensity and frequency, even if the average seasonal precipitation remains unchanged. Because pulse investigations are primarily carried out in drylands, little is known about the characteristics and extent of ecosystem plant pulse responses across the broader range of climates and biomes. Using satellite-based plant water content (from vegetation optical depth) and plant carbon uptake observations from eddy covariance towers across the continental United States climate gradient (dry to humid), we characterize large-scale plant carbon and water uptake responses to rain pulses during spring and summer months. We find that while all ecosystems in the study region show discernable plant water content and carbon flux responses to rain pulses, drier ecosystems exhibit more frequent and longer duration responses. Unlike mesic environments, drylands show significantly different responses under varying antecedent soil moisture and pulse magnitude conditions; the largest water and carbon uptakes follow large pulses on initially wet soils. We detect soil moisture thresholds primarily in drylands, which can partly explain dryland vegetation's different responses under dry and wet conditions. We conclude that vegetation responds to individual pulses of water availability across all climates and therefore a range of ecosystems are sensitive to rainfall distributions beyond simple seasonal precipitation totals.
Understanding the dependencies of the terrestrial carbon and water cycle is a prerequisite to anticipate their behaviour under climate change conditions. However, terrestrial ecosystems and the atmosphere interact via a multitude of variables, time- and space scales. Additionally the interactions might differ among vegetation types or climatic regions. Today, novel algorithms aim to disentangle the causal structure behind such interaction from empirical data. Visualising the estimated structure in networks, the nodes represent relevant meteorological determinants and land-surface fluxes, and the links dependencies among them possibly including their lag and strength. Here we show that biosphere–atmosphere interactions are strongly shaped by meteorological conditions. For example, we find that temperate and high latitude ecosystems during peak productivity exhibit very similar biosphere–atmosphere interaction networks as tropical forests. In times of anomalous conditions like drought though, both ecosystems behave more like Mediterranean ecosystems during their dry season. Our results demonstrate that ecosystems from different climate or vegetation types have similar biosphere–atmosphere interactions if their meteorological conditions are similar. We anticipate our analysis to foster the use of network approaches as they allow a more comprehensive understanding of the state of ecosystem functioning. Long term or even irreversible changes in network structure are rare and thus can be indicators of fundamental functional ecosystem shifts.
Evergreen conifer forests are the most prevalent land cover type in North America. Seasonal changes in the color of evergreen forest canopies have been documented with near‐surface remote sensing, but the physiological mechanisms underlying these changes, and the implications for photosynthetic uptake, have not been fully elucidated.
Here, we integrate on‐the‐ground phenological observations, leaf‐level physiological measurements, near surface hyperspectral remote sensing and digital camera imagery, tower‐based CO2 flux measurements, and a predictive model to simulate seasonal canopy color dynamics.
We show that seasonal changes in canopy color occur independently of new leaf production, but track changes in chlorophyll fluorescence, the photochemical reflectance index, and leaf pigmentation. We demonstrate that at winter‐dormant sites, seasonal changes in canopy color can be used to predict the onset of canopy‐level photosynthesis in spring, and its cessation in autumn. Finally, we parameterize a simple temperature‐based model to predict the seasonal cycle of canopy greenness, and we show that the model successfully simulates interannual variation in the timing of changes in canopy color.
These results provide mechanistic insight into the factors driving seasonal changes in evergreen canopy color and provide opportunities to monitor and model seasonal variation in photosynthetic activity using color‐based vegetation indices.
Plain Language Summary
The uptake of photosynthetic carbon by forests is strongly seasonal, which can be characterized by photosynthetic phenology, e.g., the start and end of the photosynthetically active season (SOS and EOS, respectively). Satellite vegetation indices (VIs) can detect photosynthesis in canopies either structurally or physiologically. Clarifying the convergence or divergence of the performance of structural and physiological VIs is therefore crucial. This study compared the capacity of three structural VIs and one physiological VI for estimating SOS and EOS. Their performances were jointly controlled by structural and physiological regulations of carbon uptake by plants. The structural and physiological controls for deciduous broadleaf forests (DBFs) were nearly synchronous during green‐up, and canopy structural changes were visible, so structural VIs were reliable for estimating SOS. Canopies, however, change slowly in evergreen needleleaf forests (ENFs) throughout the year and in DBFs during autumn, and the capacity to take up carbon is mainly limited by physiological stress, so physiological VIs outperformed structural VIs. Our study highlights the unique advantage of physiological VIs for estimating photosynthetic phenology. These findings constitute a step toward improving our understanding of the roles of the structural and physiological regulations of the dynamics of terrestrial carbon.
The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) was launched to the International Space Station on 29 June 2018 by the National Aeronautics and Space Administration (NASA). The primary science focus of ECOSTRESS is centered on evapotranspiration (ET), which is produced as Level‐3 (L3) latent heat flux (LE) data products. These data are generated from the Level‐2 land surface temperature and emissivity product (L2_LSTE), in conjunction with ancillary surface and atmospheric data. Here, we provide the first validation (Stage 1, preliminary) of the global ECOSTRESS clear‐sky ET product (L3_ET_PT‐JPL, Version 6.0) against LE measurements at 82 eddy covariance sites around the world. Overall, the ECOSTRESS ET product performs well against the site measurements (clear‐sky instantaneous/time of overpass: r² = 0.88; overall bias = 8%; normalized root‐mean‐square error, RMSE = 6%). ET uncertainty was generally consistent across climate zones, biome types, and times of day (ECOSTRESS samples the diurnal cycle), though temperate sites are overrepresented. The 70‐m‐high spatial resolution of ECOSTRESS improved correlations by 85%, and RMSE by 62%, relative to 1‐km pixels. This paper serves as a reference for the ECOSTRESS L3 ET accuracy and Stage 1 validation status for subsequent science that follows using these data.
The dynamics of biochemical processes in terrestrial ecosystems are tightly coupled to local meteorological conditions. Understanding these interactions is an essential prerequisite for predicting, e.g. the response of the terrestrial carbon cycle to climate change. However, many empirical studies in this field rely on correlative approaches and only very few studies apply causal discovery methods. Here we explore the potential for a recently proposed causal graph discovery algorithm to reconstruct the causal dependency structure underlying biosphere–atmosphere interactions. Using artificial time series with known dependencies that mimic real-world biosphere–atmosphere interactions we address the influence of non-stationarities, i.e. periodicity and heteroscedasticity, on the estimation of causal networks. We then investigate the interpretability of the method in two case studies. Firstly, we analyse three replicated eddy covariance datasets from a Mediterranean ecosystem. Secondly, we explore global Normalised Difference Vegetation Index time series (GIMMS 3g), along with gridded climate data to study large-scale climatic drivers of vegetation greenness. We compare the retrieved causal graphs to simple cross-correlation-based approaches to test whether causal graphs are considerably more informative. Overall, the results confirm the capacity of the causal discovery method to extract time-lagged linear dependencies under realistic settings. For example, we find a complete decoupling of the net ecosystem exchange from meteorological variability during summer in the Mediterranean ecosystem. However, cautious interpretations are needed, as the violation of the method's assumptions due to non-stationarities increases the likelihood to detect false links. Overall, estimating directed biosphere–atmosphere networks helps unravel complex multidirectional process interactions. Other than classical correlative approaches, our findings are constrained to a few meaningful sets of relations, which can be powerful insights for the evaluation of terrestrial ecosystem models.
The seasonal dynamic of gross primary productivity (GPP) has influences on the annual GPP (AGPP) of the terrestrial ecosystem. However, the spatiotemporal variation of the seasonal dynamic of GPP and its effects on spatial and temporal variations of AGPP are still poorly addressed. In this study, we developed a parameter, αGPP, defined as the ratio of mean daily GPP (GPPmean) to the maximum daily GPP (GPPmax) during the growing season, to analyze the seasonal dynamic of GPP based on Weibull function. The αGPP was a comprehensive parameter characterizing the shape, scale, and location of the seasonal dynamic curve of GPP. We calculated αGPP based on the data of GPP for 942 site-years from 115 flux sites in the Northern Hemisphere, and analyzed the spatiotemporal variation and influencing factors of the αGPP. We found that the αGPP of terrestrial ecosystems in the Northern Hemisphere ranged from 0.47 to 0.85, with an average of 0.62 ± 0.06. The αGPP varied significantly both among different climatic zones and different ecosystem types. The αGPP was stable on the interannual scale, while decreased as latitude increased, which was consistent across different ecosystem types. The spatial pattern of the seasonal dynamic of astronomical radiation was the dominating factor of the spatial pattern of αGPP, that was, the spatial pattern of the seasonal dynamic of astronomical radiation determined that of the seasonal dynamic of GPP by controlling that of seasonal dynamics of total radiation and temperature. In addition, we assessed the spatial variation of AGPP preliminarily based on αGPP and other seasonal dynamic parameters of GPP, indicating that the understanding of the spatiotemporal variation of αGPP could provide a new approach for studying the spatial and temporal variations of AGPP and estimating AGPP based on the seasonal dynamic of GPP.
Anthropogenic climate change is the most demanding challenge humanity has to face
in the ongoing 21st century and beyond. This dissertation delves deeper into enhancing
the knowledge on the major drivers of climate change and its mitigation. Thus, all four
articles focus on the macro-level analysis of countries over time, applying causal
inference. Specifically, the dissertation addresses the predictors of national carbon
dioxide (CO2) emissions (article 1), the controversial debate on carbon leakage from
developed to developing countries (article 2), the influence of social inequality on CO2
emissions (article 3), and the role of forests as climate solution as well as the drivers
of forest loss and its gain (article 4). Altogether, the results suggest that population
growth is a major driver of CO2 emissions and deforestation. Another key factor is
increasing wealth. However, the effect of economic growth is double-edged: On the
one hand, rising gross domestic product (GDP) almost proportionally boosts carbon
emissions so far. On the other hand, growth in GDP contributes to enhance forest
cover. Minor carbon-abating effects are observed for energy prices, technological
progress, and international environmental agreements. Designating and managing
protected areas drives forest gain. Furthermore, social inequality and international
trade are not substantially related to CO2 emissions. Particularly, there is no evidence
for carbon leakage from developed to developing countries. Given the challenge of
emissions abatement, natural climate solutions are promising for near-term and largescale
sequestration of carbon. As the fourth article highlights, dangerous climate
change could be prevented by doubling current forest cover.
