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Gross primary production is stimulated for three Populus species grown under free-air CO2 enrichment from planting through canopy closure
Abstract
How forests will respond to rising [CO2] in the long term is uncertain, most studies having involved juvenile trees in chambers prior to canopy closure. Poplar free-air CO2 enrichment (Viterbo, Italy) is one of the first experiments to grow a forest from planting through canopy closure to coppice, entirely under open-air conditions using free-air CO2 enrichment technology. Three Populus species: P. alba, P. nigra and P. x euramericana, were grown in three blocks, each containing one control and one treatment plot in which CO2 was elevated to the expected 2050 concentration of 550 ppm. The objective of this study was to estimate gross primary production (GPP) from recorded leaf photosynthetic properties, leaf area index (LAI) and meteorological conditions over the complete 3-year rotation cycle. From the meteorological conditions recorded at 30 min intervals and biweekly measurements of LAI, the microclimate of leaves within the plots was estimated with a radiation transfer and energy balance model. This information was in turn used as input into a canopy microclimate model to determine light and temperature of different leaf classes at 30 min intervals which in turn was used with the steady-state biochemical model of leaf photosynthesis to compute CO2 uptake by the different leaf classes. The parameters of these models were derived from measurements made at regular intervals throughout the coppice cycle. The photosynthetic rates for different leaf classes were summed to obtain canopy photosynthesis, i.e. GPP. The model was run for each species in each plot, so that differences in GPP between species and treatments could be tested statistically. Significant stimulation of GPP driven by elevated [CO2] occurred in all 3 years, and was greatest in the first year (223–251%), but markedly lower in the second (19–24%) and third years (5–19%). Increase in GPP in elevated relative to control plots was highest for P. nigra in 1999 and for P. x euramericana in 2000 and 2001, although in 1999 P. alba had a higher GPP than P. x euramericana. Our analysis attributed the decline in stimulation to canopy closure and not photosynthetic acclimation. Over the 3-year rotation cycle from planting to harvest, the cumulative GPP was 4500, 4960 and 4010 g C m−2 for P. alba, P. nigra and P. x euramericana, respectively, in current [CO2] and 5260, 5800 and 5000 g C m−2 in the elevated [CO2] treatments. The relative changes were consistent with independent measurements of net primary production, determined independently from biomass increments and turnover.
... When extrapolating forward in time using this regression, for each species under each CO 2 treatment, the divergence in number of days for a 50 % decline in canopy LAI was calculated inorder to estimate the number of days senescence was delayed under elevated CO 2 . Data were extracted from Wittig et al. (2005). The mean monthly GPP (g C m-2 d-1 ) of P. x euramericana exposed to elevated CO 2 at the POP/EUROFACE site and estimated in 2001 when a closed canopy existed were taken. ...
... The start of this 10 day delay can be approximated from the divergence in decline of LAI , which in 2004 was day -275 (as seen in figure 3.2). When this 10 day delay initiated on day 275 was modelled using the data from Wittig et al. (2005), a 2 % increase in GPP could be predicted. These ...
... During the first three years of growth, elevated CO 2 significantly increased the cumulative GPP (g C m-2 ) of P. nigra and P. x euramericana by 17 and 25 % respectively . Taking the daily integrated GPP obtained from a closed canopy (year 2001) from Wittig et al. (2005), a delay in senescence of 10 days for P. ...
p>Global data sets derived from remote sensing of terrestrial vegetation and phonological observations of bud set, leaf colour change and leaf drop have provided evidence for the recent extension of the growing season. Atmospheric carbon dioxide concentration has risen by ~39% since pre-industrial times and is considered a strong driver for a mean global temperature rise. This increased temperature is generally thought to be the cause of the extension in the growing seasons. In this thesis the influence increased atmospheric carbon dioxide concentration may have on the autumnal phenology of a poplar plantation was examined.
Following up to six years growth in an atmosphere enriched with carbon dioxide (CO<sub>2</sub>) using free air CO<sub>2</sub> enrichment technology, the autumnal phenology of two poplar genotypes was examined. Using remote sensing technology, at spatial and spectral resolutions varying from leaf level to airborne sensors, the changes in canopy spectral reflectance during senescence were monitored. These changes were associated with a delayed autumnal decline in canopy leaf area and leaf level chlorophyll concentration for the trees exposed to elevated CO<sub>2</sub>. Associated with this was a decrease in both specific leaf area (leaf area per unit mass) and leaf nitrogen content (on a leaf mass basis). The extension of autumnal senescence in this plantation resulting from atmospheric CO<sub>2</sub> enrichment was estimated to contribute approximately 2% to the annual gross primary production.,
The change in gene expression associated with this delay was studied using microarray technology. Delayed senescence in elevated CO<sub>2</sub> was also evident at the level of gene expression, confirming the remotely-sensed observations. For the first time, an up-regulation of genes encoding enzymes within the pathways of phenylpropanoid metabolism were identified during autumnal senescence in elevated CO<sub>2</sub> inferring increased stress tolerance.</p
... A few important, long-term CO 2 -enrichment experiments in the field do not fit these categories well, and may thus be considered special cases: (a) the longest field test to date, the flood plain study in the Chesapeake Bay (Rasse et al., 2005), a steady-state system by the above criteria, but 'open' (i.e. decoupled) in terms of nutrient supply; (b) a short rotation coppice poplar plantation (POPFACE), that is a fertile system prevented from arriving at steady state (Wittig et al., 2005); (iii) the Swiss treeline free-air CO 2 enrichment (FACE), where isolated trees (expanding canopy) are growing under strong below-ground competition with a steady-state dwarf shrub heath (Handa et al., 2005). The latter two may marginally fit type II, while the first a type I situation. ...
... Whether a CO 2 effect will be retained after the breakpoint will depend on the rate of nutrient addition (Oren et al., 2001). In any case, the timing of a biomass census will strongly affect the result (Hättenschwiler et al., 1997;Wittig et al., 2005). In the example chosen for Fig. 3(c), the total effect would be 100% at census time 1, 20% at census time 5, and 2% at census time 50. ...
... For instance, four FACE experiments with trees (two steady-state, two rapidly expanding) arrived at c. 20% increase in productivity (NPP), but there was no difference in above-ground standing crop biomass in the Oak Ridge FACE compared with controls (Norby et al., 2004(Norby et al., , 2005, and the other three differed widely in actual biomass increment. The GPP of one, a young poplar stand, reached +250% in the first year of CO 2 enrichment alone (Wittig et al., 2005). The stimulation of GBP of the Basel forest FACE trees is in the order of +40% (based on canopy photosynthesis data by Zotz et al., 2005), but the actual change in stem biomass increment was zero after 4 yr. ...
... A few important, long-term CO 2 -enrichment experiments in the field do not fit these categories well, and may thus be considered special cases: (a) the longest field test to date, the flood plain study in the Chesapeake Bay (Rasse et al., 2005), a steady-state system by the above criteria, but 'open' (i.e. decoupled) in terms of nutrient supply; (b) a short rotation coppice poplar plantation (POPFACE), that is a fertile system prevented from arriving at steady state (Wittig et al., 2005); (iii) the Swiss treeline free-air CO 2 enrichment (FACE), where isolated trees (expanding canopy) are growing under strong below-ground competition with a steady-state dwarf shrub heath (Handa et al., 2005). The latter two may marginally fit type II, while the first a type I situation. ...
... Whether a CO 2 effect will be retained after the breakpoint will depend on the rate of nutrient addition (Oren et al., 2001). In any case, the timing of a biomass census will strongly affect the result (Hättenschwiler et al., 1997;Wittig et al., 2005). In the example chosen for Fig. 3(c), the total effect would be 100% at census time 1, 20% at census time 5, and 2% at census time 50. ...
... For instance, four FACE experiments with trees (two steady-state, two rapidly expanding) arrived at c. 20% increase in productivity (NPP), but there was no difference in above-ground standing crop biomass in the Oak Ridge FACE compared with controls (Norby et al., 2004(Norby et al., , 2005, and the other three differed widely in actual biomass increment. The GPP of one, a young poplar stand, reached +250% in the first year of CO 2 enrichment alone (Wittig et al., 2005). The stimulation of GBP of the Basel forest FACE trees is in the order of +40% (based on canopy photosynthesis data by Zotz et al., 2005), but the actual change in stem biomass increment was zero after 4 yr. ...
Few of the most common assumptions used in models of responses of plants and ecosystems to elevated CO2 and climate warming have been tested under realistic life con-ditions. It is shown that some unexpected discrepancies between predictions and experimental findings exist, suggesting that a better empirical basis is required for predictions. The following ten suggestions may improve our potential to scale up from experimental scales to the real world.(1) Experiments should be timed to account for non-linearity in system responsiveness, asynchrony of responses and developmental differences. (2) By altering mineral nutrient supply, a wide range of CO2 responses can be ‘produced’, thus requiring realistic soil conditions. (3) Distinctions should be made between ‘doubling CO2 sup-ply’ and biologically effective degrees of CO2 enrichment. (4) Because of the non-linearity of plant responses to CO2, studies of at least three instead of two CO2 concentrations are necessary to describe future trends adequately. (5) Edge effects, in particular unscreened side light, may lead to allometric anomalies, strongly constraining up-scaling to stand-scale CO2 responses. (6) Variables such as growth, yield, net primary production and C turnover are often confused with carbon pools, carbon sequestration or net ecosystem production. (7) Mono- and interspecific interactions between individuals may lead to completely unpredictable CO2 responses. (8) Experiments with seedlings benefit from the absence of prehistory effects but are likely to be irrelevant for the responses of larger trees which, on the other hand, may be constrained by carry-over effects. Tree ring research indicates immediate sensitivity of large trees to environmental changes, supporting their usefulness in short-term CO2-enrichment experiments. (9) In predicting temperature responses, acclimation deserves more attention. (10) The significance of developmental responses is largely under-represented in experimental research, although these responses may overrule many of the other effects of atmospheric change. Results of more realistic experiments which account for these problems will provide a better basis for modelling the future of the biosphere.
... It also provides a means to quantitatively assign acclimatory changes in production to different underlying processes. For example, Wittig et al. (2005) used WIMOVAC to simulate the gross primary production from recorded leaf photosynthetic properties, leaf area index and meteorological conditions over a 3 year rotation cycle of a poplar forest ecosystem under both current and future elevated CO 2 conditions. The relative changes in the simulated gross primary productivity (GPP) were consistent with the estimated GPP change based on biomass increment and turnover. ...
... The relative changes in the simulated gross primary productivity (GPP) were consistent with the estimated GPP change based on biomass increment and turnover. The modelling results provided new insight showing that the decline in stimulation of gross primary production under elevated [CO 2 ] was because of earlier canopy closure, not photosynthetic acclimation (Wittig et al. 2005). This is because the responses of photosynthesis to elevated CO 2 are most pronounced in saturating light in contrast to limiting light conditions, as can be explained from the underlying biochemical/biophysical processes . ...
Windows Intuitive Model of Vegetation response to Atmosphere and Climate Change (WIMOVAC) has been used widely as a generic modular mechanistically-rich model of plant production. It can predict the responses of leaf and canopy carbon balance, as well as production in different environmental conditions, in particular those relevant to global change. Here we introduce an open source Java user-friendly version of WIMOVAC. This software is platform-independent and can be easily downloaded to a laptop and used without any prior programming skills. In this article, we describe the structure, equations, user guide and illustrate some potential applications of WIMOVAC. This article is protected by copyright. All rights reserved.
... Indeed in the natural stand, young white poplar plants showed higher rates of net assimilation, transpiration, and WUE than the adult trees, associated with greater availability of light in the regeneration openings. These results agree with those obtained by Wittig et al. (2005) in white poplar and other Populus species, in which canopy closure caused a decline in light availability and the subsequent reduction in the carbon assimilation rate and gross primary production. Elms had a lower carbon assimilation rate, transpiration, stomatal conductance, and apparent maximum quantum efficiency than white poplar trees. ...
... Elms had a lower carbon assimilation rate, transpiration, stomatal conductance, and apparent maximum quantum efficiency than white poplar trees. Wittig et al. (2005) found in white poplar and other Populus species that canopy closure caused the decline of light availability and a subsequent reduction in the carbon assimilation rate and gross primary production. Stomatal conductance and photosynthetic rates have long been recognized to be positively associated (Kozlowski and Pallardy 1997, Wang et al. 2000, Peña-Rojas et al. 2004. ...
... found no consistent support for either the resourcebased or the plant functional type response model to CO 2 . Wittig et al. (2005) evaluated GPP of fast-growing Populus species (three years from establishment to canopy closure) in response to elevated CO 2 and found that GPP increased dramatically in the first year but markedly less so in the subsequent years. Hättenschwiler and Körner (2003) similarly found accelerated growth in trees over a 30- ...
... year period of elevated CO 2 exposure, with most of the accelerated growth happening at young stages of development. The findings of Wittig et al. (2005) and Hättenschwiler and Körner (2003) suggest differing responses of trees at different developmental stages and add another obstacle to a blanket statement about the response of forest productivity to elevated CO 2 . Hättenschwiler and Körner (2003) also suggest that trees exposed to higher CO 2 levels seems to be more tolerant to drought stress. ...
... The steady-state mechanistic model of C 3 photosynthetic carbon assimilation of Farquhar et al. (1980) is fundamental for predicting leaf responses to environmental variation (Long, 1991). This model provides the basis for scaling carbon uptake from leaves to canopies (Wang and Jarvis, 1990;Amthor, 1995;Lloyd and Farquhar, 1996;dePury and Farquhar, 1997;Wittig et al., 2005), ecosystems (Field and Avissar, 1998) and landscapes (Sellers et al., 1996(Sellers et al., , 1997. The leaflevel photosynthesis model is also a key component of earth system models (Cramer et al., 2001). ...
... A complete description of the steady-state photosynthesis model was presented previously (Farquhar et al., 1980;Von Caemmerer, 2000), and an excellent review discussing a mechanistic understanding of the temperature responses of photosynthesis already exists (Sage and Kubien, 2007). Further, numerous modeling studies utilizing the photosynthesis models at multiple scales are available (Long, 1991;Harley and Baldocchi, 1995;Sellers et al., 1997;Wittig et al., 2005). ...
... When a canopy is still open, photosynthesis will be stimulated under elevated CO 2 because light-saturated photosynthesis is predominantly limited by the amount of active Rubisco (Rogers & Humphries, 2000 ). In contrast, under lightlimiting conditions, photosynthesis is limited by the regeneration of the CO 2 acceptor, and this could explain why the growth enhancement often declines after canopy closure (Wittig et al., 2005). This highlights the importance of extending experiments over several years after canopy closure. ...
... Trees were larger during the second rotation cycle and the lower R/S ratio is, thus, consistent with the normal ontogenetic development of trees. No consensus has been reached regarding the fate of the enhanced C uptake (Bernacchi et al., 2003; Wittig et al., 2005) under elevated CO 2 and its distribution among the different biomass compartments. According to the equilibrium theory (Bryant et al., 1983), increased C uptake would enhance root growth because of the imbalance between C and nutrient acquisition. ...
The quickly rising atmospheric carbon dioxide (CO2)-levels, justify the need to explore all carbon (C) sequestration possibilities that might mitigate the current CO2 increase. Here, we report the likely impact of future increases in atmospheric CO2 on woody biomass production of three poplar species (Populus alba L. clone 2AS-11, Populus nigra L. clone Jean Pourtet and Populus×euramericana clone I-214). Trees were growing in a high-density coppice plantation during the second rotation (i.e., regrowth after coppice; 2002–2004; POPFACE/EUROFACE). Six plots were studied, half of which were continuously fumigated with CO2 (FACE; free air carbon dioxide enrichment of 550 ppm). Half of each plot was fertilized to study the interaction between CO2 and nutrient fertilization. At the end of the second rotation, selective above- and belowground harvests were performed to estimate the productivity of this bio-energy plantation. Fertilization did not affect growth of the poplar trees, which was likely because of the high rates of fertilization during the previous agricultural land use. In contrast, elevated CO2 enhanced biomass production by up to 29%, and this stimulation did not differ between above- and belowground parts. The increased initial stump size resulting from elevated CO2 during the first rotation (1999–2001) could not solely explain the observed final biomass increase. The larger leaf area index after canopy closure and the absence of any major photosynthetic acclimation after 6 years of fumigation caused the sustained CO2-induced biomass increase after coppice. These results suggest that, under future CO2 concentrations, managed poplar coppice systems may exhibit higher potential for C sequestration and, thus, help mitigate climate change when used as a source of C-neutral energy.
... Nowak et al. (2004) found no consistent support for either the resourcebased or the plant functional-type response model to CO 2 . Wittig et al. (2005) evaluated GPP of fast-growing Populus species (3 years from establishment to canopy closure) and found that GPP increased dramatically in the first year but markedly less so in the subsequent years. Similar results that support the accelerated growth of trees over a 30-year period of elevated CO 2 exposure, with most of the accelerated growth occurring at young stages of development, were found by Hättenschwiler & Kö rner (2003). ...
... Similar results that support the accelerated growth of trees over a 30-year period of elevated CO 2 exposure, with most of the accelerated growth occurring at young stages of development, were found by Hättenschwiler & Kö rner (2003). The findings of Wittig et al. (2005) and Hättenschwiler & Kö rner (2003) suggest differing responses of trees at different development stages and add another obstacle to a blanket statement response of forest productivity to elevated CO 2 . Hättenschwiler & Kö rner (2003) also suggest that trees exposed to higher CO 2 levels seem to be more tolerant to drought stress. ...
Changes to forest production drivers (light, water, temperature, and site nutrient) over the last 55 years have been documented in peer-reviewed literature. The main objective of this paper is to review documented evidence of the impacts of climate change trends on forest productivity since the middle of the 20th century. We first present a concise overview of the climate controls of forest production, provide evidence of how the main controls have changed in the last 55 years, followed by a core section outlining our findings of observed and documented impacts on forest productivity and a brief discussion of the complications of interpreting trends in net primary production (NPP). At finer spatial scales, a trend is difficult to decipher, but globally, based on both satellite and ground-based data, climatic changes seemed to have a generally positive impact on forest productivity when water was not limiting. Of the 49 papers reporting forest production levels we reviewed, 37 showed a positive growth trend, five a negative trend, three reported both a positive and a negative trend for different time periods, one reported a positive and no trend for different geographic areas, and two reported no trend. Forests occupy ≈52% of the Earth's land surface and tend to occupy more temperature and radiation-limited environments. Less than 7% of forests are in strongly water-limited systems. The combined and interacting effects of temperature, radiation, and precipitation changes with the positive effect of CO2, the negative effects of O3 and other pollutants, and the presently positive effects of N will not be elucidated with experimental manipulation of one or a few factors at a time. Assessments of the greening of the biosphere depend on both accurate measurements of rates (net ecosystem exchange, NPP), how much is stored at the ecosystem level (net ecosystem production) and quantification of disturbances rates on final net biome production.
... Communication between these experts is critical for assembling and maintaining a state-of-theart capacity to predict crop responses to environmental change. The software package WIMOVAC (Humphries and Long 1995;Song et al. 2017) was an early success in this area, facilitating investigations into the response of plants to elevated atmospheric CO 2 concentrations (Rogers and Humphries 2000;Wittig et al. 2005) and eventually giving rise to a successor, BioCro, that expanded the list of available crops (Miguez et al. 2012(Miguez et al. , 2009Wang et al. 2015;Larsen et al. 2016;Jaiswal et al. 2017). ...
The central motivation for mechanistic crop growth simulation has remained the same for decades: to reliably predict changes in crop yields and water usage in response to previously unexperienced increases in air temperature and CO2 concentration across different environments, species, and genotypes. Over the years, individual process-based model components have become more complex and specialized, increasing their fidelity but posing a challenge for integrating them into powerful multiscale models. Combining models is further complicated by the common strategy of hard-coding intertwined parameter values, equations, solution algorithms, and user interfaces, rather than treating these each as separate components. It is clear that a more flexible approach is now required. Here we describe a modular crop growth simulator, BioCro II. At its core, BioCro II is a cross-platform representation of models as sets of equations. This facilitates modularity in model building and allows it to harness modern techniques for numerical integration and data visualization. Several crop models have been implemented using the BioCro II framework, but it is a general purpose tool and can be used to model a wide variety of processes.
... Despite the canopy-scale feedbacks discussed earlier, most researchers have found stimulation of both GPP and net primary productivity (NPP = the amount of carbon incorporated into plant biomass or GPP-autotrophic respiration) following exposure to elevated level of atmospheric CO [23][24][25] 2 . ...
... This method has been used to evaluate the consequences of altering Rubisco kinetic properties and changing the speed of relaxation of photoprotection on GPP (Zhu et al., 2004a, b). It has also been used to evaluate the potential changes in GPP under elevated [CO 2 ] and to dissect the factors controlling changes in GPP (Wittig et al., 2005). The sunlit-shaded model uses aggregated parameters to represent canopy architectural features. ...
Background and aims:
Understanding how climate change influences crop productivity helps identifying new options to increase crop productivity. Soybean is the most important dicotyledonous seed crop in terms of planting area. Though the impacts of elevated atmospheric [CO2] on soybean physiology, growth, and biomass accumulation have been studied extensively, the contribution of different factors to changes in season-long whole crop photosynthetic CO2 uptake (gross primary productivity - GPP) under elevated [CO2] have not been fully quantified.
Methods:
A 3D canopy model combining canopy 3D architecture, ray tracing and leaf photosynthesis model was built to: a) study the impacts of elevated [CO2] on soybean GPP across a whole growing season; b) dissect the contribution of different factors to changes in GPP; and c) determine the extent, if any, of synergism between [CO2] and light on changes in GPP. The model was parameterized from measurements of leaf physiology and canopy architectural parameters at the soybean Free Air CO2 Enrichment (SoyFACE) facility in Champaign, Illinois.
Key results:
Using this model, we showed that both CO2 fertilization effect and changes in canopy architecture contributed to the large increase in GPP while acclimation in photosynthetic physiological parameters to elevated [CO2] and altered leaf temperature played only a minor role in the changes in GPP. Furthermore, at early developmental stages, elevated CO2 increased leaf area index (LAI) which led to increased canopy light absorption and canopy photosynthesis. At later developmental stages, on days with high ambient light levels, the proportion of leaves in a canopy limited by Rubisco carboxylation increased from 12.2% to 35.6%, which led to a greater enhancement of elevated [CO2] to GPP.
Conclusions:
This study develops a new method to dissect comtribution of different factors to responses of crops under climate change. We showed that there is a synergestic effect of CO2 and light on crop growth under elevated CO2 conditions.
... Subsequently, these parameters can be used to predict plant responses to changes in environment. The FvCB model is a submodel of many larger scale representations such as crop, canopy, climate or Earth systems models (Sellers et al. 1996;Zhu et al. 2004;Lloyd et al. 1995;Bagley et al. 2015;Drewry et al. 2010;Rogers et al. 2014;Pitman 2003;Wittig et al. 2005;Cramer et al. 2001;Medvigy et al. 2009). It can be used to identify potential targets to improve CO 2 fixation (Sage and Kubien 2007;Zhu et al. 2008;Bernacchi et al. 2013;Busch and Sage 2017). ...
The arrangement of mitochondria and chloroplasts, together with the relative resistances of cell wall and chloroplast, determine the path of diffusion out of the leaf for (photo)respired CO2. Traditional photosynthesis models have assumed a tight arrangement of chloroplasts packed together against the cell wall with mitochondria located behind the chloroplasts, deep inside the cytosol. Accordingly, all (photo)respired CO2 must cross the chloroplast before diffusing out of the leaf. Different arrangements have recently been considered, where all or part of the (photo)respired CO2 diffuses through the cytosol without ever entering the chloroplast. Assumptions about the path for the (photo)respiratory flux are particularly relevant for the calculation of mesophyll conductance (gm). If (photo)respired CO2 can diffuse elsewhere besides the chloroplast, apparent gm is no longer a mere physical resistance but a flux-weighted variable sensitive to the ratio of (photo)respiration to net CO2 assimilation. We discuss existing photosynthesis models in conjunction with their treatment of the (photo)respiratory flux and present new equations applicable to the generalized case where (photo)respired CO2 can diffuse both into the chloroplast and through the cytosol. Additionally, we present a new generalized Δ¹³C model that incorporates this dual diffusion pathway. We assess how assumptions about the fate of (photo)respired CO2 affect the interpretation of photosynthetic data and the challenges it poses for the application of different models.
... 未来气候变化数据来自国家气候中心提供的"WCRP 的耦合模式比较计划-阶段 5 [29] ，输入的土地利用数据则来自于 ABM/ LUCC 模型的模拟结果。通过将模型的两个模块整合可以得到每十年的林地变化以及相 应的森林地上生物量的变化。LANDIS-II 模拟中林地的采伐方式保持当前森林管理政 策，即每十年采伐林地面积的 10%。 2.4 森林地上生物量模拟 本文利用 LANDIS-II 和 PnET-II 模型来模拟森林地上生物量的变化。LANDIS-II 模型 是一个基于栅格的空间动态森林景观模型，它可以模拟森林干扰、演替以及森林管理过 程 [30] 。模型通过输入物种生活史参数、物种建立系数 （species establishment probability, SEP） 、地表净初级生产力 （aboveground net primary production, ANPP） 以及生态分区数 据来模拟森林景观动态变化，模拟的空间尺度较为灵活，可模拟 10 4 ~10 7 ha 的森林景观 变化 [31] 。同时，模型可以用来处理气候变化对森林的影响并广泛应用于森林与气候变化 关系的研究中 [32] 。本文利用 Biomass Succession 模块、Biomass Harvest 模块和 Land Use 模 块来模拟土地利用和气候变化对森林地上生物量的影响。模型的输入主要包括空间数据 和树种属性数据，空间数据包括初始群落图、生态分区图和土地利用图，树种属性数据 主要包括物种生活史参数、ANPP 和 SEP。初始群落图来自江西省森林资源二类调查数 据。生态分区图则根据研究区地形与海拔高度分为 5 个生态区，其中生态区 1 为非林地， 生态区 2~5 分别表示低丘 （0~100 m） 、中丘 （100~250 m） 、高丘 （250~500 m） 和低山 （> 500 m）（图 5） 。树种的物种生活史参数主要来自文献、样地调查以及咨询当地林业 部门专家得到 [33][34] 策下的建设用地扩张，模型输出每十年的建设用地扩张图，同时该数据作为 LANDIS-II 模型Land Use模块的输入参数。其次，利用PnET-II模型来分别计算不同气候情景RCP 2.6、 RCP 4.5、 RCP 8 [19,22] 。 Thompson 等发现尽管气候变化有助于提升美国马萨诸塞州森林的生长，但是土地利用变 化的影响将远远抵消这种正效应 [22] 。Gustafson 等同样发现西伯利亚中南部地区的原始森 林受气候变化的影响远远不及受木材砍伐及其他人类活动干扰的影响 [19] 。本文利用 ABM/ LUCC 模型来模拟建设用地扩张，该研究方法可以更加直观地反映主体选择地块的行 为，帮助理解土地利用变化的人为驱动力。由 ANPP 的结果发现森林生产力随着气温、 降水的增加而增加，该结果与其他人在亚热带区域的结果相一致 [42] 。当然在现实的生态 系统中其他环境要素的变化也可能导致 ANPP 的变化，例如大气组成中 CO2、O3以及 N 含 量的变化都可能影响森林生产力 [43][44][45] ，而这些会在未来的研究中逐渐补充完善。在森林地 上生物量的模拟中，生物量的累积是以生态系统过程速率为基础，通过计算不同干扰下 不同树种年龄级的地上生物量得到 [46][47] 。研究结果发现，在不考虑土地利用变化的影响 下，生物量的累积与气温存在一个正相关的关系，该研究也与吴丹等人在泰和县的研究 相一致 [48] 。而在土地利用变化的干扰下，森林地上生物量结果呈现先增加然后迅速下降 的变化趋势。森林地上生物量下降的最主要原因是建设用地扩张和森林采伐导致的林地 利用类型转换和树木移除。与气候变化影响的相对滞后性相比，这些与人类紧密相关的 土地利用变化将给森林地上生物量带来更直接更快速的影响。 土地、气候和森林之间存在着相互影响、相互制约的复杂关系，这种关系涉及到土 地系统、气候系统以及森林生态系统这种多系统相互交叉的信息、能量与物质传递。由 人类活动引起的土地系统的变化会通过林地迁移、森林采伐以及林地保护与恢复等方式 改变森林景观结构 [18] 。气候系统的变化也与人类活动密切相关，同时气候和森林之间也 存在着相互影响、相互反馈的作用机制 [3] 。受土地利用和气候变化的影响，森林生态系统 的空间相互作用、森林生态系统演替、干扰、迁移和转换过程同时作用于个体、种群、 群落及生态系统的多个尺度中 [1] 。因此研究土地利用和气候变化对森林景观尺度上综合影 响及长期响应必然存在着一些不确定性。首先，数据及参数化过程的不确定。对于气候 数据，RCPs 气候数据集是来自多个全球气候模式平均后的结果，尽管比选用单独的气候 模型更具有说服力，但仍然具有很大的不确定性 [49] ...
... As expected, these models, once developed, could then be used as tools for basic research, to test particular hypothesis regarding interactions between components of a system, and the potential impact of future climate change on plant growth and productivity (Wittig et al. 2005;Zhu et al. 2005). In the context of rice breeding, these models could be used both for ideotype definition and also for identification of critical parameters. ...