Local meteorological conditions and biospheric activity are tightly coupled. Understanding these links is an essential prerequisite for predicting the Earth system under climate change conditions. However, many empirical studies on the interaction between the biosphere and the atmosphere are based on correlative approaches that are not able to deduce causal paths, and only very few studies apply causal discovery methods. Here, we use a recently proposed causal graph discovery algorithm, which aims to reconstruct the causal dependency structure underlying a set of time series. We explore the potential of this method to infer temporal dependencies in biosphere-atmosphere interactions. Specifically we address the following questions: How do periodicity and heteroscedasticity influence causal detection rates, i.e. the detection of existing and non-existing links? How consistent are results for noise-contaminated data? Do results exhibit an increased information content that justifies the use of this causal-inference method? We explore the first question using artificial time series with well known dependencies that mimic real-world biosphere-atmosphere interactions. The two remaining questions are addressed jointly in two case studies utilizing observational data. Firstly, we analyse three replicated eddy covariance datasets from a Mediterranean ecosystem at half hourly time resolution allowing us to understand the impact of measurement uncertainties. Secondly, we analyse global NDVI time series (GIMMS 3g) along with gridded climate data to study large-scale climatic drivers of vegetation greenness. Overall, the results confirm the capacity of the causal discovery method to extract time-lagged linear dependencies under realistic settings. The violation of the method's assumptions increases the likelihood to detect false links. Nevertheless, we consistently identify interaction patterns in observational data. Our findings suggest that estimating a directed biosphere-atmosphere network at the ecosystem level can offer novel possibilities to unravel complex multi-directional interactions. Other than classical correlative approaches, our findings are constrained to a few meaningful set of relations which can be powerful insights for the evaluation of terrestrial ecosystem models.
Forests are one of the most cost-effective ways to sequester carbon today. Here, I estimate the world’s land share under forests required to prevent dangerous climate change. For this, I combine newest longitudinal data of FLUXNET on forests’ net ecosystem exchange of carbon (NEE) from 78 forest sites (N = 607) with countries’ mean temperature and forest area. This straightforward approach indicates that the world’s forests sequester 8.3 GtCO2year⁻¹. For the 2 °C climate target, the current forest land share has to be doubled to 60.0% to sequester an additional 7.8 GtCO2year⁻¹, which demands less red meat consumption. This afforestation/reforestation (AR) challenge is achievable, as the estimated global biophysical potential of AR is 8.0 GtCO2year⁻¹ safeguarding food supply for 10 billion people. Climate-responsible countries have the highest AR potential. For effective climate policies, knowledge on the major drivers of forest area is crucial. Enhancing information here, I analyze forest land share data of 98 countries from 1990 to 2015 applying causal inference (N = 2494). The results highlight that population growth, industrialization, and increasing temperature reduce forest land share, while more protected forest and economic growth generally increase it. In all, this study confirms the potential of AR for climate change mitigation with a straightforward approach based on the direct measurement of NEE. This might provide a more valid picture given the shortcomings of indirect carbon stock-based inventories. The analysis identifies future regional hotspots for the AR potential and informs the need for fast and forceful action to prevent dangerous climate change.
List of sites used in this study.
DBF = Deciduous broadleaf forest, DNF = deciduous needleleaf forest, EBF = evergreen broadleaf forest, ENF = evergreen needleleaf forest, MF = mixed forest, WSA = woody savanna, and SAV = savanna.
(PDF)
We develop and evaluate a method to estimate O3
deposition and stomatal O3 uptake across networks of eddy
covariance flux tower sites where O3 concentrations and
O3 fluxes have not been measured. The method combines standard
micrometeorological flux measurements, which constrain O3
deposition velocity and stomatal conductance, with a gridded dataset of
observed surface O3 concentrations. Measurement errors are
propagated through all calculations to quantify O3 flux
uncertainties. We evaluate the method at three sites with O3 flux
measurements: Harvard Forest, Blodgett Forest, and Hyytiälä Forest.
The method reproduces 83 % or more of the variability in daily stomatal
uptake at these sites with modest mean bias (21 % or less). At least
95 % of daily average values agree with measurements within a factor of 2
and, according to the error analysis, the residual differences from measured
O3 fluxes are consistent with the uncertainty in the
underlying measurements.The product, called synthetic O3 flux or SynFlux, includes 43
FLUXNET sites in the United States and 60 sites in Europe, totaling 926 site
years of data. This dataset, which is now public, dramatically expands the
number and types of sites where O3 fluxes can be used for ecosystem
impact studies and evaluation of air quality and climate models. Across these
sites, the mean stomatal conductance and O3 deposition velocity is
0.03–1.0 cm s−1. The stomatal O3 flux during the growing
season (typically April–September) is 0.5–11.0 nmol
O3 m−2 s−1 with a mean of 4.5 nmol
O3 m−2 s−1 and the largest fluxes generally occur where
stomatal conductance is high, rather than where O3 concentrations
are high. The conductance differences across sites can be explained by
atmospheric humidity, soil moisture, vegetation type, irrigation, and land
management. These stomatal fluxes suggest that ambient O3 degrades
biomass production and CO2 sequestration by 20 %–24 % at
crop sites, 6 %–29 % at deciduous broadleaf forests, and
4 %–20 % at evergreen needleleaf forests in the United States and
Europe.
We develop and evaluate a method to estimate O3 deposition and stomatal O3 uptake across networks of eddy covariance flux tower sites where O3 concentrations and O3 fluxes have not been measured. The method combines standard micrometeorological flux measurements, which constrain O3 deposition velocity and stomatal conductance, with a gridded dataset of observed surface O3 concentrations. Measurement errors are propagated through all calculations to quantify O3 flux uncertainties. We evaluate the method at three sites with O3 flux measurements: Harvard Forest, Blodgett Forest, and Hyytiälä Forest. The method reproduces 83 % or more of the variability in daily stomatal uptake at these sites with modest mean bias (21 % or less). At least 95 % of daily average values agree with measurements within a factor of two and, according to the error analysis, the residual differences from measured fluxes are consistent with the uncertainty in the underlying measurements.
The product, called synthetic O3 flux or SynFlux, includes 43 FLUXNET sites in the United States and 60 sites in Europe, totaling 926 site-years of data. This dataset, which is now public, dramatically expands the number and types of sites where O3 fluxes can be used for ecosystem impact studies and evaluation of air quality and climate models. Across these sites, the mean stomatal conductance and O3 deposition velocity is 0.03–1.0 cm s−1. The stomatal O3 flux during the growing season (April–September) is 0.5–11.0 nmol m−2 s−1 with a mean of 4.5 nmol m−2 s−1 and the largest fluxes generally occur where stomatal conductance is high, rather than where O3 concentrations are high. The conductance differences across sites can be explained by atmospheric humidity, soil moisture, vegetation type, irrigation, and land management. These stomatal fluxes suggest that ambient O3 degrades biomass production and CO2 sequestration by 20–24 % at crop sites, 6–29 % at deciduous broadleaf forests, and 4–20 % at evergreen needleleaf forests in the United States and Europe.
Extreme climatic events, such as droughts and heat stress, induce anomalies in ecosystem–atmosphere CO2 fluxes, such as gross primary production (GPP) and ecosystem respiration (Reco), and, hence, can change the net ecosystem carbon balance. However, despite our increasing understanding of the underlying mechanisms, the magnitudes of the impacts of different types of extremes on GPP and Reco within and between ecosystems remain poorly predicted.
Here we aim to identify the major factors controlling the amplitude of extreme-event impacts on GPP, Reco, and the resulting net ecosystem production (NEP). We focus on the impacts of heat and drought and their combination. We identified hydrometeorological extreme events in consistently downscaled water availability and temperature measurements over a 30-year time period. We then used FLUXNET eddy covariance flux measurements to estimate the CO2 flux anomalies during these extreme events across dominant vegetation types and climate zones.
Overall, our results indicate that short-term heat extremes increased respiration more strongly than they downregulated GPP, resulting in a moderate reduction in the ecosystem's carbon sink potential. In the absence of heat stress, droughts tended to have smaller and similarly dampening effects on both GPP and Reco and, hence, often resulted in neutral NEP responses. The combination of drought and heat typically led to a strong decrease in GPP, whereas heat and drought impacts on respiration partially offset each other. Taken together, compound heat and drought events led to the strongest C sink reduction compared to any single-factor extreme. A key insight of this paper, however, is that duration matters most: for heat stress during droughts, the magnitude of impacts systematically increased with duration, whereas under heat stress without drought, the response of Reco over time turned from an initial increase to a downregulation after about 2 weeks. This confirms earlier theories that not only the magnitude but also the duration of an extreme event determines its impact.
Our study corroborates the results of several local site-level case studies but as a novelty generalizes these findings on the global scale. Specifically, we find that the different response functions of the two antipodal land–atmosphere fluxes GPP and Reco can also result in increasing NEP during certain extreme conditions. Apparently counterintuitive findings of this kind bear great potential for scrutinizing the mechanisms implemented in state-of-the-art terrestrial biosphere models and provide a benchmark for future model development and testing.