A large number of genes related to source, sink and flow have been identified after decades of research in plant genetics. Unfortunately, these genes have not been effectively utilized in modern crop breeding. This perspective paper aims to examine the reasons behind such a phenomenon and propose a strategy to resolve this situation. Specifically, we first systematically survey the currently cloned genes related to source, sink and flow; then we discuss three factors hindering effective application of these identified genes, which include the lack of effective methods to identify limiting or critical steps in a signaling network, the misplacement of emphasis on properties, at the leaf, instead of the whole canopy level, and the non‐linear complex interaction between source, sink and flow. Finally, we propose the development of systems models of source, sink and flow, together with detailed simulation of interactions between them and their surrounding environments, to guide effective use the identified elements in modern rice breeding. These systems models will contribute directly to the definition of crop ideotype and also identification of critical features and parameters that limit the yield potential in current cultivars.
... Except for temperature and precipitation, change in other environmental factors also leads to ANPP change. For example, the changing atmospheric concentrations, such as COB 2 B (Wittig et al., 2005), OB 3 B (Cojocariu et al., 2005) andN (Huang et al., 2014), may also affect forest productivity. In our study, we incorporated the effect of COB 2 B concentrations on the stomata conductance of different species. ...
Global and regional environmental change such as land use and climate change have significant and interactive effects on forest. These integrated effects will undoubtedly alter the distribution, function and succession processes of forest ecosystems. In order to respond and adapt to these changes, it is necessary to understand their individual and integrated effects. In this study, we proposed a framework by using coupling models to gain a better understanding of the complex ecological processes. We combined an agent-based model for land use and land cover change (ABM/LUCC), an ecosystem process model (PnET-II), and a forest dynamic landscape model (LANDIS-II) to simulate the change of forest total aboveground biomass (AGB), which was driven by land use and climate change factors for the period 2010-2050 in Taihe County of southern China, where subtropical coniferous plantations dominate. We conducted a series of land use and climate change scenarios to compare the differences in forest total AGB. The results show that: (1) land use, including town expansion, deforestation and forest conversion, and climate change are likely to influence forest total AGB in the near future in Taihe County. (2) Although climate change will make a contribution to an increase in the forest total AGB, land use change can result in a rapid decrease in forest total AGB and play a vital role in the integrated simulation. The forest total AGB under the integrated scenario decreased by 33.13% (RCP2.6+land use), 32.92% (RCP4.5+land use), and 32.42% (RCP8.5+land use) by 2050, which is in comparison to the results under separate RCPs without land use disturbance. (3) The framework can offer a coupled method to better understand the complex and interactive ecological processes, which may provide some supports for adapting to land use and climate change, improving and optimizing plantation structure and function, and developing measures for sustainable forest management.
... The FvCB leaf photosynthesis model for C 3 plants [1,2] is fundamental for the prediction of leaf responses to environmental variation [3]. This model has been widely used to simulate CO 2 assimilation and the response of plants to climate change for different spatiotemporal scales [4][5][6][7][8][9][10][11], due to its solid theoretical basis and simplicity [12]. It is also frequently used in reverse to quantify the ...
The Farquhar-von Caemmerer-Berry (FvCB) biochemical model of photosynthesis, commonly used to estimate CO2 assimilation at various spatial scales from leaf to global, has been used to assess the impacts of climate change on crop and ecosystem productivities. However, it is widely known that the parameters in the FvCB model are difficult to accurately estimate. The objective of this study was to assess the methods of Sharkey et al. and Gu et al., which are often used to estimate the parameters of the FvCB model. We generated An/Ci datasets with different data accuracies, numbers of data points, and data point distributions. The results showed that neither method accurately estimated the parameters; however, Gu et al.'s approach provided slightly better estimates. Using Gu et al.'s approach and datasets with measurement errors and the same accuracy as a typical open gas exchange system (i.e., Li-6400), the majority of the estimated parameters-Vcmax (maximal Rubisco carboxylation rate), Kco (effective Michaelis-Menten coefficient for CO2), gm (internal (mesophyll) conductance to CO2 transport) and Γ* (chloroplastic CO2 photocompensation point)-were underestimated, while the majority of Rd (day respiration) and a (the non-returned fraction of the glycolate carbon recycled in the photorespiratory cycle) were overestimated. The distributions of Tp (the rate of triose phosphate export from the chloroplast) were evenly dispersed around the 1:1 line using both approaches. This study revealed that a high accuracy of leaf gas exchange measurements and sufficient data points are required to correctly estimate the parameters for the biochemical model. The accurate estimation of these parameters can contribute to the enhancement of food security under climate change through accurate predictions of crop and ecosystem productivities. A further study is recommended to address the question of how the measurement accuracies can be improved.
... Photosynthesis, calculated in μmol m −2 s −1 was converted to biomass equivalent, assuming that 1 mol of CO 2 assimilated would result in 30 g of carbohydrate. Carbohydrate mass gain was predicted over the whole growing season, assuming that 40% of photosynthate is lost in whole-plant respiration [19,20]. All simulations were conducted in MATLAB (R2014b, Mathworks Inc.) ...
Sugarcane has emerged as the second largest source of biofuel, primarily as ethanol produced in Brazil. Dual row planting using asymmetric spacing of rows can decrease damage to plants and soil structure from harvest equipment though potentially can cause some loss of productivity due to increased shading. Can we assess this loss, without experimental testing of the thousands of potential permutations of planting design and cultivar canopy form? Here we develop a computational framework which couples 3D canopy architectural information, a ray-tracing algorithm, and a steady-state C4 photosynthesis model to study this question. We demonstrate the utility of the model by comparing evenly spaced rows at 100 cm to alternating row spacing of 45 and 155 cm. Asymmetric planting caused a 9% decrease in predicted net canopy carbon uptake over the growing season for a major current cultivar. The loss was greater at lower leaf area indices, when leaves were more vertical and when rows were oriented east-west, suggesting agronomic approaches to minimize loss. This study demonstrates the utility of this computational framework, which could also be used to aid breeding by identifying ideotypes for different environments and objectives, and to assess impacts of environmental change.
... Under canopy closure, stimulation depended on lightuse efficiency. At POP-EUROFACE, NPP stimulation of three poplar species under eCO 2 (550 ppm) gradually declined due to canopy closure rather than acclimation of photosynthesis ( Wittig et al. 2005). Soil C was enriched by new root litter at stable NEP, as old SOM declined ( Gielen et al. 2005). ...
Forests store the largest terrestrial pools of carbon (C), helping to stabilize the global climate system, yet are threatened by climate change (CC) and associated air pollution (AP, highlighting ozone (O3) and nitrogen oxides (NOx)). We adopt the perspective that CC-AP drivers and physiological impacts are universal, resulting in consistent stress responses of forest ecosystems across zonobiomes. Evidence supporting this viewpoint is presented from the literature on ecosystem gross/net primary productivity and water cycling. Responses to CC-AP are compared across evergreen/deciduous foliage types, discussing implications of nutrition and resource turnover at tree and ecosystem scales. The availability of data is extremely uneven across zonobiomes, yet unifying patterns of ecosystem response are discernable. Ecosystem warming results in trade-offs between respiration and biomass production, affecting high elevation forests more than in the lowland tropics and low-elevation temperate zone. Resilience to drought is modulated by tree size and species richness. Elevated O3 tends to counteract stimulation by elevated carbon dioxide (CO2). Biotic stress and genomic structure ultimately determine ecosystem responsiveness. Aggrading early- rather than mature late-successional communities respond to CO2 enhancement, whereas O3 affects North American and Eurasian tree species consistently under free-air fumigation. Insect herbivory is exacerbated by CC-AP in biome-specific ways. Rhizosphere responses reflect similar stand-level nutritional dynamics across zonobiomes, but are modulated by differences in tree-soil nutrient cycling between deciduous and evergreen systems, and natural versus anthropogenic nitrogen (N) oversupply. The hypothesis of consistency of forest responses to interacting CC-AP is supported by currently available data, establishing the precedent for a global network of long-term coordinated research sites across zonobiomes to simultaneously advance both bottom-up (e.g., mechanistic) and top-down (systems-level) understanding. This global, synthetic approach is needed because high biological plasticity and physiographic variation across individual ecosystems currently limit development of predictive models of forest responses to CC-AP. Integrated research on C and nutrient cycling, O3-vegetation interactions and water relations must target mechanisms' ecosystem responsiveness. Worldwide case studies must be subject to biostatistical exploration to elucidate overarching response patterns and synthesize the resulting empirical data through advanced modelling, in order to provide regionally coherent, yet globally integrated information in support of internationally coordinated decision-making and policy development.
... Except for temperature and precipitation, change in other environmental factors also leads to ANPP change. For example, the changing atmospheric concentrations, such as COB 2 B (Wittig et al., 2005), OB 3 B (Cojocariu et al., 2005) andN (Huang et al., 2014), may also affect forest productivity. In our study, we incorporated the effect of COB 2 B concentrations on the stomata conductance of different species. ...
Global and regional environmental changes such as land use and climate change have significantly integrated and interactive effects on forest. These integrated effects will undoubtedly alter the distribution, function and succession processes of forest ecosystems. In order to adapt to these changes, it is necessary to understand their individual and integrated effects. In this study, we proposed a framework by using coupling models to gain a better understanding of the complex ecological processes. We combined an agent-based model for land use and land cover change (ABM/LUCC), an ecosystem process model (PnET-II), and a forest dynamic landscape model (LANDIS-II) to simulate the change of forest aboveground biomass (AGB) which was driven by land use and climate change factors for the period of 2010–2050 in Taihe County of southern China, where subtropical coniferous plantations dominate. We conducted a series of land use and climate change scenarios to compare the differences in forest AGB. The results show that: (1) land use, including town expansion, deforestation and forest conversion and climate change are likely to influence forest AGB in the near future in Taihe County. (2) Though climate change will make a good contribution to an increase in forest AGB, land use change can result in a rapid decrease in the forest AGB and play a vital role in the integrated simulation. The forest AGB under the integrated scenario decreased by 53.7% (RCP2.6 + land use), 57.2% (RCP4.5 + land use), and 56.9% (RCP8.5 + land use) by 2050, which is in comparison to the results under separate RCPs without land use disturbance. (3) The framework can offer a coupled method to better understand the complex and interactive ecological processes, which may provide some supports for adapting to land use and climate change, improving and optimizing plantation structure and function, and developing measures for sustainable forest management.
... An early work in European forests provided an observation-based evidence that forest productivity exerts stronger influence on SR than climatic factors do (Janssens et al., 2001), and their finding was evaluated indirectly via litter manipulation experiments in China, which found that SR was more sensitive to changes in litterfall than to climatic factors (Xu et al., 2013;Han et al., 2015). Furthermore, the tight relationship between component fluxes and GPP are explicitly demonstrated in Free-Air CO 2 Enrichment (FACE) experiments, which indicated that increased atmospheric CO 2 concentration stimulates GPP, which in turn accompanies increases in component fluxes such as NPP and SR; in comparison, climatic factors are less closely related to those increases (e.g., DeLucia et al., 1999;Ainsworth and Long, 2005;King et al., 2004;Norby et al., 2005;Wittig et al., 2005). These results, in conjunction with the results of the present study, indicate that climatic factors certainly influence carbon fluxes, but that their influences are generally overshadowed by the variability in GPP. ...
Forest ecosystems sequester large amounts of atmospheric CO2, and the contribution from forests in Asia is not negligible. Previous syntheses of carbon fluxes in Asian ecosystems mainly employed estimates of eddy covariance measurements, net ecosystem production (NEP), gross primary production (GPP), and ecosystem respiration (RE); however, to understand the variability within carbon cycles, fluxes such as autotropic respiration (AR), net primary production (NPP), litterfall, heterotrophic respiration (HR), and soil respiration (SR) need to be analyzed comprehensively in conjunction with NEP, GPP, and RE. Here we investigated the spatial variability of component fluxes of carbon balance (GPP, AR, NPP, litterfall, HR, SR, and RE) in relation to climate factors, between carbon fluxes, and to NEP using observations compiled from the literature for 22 forest sites in monsoon Asia. We found that mean annual temperature (MAT) largely relates to the spatial variability of component fluxes in monsoon Asian forests, with stronger positive effect in the mid–high latitude forests than in the low latitude forests, but even stronger relationships were identified between component fluxes regardless of regions. This finding suggests that the spatial variability of carbon fluxes in monsoon Asia is certainly influenced by climatic factors such as MAT, but that the overall spatial variability of AR, NPP, litterfall, HR, SR, and RE is rather controlled by that of productivity (i.e., GPP). Furthermore, component fluxes of the mid–high and low latitude forests showed positive and negative relationships, respectively, with NEP. Further investigation identified a common spatial variability in NEP and annual aboveground biomass changes with respect to GPP. The relationship between GPP and NEP in the mid–high latitudes implies that productivity and net carbon sequestration increase simultaneously in boreal and temperate forests. Meanwhile, the relationship between GPP and NEP in the low latitudes indicates that net carbon sequestration decreases with productivity, potentially due to the regional contrast in nitrogen depositions and stand age within sub-tropical and tropical forests; however, it requires further data syntheses or modelling investigations for confirmation of its general validity. These unique features of monsoon Asian forest carbon fluxes provide useful information for improving ecosystem model simulations, which still differ in their predictability of carbon flux variability.
... Observed increase of above-ground production in C 3 pastures is about +10% (Nowak et al., 2004;Ainsworth and Long, 2005). For commercial forestry, slowgrowing trees may respond little to elevated CO 2 (e.g., Vanhatalo et al., 2003), and fast-growing trees more strongly, with harvestable wood increases of +15-25% at 550 ppm and high N (Calfapietra et al., 2003;Liberloo et al., 2005;Wittig et al., 2005). Norby et al. (2005) found a mean tree net primary production (NPP) response of 23% in young tree stands; however in mature tree stands Korner et al. (2005) reported no stimulation. ...
W.E.Easterling, P.K. Aggarwal, P. Batima, K.M. Brander, L. Erda, S.M. Howden, A. Kirilenko, J. Morton, J.-F. Soussana, J. Schmidhuber and F.N. Tubiello, 2007: Food, fibre and forest products. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani,
J.P. Palutikof, P.J. van der Linden,C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 273-313.
... Observed increase of above-ground production in C 3 pastures is about +10% (Nowak et al., 2004;Ainsworth and Long, 2005). For commercial forestry, slowgrowing trees may respond little to elevated CO 2 (e.g., Vanhatalo et al., 2003), and fast-growing trees more strongly, with harvestable wood increases of +15-25% at 550 ppm and high N (Calfapietra et al., 2003;Liberloo et al., 2005;Wittig et al., 2005). Norby et al. (2005) found a mean tree net primary production (NPP) response of 23% in young tree stands; however in mature tree stands Korner et al. (2005) reported no stimulation. ...
In mid- to high-latitude regions,moderatewarming benefits crop and pasture yields, but even slightwarming decreases yields in seasonally dry and low-latitude regions (medium confidence). Modelling results for a range of sites find that, in mid- to high- latitude regions, moderate to medium local increases in temperature (1-3ºC), along with associated carbon dioxide (CO2) increase and rainfall changes, can have small beneficial impacts on crop yields. In low-latitude regions, even moderate temperature increases (1-2°C) are likely to have negative yield impacts for major cereals. Further warming has increasingly negative impacts in all regions (medium to low confidence) [Figure 5.2]. These results, on the whole, project the potential for global food production to increase with increases in local average temperature over a range of 1 to 3ºC, but above this range to decrease [5.4, 5.6]. The marginal increase in the number of people at risk of hunger due to climate change must be viewed within the overall large reductions due to socio-economic development (medium confidence). Compared to 820 million undernourished today, the IPCC Special Report on Emissions Scenarios (SRES) scenarios of socio-economic development without climate change project a reduction to 100-230 million (range is over A1, B1, B2 SRES scenarios) undernourished by 2080 (or 770 million under the A2 SRES scenario) (medium confidence). Scenarios with climate change project 100-380 million (range includes with and without CO2 effects and A1, B1, B2 SRES scenarios) undernourished by 2080 (740-1,300 million underA2) (low to medium confidence). Climate and socio-economic changes combine to alter the regional distribution of hunger, with large negative effects on sub-Saharan Africa (low to medium confidence) [Table 5.6]. Projected changes in the frequency and severity of extreme climate events have significant consequences for food and forestry production, and food insecurity, in addition to impacts of projected mean climate (high confidence). Recent studies indicate that climate change scenarios that include increased frequency of heat stress, droughts and flooding events reduce crop yields and livestock productivity beyond the impacts due to changes in mean variables alone, creating the possibility for surprises [5.4.1, 5.4.2]. Climate variability and change also modify the risks of fires, and pest and pathogen outbreaks, with negative consequences for food, fibre and forestry (FFF) (high confidence) [5.4.1 to 5.4.5]. Simulations suggest rising relative benefits of adaptation with low to moderate warming (medium confidence), although adaptation stresses water and environmental resources as warming increases (low confidence). There are multiple adaptation options that imply different costs, ranging fromchanging practices in place to changing locations of FFF activities [5.5.1]. Adaptation effectiveness varies from Food, Fibre and Forest Products only marginally reducing negative impacts to changing a negative impact into a positive one. On average, in cereal cropping systems worldwide, adaptations such as changing varieties and planting times enable avoidance of a 10-15% reduction in yield corresponding to 1-2°C local temperature increase. The benefit from adapting tends to increase with the degree of climate change up to a point [Figure 5.2]. Adaptive capacity in low latitudes is exceeded at 3°C local temperature increase [Figure 5.2, Section 5.5.1]. Changes in policies and institutions will be needed to facilitate adaptation to climate change. Pressure to cultivate marginal land or to adopt unsustainable cultivation practices as yields drop may increase land degradation and resource use, and endanger biodiversity of both wild and domestic species [5.4.7]. Adaptation measures must be integrated with development strategies and programmes, country programmes and Poverty Reduction Strategies [5.7]. Smallholder and subsistence farmers, pastoralists and artisanal fisherfolkwill suffer complex, localised impacts of climate change (high confidence). These groups, whose adaptive capacity is constrained, will experience the negative effects on yields of low-latitude crops, combined with a high vulnerability to extreme events. In the longer term, there will be additional negative impacts of other climate-related processes such as snow-pack decrease (especially in the Indo-Gangetic Plain), sea level rise, and spread in prevalence of human diseases affecting agricultural labour supply. [5.4.7] Globally, commercial forestry productivity rises modestly with climate change in the short and medium term, with large regional variability around the global trend (medium confidence). The change in the output of global forest products ranges from a modest increase to a slight decrease, although regional and local changes will be large [5.4.5.2]. Production increase will shift from low-latitude regions in the short-term, to high- latitude regions in the long-term [5.4.5]. Local extinctions of particular fish species are expected at edges of ranges (high confidence). Regional changes in the distribution and productivity of particular fish species are expected due to continued warming and local extinctions will occur at the edges of ranges, particularly in freshwater and diadromous species (e.g., salmon, sturgeon). In some cases ranges and productivity will increase [5.4.6]. Emerging evidence suggests that meridional overturning circulation is slowing, with serious potential consequences for fisheries (medium confidence) [5.4.6]. Food and forestry trade is projected to increase in response to climate change, with increased dependence on food imports for most developing countries (medium to low confidence). While the purchasing power for food is reinforced in the period to 2050 by declining real prices, it would be adversely affected by higher real prices for food from2050 to 2080. [5.6.1, 5.6.2]. 275
... These results suggest that increasing temperature and precipitation may have a negative impact on forest productivity in the near future. In addition, changing atmosphere concentration (e.g., CO 2 (Nowak et al., 2004;Wittig et al., 2005), O 3 (Cojocariu et al., 2005;Oksanen et al., 2005) and N (Aber et al., 1995a;Huang et al., 2014)) may also affect forest's productivity. In our study, we incorporated the effect of CO 2 concentrations on the stomata conductance of different species (Saxe et al., 1988;Medlyn et al., 2001). ...
In the past three decades, our global climate has been experiencing unprecedented warming. This warming has and will continue to significantly influence the structure and function of forest ecosystems. While studies have been conducted to explore the possible responses of forest landscapes to future climate change, the representative concentration pathways (RCPs) scenarios under the framework of the Coupled Model Intercomparison Project Phase 5 (CMIP5) have not been widely used in quantitative modeling research of forest landscapes. We used LANDIS-II, a forest dynamic landscape model, coupled with a forest ecosystem process model (PnET-II), to simulate spatial interactions and ecological succession processes under RCP scenarios, RCP2.6, RCP4.5 and RCP8.5, respectively. We also modeled a control scenario of extrapolating current climate conditions to examine changes in distribution and aboveground biomass (AGB) among five different forest types for the period of 2010–2100 in Taihe County in southern China, where subtropical coniferous plantations dominate. The results of the simulation show that climate change will significantly influence forest distribution and AGB. (i) Evergreen broad-leaved forests will expand into Chinese fir and Chinese weeping cypress forests. The area percentages of evergreen broad-leaved forests under RCP2.6, RCP4.5, RCP8.5 and the control scenarios account for 18.25%, 18.71%, 18.85% and 17.46% of total forest area, respectively. (ii) The total AGB under RCP4.5 will reach its highest level by the year 2100. Compared with the control scenarios, the total AGB under RCP2.6, RCP4.5 and RCP8.5 increases by 24.1%, 64.2% and 29.8%, respectively. (iii) The forest total AGB increases rapidly at first and then decreases slowly on the temporal dimension. (iv) Even though the fluctuation patterns of total AGB will remain consistent under various future climatic scenarios, there will be certain responsive differences among various forest types.
... Pellston, Michigan, USA OTC (OBRB) Cuttings 1 growing season N Curtis et al., 1995;Pregitzer et al., 1995;Randlett et al., 1996;Lussenhop et al., 1998 P. Ceulemans et al., 1995aCeulemans et al., ,b,c, 1996Ceulemans et al., , 1997Kalina and Ceulemans, 1997;Will and Ceulemans, 1997 Calfapietra et al., 2001Calfapietra et al., , 2003aCalfapietra et al., ,b, 2005Ferris et al., 2001;Gielen et al., , 2002Gielen et al., , 2003aMiglietta et al., 2001;Taylor et al., 2001Taylor et al., , 2003Taylor et al., , 2005Bernacchi et al., 2003;Lukac et al., 2003;Hoosbeek et al., 2004;King et al., 2004;Liberloo et al., 2004Liberloo et al., , 2005Tricker et al., 2004Tricker et al., , 2005Cotrufo et al., 2005;Luo et al., 2005;Moscatelli et al., 2005a,b;Wittig et al., 2005 P. King et al., 2001b;Noormets et al., 2001;Oksanen et al., 2001;Takeuchi et al., 2001;Karnosky et al., 2002Karnosky et al., , 2003bKarnosky et al., , 2005 increase in biomass. Soil respiration will likely increase, as has been observed in large-scale field experiments (King et al., 2004) and at the Biosphere 2 Laboratory in Arizona, USA . ...
This book entitled 'Poplars and Willows: Trees for Society and the Environment' contains twelve chapters. The poplars could equally well be willows, since they are clearly of a single, identified taxon (Chapter 2), selected originally from naturally occurring genetic resources (Chapter 3), but having undergone a process of domestication (Chapter 4) to enhance productivity and perhaps resistance to diseases (Chapter 8) and damaging insects (Chapter 9). The procedures for operationally producing poplar planting material, and for ensuring successful establishment and growth once planted, have been developed, honed and adapted to different regions of the world (Chapter 5). The trees provide shelter, an environmental benefit, to the field crop (Chapter 6). The scientist needs to be aware of the stresses placed on the agroforestry ecosystem by abiotic factors such as drought, salinity and the changing global climate (Chapter 7). The trees in the older plantation in the photo will soon be ready to harvest for a variety of products (Chapter 10) and the person managing this agroforestry system will need to consider the market trends and future outlook for different poplar products (Chapter 11), as well as for the field crops. By its very nature, the scene is one of support for rural livelihoods and sustainable development (Chapter 12).
... Negative impacts of drought on deciduous forests are also likely (Broadmeadow et al., 2005). Water stress in the south may be partially compensated by increased water-use efficiency (Magnani et al., 2004), elevated CO 2 (Wittig et al., 2005) and increased leaf area index (Kull et al., 2005), although this is currently under debate (Medlyn et al., 2001;Ciais et al., 2004). ...
... Negative impacts of drought on deciduous forests are also likely (Broadmeadow et al., 2005). Water stress in the south may be partially compensated by increased water-use efficiency (Magnani et al., 2004), elevated CO 2 (Wittig et al., 2005) and increased leaf area index (Kull et al., 2005), although this is currently under debate (Medlyn et al., 2001;Ciais et al., 2004). ...
... Despite the canopy-scale feedbacks discussed earlier, most researchers have found stimulation of both GPP and net primary productivity (NPP = the amount of carbon incorporated into plant biomass or GPP-autotrophic respiration) following exposure to elevated level of atmospheric CO 2 . [23][24][25] ...
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... 7. For commercial forestry this would mean that slow-growing trees may respond li le to elevated CO 2 (e.g., Vanhatalo et al. 2003), while fast-growing trees would possibly do so more strongly (Calfapietra et al. 2003;Liberloo et al. 2005;Wi ig et al. 2005). ...
The Amazon basin is a key component of the global carbon cycle, which is itself a
determining factor for global climate. The rainforests in the basin store about 120 billion
metric tons of carbon in their biomass. Indeed, the Amazon rainforest is considered
to be a net carbon sink or reservoir because vegetation growth on average exceeds mortality,
resulting in an annual net sink of between 0.8 to 1.1 billion metric tons of carbon.
In Brazil, the basin is the home of about 25 million people, most in urban areas, but
it also includes several unique indigenous and traditional cultures, and is the largest
repository of global biodiversity. It is larger than the European Union (around 5.2 million
square kilometers) and produces about 20 percent of the world’s fl ow of fresh water
into the oceans.
Current climate trends may be unbalancing this well-regulated system and, in association
with land use changes, may be shifting the region from a carbon sink to a carbon
source. Changing forest structure and behavior would have signifi cant implications for
the local, regional and global carbon and water cycles. Amazon forest dieback would be a
massive event, aff ecting all life-forms that rely on this diverse ecosystem, including humans,
and producing ramifi cations for the entire planet. Clearly, with changes at a global scale at
stake, there is a need to be er understand the risk, and dynamics of Amazon dieback.
Thus, the goal of this study is to assist in understanding the risk of a potential reduction
in biomass density in the Amazon basin induced by climate change impacts (Amazon
dieback) and its implications.
Feedback and fertilization eff ects of elevated atmospheric carbon dioxide levels on
forest ecosystems like the Amazon have proven to be a key unknown in assessing the
risk of Amazon forest dieback under 21st century climate change scenarios. Reducing
this uncertainty ought to be a key priority going forward. In the absence of robust information,
the precautionary principle applies, which in this case suggests that the assumption
of carbon dioxide fertilization being an important factor positively aff ecting
ecosystem resilience of the Amazon cannot be used as a basis for sound policy advice.
Therefore, the study concludes that during this century, the probability of Amazon
dieback is highest in the Eastern Amazon and lowest in the Northwest, but that its severity
increases over time and also is a function of the global greenhouse gas emission
trajectory considered. These results point to the need to avoid reaching a point in global
emissions that would result in an induced loss of Amazon forests. Therefore, Amazon
dieback should be considered a threshold for dangerous climate change.
... Total densities of thrips, hemipterans and lepidopterans were also higher in pitfall traps for the last three years of the study. Several other long-term studies have reported sustained biomass stimulation under long-term exposure to elevated CO 2 (Rasse et al. 2005, Wittig et al. 2005, Kimball et al. 2007) which suggests increased herbivore densities might be a general phenomenon, though such data have not been collected. Thus, in a future world of increased CO 2 some insect herbivores may increase in density. ...
... Consequently, more water is left on the land surface as runoff due to increased plant water use efficiency (Wigley and Jones, 1985). On the other hand, increasing CO 2 also has the potential to increase foliage cover (Betts et al., 1997;Betts, 1999;Wittig et al., 2005). This can consequently lead to increases in the transpiration flux to the atmosphere as well as increased interception losses, resulting in reductions in runoff (Murray, in press). ...
Assessments of water resources by using macro‐scale models tend to be conducted at the continental or large catchment scale. However, security of freshwater supplies is a local issue and thus necessitates study at such a scale. This research aims to evaluate the suitability of the Land Processes and eXchanges dynamic global vegetation model (LPX‐DGVM) for simulating runoff for small catchments in the UK. Simulated annual and monthly runoff is compared against the National River Flow Archive streamflow observations from 12 catchments of varying size (500–10 000 km ² ) and climate regimes.
Results show that LPX reproduces observed inter‐annual and intra‐annual runoff variability successfully in terms of both flow timings and magnitudes. Inter‐annual variability in flow timings is simulated particularly well (as indicated by Willmott's index of agreement values of ≥0.7 for the majority of catchments), whereas runoff magnitudes are generally slightly overestimated. In the densely populated Thames catchment, these overestimations are partly accounted for by water consumption. Seasonal variability in runoff is also modelled well, as shown by Willmott's index of agreement values of ≥0.9 for all but one catchment. Absence of river routing and storage from the model, in addition to precipitation uncertainties, is also suggested as contributing to simulated runoff discrepancies.