Several global gross primary production (GPP) and evapotranspiration (ET) remote sensing products exist, mainly provided by machine-learning (e.g. MPI-BGC) and semi-empirical (e.g. MODIS) approaches. Process-based approaches have the advantage of representing the atmosphere-vegetation-soil system and associated fluxes as an organic integration, but their sophistication results in a lack of high spatiotemporal resolution continuous products. Targeting this gap, we reported a new set of global 8-day composite 1-km resolution GPP and ET products from 2000 to 2015, using a simplified process-based model, the Breathing Earth System Simulator (BESS). BESS couples atmosphere and canopy radiative transfer, photosynthesis and evapotranspiration, and uses MODIS atmosphere and land data and other satellite data sources as inputs. We evaluated BESS products against FLUXNET observations at site scale (total of 113 sites, 742 site years), and against MPI-BGC products at global scale. At site scale, BESS 8-day products agreed with FLUXNET observations with R² = 0.67 and RMSE = 2.58 gC m− 2 d− 1 for GPP, and R² = 0.62 and RMSE = 0.78 mm d− 1 for ET, respectively, and they captured a majority of seasonal variability, about half of spatial variability, and a minority of interannual variability in FLUXNET observations. At global scale, BESS mean annual sum GPP and ET maps agreed with MPI-BGC products with R² = 0.93 and RMSE = 229 gC m− 2 y− 1 for GPP, and R² = 0.90 and RMSE = 118 mm y− 1 for ET, respectively. Over the period of 2001–2011, BESS quantified the mean global GPP and ET as 122 ± 25 PgC y− 1 and 65 × 10³ ± 11 × 10³ km³ y− 1, respectively, with a significant ascending GPP trend by 0.27 PgC y− 2 (p < 0.05), similar to MPI-BGC products as well. Overall, BESS GPP and ET estimates were comparable with FLUXNET observations and MPI-BGC products. The process-based BESS can serve as a set of independent GPP and ET products from official MODIS GPP and ET products.
The Global Land Evaporation Amsterdam Model (GLEAM) is a set of algorithms dedicated to the estimation of terrestrial evaporation and root-zone soil moisture from satellite data. Ever since its development in 2011, the model has been regularly revised aiming at the optimal incorporation of new satellite-observed geophysical variables, and improving the representation of physical processes. In this study, the next version of this model (v3) is presented. Key changes relative to the previous version include: (1) a revised formulation of the evaporative stress, (2) an optimized drainage algorithm, and (3) a new soil moisture data assimilation system. GLEAM v3 is used to produce three new data sets of terrestrial evaporation and root-zone soil moisture, including a 35-year data set spanning the period 1980–2014 (v3.0a, based on satellite-observed soil moisture, vegetation optical depth and snow water equivalents, reanalysis air temperature and radiation, and a multi-source precipitation product), and two fully satellite-based data sets. The latter two share most of their forcing, except for the vegetation optical depth and soil moisture products, which are based on observations from different passive and active C- and L-band microwave sensors (European Space Agency Climate Change Initiative data sets) for the first data set (v3.0b, spanning the period 2003–2015) and observations from the Soil Moisture and Ocean Salinity satellite in the second data set (v3.0c, spanning the period 2011–2015). These three data sets are described in detail, compared against analogous data sets generated using the previous version of GLEAM (v2), and validated against measurements from 64 eddy-covariance towers and 2338 soil moisture sensors across a broad range of ecosystems. Results indicate that the quality of the v3 soil moisture is consistently better than the one from v2: average correlations against in situ surface soil moisture measurements increase from 0.61 to 0.64 in case of the v3.0a data set and the representation of soil moisture in the second layer improves as well, with correlations increasing from 0.47 to 0.53. Similar improvements are observed for the two fully satellite-based data sets. Despite regional differences, the quality of the evaporation fluxes remains overall similar as the one obtained using the previous version of GLEAM, with average correlations against eddy-covariance measurements between 0.78 and 0.80 for the three different data sets. These global data sets of terrestrial evaporation and root-zone soil moisture are now openly available at http://GLEAM.eu and may be used for large-scale hydrological applications, climate studies and research on land-atmosphere feedbacks.
Precipitation patterns are expected to change in the Mediterranean region within the next decades, with projected decreases in total rainfall and increases in extreme events. We manipulated precipitation patterns in a Mediterranean woodland, dominated by Arbutus unedo L., to study the effects of changing precipitation regimes on above-ground net primary production (ANPP) and soil C dynamics, specifically plant-derived C input to soil and soil respiration (SR). Experimental plots were exposed to either a 20 % reduction of throughfall or to water addition targeted at maintaining soil water content above a minimum of 10 % v/v. Treatments were compared to control plots which received ambient precipitation. Enhanced soil moisture during summer months highly stimulated annual stem primary production, litter fall, SR and net annual plant-derived C input to soil which on average increased by 130 %, 26 %, 58 % and 220 %, respectively, as compared to the control. In contrast, the 20 % reduction in throughfall (equivalent to 10 % reduction in precipitation) did not significantly change soil moisture at the site, and therefore did not significantly affect ANPP or SR. We conclude that minor changes (around 10 % reduction) in precipitation amount are not likely to significantly affect ANPP or soil C dynamics in Mediterranean woodlands. However, if summer rain increases, C cycling will significantly accelerate but soil C stocks are not likely to be changed in the short-term. More studies involving modelling of long-term C dynamics are needed to predict if the estimated increases in soil C input under wet conditions is going to be sustained and if labile C is being substituted to stable C, with a negative effect on long-term soil C stocks.
This rigorous yet accessible text introduces the key physical and biochemical processes involved in plant interactions with the aerial environment. It is designed to make the more numerical aspects of the subject accessible to plant and environmental science students, and will also provide a valuable reference source to practitioners and researchers in the field. The third edition of this widely recognised text has been completely revised and updated to take account of key developments in the field. Approximately half of the references are new to this edition and relevant online resources are also incorporated for the first time. The recent proliferation of molecular and genetic research on plants is related to whole plant responses, showing how these new approaches can advance our understanding of the biophysical interactions between plants and the atmosphere. Remote sensing technologies and their applications in the study of plant function are also covered in greater detail.
Abstract El Niño–La Niña cycles strongly influence dry and wet seasons in the tropics and consequently nitrous oxide (N2O) emissions from tropical rainforest soils. We monitored whole-system and soil chamber N2O fluxes during 5-month-long droughts in the Biosphere 2 tropical forest to determine how rainfall changes N2O production. A consistent pattern of N2O flux changes during drought and subsequent wetting emerged from our experiments. Soil surface drying during the first days of drought, presumably increased gas transport out of the soil, which increased N2O fluxes. Subsequent drying caused an exponential decrease in whole-system (4.0±0.1% day−1) and soil chamber N2O flux (8.9±0.8% day−1; south chamber; and 13.7±1.1% day−1; north chamber), which was highly correlated with soil moisture content. Soil air N2O concentration ([N2O]) and flux measurements revealed that surface N2O production persisted during drought. The first rainfall after drought triggered a N2O pulse, which amounted to 25% of drought-associated reduction in N2O flux and 1.3±0.4% of annual N2O emissions. Physical displacement of soil air by water and soil chemistry changes during drought could not account for the observed N2O pulse. We contend that osmotic stress on the microbial biomass must have supplied the N source for pulse N2O, which was produced at the litter–soil interface. After the pulse, N2O fluxes were consistently 90% of predrought values. Nitrate change rate, nutrient, [N2O], and flux analyses suggested that nitrifiers dominated N2O production during the pulse and denitrifiers during wet conditions. N2O flux measurements in Biosphere 2, especially during the N2O pulse, demonstrate that large-scale integration methods, such as flux towers, are essential for improving ecosystem N2O flux estimates.
Precipitation patterns are expected to change in the Mediterranean region within the next decades, with projected decreases in total rainfall and increases in ex-treme events. We manipulated precipitation patterns in a Mediterranean woodland, dominated by Arbutus unedo L., to study the effects of changing precipitation regimes on above-ground net primary production (ANPP) and soil C dynam-ics, specifically plant-derived C input to soil and soil res-piration (SR). Experimental plots were exposed to either a 20 % reduction of throughfall or to water addition targeted at maintaining soil water content above a minimum of 10 % v/v. Treatments were compared to control plots which re-ceived ambient precipitation. Enhanced soil moisture during summer months highly stimulated annual stem primary pro-duction, litter fall, SR and net annual plant-derived C input to soil which on average increased by 130 %, 26 %, 58 % and 220 %, respectively, as compared to the control. In con-trast, the 20 % reduction in throughfall (equivalent to 10 % reduction in precipitation) did not significantly change soil moisture at the site, and therefore did not significantly af-fect ANPP or SR. We conclude that minor changes (around 10 % reduction) in precipitation amount are not likely to sig-nificantly affect ANPP or soil C dynamics in Mediterranean woodlands. However, if summer rain increases, C cycling will significantly accelerate but soil C stocks are not likely to be changed in the short-term. More studies involving mod-elling of long-term C dynamics are needed to predict if the estimated increases in soil C input under wet conditions is going to be sustained and if labile C is being substituted to stable C, with a negative effect on long-term soil C stocks.
A 6-year field experiment of drought manipulation was performed in a Mediterranean forest with the aim of determining the effects of the drought predicted by most climate and ecophysiological models on the C and N concentration, accumulation, and stoichiometry in plants and soil. Drought had different effects among dominant species. In Quercus ilex, it increased the C/P ratio in wood and roots, N concentrations in roots and litter, and N/P ratio in wood and roots and decreased the C concentration in roots, C/N ratio in roots and litter, and C/P ratio in litter. In Arbutus unedo, drought increased the N concentration in litter and decreased the N concentration in leaves, thus decreasing N leaf reabsorption. No significant changes in C and N concentrations were found in Phillyrea lat folia. Drought affected the P plant absorption capacity more than that of N (more mobile). There was a general decreasing trend of C and N accumulation in aboveground biomass, with this effect being significant in A. unedo, which accumulated 80% less C and lost 2 kg ha(-1) of N in aboveground biomass in drought plots in the period 1999-2005, whereas in this same period it accumulated 9 kg ha-1 of N in total aboveground biomass in control plots. Total soil N and soil organic C increased in droughted soils. The effects of drought on C/N/P stoichiometry and N uptake capacity were different among the three dominant plant species. Q. ilex and A. unedo were more sensitive to drought than P. latfolia. The increase in C/P and N/P ratios in Q. ilex and the decrease in N uptake in A. unedo might decrease their competitive capacity under drought by decreasing water use efficiency. These results altogether indicated slower C and N mineralization and lower N plant capture, N leaf reabsorption, and N accumulation in some dominant plant species in response to drought, thus reducing C and N soil turnover, increasing C and N accumulation in soil, and reducing C and N at the stand level.