Overall, the results show that the LPX‐DGVM can successfully simulate runoff processes for small catchments in the UK. This study offers promising insights into the use of global‐scale models and datasets for local‐scale studies of water resources, with the eventual aim of providing local‐scale projections of future water distributions. Copyright © 2013 John Wiley & Sons, Ltd.
... The [CO 2 ] in FACE may be below the threshold to trigger photosynthetic down-regulation. The fact that photosynthetic down-regulation occurs in some species and (or) genotypes (Atkinson et al. 1997;Rey and Jarvis 1998;Roberntz and Stockfors 1998;Tissue et al. 1999;Urban and Marek 1999;Laitinen et al. 2000;Kubiske et al. 2002) but not in others (Liu and Teskey 1995;Ceulemans et al. 1997;Maier et al. 2002;Nowak et al. 2004;Wittig et al. 2005) suggests that the threshold may vary with species and genotype. ...
To investigate the genetic variation of trembling aspen (Populus tremuloides Michx.) in ecophysiological responses to [CO2] elevation, 1-year-old seedlings of four provenances (three families per provenance) from northwestern Ontario were exposed to three [CO2] levels in the greenhouse: ambient (360 ppm), 1.5 x ambient (540 ppm), and 2 x ambient (720 ppm). Biomass and foliage gas exchange were examined after 60 d of treatment. [CO2] elevation significantly increased the rate of net CO 2 assimilation and photosynthetic water use efficiency. The stimulation was generally greater in the 540 ppm [CO2] than in 720 ppm [CO2]. The 720 ppm [CO2] resulted in a 10% photosynthetic down-regulation, but no down-regulation was detected in the 540 ppm CO2 treatment. The 540 ppm [CO2] (but not the 720 ppm) treatment significantly decreased stomatal conductance and transpiration rate in the provenances from the Great Lakes - St. Lawrence Region but not in those from the Boreal Region. The intercellular to ambient CO2 concentration ratio (Ci/Ca) was significantly higher under 720 ppm [CO 2] than under the other two [CO2]. The CO2 elevations generally increased the total and root biomass, and the stimulation was greater in the 540 ppm [CO2] than in the 720 ppm [CO2] treatment. The two provenances from the Great Lakes - St. Lawrence region generally had greater biomasses than those from the boreal region, while there were no significant differences between them in the physiological variables. However, we did not find any significant differences between provenances in the responses of biomass to [CO2] treatments.
... Oren et al. (2001) reported such a transitory stem growth response for P. taeda. A decreasing growth stimulation with time was also observed in the POPFACE experiment in Italy with Populus saplings (Miglietta et al., 2001, Wittig et al., 2005. In this fast growing, young plantation, gross primary production was greatest in the first year (223-251%), but markedly lower in the second (19-24%) and third year (5-19%). ...
Are mature forest trees carbon limited at current CO2 concentrations? Will ‘mid-life’, 35 m tall deciduous trees grow faster in a CO2-enriched atmosphere? To answer these questions we exposed ca. 100-year-old temperate forest trees at the Swiss Canopy Crane site near Basel, Switzerland to a ca. 540 ppm CO2 atmosphere using web-FACE technology. Here, we report growth responses to elevated CO2 for 11 tall trees (compared with 32 controls) of five species during the initial four treatment years. Tested across all trees, there was no CO2 effect on stem basal area (BA) increment (neither when tested per year nor cumulatively for 4 years). In fact, the 4th year means were almost identical for the two groups. Stem growth data were standardized by pretreatment growth (5 years) in order to account for a priori individual differences in vigor. Although this experiment was not designed to test species specific effects, one species, the common European beech, Fagus sylvatica, showed a significant growth enhancement in the first year, which reoccurred during a centennial drought in the third year. None of the other dominant species (Quercus petraea, Carpinus betulus) showed a growth response to CO2 in any of the 4 years or for all years together. The inclusion or exclusion of single individuals of Prunus avium and Tilia platyphyllos did not change the picture. In elevated CO2, lateral branching in terminal shoots was higher in Fagus in 2002, when shoots developed from buds that were formed during the first season of CO2 enrichment (2001), but there was no effect in later years and no change in lateral branching in any of the other species. In Quercus, there was a steady stimulation of leading shoot length in high-CO2 trees. Phenological variables (bud break, leaf fall, leaf duration) were highly species specific and were not affected by elevated CO2 in any consistent way. Our 4-year data set reflects a very dynamic and species-specific response of tree growth to a step change in CO2 supply. Stem growth after 4 years of exposure does not support the notion that mature forest trees will accrete wood biomass at faster rates in a future CO2-enriched atmosphere.
... Tel: +1-217-333-8048;e-mail: bernacch@illinois.edu modelling schemes that consider larger spatiotemporal scales, including canopies (Wang & Jarvis 1990;Amthor 1994;Lloyd & Farquhar 1996;dePury & Farquhar 1997;Wittig et al. 2005;Drewry et al. 2010a,b), ecosystems (Field & Avissar 1998) and landscapes (Sellers et al. 1996(Sellers et al. , 1997. The leaf A model is also a key component of earth system models (Cramer et al. 2001;Medvigy et al. 2009). ...
Globally, photosynthesis accounts for the largest flux of CO2 from the atmosphere into ecosystems and is the driving process for terrestrial ecosystem function. The importance of accurate predictions of photosynthesis over a range of plant growth conditions led to the development of a C3 photosynthesis model by Farquhar, von Caemmerer & Berry (1980) that has become increasingly important as society places greater pressures on vegetation. The photosynthesis model has played a major role in defining the path toward scientific understanding of photosynthetic carbon uptake and the role of photosynthesis on regulating the earth's climate and biogeochemical systems. In this review, we summarize the photosynthesis model, including its continued development and applications. We also review the implications these developments have on quantifying photosynthesis at a wide range of spatial and temporal scales, and discuss the model's role in determining photosynthetic responses to changes in environmental conditions. Finally, the review includes a discussion of the larger-scale modeling and remote sensing applications that rely on the leaf photosynthesis model and are likely to open new scientific avenues to address the increasing challenges to plant productivity over the next century.
Supplementary resource:
http://demonstrations.wolfram.com/author.html?author=Carl+J.+Bernacchi%2C+Justin+E.+Bagley%2C+Shawn+P.+Serbin%2C+Ursula+M.+Ruiz-Vera%2C+David+M.+Rosenthal%2C+and+Andy+VanLoocke
... [55] The electron transport rate J is related to absorbed photosynthetically active radiation, but the equation varies among models. We use a common form from von Caemmerer [2000], used also in the plant canopy models of de Pury and Farquhar [1997], Wittig et al. [2005], and Mercado et al. [2009a]. The photosynthetically active radiation varies between sunlit leaves ( sun ) and shaded leaves ( sha ). ...
The Community Land Model version 4 (CLM4) overestimates gross primary production (GPP) compared with data-driven estimates and other process models. We use global, spatially gridded GPP and latent heat flux upscaled from the FLUXNET network of eddy covariance towers to evaluate and improve canopy processes in CLM4. We investigate differences in GPP and latent heat flux arising from model parameterizations (termed model structural error) and from uncertainty in the photosynthetic parameter V(c max) (termed model parameter uncertainty). Model structural errors entail radiative transfer, leaf photosynthesis and stomatal conductance, and canopy scaling of leaf processes. Model structural revisions reduce global GPP over the period 1982-2004 from 165 Pg C yr(-1) to 130 Pg C yr(-1), and global evapotranspiration decreases from 68,000 km(3) yr(-1) to 65,000 km(3) yr(-1), within the uncertainty of FLUXNET-based estimates. Colimitation of photosynthesis is a cause of the improvements, as are revisions to photosynthetic parameters and their temperature dependency. Improvements are seen in all regions and seasonally over the course of the year. Similar improvements occur in latent heat flux. Uncertainty in V(c max) produces effects of comparable magnitude as model structural errors, but of offsetting sign. This suggests that model structural errors can be compensated by parameter adjustment, and this may explain the lack of consensus in values for V(c max) used in terrestrial biosphere models. Our analyses show that despite inherent uncertainties global flux fields empirically inferred from FLUXNET data are a valuable tool to guide terrestrial biosphere model development and evaluation.
... Several other long-term field studies have reported sustained biomass stimulation (Rasse et al., 2005; Wittig et al., 2005; Erickson et al., 2007; Kimball et al., 2007). Our results are unique in which stimulation was sustained after the community reached canopy closure (Ainsworth et al., 2002; Li et al., 2007b), root closure (Day et al., 2006), and displayed nitrogen limitation (Hungate et al., 2006): three critical constraints predicted to limit CO 2 response (Norby et al., 1999; Kö rner, 2006). ...
This study reports the aboveground biomass response of a fire-regenerated Florida scrub-oak ecosystem exposed to elevated CO2 (1996–2007), from emergence after fire through canopy closure. Eleven years exposure to elevated CO2 caused a 67% increase in aboveground shoot biomass. Growth stimulation was sustained throughout the experiment; although there was significant variability between years. The absolute stimulation of aboveground biomass generally declined over time, reflecting increasing environmental limitations to long-term growth response. Extensive defoliation caused by hurricanes in September 2004 was followed by a strong increase in shoot density in 2005 that may have resulted from reopening the canopy and relocating nitrogen from leaves to the nutrient-poor soil. Biomass response to elevated CO2 was driven primarily by stimulation of growth of the dominant species, Quercus myrtifolia, while Quercus geminata, the other co-dominant oak, displayed no significant CO2 response. Aboveground growth also displayed interannual variation, which was correlated with total annual rainfall. The rainfall × CO2 interaction was partially masked at the community level by species-specific responses: elevated CO2 had an ameliorating effect on Q. myrtifolia growth under water stress. The results of this long-term study not only show that atmospheric CO2 concentration had a consistent stimulating effect on aboveground biomass production, but also showed that available water is the primary driver of interannual variation in shoot growth and that the long-term response to elevated CO2 may have been caused by other factors such as nutrient limitation and disturbance.
... Curtis et al., 2000) studies. Work at the Aspen FACE site in northern Wisconsin, USA, has sought to assess the response of aspen genotypes to CO 2 and ozone enrichment (Karnosky et al., 2003), while parallel work at POPFACE in Italy has evaluated responses of three species of Populus to CO 2 (Wittig et al., 2005). Both studies document enhanced primary production in trees exposed to enriched CO 2 ; in the Aspen FACE experiment, for example, elevating CO 2 by 55% increased aspen growth rates by 40% (Karnosky et al., 2003). ...
As atmospheric CO2 levels rise, temperate and boreal forests in the Northern Hemisphere are gaining importance as carbon sinks. Quantification of that role, however, has been difficult due to the confounding effects of climate change. Recent large-scale experiments with quaking aspen (Populus tremuloides), a dominant species in many northern forest ecosystems, indicate that elevated CO2 levels can enhance net primary production. Field studies also reveal that droughts contribute to extensive aspen mortality. To complement this work, we analyzed how the growth of wild aspen clones in Wisconsin has responded to historical shifts in CO2 and climate, accounting for age, genotype (microsatellite heterozygosity), and other factors. Aspen growth has increased an average of 53% over the past five decades, primarily in response to the 19.2% rise in ambient CO2 levels. CO2-induced growth is particularly enhanced during periods of high moisture availability. The analysis accounts for the highly nonlinear changes in growth rate with age, and is unaffected by sex or location sampled. Growth also increases with individual heterozygosity, but this heterozygote advantage has not changed with rising levels of CO2 or moisture. Thus, increases in future growth predicted from previous large-scale, common-garden work are already evident in this abundant and ecologically important tree species. Owing to aspen's role as a foundation species in many North American forest ecosystems, CO2-stimulated growth is likely to have repercussions for numerous associated species and ecosystem processes.
Carbon dioxide (CO2) is an important heat-trapping/greenhouse gas that comes from the extraction and burning of fossil fuels (such as coal, oil and natural gas), wildfires and natural processes like volcanic eruptions. A higher concentration of atmospheric CO2 above the ambient is called elevated CO2. Open top chamber (OTC) and free air CO2 enrichment (FACE) technology are used to elevate the atmospheric concentration of CO2. A change in atmospheric CO2 could affect soil carbon storage through changes in plant and microbial activities. Elevated atmospheric CO2 consistently stimulates plant growth, thereby increasing inputs of carbon into soil, mainly through increased detrital production and root exudation. Most of the additional carbon released into the soil in response to elevated CO2 is labile and decomposes quickly. Accelerated decomposition of soil organic matter by stimulated microbial activity as a result of the higher addition of easily degradable root exudates in response to elevated CO2 conditions, is termed the priming effect. Elevated CO2 increases the yield of C3 plants and also increases the labile pool in the soil, but reduces the old organic carbon pools due to the priming effect. Moisture content at field capacity and moderate temperature (35 °C) along with elevated CO2 (up to 650 ppm) increase soil organic carbon. The SRI method of rice cultivation followed by raised bed wheat cultivation increases total soil organic carbon. Soil carbon sequestration under elevated CO2 can only be increased when additional nutrients are supplied.
Carbon dioxide (CO2) is an important heat-trapping/greenhouse gas that comes from the extraction and burning of fossil fuels (such as coal, oil and natural gas), wildfires and natural processes like volcanic eruptions. A higher concentration of atmospheric CO2 above the ambient is called elevated CO2. Open top chamber (OTC) and free air CO2 enrichment (FACE) technology are used to elevate the atmospheric concentration of CO2. A change in atmospheric CO2 could affect soil carbon storage through changes in plant and microbial activities. Elevated atmospheric CO2 consistently stimulates plant growth, thereby increasing inputs of carbon into soil, mainly through increased detrital production and root exudation. Most of the additional carbon released into the soil in response to elevated CO2 is labile and decomposes quickly. Accelerated decomposition of soil organic matter by stimulated microbial activity as a result of the higher addition of easily degradable root exudates in response to elevated CO2 conditions, is termed the priming effect. Elevated CO2 increases the yield of C3 plants and also increases the labile pool in the soil, but reduces the old organic carbon pools due to the priming effect. Moisture content at field capacity and moderate temperature (35 °C) along with elevated CO2 (up to 650 ppm) increase soil organic carbon. The SRI method of rice cultivation followed by raised bed wheat cultivation increases total soil organic carbon. Soil carbon sequestration under elevated CO2 can only be increased when additional nutrients are supplied.
Aerosols can affect crop photosynthesis by altering radiation and meteorological conditions. By combining field observations, mechanistic modeling, and satellite‐retrieved solar‐induced chlorophyll fluorescence (SIF), we assessed aerosols' impacts on crop photosynthesis from leaf to regional scale. We found that the initial increase in aerosol optical depth (AOD) enhanced photosynthesis of sun leaves, shade leaves, and canopy, which reached their maximum at AOD=0.76, 1.13, and 0.93, respectively, and then decreased. Aerosol‐induced changes in radiation regime and the concurrent high relative humidity led to such nonlinear responses. Similarly, the SIF of croplands in the North China Plain also showed a bell‐shaped response to aerosols. The optimal AOD level at which SIF reached the maximum value varied from 0.56 to 1.04, depending on the background meteorological conditions. Approximately 76‐90% of the North China Plain exceeded the optimal AOD level, suggesting that stringent aerosol pollution control could promote cropland productivity in this region.
Determining whether the terrestrial biosphere will be a source or sink of carbon
Mixed results are shown for the leaf area index of small, dense stands of young deciduous trees in soil-litter-plant enclosures (model ecosystems) at elevated [CO2]. More living biomass is accumulated each year over up to 6 years of growth at almost doubled ambient [CO2]. Examples of daily courses of CO2 gas exchange rates of these stands and canopy gross photosynthesis are calculated by means of the net CO2 gas exchange and dark respiration rates of the whole soil-litter-plant systems under unchanged ambient and elevated [CO2]. System total respiration is shown in response to changes in soil temperature, and selected daily courses illustrate the comparison of total system CO2 gross uptake with the total system and leaf dark respiration. All measured daily courses of system CO2 net assimilation are combined into monthly averages for one experimental year. This shows that the effect of elevated [CO2] on the overall CO2 gas exchange balance of the small tree groups is significantly positive early in the growth season. Water use efficiency is calculated for the whole system using special mathematical formulas (see Chap. 2). A clear reduction in water use at elevated [CO2] occurs at the stand level.
Twenty-five percent of the world’s forests are in the temperate biome. They include a wide range of forest types, and the exact boundaries with boreal forests to the north and tropical forests to the south are not always clear. There is a great variety of species, soil types, and environmental conditions which lead to a diversity of factors affecting carbon storage and flux. Temperate forests have been severely impacted by human use – throughout history, all but about 1% have been logged-over, converted to agriculture, intensively managed, grazed, or fragmented by sprawling development. Nevertheless, they have proven to be resilient – mostly second growth forests now cover about 40–50% of the original extent of the biome. Although remaining intact temperate forests continue to be fragmented by development, particularly in North America, there is no large-scale deforestation at present, nor is there likely to be in the future. The status of the temperate biome as a carbon reservoir and atmospheric CO2 sink rests mainly on strong productivity and resilience in the face of disturbance. The small “sink” status of temperate forests could change to a “source” status if the balance between photosynthesis and respiration shifts.
The alarming and unprecedented rise in the atmo-spheric concentration of greenhouse gases under global climate change warrants an urgent need to understand the synergistic and holistic mechanisms associated with plant growth and productivity. Photosynthesis is a major process of sequestration and turnover of the total carbon on the planet. The extensive literature on the impacts of climate change demonstrates both posi-tive and negative effects of rising CO 2 on photosynthe-sis in different groups of higher plants. Significant variation exists in the physiological, biochemical and molecular responsiveness to elevated CO 2 atmosphere, among terrestrial plant species including those with C 3 , C 4 and crassulacean acid metabolic (CAM) path-ways. However, the regulatory events associated with the inter-and intraspecific metabolic plasticity gov-erned by genetic organization in different plants are little understood. The adaptive acclimation responses of plants to changing climate remain contradictory. This review focuses primarily on the impacts of global climate change on plant growth and productivity with special reference to adaptive photosynthetic acclima-tive responses to elevated CO 2 concentration. The effects of elevated CO 2 concentration on plant growth and development, source–sink balance as well as its interactive mechanisms with other environmental factors including water availability, temperature and mineral nutrition are discussed.
The economy implications that climate change holds for agricultural sectors in 2050 are estimated using a static computable general equilibrium model. A peculiar feature of this exercise is the interfacing of the economic model with a climate model forecasting temperature increase in the relevant year and a crop-growth model estimating climate change impact on cereal productivity. The main results of the study show, on one hand, the limited influence of climate change on world food supply and welfare, but, on the other hand, its important distributional consequences, the negative effects being concentrated mainly on the developing countries. The simulation exercise is introduced by a broad overview of the relevant literature.
Jatropha (Jatropha curcas) is a non-edible oil producing plant which is being advocated as an alternative biofuel energy resource. Its ability to grow in diverse soil conditions and minimal requirements of essential agronomical inputs compared to other oilseed crops makes it viable for cost-effective advanced biofuel production. We designed a study to investigate the effects of elevated [CO2] (550 ppm) on the growth, reproductive development, source-sink relationships, fruit and seed yield of Jatropha curcas. We report, for the first time that elevated CO2 significantly influences reproductive characteristics of Jatropha and improve its fruit and seed yields. Net photosynthetic rate of Jatropha was 50% higher in plants grown in elevated CO2 compared with field and ambient CO2-grown plants. The study also revealed that elevated CO2 atmosphere significantly increased female to male flower ratio, above ground biomass and carbon sequestration potential in Jatropha (24 kg carbon tree−1) after one year. Our data demonstrate that Jatropha curcas was able to sustain enhanced rates of photosynthesis in elevated CO2 conditions as it had sufficient sink strength to balance the increased biomass yields. Our study also elucidates that the economically important traits including fruit and seed yield in elevated CO2 conditions were significantly high in Jatropha curcas that holds great promise as a potential biofuel tree species for the future high CO2 world.
Despite experimental evidence showing that elevated CO2 levels increase growth in most plants, the isolation of a signal consistent with anthropogenically caused increases in atmospheric CO2 from the dendrochronological record has shown mixed results. Our extensive sets of tree ring data from the Ozark Mountains in Missouri showed that since 1850, Quercus velutina Lam., Quercus coccinea Muench., and Pinus echinata Mill. trees increased in stem growth coincidently with increases in atmospheric CO2. Those long-term increases in radial growth appear unrelated to historical disturbance levels for the region, to long-term changes in relevant climatic variables, or to productivity of sites sampled for the purpose of creating a time sequence of tree ring growth. It is still unclear what the potential role of nitrogen deposition might have been for tree growth. We cross-dated a large number of increment cores and aligned the ring width data by pith date for accurate age constant assessments of growth over the past 150 years. Thus, we circumvented changes in growth trend associated with differences in physiological functioning during development, as well as the need for statistical detrending that removes an unknown degree of long-term environmental signal, the so called segment length curse that applies to standard dendrochronological investigations. When the positive relationship between CO2 and ring width was examined at different ages, an ontogenetic decline in the rate of growth stimulation was found. Specifically, both the pooled Quercus spp. and P. echinata were characterized by a negative exponential pattern of response over a developmental sequence through age 50. Further knowledge of an intrinsic decline in CO2 sensitivity with tree age or size such as this may be important for increased accuracy in estimating terrestrial carbon stocks across successional landscapes.
1] Atmospheric CO 2 and China's climate have changed greatly during 1961–2000. The influence of increased CO 2 and changing climate on the carbon cycle of the terrestrial ecosystems in China is still unclear. In this article we used a process-based ecosystem model, Biome-BGC, to assess the effects of changing climate and elevated atmospheric CO 2 on terrestrial China's carbon cycle during two time periods: (1) the present (1961– 2000) and (2) a future with projected climate change under doubled CO 2 (2071–2110). The effects of climate change alone were estimated by driving Biome-BGC with a fixed CO 2 concentration and changing climate, while the CO 2 fertilization effects were calculated as the difference between the results driven by both increasing CO 2 and changing climate and those of variable climate alone. Model simulations indicate that during 1961–2000 at the national scale, changes in climate reduced carbon storage in China's ecosystems, but increasing CO 2 compensated for these adverse effects of climate change, resulting in an overall increase in the carbon storage of China's ecosystems despite decreases in soil carbon. The interannual variability of the carbon cycle was associated with climate variations. Regional differences in climate change produced differing regional carbon uptake responses. Spatially, reductions in carbon in vegetation and soils and increases in litter carbon were primarily caused by climate change in most parts of east China, while carbon in vegetation, soils, and litter increased for much of west China. Under the future scenario (2071–2110), with a doubling CO 2 , China will experience higher precipitation and temperature as predicted by the Hadley Centre HadCM3 for the Intergovernmental Panel on Climate Change Fourth Assessment. The concomitant doubling of CO 2 will continue to counteract the negative effects of climate change on carbon uptake in the future, leading to an increase in carbon storage relative to current levels. This study highlights the role of CO 2 fertilization in the carbon budget of China's ecosystems, although future studies should include other important processes such as land use change, human management (e.g., fertilization and irrigation), environmental pollution, etc.
The physiological characteristics of holm oak (Quercus ilex L.) resprouts originated from plants grown under current CO2 concentration (350 μl l−1) (A-resprouts) were compared with those of resprouts originated from plants grown under elevated CO2 (750 μl l−1) (E-resprouts). At their respective CO2 growth concentration, no differences were observed in photosynthesis and chlorophyll fluorescence parameters between the two kinds of resprout. E-resprouts appeared earlier and showed lower stomatal conductance, higher water-use efficiency and increased growth (higher leaf, stem and root biomass and increased height). Analyses of leaf chemical composition showed the effect of elevated [CO2] on structural polysaccharide (higher cellulose content), but no accumulation of total non-structural carbohydrate on area or dry weight basis was seen. Four months after appearance, downregulation of photosynthesis and electron transport components was observed in E-resprouts: lower photosynthetic capacity, photosystem II quantum efficiency, photochemical quenching of fluorescence and relative electron transport rate. Reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo) activity, deduced from the maximum carboxylation velocity of RuBisCo, accounts for the observed acclimation. Increased susceptibility of photosynthetic apparatus to increasing irradiance was detected in E-resprouts.
Various aspects of the biochemistry of photosynthetic carbon assimilation in C3 plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylase-oxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO2(p(CO2)) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O2(p(O2)), the dependence of net CO2 assimilation rate on p(CO2) and irradiance, and the influence of p(CO2) and irradiance on the temperature dependence of assimilation rate.
CO{sub 2} concentration was elevated throughout 3 years around stands of the C{sub 3} sedge Scirpus olneyi on a tidal marsh of the Chesapeake Bay. The hypothesis that tissues developed in an elevated CO{sub 2} atmosphere will show an acclimatory decrease in photosynthetic capacity under light-limiting conditions was examined. The absorbed light quantum yield of CO{sub 2} uptake ({phi}{sub abs}) and the efficiency of photosystem II photochemistry were determined for plants which had developed in open top chambers with CO{sub 2} concentrations in air of 680 micromoles per mole, and of 351 micromoles per mole as controls. When measured in an atmosphere with 10 millimoles per mole O{sub 2} to suppress photorespiration, shoots showed a {phi}{sub abs} of 0.093 {plus minus} 0.003, with no statistically significant difference between shoots grown in elevated or control CO{sub 2} concentration. Efficiency of photosystem II photochemistry was also unchanged by development in an elevated CO{sub 2} atmosphere. Shoots grown and measured in 680 micromoles per mole of CO{sub 2} in air showed a {phi}{sub abs} of 0.078 {plus minus} 0.004 compared with 0.065 {plus minus} for leaves grown and measured in 351 micromoles per mole CO{sub 2} in air; a highly significant increase. In accordance with the change in {phi}{sub abs}, the light compensation point of photosynthesis decreased from 51 {plus minus} 3 to 31 {plus minus} 3 micromoles per square meter per second for stems grown and measured in 351 and 680 micromoles per mole of CO{sub 2} in air, respectively.
Free-air CO2 enrichment (FACE) technology was used to expose a loblolly pine (Pinus taeda L.) forest to elevated atmospheric CO2 (ambient + 200 µl l–1). After 4 years, basal area of pine trees was 9.2% larger in elevated than in ambient CO2 plots. During the first 3 years the growth rate of pine was stimulated by ~26%. In the fourth year this stimulation declined to 23%. The average net ecosystem production (NEP) in the ambient plots was 428 gC m–2 year–1, indicating that the forest was a net sink for atmospheric CO2. Elevated atmospheric CO2 stimulated NEP by 41%. This increase was primarily an increase in plant biomass increment (57%), and secondarily increased accumulation of carbon in the forest floor (35%) and fine root increment (8%). Net primary production (NPP) was stimulated by 27%, driven primarily by increases in the growth rate of the pines. Total heterotrophic respiration (R
h) increased by 165%, but total autotrophic respiration (R
a) was unaffected. Gross primary production was increased by 18%. The largest uncertainties in the carbon budget remain in separating belowground heterotrophic (soil microbes) and autotrophic (root) respiration. If applied to temperate forests globally, the increase in NEP that we measured would fix less than 10% of the anthropogenic CO2 projected to be released into the atmosphere in the year 2050. This may represent an upper limit because rising global temperatures, land disturbance, and heterotrophic decomposition of woody tissues will ultimately cause an increased flux of carbon back to the atmosphere.
The mixed model approach to the analysis of repeated measurements allows users to model the covariance structure of their data. That is, rather than using a univariate or a multivariate test statistic for analyzing effects, tests that assume a particular form for the covariance structure, the mixed model approach allows the data to determine the appropriate structure. Using the appropriate covariance structure should result in more powerful tests of the repeated measures effects according to advocates of the mixed model approach. SAS' (SAS Institute, 1996) mixed model program, PROC MIXED, provides users with two information criteria for selecting the `best' covariance structure, Akaike (1974) and Schwarz (1978). Our study compared these log likelihood tests to see how effective they would be for detecting various population covariance structures. In particular, the criteria were compared in nonspherical repeated measures designs having equal/unequal group sizes and covariance matrices when data were both normally and nonnormally distributed. The results indicate that neither criterion was effective in finding the correct structure. On average, for the 26 investigated distributions, the Akaike criterion only resulted in the correct structure being selected 47 percent of the time while the Schwarz criterion resulted in the correct structure being selected just 35 percent of the time.
Stem diameter, total plant height and number of sylleptic branches of three poplar (Populus) genotypes were followed during the first growing season of a high density intensively cultured plantation (in Central Italy) both under ambient CO2 (Control) and under elevated atmospheric CO2 (550 ppm) using the FACE technique. The three poplar genotypes belonged to different species of Populus alba L., Populus nigra L. and Populus x euramericana Dode (Guinier). All three genotypes responded by an enhanced growth performance but the extent of their response to the FACE treatment was different. A stem volume index was calculated considering the stem composed by a truncated cone in the lower part and by a cone in the upper part. At the end of the first growing season, stem volume index was increased in the FACE treatment by 54% to 79% as compared to Control treatment, depending on the genotype. This increased stem volume index was caused by an increase of basal stem diameter rather than by an enhancement of plant height. Number of sylleptic branches was stimulated by more than 35% in the P. nigra genotype. The results confirm the optimal performance of this new POPFACE experiment and show the positive response of this fast-growing tree species to elevated CO2 conditions at an ecosystem scale even if considering the genotypic differences.