In the last decade, our understanding of the processes underlying plant response to drought, at the molecular and whole-plant levels, has rapidly progressed. Here, we review that progress. We draw attention to the perception and signalling processes (chemical and hydraulic) of water deficits. Knowledge of these processes is essential for a holistic understanding of plant resistance to stress, which is needed to improve crop management and breeding techniques. Hundreds of genes that are induced under drought have been identified. A range of tools, from gene expression patterns to the use of transgenic plants, is being used to study the specific function of these genes and their role in plant acclimation or adaptation to water deficit. However, because plant responses to stress are complex, the functions of many of the genes are still unknown. Many of the traits that explain plant adaptation to drought — such as phenology, root size and depth, hydraulic conductivity and the storage of reserves — are associated with plant development and structure and are constitutive rather than stress induced. But a large part of plant resistance to drought is the ability to get rid of excess radiation, a concomitant stress under natural conditions. The nature of the mechanisms responsible for leaf photoprotection, especially those related to thermal dissipation, and oxidative stress are being actively researched. The new tools that operate at molecular, plant and ecosystem levels are revolutionising our understanding of plant response to drought, and our ability to monitor it. Techniques such as genome-wide tools, proteomics, stable isotopes and thermal or fluorescence imaging may allow the genotype–phenotype gap to be bridged, which is essential for faster progress in stress biology research.
The strength of coupling between canopy gas exchange and root respiration was examined in ~15-yr-old ponderosa pine (Pinus ponderosa Doug. Ex Laws.) growing under seasonally drought stressed conditions. By regularly watering part of the root system to reduce tree water stress and measuring soil CO2 efflux on the dry, distant side of the tree, we were able to determine the strength of the relationship between soil autotrophic (root and rhizosphere) respiration and changes in canopy carbon uptake and water loss by comparison with control trees (no watering). After ~40 days the soil CO2 efflux rate, relative to pre-treatment conditions, was twice that of the controls. This difference, attributable to root and rhizosphere respiration, was strongly correlated with differences in transpiration rates between treatments (r
2 = 0.73, p<0.01). By the end of the period, transpiration of the irrigated treatment was twice that of controls. Periodic measurements of photosynthesis under non-light limited conditions paralleled the patterns of transpiration and were systematically higher in the irrigated treatment. We observed no evidence for a greater sensitivity of soil autotrophic respiration to temperature compared to the response of heterotrophic respiration to temperature; the Q
10 for total soil respiration was 1.6 (p>0.99) for both treatments. At the ecosystem scale, daily soil CO2 efflux rate was linearly related to gross primary productivity (GPP) as measured by eddy-covariance technique (r
2 = 0.55, p<0.01), suggesting patterns of soil CO2 release appear strongly correlated to recent carbon assimilation in this young pine stand. Collectively the observed relationships suggest some consideration should be given to the inclusion of canopy processes in future models of soil respiration.
Leaf gas exchange, transpiration, water potential and xylem water flow measurements were used in order to investigate the daily water balance of intact, naturally growing, adult Larix and Picea trees without major injury. The total daily water use of the tree was very similar when measured as xylem water flow at breast height or at the trunk top below the shade branches, or as canopy transpiration by a porometer or gas exchange chamber at different crown positions. The average canopy transpiration is about 12% lower than the transpiration of a single twig in the sun crown of Larix and Picea. Despite the similarity in daily total water flows there are larger differences in the actual daily course. Transpiration started 2 to 3 h earlier than the xylem water flow and decreased at noon before the maximum xylem water flow was reached, and stopped in the evening 2 to 3 h earlier than the water flow though the stem. The daily course of the xylem water flow was very similar at the trunk base and top below the lowest branches with shade needles. The difference in water efflux from the crown via transpiration and the water influx from the trunk is caused by the use of stored water. The specific capacitance of the crown wood was estimated to be 4.7 x 10-8 and 6.3 x 10-8 kg kg-1 Pa-1 and the total amount of available water storage was 17.8 and 8.7 kg, which is 24% and 14% of the total daily transpiration in Larix and Picea respectively. Very little water was used from the main tree trunk. With increasing transpiration and use of stored water from wood in the crown, the water potential in the foliage decreases. Plant water status recovers with the decrease of transpiration and the refilling of the water storage sites. The liquid flow conductance in the trunk was 0.45 x 10-9 and 0.36 x 10-9 mol m-2s-1 Pa-1 in Larix and Picea respectively. The role of stomata and their control by environmental and internal plant factors is discussed.
Throughout a 13year period, the Throughfall Displacement Experiment sustained both increased (+33; wet) and decreased (−33%;
dry) throughfall into an upland oak forest in Tennessee. Organic (O) horizon carbon (C) stocks were measured at several occasions
before, during and after the experiment and mineral soil C stocks before and after the experiment. In the O horizon, higher
C stocks were observed in the dry treatment compared to the ambient and wet, attributable to a combination of enhanced litter
inputs and reduced decomposition. No precipitation treatment effects on mineral soil C stocks were found to a depth of 60cm.
Conversely, long-term reductions in surface mineral soil C stocks were surprisingly high for all treatments (3.5–2.7% C in
the 0–15cm layer and from 0.6 to 0.5% in the 15–30cm layer) over the duration of the experiment. A clear explanation for
this temporal trend in C storage was not readily apparent.
We reviewed the literature to examine the vulnerability to water stress-induced embolism of Pinaceae relative to other conifers and to study the inter-relationships among the main traits involved in the hydraulic function within the Pinaceae. Results showed that Pinaceae (particularly the genus Pinus) are more vulnerable to xylem embolism, and show less variability in this character, than other conifers. Detailed data from 12 populations of Pinaceae (11 species) from three different areas (Piñol and Sala 2000; Martínez-Vilalta and Piñol 2002; Oliveras et al. 2003) was used to study the relationships among hydraulic properties of stems. These included: leaf-to-wood area ratio (AL:A W), wood- and leaf-specific hydraulic conductivity (KW and KL, respectively), vulnerability to xylem embolism (Ψ50PLC), carbon isotope composition of needles (δ13C) and minimum needle water potential (minimum ΨL). Results showed that hydraulic properties tended to be more correlated among each other than with indicators of environmental (precipitation to potential evapotranspiration ratio, P/E) or physiological water stress (minimum ΨL). The only exception was an increase of δ13C with decreasing minimum ΨL and P/E. Overall, AL:A W ratio decreased with increasing vulnerability to xylem embolism, and with increasing KW and KL (P
Respiration has been proposed to be the main determinant of the carbon balance in European forests and is thus essential for our understanding of the carbon cycle. However, the choice of experimental design strongly affects estimates of annual respiration and of the contribution of soil respiration to total ecosystem respiration. In a detailed study of ecosystem and soil respiration fluxes in an old unmanaged deciduous forest in Central Germany over 3 years (2000–2002), we combined soil chamber and eddy covariance measurements to obtain a comprehensive picture of respiration in this forest. The closed portable chambers offered to investigate spatial variability of soil respiration and its controls while the eddy covariance system offered continuous measurements of ecosystem respiration. Over the year, both fluxes were mainly correlated with temperature. However, when soil moisture sank below 23 vol.% in the upper 6 cm, water limitations also became apparent. The temporal resolution of the eddy covariance system revealed that relatively high respiration rates occurred during budbreak due to increased metabolic activity and after leaf fall because of increased decomposition. Spatial variability in soil respiration rates was large and correlated with fine root biomass (r
2 = 0.56) resulting in estimates of annual efflux varying across plots from 730 to 1,258 (mean 898) g C m−2 year−1. Power function calculations showed that achieving a precision in the soil respiration estimate of 20% of the full population mean at a confidence level of 95%, requires about eight sampling locations. Our results can be used as guidelines to improve the representativeness of soil respiration measurements by nested sampling designs, being applied in long-term and large-scale carbon sequestration projects such as FLUXNET and CarboEurope.
Drought stress plays an important role in determining both the structure and function of forest ecosystems, because of the close association between the carbon (C) and hydrological cycles. We used a detailed model of the soil-plant-atmosphere continuum to investigate the links between carbon uptake and the hydrological cycle in a mature, open stand of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) at the Metolius river in eastern Oregon over a 2-year period (1996-1997). The model was parameterized from local measurements of vegetation structure, soil properties and meteorology, and tested against independent measurements of ecosystem latent energy (LE) and carbon fluxes and soil water content. Although the 2 years had very different precipitation regimes, annual uptake of C and total transpiration were similar in both years, according to both direct observation and simulations. There were important differences in ratios of evaporation to transpiration, and in the patterns of water abstraction from the soil profile, depending on the frequency of summer storms. Simulations showed that, during periods of maximum water limitation in late summer, plants maintained a remarkably constant evapotranspirative flux because of deep rooting, whereas changes in rates of C accumulation were determined by interactions between atmospheric vapor pressure deficit and stomatal conductance. Sensitivity analyses with the model suggest a highly conservative allocation strategy in the vegetation, focused belowground on accessing a soil volume large enough to buffer summer droughts, and optimized to account for interannual variability in precipitation. The model suggests that increased allocation to leaf area would greatly increase productivity, but with the associated risk of greater soil water depletion and drought stress in some years. By constructing sparse canopies and deep rooting systems, these stands balance reduced productivity in the short term with risk avoidance over the long term.