A new design of free‐air CO 2 enrichment (FACE) is presented that has been used to expose a poplar plantation to elevated atmospheric CO 2 concentrations in other‐wise unaltered conditions, in the open.
This system releases pure CO 2 at high velocity, through a large number of small gas jets, causing rapid mixing between CO 2 and air. The theoretical and practical aspects of this design are described, with emphasis on the fluid mechanics of air–CO 2 mixing in sonic jets. Field performance data, including spectral analysis of short‐term fluctuations in CO 2 concentrations as well as temporal and spatial CO 2 control, are reported for the European project POPFACE facility.
Temporal and spatial performances of the operational POPFACE systems were adequate with average long‐term CO 2 mole fractions on target. Averages over 1 min measured in the centre of the rings were within ±20% and ±10% of the target concentration for > 91% and > 75% of the time, respectively.
The data presented provide convincing evidence that a pure‐CO 2 FACE system can achieve reliable control, in terms of the quality of the CO 2 control, with significant simplification of construction and reduced capital cost.
Short- and long-term effects of elevated CO2 concentration and temperature on whole plant respiratory relationships are examined for wheat grown at four constant temperatures and at two CO2 concentrations. Whole plant CO2 exchange was measured on a 24 h basis and measurement conditions varied both to observe short-term effects and to determine the growth respiration coefficient (rg), dry weight maintenance coefficient (rm), basal (i.e. dark acclimated) respiration coefficient (rg), and 24 h respiration:photosynthesis ratio (R:P). There was no response of rg to short-term variation in CO2 concentration. For plants with adequate N supply, rg was unaffected by the growth-CO2 despite a 10% reduction in the plant's N concentration (%N). However, rm was decreased 13%, and rb was decreased 20% by growth in elevated CO2 concentration relative to ambient. Nevertheless, R:P was not affected by growth in elevated CO2. Whole plant respiration responded to short-term variation of ± 5 °C around the growth temperature with low sensitivity (Q10= 1.8 at 15 °C, 1.3 at 30 °C). The shape of the response of whole plant respiration to growth temperature was different from that of the short term response, being a slanted S-shape declining between 25 and 30 °C. While rm, increased, rg decreased when growth temperature increased between 15 and 20 °C. Above 20 °C rm became temperature insensitive while rg increased with growth temperature. Despite these complex component responses, R:P increased only from 0.40 to 0.43 between 15° and 30 °C growth temperatures. Giving the plants a step increase in temperature caused a transient increase in R:P which recovered to the pre-transient value in 3 days. It is concluded that use of a constant R:P with respect to average temperature and CO2 concentration may be a more simple and accurate way to model the responses of wheat crop respiration to ‘climate change’ than the more complex and mechanistically dubious functional analysis into growth and maintenance components.
A free-air CO2 enrichment (FACE) system was designed to permit the experimental exposure of tall vegetation such as stands of forest trees to elevated atmospheric CO2 concentrations ([CO2]a) without enclosures that alter tree microenvironment. We describe a prototype FACE system currently in operation in forest plots in a maturing loblolly pine (Pinus taeda L.) stand in North Carolina, USA. The system uses feedback control technology to control [CO2] in a 26 m diameter forest plot that is over 10 m tall, while monitoring the 3D plot volume to characterize the whole-stand CO2 regime achieved during enrichment. In the second summer season of operation of the FACE system, atmospheric CO2 enrichment was conducted in the forest during all daylight hours for 96.7% of the scheduled running time from 23 May to 14 October with a preset target [CO2] of 550 μmol mol–1, ≈ 200 μmol mol–1 above ambient [CO2]. The system provided spatial and temporal control of [CO2] similar to that reported for open-top chambers over trees, but without enclosing the vegetation. The daily average daytime [CO2] within the upper forest canopy at the centre of the FACE plot was 552 ± 9 μmol mol–1 (mean ± SD). The FACE system maintained 1-minute average [CO2] to within ± 110 μmol mol–1 of the target [CO2] for 92% of the operating time. Deviations of [CO2] outside of this range were short-lived (most lasting < 60 s) and rare, with fewer than 4 excursion events of a minute or longer per day. Acceptable spatial control of [CO2] by the system was achieved, with over 90% of the entire canopy volume within ± 10% of the target [CO2] over the exposure season. CO2 consumption by the FACE system was much higher than for open-top chambers on an absolute basis, but similar to that of open-top chambers and branch bag chambers on a per unit volume basis. CO2 consumption by the FACE system was strongly related to windspeed, averaging 50 g CO2 m–3 h–1 for the stand for an average windspeed of 1.5 m s–1 during summer. The [CO2] control results show that the free-air approach is a tractable way to study long-term and short-term alterations in trace gases, even within entire tall forest ecosystems. The FACE approach permits the study of a wide range of forest stand and ecosystem processes under manipulated [CO2]a that were previously impossible or intractable to study in true forest ecosystems.
Free-air CO2 enrichment (FACE) technology was used to expose a loblolly pine (Pinus taeda L.) forest to elevated atmospheric CO2 (ambient + 200 l l-1). After 4 years, basal area of pine trees was 9.2% larger in elevated than in ambient CO2 plots. During the first 3 years the growth rate of pine was stimulated by ~26%. In the fourth year this stimulation declined to 23%. The average net ecosystem production (NEP) in the ambient plots was 428 gC m-2 year-1, indicating that the forest was a net sink for atmospheric CO2. Elevated atmospheric CO2 stimulated NEP by 41%. This increase was primarily an increase in plant biomass increment (57%), and secondarily increased accumulation of carbon in the forest floor (35%) and fine root increment (8%). Net primary production (NPP) was stimulated by 27%, driven primarily by increases in the growth rate of the pines. Total heterotrophic respiration (Rh) increased by 165%, but total autotrophic respiration (Ra) was unaffected. Gross primary production was increased by 18%. The largest uncertainties in the carbon budget remain in separating belowground heterotrophic (soil microbes) and autotrophic (root) respiration. If applied to temperate forests globally, the increase in NEP that we measured would fix less than 10% of the anthropogenic CO2 projected to be released into the atmosphere in the year 2050. This may represent an upper limit because rising global temperatures, land disturbance, and heterotrophic decomposition of woody tissues will ultimately cause an increased flux of carbon back to the atmosphere.
Atmospheric CO(2) concentration ([CO(2)]) is now higher than it was at any time in the past 26 million years and is expected to nearly double during this century. Terrestrial plants with the C(3) photosynthetic pathway respond in the short term to increased [CO(2)] via increased net photosynthesis and decreased transpiration. In the longer term this increase is often offset by downregulation of photosynthetic capacity. But much of what is currently known about plant responses to elevated [CO(2)] comes from enclosure studies, where the responses of plants may be modified by size constraints and the limited life-cycle stages that are examined. Free-Air CO(2) Enrichment (FACE) was developed as a means to grow plants in the field at controlled elevation of CO(2) under fully open-air field conditions. The findings of FACE experiments are quantitatively summarized via meta-analytic statistics and compared to findings from chamber studies. Although trends agree with parallel summaries of enclosure studies, important quantitative differences emerge that have important implications both for predicting the future terrestrial biosphere and understanding how crops may need to be adapted to the changed and changing atmosphere.
In the past, stomatal responses have generally been considered in relation to single environmental variables in part because the interactions between factors have appeared difficult to quantify in a simple way. A linear correlation between stomatal conductance (g) and CO2 assimilation rate (A) has been reported when photon fluence was varied and when the photosynthetic capacity of leaves was altered by growth conditions, provided CO2, air humidity and leaf temperature were constant (1). Temperature and humidity are, however, not consistent in nature. Lack of a concise description of stomatal responses to combinations of environmental factors has limited attempts to integrate these responses into quantitative models of leaf energy balance, photosynthesis, and transpiration. Moreover, this lack has hindered progress toward understanding the stomatal mechanism. We have taken a multi-variant approach to the study of stomatal conductance and we show that under many conditions the responses of stornata can be described by a set of linear relationships. This model can be linked to models of leaf carbon metabolism and the environment to predict fluxes of CO2, H2O and energy. In this paper, we show how the model of conductance can be linked to a description of CO2 assimilation as a function of intercellular CO2 (whether empirical or the output of a model) to predict the distribution of flux control between the stornata and leaf “biochemistry” under conditions in a gas-exchange cuvette.
Leaf photosynthesis (Ps), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free-air CO 2 enrichment in Wis-consin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO 2 , respectively , at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloud-free days (PPFD day,sat) around the lowermost branches was 16·8 ± ± ± ± 0·8 and 8·7 ± ± ± ± 0·2% of values at the top for the ambient and elevated CO 2 canopies, respectively. Elevated CO 2 significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFD day,sat throughout the canopies, and elevated CO 2 did not affect that relationship. Leaf Ps light-response curves responded differently to elevated CO 2 , depending upon canopy position. Elevated CO 2 increased Ps sat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO 2. Consequently, estimated daily integrated Ps on cloud-free days (Ps day,sat) was stimulated by elevated CO 2 only in the upper canopy. Ps day,sat of the lowermost branches was actually lower with elevated CO 2 because of the darker light environment. The lack of CO 2 stimulation at the mid-and lower canopy was probably related to significant down-regulation of photosynthetic capacity; there was no down-regulation of Ps in the upper canopy. The relationship between Ps day,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole-canopy Ps or photosynthetic N use efficiency. Elevated CO 2 had no effect on the optimization of canopy N allocation.
The response of photosynthesis was analyzed during canopy closure in a Florida scrub-oak ecosystem exposed to elevated [CO2] (704 mumol CO2/mol air; concentration Of CO2). The species were measured on six occasions, covering different seasons, during the third. and fourth year of exposure to elevated [CO2]. The entire regrowth cycle of this community has been under elevated [CO2], providing a rare opportunity to assess the differential responses of species during the critical phase of canopy closure. Measurements were taken in order to determine both season-specific and species-specific differences' in the response of photosynthesis to elevated [CO2]. Photosynthesis was measured with an open-gas exchange system, and in vivo rates of Rubisco carboxylation (V-c,V-max) and electron transport (J(max)) were derived to assess changes in the photosynthetic capacity in the co-dominant, evergreen oak species. Quercus myrtifolia did not show any change in photosynthetic capacity with prolonged exposure to elevated [CO2] during any season, and as a result the increase in photosynthesis due to the increased Supply Of CO2 was sustained at 72%. The codominant, Q. geminata, showed a loss of photosynthetic capacity with growth at elevated [CO2], such that during most measurement periods light-saturated photosynthesis in leaves grown and measured at elevated [CO2] was no higher than in leaves grown and measured at ambient CO2. A third oak, Q. chapmanii, showed a response similar to that of Q. myrtifolia. This suggests that at the critical phase of canopy closure in a woody community,, elevation Of [CO2] causes a species-dependent and time-dependent change in the capacity of the codominants to acquire carbon and energy.
The elements of climate (radiation, temperature, wind and moisture) determine to a large extent the type of vegetation and development of soil in a biome [25]. The driving force behind the Earth’s climate is radiation from the sun. Incident solar radiation on the Earth’s surface produces differential temperature regimes that, when coupled with the Earth’s rotation around its axis and orbit around the sun, generate wind patterns and ocean currents. These air and water movements in turn influence global patterns of rainfall and heat distribution, generating the diurnal, seasonal and latitudinal changes in climate that occur on the Earth’s surface. In addition to its effects on climate, solar energy is the ultimate source of energy for all life on Earth. Thus, the ability to calculate solar irradiance on a surface, whether it is plant, animal, or non-living is a valuable tool in physiological ecology. The first part of this chapter presents relationships that may be used to calculate solar radiation on idealized surfaces under clear sky conditions.
Previous studies of photosynthetic acclimation to elevated CO2 have focused on the most recently expanded, sunlit leaves in the canopy. We examined acclimation in a vertical profile of leaves through a canopy of wheat (Triticum aestivum L.). The crop was grown at an elevated CO2 partial pressure of 55 Pa within a replicated field experiment using free-air CO2 enrichment. Gas exchange was used to estimate in vivo carboxylation capacity and the maximum rate of ribulose-1,5-bisphosphate-limited photosynthesis. Net photosynthetic CO2 uptake was measured for leaves in situ within the canopy. Leaf contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), light-harvesting-complex (LHC) proteins, and total N were determined. Elevated CO2 did not affect carboxylation capacity in the most recently expanded leaves but led to a decrease in lower, shaded leaves during grain development. Despite this acclimation, in situ photosynthetic CO2 uptake remained higher under elevated CO2. Acclimation at elevated CO2 was accompanied by decreases in both Rubisco and total leaf N contents and an increase in LHC content. Elevated CO2 led to a larger increase in LHC/Rubisco in lower canopy leaves than in the uppermost leaf. Acclimation of leaf photosynthesis to elevated CO2 therefore depended on both vertical position within the canopy and the developmental stage.
The concentration of atmospheric carbon dioxide was increased by 200 microliters per liter in a forest plantation, where competition
between organisms, resource limitations, and environmental stresses may modulate biotic responses. After 2 years the growth
rate of the dominant pine trees increased by about 26 percent relative to trees under ambient conditions. Carbon dioxide enrichment
also increased litterfall and fine-root increment. These changes increased the total net primary production by 25 percent.
Such an increase in forest net primary production globally would fix about 50 percent of the anthropogenic carbon dioxide
projected to be released into the atmosphere in the year 2050. The response of this young, rapidly growing forest to carbon
dioxide may represent the upper limit for forest carbon sequestration.
Length of exposure, degree of maturity and type of tissue all affect the results obtained in response to elevated CO2 treatment of trees. Seedlings are most responsive and, in many cases, the first few weeks or months of exposure may set the pattern for future growth. Measurements of leaf photosynthesis and respiration are not good predictors for incorporation of carbon into tissue. Seasonal changes in non-structural carbohydrates, emissions of isoprenes from leaves and exudation from roots can 'waste' photosynthate. However, these are difficult or impossible to quantify. Currently, the only generalization that can be made is that growth will be accelerated but the magnitude of this depends on tissue type, nutrition and environmental conditions. The implications of this for a future elevated atmospheric CO2 world are complex. Interactions and competition between species should be incorporated into long-term studies. These studies must, themselves, be incorporated into appropriate models which take into account regional soils and climates for use in prediction of the effects of global climate change on trees and forests.