Five years of eddy-covariance and other measurements at a mature ponderosa pine forest and a nearby young plantation are used to contrast the carbon fluxes for long-term averages, seasonal patterns, diel patterns and interannual variability, and to examine the differing responses to water-stress. The mature forest with larger leaf area and wetter and cooler soils has a net uptake of carbon 3.3 times that of the young plantation. In the spring, photosynthesis is larger at the mature site as expected based on the difference in leaf area, however, another important factor is the reduction in springtime respiration at the mature site due to lower soil temperatures because of more shade from the canopy. Patterns of photosynthesis, inherent water-use efficiency (IWUE) and tree transpiration indicate that the young plantation responds to the seasonal drought sooner and to a more severe degree. Lower sensitivity to seasonal drought at the mature site is likely due to higher soil moisture reserves year round and a deeper root system that can access more water. Outside the seasonal drought period, the IWUE is the same at both sites, suggesting a species-specific value. Larger interannual variability at the plantation is associated with water-year drought and aggrading. Keywords: carbon fluxes; net ecosystem exchange of carbon; eddy-covariance; water-stress; net ecosystem production; water-use efficiency (Published: 13 February 2012) Citation: TellusB 2012, 64 , 17159, DOI: 10.3402/tellusb.v64i0.17159
Responses of stomatal conductance (gs) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical func- tions. However, the magnitude of the decrease - the stom- atal sensitivity - varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of gs to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of gs at low D (£ 1k Pa). To test this relationship we used the function gs = gsref - m ·l nD where m is the stomatal sensitivity and gsref = gs at D = 1 kPa. Regardless of species or methodology, m was highly correlated with gsref (average r 2 = 0·75) with a slope of approximately 0·6. We demonstrate that this empirical slope is consistent with the theoretical slope derived from a simple hydraulic model that assumes stomatal regulation of leaf water potential. The theoretical slope is robust to deviations from underlying assumptions and variation in model parameters. The relationships within and among species are close to theoretical predictions, regardless of whether the analysis is based on porometric measurements of gs in relation to leaf-surface D (Ds), or on sap flux-based stomatal conductance of whole trees (GSi), or stand-level stomatal conductance (GS) in relation to D. Thus, individ- uals, species, and stands with high stomatal conductance at low D show a greater sensitivity to D, as required by the role of stomata in regulating leaf water potential.
To partition evapotranspiration between canopy and subcanopy components in a 12-m-tall Pinus taeda forest and to assess certain aspects of environmental regulation of canopy transpiration, we quantified water flux in a forest using three approaches. 1) measuring water flux in xylem of trees, and scaling to stand transpiration of canopy trues (E-C); 2) measuring soil water content and saturated conductivity, and modeling drainage to estimate total evapotranspiration (E-T) during rainless days based on a local water balance (LWB); and 3) using an eddy correlation approach to estimate total E-T. We calculated variances for each estimate, and proposed an approach to test for differences between estimates of E-C and E-T. Diurnal 'patterns' in water uptake were similar using direct measurements in stem xylem and LWB. However, LWB was found to be inappropriate for estimating 'absolute' E-T diurnally when changes in soil moisture between consecutive measurements were small. Eddy correlation estimates of E-T are of a higher temporal resolution than xylem flux measurements made in branches. Diurnal flux patterns in branches are more similar to the pattern generated by eddy correlation than those in stems. However, differences between the patterns indicate that patchiness in branch transpiration may preclude using branch xylem flux measurements to estimate canopy conductance. In one stand, daily E-C accounted for ca 70 % of total E-T estimated by either LWB tin a separate study) or the eddy correlation approach; the difference between E-T and E-C was significant based on variances calculated to account for spatial variation in each. Regardless of the vapor pressure deficit, E-C decreased linearly with soil moisture from 2.5 to 1.5 mm d(-1) over a 9-d drying cycle, as soil moisture in the rooting zone (ca 0.35 m depth) declined by 23 mm. ((C) Inra/Elsevier, Paris.).
Pioneering work in the last century has resulted in a widely accepted
paradigm that primary production is strongly positively related to
temperature and water availability such that the northern hemispheric
forest carbon sink may increase under conditions of global warming.
However, the terrestrial carbon sink at the ecosystem level (i.e. net
ecosystem productivity, NEP) depends on the net balance between gross
primary productivity (GPP) and ecosystem respiration (TER). Through an
analysis of European eddy covariance flux data sets, we find that the
common climate relationships for primary production do not hold for NEP.
This is explained by the fact that decreases in GPP are largely
compensated by parallel decreases in TER when climatic factors become
more limiting. Moreover, we found overall that water availability was a
significant modulator of NEP, while the multivariate effect of mean
annual temperature is small and not significant. These results indicate
that climate- and particularly temperature-based projections of net
carbon balance may be misleading. Future research should focus on
interactions between the water and carbon cycles and the effects of
disturbances on the carbon balance of terrestrial ecosystems.
Observations on the net carbon exchange of forests in the European Mediterranean region, measured recently by the eddy covariance
method, have revived interest in a phenomenon first characterized on agricultural and forest soils in East Africa in the 1950s
and 1960s by H. F. Birch and now often referred to as the “Birch effect.” When soils become dry during summer because of lack
of rain, as is common in regions with Mediterranean climate, or are dried in the laboratory in controlled conditions, and
are then rewetted by precipitation or irrigation, there is a burst of decomposition, mineralization and release of inorganic
nitrogen and CO2. In forests in Mediterranean climates in southern Europe, this effect has been observed with eddy covariance techniques and
soil respiration chambers at the stand and small plot scales, respectively. Following the early work of Birch, laboratory
incubations of soils at controlled temperatures and water contents have been used to characterize CO2 release following the rewetting of dry soils. A simple empirical model based on laboratory incubations demonstrates that
the amount of carbon mineralized over one year can be predicted from soil temperature and precipitation regime, provided that
carbon lost as CO2 is taken into account. We show that the amount of carbon returned to the atmosphere following soil rewetting can reduce significantly
the annual net carbon gain by Mediterranean forests.
Copyrighted by American Geophysical Union. We analyzed 7 years (2002–2008) of micrometeorological and concurrent biological observations of carbon and water fluxes at a mature ponderosa pine forest in central Oregon in a semiarid climate. We sought to evaluate the extent that gross primary productivity, net ecosystem exchange, ecosystem respiration, net primary productivity, net ecosystem productivity, tree transpiration, and evapotranspiration varied seasonally and interannually in this ecosystem subjected to varying periods and severity of droughts. To explain variation, we found it necessary to define seasons functionally within a hydroecological year rather than by fixed calendar dates. The interannual variability in growing season length was large (45 days), and the end date was more variable than the onset. Plant-available soil water was the main determinant of carbon fluxes. Spring evapotranspiration primarily used shallow water, whereas summer and fall evapotranspiration drew water from deeper in the soil profile. A multiyear drought (2001–2003) had a more severe and fundamentally different impact on carbon and water cycles than a single-year (2005) drought because of carryover effects in soil water and carbohydrate reserves in plant tissue. Calendar year–based analysis was inadequate to diagnose drought years in precipitation and ecosystem drought response. Extension of meteorological records back to 1982 showed that anomalies were coherent across the region and that the observations represented below-average precipitation and above-average temperatures coherent with a warm-phase Pacific Decadal Oscillation. The carbon sink of this seasonally water-limited ecosystem is anticipated to increase with increasing available soil water during the growing season.
Publisher Summary The study of leaf anatomy and of the mechanisms of the opening and closing of stomatal guard cells leads one to suppose that the stomata constitute the main or even the sole regulating system in leaf transpiration. Meteorologists have developed a wide variety of formulae for estimating evaporation from vegetation that are based entirely on weather variables and take no account at all of the species composition or stomatal properties of the transpiring vegetation. These “potential evaporation” formulae are widely and, to a large degree, successfully used for estimating evaporation from vegetation that is not water-stressed. Transpiration depends on stomatal conductance, net radiation receipt and upon air saturation deficit, temperature, and wind speed. Saturation deficit and wind speed vary through leaf boundary layers, through canopies, and through the atmosphere above the canopies. The sensitivity of saturation deficit to changes in stomatal conductance depends on where the saturation deficit is measured. If all of the stomata on a single leaf change aperture in unison, there may be a substantial change in saturation deficit measured at the leaf surface but a negligible change in saturation deficit measured a centimetre or two away, outside the leaf boundary layer.
PREFACE TO THE SECOND EDITION LIST OF SYMBOLS 1. SCOPE OF ENVIRONMENTAL PHYSICS 2. GAS LAWS Pressure, volume and temperature Specific heats Lapse rate Water and water vapour Other gases 3. TRANSPORT LAWS General transfer equation Molecular transfer processes Diffusion coefficients Radiation laws 4. RADIATION ENVIRONMENT Solar radiation Terrestrial radiation Net radiation 5. MICROCLIMATOLOGY OF RADIATION (i) Interception Direct solar radiation Diffuse radiation Radiation in crop canopies 6. MICROCLIMATOLOGY OF RADIATION (ii) Absorption and reflection Radiative properties of natural materials Net radiation 7. MOMENTUM TRANSFER Boundary layers Wind profiles and drag on uniform surfaces Lodging and windthrow 8. HEAT TRANSFER Convection Non-dimensional groups Measurements of convection Conduction Insulation of animals 9. MASS TRANSFER (i) Gases and water vapour Non-dimensional groups Measurement of mass transfer Ventilation Mass transfer through pores Coats and clothing 10.MASS TRANSFER (ii) Particles Steady motion 11.STEADY STATE HEAT BALANCE (i) Water surfaces and vegetation Heat balance equation Heat balance of thermometers Heat balance of surfaces Developments from the Penman Equation 12.STEADY STATE HEAT BALANCE (ii) Animals Heat balance components The thermo-neutral diagram Specification of the environment Case studies 13.TRANSIENT HEAT BALANCE Time constant General cases Heat flow in soil 14.CROP MICROMETEOROLOGY (i) Profiles and fluxes Profiles Profile equations and stability Measurement of flux above the canopy 15.CROP MICROMETEOROLOGY (ii) Interpretation of measurements Resistance analogues Case studies: Water vapour and transpiration Carbon dioxide and growth Sulphur dioxide and pollutant fluxes to crops Transport within canopies APPENDIX BIBLIOGRAPHY REFERENCES INDEX
Respiration rates of heterogeneous forest canopies arise from needles, stems, roots and soil microbes. To assess the temporal and spatial variation in respiration rates of these components in a heterogeneous ponderosa pine forest canopy, and the processes that control these fluxes, we conducted an intensive field study during the summer of 2000. We employed a combination of biological and micrometeorological measurements to assess carbon respiratory fluxes at the soil surface, within and above a 4-m-tall ponderosa pine forest. We also conducted manipulation studies to examine the carbon fluxes from the roots and heteorotrophs.Spatial variation in soil CO2 efflux was large, averaging 40% of the mean, which varied by nearly a factor of two between minima for bare soil to maxima beneath dense patches of understorey vegetation. The estimated vertical profile of respiration from chamber data, and the profile of nocturnal fluxes measured by the three eddy flux systems suggested that >70% of the ecosystem respiration was coming from below the 1.75-m measurement height of one of the flux systems, and 71% of photosynthetic carbon uptake in July was released by soil processes, thus there was a strong vertical gradient in respiration relatively close to the soil surface in this young forest. These results stress the importance of understanding spatial and temporal variation in soil processes when interpreting nocturnal eddy covariance data.
The measured net ecosystem exchange (NEE) of CO2 between the ecosystem and the atmosphere
reflects the balance between gross CO2 assimilation [gross primary production (GPP)]
and ecosystem respiration (Reco). For understanding the mechanistic responses of ecosystem
processes to environmental change it is important to separate these two flux components. Two
approaches are conventionally used: (1) respiration measurements made at night are extrapolated
to the daytime or (2) light–response curves are fit to daytime NEE measurements and
respiration is estimated from the intercept of the ordinate, which avoids the use of potentially
problematic nighttime data.We demonstrate that this approach is subject to biases if the effect
of vapor pressure deficit (VPD) modifying the light response is not included.We introduce an
algorithm for NEE partitioning that uses a hyperbolic light response curve fit to daytime NEE,
modified to account for the temperature sensitivity of respiration and the VPD limitation of
photosynthesis. Including the VPD dependency strongly improved the model’s ability to
reproduce the asymmetric diurnal cycle during periods with high VPD, and enhances the
reliability of Reco estimates given that the reduction of GPP by VPD may be otherwise
incorrectly attributed to higher Reco. Results from this improved algorithm are compared
against estimates based on the conventional nighttime approach. The comparison demonstrates that the uncertainty arising from systematic errors dominates the overall uncertainty of annual sums (median absolute deviation of GPP: 47 gCm�2 yr�1), while errors arising from the random error (median absolute deviation: � 2gCm�2 yr�1) are negligible. Despite sitespecific differences between the methods, overall patterns remain robust, adding confidence to statistical studies based on the FLUXNET database. In particular, we show that the strong correlation between GPP and Reco is not spurious but holds true when quasi-independent, i.e. daytime and nighttime based estimates are compared.
Measurements of leaf transpiration and calculations of leaf conductance to water vapor are important in almost all investigations of plant water relations. Transpiration is a primary determinant of leaf energy balance (Chapter 7) and plant water status (Chapter 9). Together with the exchange of CO2 it determines the water use efficiency. The close linkage between CO2 uptake and H2O via the stomatal pore has allowed for separation of stomatal and biochemical limitations to photosynthesis through calculation of intercellular CO2 concentrations. In this chapter we will cover the principles and instruments necessary for measurement of leaf transpiration and the calculation of leaf conductances to water vapor exchange. We will also consider the methodology and problems involved in determining whole-plant and canopy transpiration rates.
Root minirhizotron tubes were installed at two sites around three different age classes of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) to follow patterns of fine root (≤2 mm diameter) dynamics during a 4 year study. Both sites were old-growth forests until 1978, when one site was clear-cut and allowed to regenerate naturally. The other site had both intermediate-aged trees (50–60 years) and old-growth trees (>250 years old). Estimates of fine root standing crop were greatest around young trees and least around intermediate-aged trees. Root production was highly synchronized in all age classes, showing a single peak in late May – early June each year. Root production and mortality were proportional to standing root crop (biomass), suggesting that allocation to new root growth was proportional to root density regardless of tree age. The turnover index (mortality/maximum standing crop) varied from 0.62 to 0.89·year–1, indicating root life spans in excess of 1 year. It appears that young ponderosa pine stands have greater rates of fine root production than older stands but lose more fine roots each year through mortality. The results indicate that soil carbon may accumulate faster in younger than in older stands.
Net carbon flux partitioning was used to disentangle abiotic and biotic drivers of all important component fluxes influencing the overall sink strength of a Mediterranean ecosystem during a rapid spring to summer transition. Between May and June 2006 we analyzed how seasonal drought affected ecosystem assimilation and respiration fluxes in an evergreen oak woodland and attributed variations in the component fluxes (trees, understory, soil microorganisms and roots) to observations at the ecosystem scale. We observed a two thirds decrease in both ecosystem carbon assimilation and respiration (Reco) within only 15 days time. The impact of decreasing Reco on the ecosystem carbon balance was smaller than the impact of decreasing primary productivity. Flux partitioning of GPP and Reco into their component fluxes from trees, understory, soil microorganisms and roots showed that declining ecosystem sink strength was due to a large drought and temperature-induced decrease in understory carbon uptake (from 56% to 21%). Hence, the shallow-rooted annuals mainly composing the understory have a surprisingly large impact on the source/sink behavior of this open evergreen oak woodland during spring to summer transition and the timing of the onset of drought might have a large effect on the annual carbon budget. In response to seasonal drought Reco was increasingly dominated by respiration of heterotrophic soil microorganisms, while the root flux was found to be of minor importance. Soil respiration flux decreased with drought but its contribution to total daily CO2-exchange increased by 11.5%. This partitioning approach disentangled changes in respiratory and photosynthetic ecosystem fluxes that were not apparent from the eddy-covariance or the soil respiration data alone. By the novel combination of understory vs. overstory carbon flux partitioning with soil respiration data from trenched and control plots, we gained a detailed understanding of factors controlling net carbon exchange of Mediterranean ecosystems.
Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top-down approaches in conjunction with a mechanistic ecosystem gas-exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange. The three sites include two nearly monospecific Quercus ilex L. forests – one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) – and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO2 exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO2 fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman-Monteith equation. In a top-down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity (Q10) in response to drought. Soil temperature and soil water content explained 70–80% of the seasonal variability of ecosystem respiration. During the drought, light-saturated ecosystem gross carbon uptake and day-time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water-use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water-use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO2, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.
In arid ecosystems, as much as 75% of solar radiation that penetrates the atmosphere hits the surface of the soil. The combination of high irradiance, high temperature, and low moisture puts constraints on the activity and organization of microbial communities. To separate the direct effects of UV absorbance on litter decomposition from the indirect effects of microbial selection, we placed mixed cohorts of senescent piñon (Pinus edulis) and juniper (Juniperus monosperma) litter into triplicate microcosms assigned to four treatments: UV irradiated (0.6 mW/cm2 UV-A and UV-B for 12 h/day), with and without water additions, and non-irradiated with and without water additions. After 26 weeks, mass loss rates did not differ significantly among treatments with 90% organic matter remaining for piñon litter and 60% for juniper. The distribution and abundance of functional groups as assessed by FTIR spectra did not reveal any differences in relation to treatment, but differences were observed in the quantity and quality of dissolved organic carbon (DOC) extracted from the samples. The amount of DOC extracted increased by 56% for piñon litter and decreased by 32% for juniper in the UV-only treatment compared to initial values. Litter treated with UV and water had the lowest concentration of DOC with a decrease of 69% for juniper litter and 28% for piñon. The largest concentration of reactive phenols was found in the UV-only treatment with a 309% increase for piñon litter and a 10% reduction for juniper when compared to initial values. The treatments receiving water in the presence or absence of UV showed a similar response for both litter types, a reduction of 20–30% in phenolic concentration. Five extracellular enzyme activities, used, as indicators of microbial activity, were higher in the treatments that received water, but activities did not show an interaction with UV irradiation. The results suggest that UV radiation alone, or in combination with microbial activity, was as effective at decomposing litter as microbial activity alone. Thus, solar radiation can be an important contributor to litter degradation in arid systems.
Climate change will increase the recurrence of extreme weather events such as drought and heavy rainfall. Evidence suggests that modifications in extreme weather events pose stronger threats to ecosystem functioning than global trends and shifts in average conditions. As ecosystem functioning is connected with ecological services, this has far-reaching effects on societies in the 21st century. Here, we: (i) present the rationale for the increasing frequency and magnitude of extreme weather events in the near future; (ii) discuss recent findings on meteorological extremes and summarize their effects on ecosystems and (iii) identify gaps in current ecological climate change research.
To estimate how tree photosynthesis modulates soil respiration, we simultaneously and continuously measured soil respiration and canopy photosynthesis over an oak-grass savanna during the summer, when the annual grass between trees was dead. Soil respiration measured under a tree crown reflected the sum of rhizosphere respiration and heterotrophic respiration; soil respiration measured in an open area represented heterotrophic respiration. Soil respiration was measured using solid-state CO2 sensors buried in soils and the flux-gradient method. Canopy photosynthesis was obtained from overstory and understory flux measurements using the eddy covariance method. We found that the diurnal pattern of soil respiration in the open was driven by soil temperature, while soil respiration under the tree was decoupled with soil temperature. Although soil moisture controlled the seasonal pattern of soil respiration, it did not influence the diurnal pattern of soil respiration. Soil respiration under the tree controlled by the root component was strongly correlated with tree photosynthesis, but with a time lag of 7–12 h. These results indicate that photosynthesis drives soil respiration in addition to soil temperature and moisture.
Fluxes of CO2 and N2O were measured along a microclimatic gradient stretching from the centre of a gap into a mature beech stand using an automated chamber method. Simultaneously the regulating factors like soil water tensions, soil temperatures, nitrate concentrations were measured along the gradient. The daily mean values of the fluxes of CO2 and N2O were divided into classes of temperature and furthermore subdivided into classes of soil water tension to assess the significance of each regulating factor.
Soil respiration at the centre of the gap was 40% lower compared to the rooted mature stand. The difference was explained by root respiration. At both sites soil respiration was primarily controlled by the soil temperature with an average Q10 value of 2.3 over the different classes of temperature and soil water tension. Soil water tension reduced the soil respiration by up to 20% only by soil water tension above 400–600 hPa at the mature stand. The formation of N2O was reduced when the soil temperature was below 10°C or the soil water tension exceeded 200 hPa. Therefore the N2O emission was 6 times higher at the unrooted centre of the gap due to the high moisture content in the growing season. Higher nitrate concentration doubled the N2O emission at the unrooted edge of the canopy and resulted in losses of 6.4 kg N ha-1 within six months. Above 10°C and below 200 hPa the N2O emission depended strongly upon the temperature with varying Q10 values over the different classes of temperature and soil water tension. High Q10 values up to 14.4 have been calculated below 14°C and were explained by several processes with synergetic effects.
Interactive effects of multiple global change factors on ecosystem processes are complex. It is relatively expensive to explore those interactions in manipulative experiments. We conducted a modeling analysis to identify potentially important interactions and to stimulate hypothesis formulation for experimental research. Four models were used to quantify interactive effects of climate warming (T), altered precipitation amounts [doubled (DP) and halved (HP)] and seasonality (SP, moving precipitation in July and August to January and February to create summer drought), and elevated [CO₂] (C) on net primary production (NPP), heterotrophic respiration (Rh), net ecosystem production (NEP), transpiration, and runoff. We examined those responses in seven ecosystems, including forests, grasslands, and heathlands in different climate zones. The modeling analysis showed that none of the three-way interactions among T, C, and altered precipitation was substantial for either carbon or water processes, nor consistent among the seven ecosystems. However, two-way interactive effects on NPP, Rh, and NEP were generally positive (i.e. amplification of one factor's effect by the other factor) between T and C or between T and DP. A negative interaction (i.e. depression of one factor's effect by the other factor) occurred for simulated NPP between T and HP. The interactive effects on runoff were positive between T and HP. Four pairs of two-way interactive effects on plant transpiration were positive and two pairs negative. In addition, wet sites generally had smaller relative changes in NPP, Rh, runoff, and transpiration but larger absolute changes in NEP than dry sites in response to the treatments. The modeling results suggest new hypotheses to be tested in multifactor global change experiments. Likewise, more experimental evidence is needed for the further improvement of ecosystem models in order to adequately simulate complex interactive processes.
The drought of 2003 was exceptionally severe in many regions of Europe, both in duration and in intensity. In some areas, especially in Germany and France, it was the strongest drought for the last 50 years, lasting for more than 6 months.We used continuous carbon and water flux measurements at 12 European monitoring sites covering various forest ecosystem types and a large climatic range in order to characterise the consequences of this drought on ecosystems functioning.As soil water content in the root zone was only monitored in a few sites, a daily water balance model was implemented at each stand to estimate the water balance terms: trees and understorey transpiration, rainfall interception, throughfall, drainage in the different soil layers and soil water content. This model calculated the onset date, duration and intensity of the soil water shortage (called water stress) using measured climate and site properties: leaf area index and phenology that both determine tree transpiration and rainfall interception, soil characteristics and root distribution, both influencing water absorption and drainage. At sites where soil water content was measured, we observed a good agreement between measured and modelled soil water content.Our analysis showed a wide spatial distribution of drought stress over Europe, with a maximum intensity within a large band extending from Portugal to NE Germany.Vapour fluxes in all the investigated sites were reduced by drought, due to stomatal closure, when the relative extractable water in soil (REW) dropped below ca. 0.4. Rainfall events during the drought, however, typically induced rapid restoration of vapour fluxes.Similar to the water vapour fluxes, the net ecosystem production decreased with increasing water stress at all the sites. Both gross primary production (GPP) and total ecosystem respiration (TER) also decreased when REW dropped below 0.4 and 0.2, for GPP and TER, respectively.A higher sensitivity to drought was found in the beech, and surprisingly, in the broadleaved Mediterranean forests; the coniferous stands (spruce and pine) appeared to be less drought-sensitive.The effect of drought on tree growth was also large at the three sites where the annual tree growth was measured. Especially in beech, this growth reduction was more pronounced in the year following the drought (2004). Such lag effects on tree growth should be considered an important feature in forest ecosystems, which may enhance vulnerability to more frequent climate extremes.
The ongoing changes in the global climate expose the world's ecosystems not only to increasing CO₂ concentrations and temperatures but also to altered precipitation (P) regimes. Using four well-established process-based ecosystem models (LPJ, DayCent, ORCHIDEE, TECO), we explored effects of potential P changes on water limitation and net primary production (NPP) in seven terrestrial ecosystems with distinctive vegetation types in different hydroclimatic zones. We found that NPP responses to P changes differed not only among sites but also within a year at a given site. The magnitudes of NPP change were basically determined by the degree of ecosystem water limitation, which was quantified here using the ratio between atmospheric transpirational demand and soil water supply. Humid sites and/or periods were least responsive to any change in P as compared with moderately humid or dry sites/periods. We also found that NPP responded more strongly to doubling or halving of P amount and a seasonal shift in P occurrence than that to altered P frequency and intensity at constant annual amounts. The findings were highly robust across the four models especially in terms of the direction of changes and largely consistent with earlier P manipulation experiments and modelling results. Overall, this study underscores the widespread importance of P as a driver of change in ecosystems, although the ultimate response of a particular site will depend on the detailed nature and seasonal timing of P change.
Linking sap flow in tree boles to plant transpiration continues to be a fundamental and practical research problem in physiological ecology and forest hydrology. Many models have been proposed to describe water movement within trees with varying degrees of success. The prevailing resistance–capacitance (RC)-circuit models have the advantage of being easy to implement. However, RC models are ordinary differential equation (ODE) models that reduce the spatial–temporal dynamics of a tree hydraulic system to the temporal variation of simplified quantities; thus, the RC parameterization is more empirical and open to various interpretations. For coniferous trees, a reasonable alternative to RC circuit models is a porous media (PM) model, which is a partial differential equation (PDE) model that describes the spatial–temporal dynamics. The model more closely represents the physical elements of the conifer hydraulic system but also requires a direct estimation of its properties. Our proposed PM model is original in that it formulates a theoretical link between measured quantities (i.e., sap flux density and tree structure) and model parameters, obtained during nighttime, which permits direct numerical conversion of sap flow to transpiration rate during daytime. In addition to fully simulating the PDE, we propose an alternative method to transform the PDE into a set of ODEs, to significantly reduce computational demands. Although the ODE results are noisy, the transpiration pattern produced by the ODE, once filtered, is similar to that of the PDE. We demonstrate that measurements of the sap flux in multiple positions below and within the crown can be used to compute the height-dependent transpiration rate; but if rates of bulk crown transpiration are of primary interest, readily obtainable measurements at two heights, at the base of the tree and below the crown, are sufficient for the computation.
Measurements of sap flow, vapour fluxes, throughfall and soil water content were conducted for 19 months in a young beech stand growing at low elevation, in the Hesse forest. This experiment is part of the Euroflux network, covering 15 representative European forests. Study of the radial variation of sap flow within tree trunks, showed a general pattern of sap flux density in relation to the depth below cambium. Among-tree variation of sap flow was also assessed, in order to determine the contribution of the different crown classes to the total stand transpiration. Stand sap flow and vapour flux, measured with eddy covariance technique, were well correlated, for half hourly as well for daily values, the ratios of the fluxes for both averaging periods being 0.77. A strong canopy coupling to the atmosphere was found, omega factor ranging between 0.05 and 0.20 relative to the windspeed. Canopy conductance variation was related to a range of environmental variables: global radiation, vapour pressure deficit, air temperature and soil water deficit. In addition to the effect of radiation and of vapour pressure deficit often found in various other tree species, here beech exhibited a strong reduction in canopy conductance when air temperature decreased below 17°C. The model of transpiration was calibrated using data measured in the Hesse forest and applied to another beech stand under mountainous conditions in the Vosges mountains (east France). Measured and modelled stand transpiration were in good agreement.
Fluxes of CO2, water vapor, and sensible heat were measured by the eddy covariance method above a young ponderosa pine plantation in the Sierra Nevada Mountains (CA) over two growing seasons (1 June–10 September 1997 and 1 May–30 October 1998). The Mediterranean-type climate of California is characterized by a protracted summer drought, with precipitation occurring mainly from October through May. While drought stress increased continuously over both summer growing seasons, 1998 was wetter and cooler than average due to El Niño climate patterns and 1997 was hotter and drier than average. One extreme 3-day heat wave in 1997 (Days 218–221) caused a step change in the relationship between H2O flux and vapor pressure deficit, resulting in a change in canopy conductance, possibly due to cavitation of the tree xylem. This step change was also correlated with decreased rates of C sequestration and evapotranspiration; we estimate that this extreme climatic event decreased gross ecosystem production (GEP) by roughly 20% (4 μmol C m−2 s−1) for the rest of the growing season. In contrast, a cooler, wetter spring in 1998 delayed the onset of photosynthesis by about 3 weeks, resulting in roughly 20% lower GEP relative to the spring of 1997. We conclude that the net C balance of Mediterranean-climate pine ecosystems is sensitive to extreme events under low soil moisture conditions and could be altered by slight changes in the climate or hydrologic regime.
Heightened awareness of global change issues within both science and political communities has increased interest in using the global network of eddy covariance flux towers to more fully understand the impacts of natural and anthropogenic phenomena on the global carbon balance. Comparisons of net ecosystem exchange (FNEE) responses are being made among biome types, phenology patterns, and stress conditions. The comparisons are usually performed on annual sums of FNEE; however, the average data coverage during a year is only 65%. Therefore, robust and consistent gap filling methods are required.We review several methods of gap filling and apply them to data sets available from the EUROFLUX and AmeriFlux databases. The methods are based on mean diurnal variation (MDV), look-up tables (LookUp), and nonlinear regressions (Regr.), and the impact of different gap filling methods on the annual sum of FNEE is investigated. The difference between annual FNEE filled by MDV compared to FNEE filled by Regr. ranged from −45 to +200 g C m−2 per year (MDV−Regr.). Comparing LookUp and Regr. methods resulted in a difference (LookUp−Regr.) ranging from −30 to +150 g C m−2 per year.We also investigated the impact of replacing measurements at night, when turbulent mixing is insufficient. The nighttime correction for low friction velocities (u∗) shifted annual FNEE on average by +77 g C m−2 per year, but in certain cases as much as +185 g C m−2 per year.Our results emphasize the need to standardize gap filling-methods for improving the comparability of flux data products from regional and global flux networks.
We examined a 6-year record of automated chamber-based soil CO2 efflux (F-s) and the underlying processes in relation to climate and canopy gas exchange at an AmeriFlux site in a seasonally drought-stressed pine forest. Interannual variability of F-s was large (CV=17%) with a range of 427 g C m(-2) yr(-1) around a mean annual F-s of 811 g C m(-2) yr(-1). On average, 76% of the variation of daily mean F-s could be quantified using an empirical model with year-specific basal respiration rate that was a linear function of tree basal area increment (BAI) and modulated by a common response to soil temperature and moisture. Interannual variability in F-s could be attributed almost equally to interannual variability in BAI (a proxy for above-ground productivity) and interannual variability in soil climate. Seasonal total F-s was twice as sensitive to soil moisture variability during the summer months compared with temperature variability during the same period and almost insensitive to the natural range of interannual variability in spring temperatures. A strong seasonality in both root respiration (R-r) and heterotrophic respiration (R-h) was observed with the fraction attributed to R-r steadily increasing from 18% in mid-March to 50% in early June through early July before dropping rapidly to 10% of F-s by mid-August. The seasonal pattern in R-r (10-day averages) was strongly linearly correlated with tree transpiration (r(2)=0.90, P < 0.01) as measured using sap flux techniques and gross ecosystem productivity (GEP, r(2)=0.83, P < 0.01) measured by the eddy-covariance approach. R-r increased by 0.43 g C m(-2) day(-1) for every 1 g C m(-2) day(-1) increase in GEP. The strong linear correlation of R-r to seasonal changes in GEP and transpiration combined with longer-term interannual variability in the base rate of F-s, as a linear function of BAI (r(2)=0.64, P=0.06), provides compelling justification for including canopy processes in future models of F-s.
1. From previously published measurements of soil respiration rate (R) and temperature (T) the goodness of fit of various R vs T relationships was evaluated. 2. Exponential (Q10) and conventional Arrhenius relationships between T and R cannot provide an unbiased estimate of respiration rate. Nor is a simple linear relationship appropriate. 3. The relationship between R and T can, however, be accurately represented by an Arrhenius type equation where the effective activation energy for respiration varies inversely with temperature. An empirical equation is presented which yields an unbiased estimator of respiration rates over a wide range of temperatures. 4. When combined with seasonal estimates of Gross Primary Productivity (GPP) the empirical relationship derived provides representative estimates of the seasonal cycle of net ecosystem productivity and its effects on atmospheric CO2. The predicted seasonal cycle of net ecosystem productivity is very sensitive to the assumed respiration vs temperature relationship. 5. For biomes in areas where soil temperatures are low, soil respiration rate is relatively more sensitive to fluctuations in temperature. Nevertheless, more information is required before any predictions can be made about changes in soil carbon pools in response to future temperature changes.
Leaf area and its spatial distribution are key parameters in describing canopy characteristics. They determine radiation regimes and influence mass and energy exchange with the atmosphere. The evaluation of leaf area in conifer stands is particularly challenging due to their open nature and clumping on the needle, shoot and tree scale. The overall objective of our study was to characterize leaf area index (LAI) (Lh, m2 half-surface area foliage m−2 ground) in the vicinity of our old-growth and 14-year-old ponderosa pine (Pinus ponderosa, var. Laws) eddy covariance flux sites, with future plans to scale from the flux sites to the pine region using ecosystem models and remote sensing. From the combination of optical and canopy geometry measurements, sapwood and litter-fall measurements, and one- and three-dimensional (3-D) models, we evaluated the variation in estimates of Lh in a mixed-age stand at the old-growth flux site. We also compared sapwood area estimates from a local allometric equation with LAI-2000 estimates that have been corrected for clumping and the interception of light by stems and branches (Lhc, m2 half-surface area m−2 ground) across a range of age classes and stand densities of ponderosa pine forests along a 15 km swath in Central Oregon that encompassed the flux sites. In the old-growth stand, the litter-fall and sapwood estimates tended to be higher than the optical and 3-D radiative transfer model estimates. Across the 15 km east–west gradient from the crest of the Cascade Mountains, Lhc was typically lower than the sapwood estimates (Lhsw; slope 0.38). The Lhc data, as well as aboveground production estimates for the 17 pine plots will be useful for scaling flux measurements to the region using ecosystem models that have been validated with these data.
Summer drought is a feature of the semi-arid region of central Oregon, USA, where vegetation naturally develops into ponderosa pine (Pinus ponderosa var. Laws) forest. Forest management consists of clearcut harvest and natural regeneration. Soil microbial activity is interconnected with forest processes because substrate quality and availability can be important driving variables. Stand development influences the soil water regime, and water availability may also limit microbial activity. We determined factors limiting litter and mineral soil carbon (C) mineralisation rates in undisturbed old growth and regenerating (hereafter, young) ponderosa pine stands under a semi-arid climate. Mass of litter and dead fine roots did not differ significantly between the stands, but litter substrate quality was different. Young stand litter had significantly higher concentrations of total nitrogen (N), extractable organic N, extractable C, and microbial C and N than that from the old stand, probably because of litter fall from the broadleaved shrub understorey, including the N-fixing species Purshia tridentata (Pursch) DC, that comprised 40% of the young stand’s leaf area. The old stand contained no understorey. For litter samples from the two stands, wetted to 60% of water-holding capacity (WHC), net mineral-N and CO2–C mineralisation rates were similar despite the substrate quality differences. Mineral soil properties at 0–0.1m depth were similar in the two stands, except for lower CO2–C production in samples from the young stand; at 0.1–0.5m depth, total C and N and microbial N concentrations were higher in the young stand. Net mineral-N production in field-moist soil, sampled during a typical summer drought and incubated at 25°C for 56 days, was generally 3–6mgkg−1 soil at both sites, but increased up to 29mgkg−1 upon wetting to 60% of water-holding capacity. Over 56-day-long incubations, wetting also increased litter and soil microbial respiration rates by factors of about 500 and 3, respectively. The incubations yielded a proportionality between respiration rate and water content that was supported by in situ measurements of soil respiration in the young stand, before and after irrigation. A hypothetically wet year without soil water deficit caused a 2.5-fold increase in a modelled estimate of the young stand’s annual soil respiration rate. Litter and soil C mineralisation rates in these ponderosa pine forests thus appeared to be limited much more by the availability of water than by a lack of available C or N substrates.
Soil moisture affects microbial decay of SOM and rhizosphere respiration (RR) in temperate forest soils, but isolating the response of soil respiration (SR) to summer drought and subsequent wetting is difficult because moisture changes are often confounded with temperature variation. We distinguished between temperature and moisture effects by simulation of prolonged soil droughts in a mixed deciduous forest at the Harvard Forest, Massachusetts. Roofs constructed over triplicate 5 × 5 m² plots excluded throughfall water during the summers of 2001 (168 mm) and 2002 (344 mm), while adjacent control plots received ambient throughfall and the same natural temperature regime. In 2003, throughfall was not excluded to assess the response of SR under natural weather conditions after two prolonged summer droughts. Throughfall exclusion significantly decreased mean SR rate by 53 mg C m⁻² h⁻¹ over 84 days in 2001, and by 68 mg C m⁻² h⁻¹ over 126 days in 2002, representing 10–30% of annual SR in this forest and 35–75% of annual net ecosystem exchange (NEE) of C. The differences in SR were best explained by differences in gravimetric water content in the Oi horizon (r²=0.69) and the Oe/Oa horizon (r²=0.60). Volumetric water content of the A horizon was not significantly affected by throughfall exclusion. The radiocarbon signature of soil CO2 efflux and of CO2 respired during incubations of O horizon, A horizon and living roots allowed partitioning of SR into contributions from young C substrate (including RR) and from decomposition of older SOM. RR (root respiration and microbial respiration of young substrates in the rhizosphere) made up 43–71% of the total C respired in the control plots and 41–80% in the exclusion plots, and tended to increase with drought. An exception to this trend was an interesting increase in CO2 efflux of radiocarbon-rich substrates during a period of abundant growth of mushrooms.
Abstract The intensification of the hydrological cycle, with an observed and modeled increase in drought incidence and severity, underscores the need to quantify drought effects on carbon cycling and the terrestrial sink. FLUXNET, a global network of eddy covariance towers, provides dense data streams of meteorological data, and through flux partitioning and gap filling algorithms, estimates of net ecosystem productivity (FNEP), gross ecosystem productivity (P), and ecosystem respiration (R). We analyzed the functional relationship of these three carbon fluxes relative to evaporative fraction (EF), an index of drought and site water status, using monthly data records from 238 micrometeorological tower sites distributed globally across 11 biomes. The analysis was based on relative anomalies of both EF and carbon fluxes and focused on drought episodes by biome and climatic season. Globally P was ≈50% more sensitive to a drought event than R. Network-wide drought-induced decreases in carbon flux averaged −16.6 and −9.3 g C m−2 month−1 for P and R, i.e., drought events induced a net decline in the terrestrial sink. However, in evergreen forests and wetlands drought was coincident with an increase in P or R during parts of the growing season. The most robust relationships between carbon flux and EF occurred during climatic spring for FNEP and in climatic summer for P and R. Upscaling flux sensitivities to a global map showed that spatial patterns for all three carbon fluxes were linked to the distribution of croplands. Agricultural areas exhibited the highest sensitivity whereas the tropical region had minimal sensitivity to drought. Combining gridded flux sensitivities with their uncertainties and the spatial grid of FLUXNET revealed that a more robust quantification of carbon flux response to drought requires additional towers in all biomes of Africa and Asia as well as in the cropland, shrubland, savannah, and wetland biomes globally.