We linked a leaf-level CO 2 assimilation model with a model that accounts for light attenuation in the canopy and measurements of sap-flux-based canopy conductance into a new canopy conductance-constrained carbon assimilation (4C-A) model. We estimated canopy CO 2 uptake (A nC) at the Duke Forest free-air CO 2 enrichment (FACE) study. Rates of A nC estimated from the 4C-A model agreed well with leaf gas exchange measurements (A net) in both CO 2 treatments. Under ambient conditions, monthly sums of net CO 2 uptake by the canopy (A nC) were 13% higher than estimates based on eddy-covariance and chamber measurements. Annual estimates of A nC were only 3% higher than carbon (C) accumulations and losses estimated from ground-based measurements for the entire stand. The C budget for the Pinus taeda component was well constrained (within 1% of ground-based measurements). Although the closure of the C budget for the broadleaf species was poorer (within 20%), these species are a minor component of the forest. Under elevated CO 2 , the C used annually for growth, turnover, and respiration balanced only 80% of the A nC . Of the extra 700 g C m À 2 a À 1 (1999 and 2000 average), 86% is attributable to surface soil CO 2 efflux. This suggests that the production and turnover of fine roots was underestimated or that mycorrhizae and rhizodeposition became an increasingly important component of the C balance. Under elevated CO 2 , net ecosystem production increased by 272 g C m À 2 a À 1 : 44% greater than under ambient CO 2 . The majority (87%) of this C was sequestered in a moderately long-term C pool in wood, with the remainder in the forest floor–soil subsystem.
Abstract We linked a leaf-level CO2 assimilation model with a model that accounts for light attenuation in the canopy and measurements of sap-flux-based canopy conductance into a new canopy conductance-constrained carbon assimilation (4C-A) model. We estimated canopy CO2 uptake (AnC) at the Duke Forest free-air CO2 enrichment (FACE) study. Rates of AnC estimated from the 4C-A model agreed well with leaf gas exchange measurements (Anet) in both CO2 treatments. Under ambient conditions, monthly sums of net CO2 uptake by the canopy (AnC) were 13% higher than estimates based on eddy-covariance and chamber measurements. Annual estimates of AnC were only 3% higher than carbon (C) accumulations and losses estimated from ground-based measurements for the entire stand. The C budget for the Pinus taeda component was well constrained (within 1% of ground-based measurements). Although the closure of the C budget for the broadleaf species was poorer (within 20%), these species are a minor component of the forest. Under elevated CO2, the C used annually for growth, turnover, and respiration balanced only 80% of the AnC. Of the extra 700 g C m−2 a−1 (1999 and 2000 average), 86% is attributable to surface soil CO2 efflux. This suggests that the production and turnover of fine roots was underestimated or that mycorrhizae and rhizodeposition became an increasingly important component of the C balance. Under elevated CO2, net ecosystem production increased by 272 g C m−2 a−1: 44% greater than under ambient CO2. The majority (87%) of this C was sequestered in a moderately long-term C pool in wood, with the remainder in the forest floor–soil subsystem.
Because of their prominent role in the global carbon balance and their possible carbon sequestration, trees are very important organisms in relation to global climatic changes. Knowledge of these processes is the key to understanding the functioning of the whole forest ecosystem which can he modelled and predicted based on the physiological process information. This paper reviews the major methods and techniques used to examine the likely effects of elevated CO 2 on woody plants, as well as the major physiological responses of trees to elevated CO 2 . The available exposure techniques and approaches are described. An overview table with all relevant literature data over the period 1989‐93 summarizes the percent changes in biomass, root/shoot ratio, photosynthesis, leaf area and water use efficiency under elevated CO 2 . Interaction between growth, photosynthesis and nutrition is discussed with a special emphasis on downward regulation of photosynthesis. The stimulation or reduction found in the respiratory processes of woody plants are reviewed, as well as the effect of elevated CO 2 on stomatal density, conductance and water use efficiency. Changes in plant quality and their consequences are examined. Changes in underground processes under elevated CO 2 are especially emphasized and related to the functioning of the ecosystem. Some directions for future research are put forward.
Contents
Summary
425
I.
Introduction
426
II.
The special case of trees
426
III.
Methodologies, strategies and techniques
427
IV.
Physiological responses of trees to elevated CO 2
428
V.
Conclusions: Looking forward to the future at the ecosystem level
441
Acknowledgements
442
References
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Photosynthesis of tree seedlings is generally enhanced during short-term exposure to elevated atmospheric CO2, but longer-term photosynthetic responses are often more variable because they are affected by morphological, biochemical and physiological feedback mechanisms that regulate carbon assimilation to meet sink demand. To examine biochemical and morphological factors that might regulate the long-term photosynthetic response of field-grown trees to elevated CO2, we grew ponderosa pine (Pinus ponderosa Dougl. ex Laws.) trees in open-top chambers for six years in native soil at ambient CO2 (35 Pa) and elevated CO2 (70 Pa) at a site near Placerville, CA. Trees were well watered and exposed to natural light and ambient temperature. At the end of the sixth growing season at elevated CO2, net photosynthesis was enhanced 53%, despite reductions in photosynthetic capacity. The positive net photosynthetic response to elevated CO2 reflected greater relative increases in Rubisco sensitivity compared with the decreases resulting from biochemical adjustments. Analyses of net photosynthetic rate versus internal CO2 partial pressure curves indicated that reductions in photosynthetic capacity in response to elevated CO2 were the result of significant reductions in maximum photosynthetic rate (20%), Rubisco carboxylation capacity (36%), and electron transport capacity (21%). Decreased photosynthetic capacity was accompanied by reductions in various photosynthetic components, including total chlorophyll (24%), Rubisco protein content (38%), and mass-based leaf nitrogen concentration (14%). Net photosynthesis was unaffected by morphological adjustments because there was no change in leaf mass per unit area at elevated CO2. An apparent positive response of photosynthetic adjustment in the elevated CO2 treatment was the redistribution of N within the photosynthetic system to balance Rubisco carboxylation and electron transport capacities. We conclude that trees, without apparent limitations to root growth, may exhibit photosynthetic adjustment responses in the field after long-term exposure to elevated CO2.
The response of photosynthesis was analyzed during canopy closure in a Florida scrub-oak ecosystem exposed to elevated [CO2] (704 μmol CO2/mol air; concentration of CO2). The species were measured on six occasions, covering different seasons, during the third and fourth year of exposure to elevated [CO2]. The entire regrowth cycle of this community has been under elevated [CO2], providing a rare opportunity to assess the differential responses of species during the critical phase of canopy closure. Measurements were taken in order to determine both season-specific and species-specific differences in the response of photosynthesis to elevated [CO2]. Photosynthesis was measured with an open-gas exchange system, and in vivo rates of Rubisco carboxylation (Vc,max) and electron transport (Jmax) were derived to assess changes in the photosynthetic capacity in the codominant, evergreen oak species. Quercus myrtifolia did not show any change in photosynthetic capacity with prolonged exposure to elevated [CO2] during any season, and as a result the increase in photosynthesis due to the increased supply of CO2 was sustained at 72%. The codominant, Q. geminata, showed a loss of photosynthetic capacity with growth at elevated [CO2], such that during most measurement periods light-saturated photosynthesis in leaves grown and measured at elevated [CO2] was no higher than in leaves grown and measured at ambient CO2. A third oak, Q. chapmanii, showed a response similar to that of Q. myrtifolia. This suggests that at the critical phase of canopy closure in a woody community, elevation of [CO2] causes a species-dependent and time-dependent change in the capacity of the codominants to acquire carbon and energy.
In big-leaf models of canopy photosynthesis, the Rubisco activity per unit ground area is taken as the sum of activities per unit leaf area within the canopy, and electron transport capacity is similarly summed. Such models overestimate rates of photosynthesis and require empirical curvature factors in the response to irradiance. We show that, with any distribution of leaf nitrogen within the canopy (including optimal), the required curvature factors are not constant but vary with canopy leaf area index and leaf nitrogen content. We further show that the underlying reason is the difference between the time-averaged and instantaneous distributions of absorbed irradiance, caused by penetration of sunflecks and the range of leaf angles in canopies.
These errors are avoided in models that treat the canopy in terms of a number of layers – the multi-layer models. We present an alternative to the multi-layer model: by separately integrating the sunlit and shaded leaf fractions of the canopy, a single layered sun/shade model is obtained, which is as accurate and simpler. The model is a scaled version of a leaf model as distinct from an integrative approach.
The maximum quantum yields (?a,c) for CO2 uptake in low-oxygen atmospheres were determined for 11 species of C3 vascular plants of diverse taxa, habitat and life form using an Ulbricht-sphere leaf chamber. Comparisons were also made between tissues of varied age within species. The species examined were Psilotum nudum (L.) P. Beauv., Davallia bullata Wall. ex Hook., Cycas revoluta Thunb., Araucaria heterophylla (Salisb.) Franco, Picea abies (L.) Karst., Nerium oleander L., Ruellia humilis Nutt., Pilea microphylla (L.) Karst., Beaucarnea stricta Lem., Oplismenus hirtellus (L.) P. Beauv. and Poa annua L. Quantum yields were calculated from the initial slopes of the response of CO2 uptake to the quantity of photons absorbed in conditions of diffuse lighting. Regression analysis of variance of the initial slopes of the response of CO2 uptake to photon absorption failed to show any statistically significant differences between age classes within species or between the mature photosynthetic organs of different species. The constancy of ?a,c was apparent despite marked variation in the light-saturated rates of CO2 uptake within and between species. The mean ?a,c was 0.093±0.003 for 11 species. By contrast, surface absorptance varied markedly between species from 0.90 to 0.60, producing proportional variation in the quantum yield calculated on an incidentlight basis. The ratio of variable to maximum fluorescence emission at 695 nm for the same tissues also failed to show any statistically significant variation between species, with a mean of 0.838±0.008. Mean values of ?a,c reported here for C3 species, in the absence of photorespiration, are higher than reported in previous surveys of vascular plants, but consistent with recent estimates of the quantum yields of O2 evolution.
A central question concerning the response of terrestrial ecosystems to a changing atmosphere is whether increased uptake of carbon in response to increasing at- mospheric carbon dioxide concentration results in greater plant biomass and carbon storage or, alternatively, faster cycling of C through the ecosystem. Net primary productivity (NPP) of a closed-canopy Liquidambar styraciflua (sweetgum) forest stand was assessed for three years in a free-air CO2-enrichment (FACE) experiment. NPP increased 21% in stands ex- posed to elevated CO2, and there was no loss of response over time. Wood increment increased significantly during the first year of exposure, but subsequently most of the extra C was allocated to production of leaves and fine roots. These pools turn over more rapidly than wood, thereby reducing the potential of the forest stand to sequester additional C in response to atmospheric CO2 enrichment. Hence, while this experiment provides the first evidence that CO2 enrichment can increase productivity in a closed-canopy deciduous forest, the implications of this result must be tempered because the increase in productivity resulted in faster cycling of C through the system rather than increased C storage in wood. The fate of the additional C entering the soil system and the environmental interactions that influence allocation need further investigation.
A fast growing high density Populus plantation located in central Italy was exposed to elevated carbon dioxide for a period of three years. An elevated CO2 treatment (550 ppm), of 200 ppm over ambient (350 ppm) was provided using a FACE technique. Standing root biomass, fine root turnover and mycorrhizal colonization of the following Populus species was examined: Populus alba L., Populus nigra L., Populus x euramericana Dode (Guinier). Elevated CO2 increased belowground allocation of biomass in all three species examined, standing root biomass increased by 47–76% as a result of FACE treatment. Similarly, fine root biomass present in the soil increased by 35–84%. The FACE treatment resulted in 55% faster fine root turnover in P. alba and a 27% increase in turnover of roots of P. nigra and P. x euramericana. P. alba and P. nigra invested more root biomass into deeper soil horizon under elevated CO2. Response of the mycorrhizal community to elevated CO2 was more varied, the rate of infection increased only in P. alba for both ectomycorrhizal (EM) and arbuscular mycorrhizas (AM). The roots of P. nigra showed greater infection only by AM and the colonization of the root system of P. x euramericana was not affected by FACE treatment. The results suggest that elevated atmospheric CO2 conditions induce greater belowground biomass investment, which could lead to accumulation of assimilated C in the soil profile. This may have implications for C sequestration and must be taken into account when considering long-term C storage in the soil.
Simultaneous measurements of net ecosystem CO2 exchange (NEE) were made in a Florida scrub-oak ecosystem in August 1997 and then every month between April 2000 to July 2001, using open top chambers (NEEO) and eddy covariance (NEEE). This study provided a cross validation of these two different techniques for measuring NEE. Unique characteristics of the comparison were that the measurements were made simultaneously, in the same stand, with large replicated chambers enclosing a representative portion of the ecosystem (75 m2, compared to approximately 1–2 ha measured by the eddy covariance system). The value of the comparison was greatest at night, when the microclimate was minimally affected by the chambers. For six of the 12 measurement periods, night NEEO was not significantly different to night NEEE, and for the other periods the maximum difference was 1.1 µmol m−2s−1, with an average of 0.72 ± 0.09 µmol m−2s−1. The comparison was more difficult during the photoperiod, because of differences between the microclimate inside and outside the chambers. During the photoperiod, air temperature (Tair) and air vapour pressure deficits (VPD) became progressively higher inside the chambers until mid-afternoon. In the morning NEEO was higher than NEEE by about 26%, consistent with increased temperature inside the chambers. Over the mid-day period and the afternoon, NEEO was 8% higher that NEEE, regardless of the large differences in microclimate. This study demonstrates both the uses and difficulties associated with attempting to cross validate NEE measurements made in chambers and using eddy covariance. The exercise was most useful at night when the chamber had a minimal effect on microclimate, and when the measurement of NEE is most difficult.
The chapter focuses on the complex interactions between CO2 concentration, photosynthesis, and other environmental variables along with more complex relationships between carbon assimilation and plant respiration, growth, and yield. The chapter explores these relationships in trees and attempts to predict the consequences of increase in CO2 concentration on temperate zone forests. The complexity of forest ecosystems can result in many consequences of long-term changes in the rates of carbon gain and water loss by trees. The chapter considers four main reasons for being concerned about the rise in CO2 and its effect on trees and forests. These include (1) the enhancement of biological knowledge about the functioning of tree species of major ecological and economic importance, (2) the impact on the productivity and value of the economic product, (3) the impact on the ecology and environment of woods and forests, and (4) the downstream, socio-economic consequences. The chapter also examines the effects of the projected increase in global atmosphere CO2 concentration based on spatial and temporal scales.
Modelling is used to predict long-term forest responses to increased atmospheric CO2 concentrations. Although productivity models are based on light intercepted by the canopy, very little experimental data are available for closed forest stands. Nevertheless, the relationships between light inside a canopy, leaf area, canopy structure, and individual leaf characteristics may be affected by elevated CO2, affecting in turn carbon gain.
Using a free-air CO2 enrichment (FACE) design in a high-density plantation of Populus spp., we studied the effects of increased CO2 concentrations on transmittance (tau) of photosynthetic photon flux density (Q(p)), on ratios of red/far-red light (R/FR), on leaf area index (LAI), on leaf inclination, on leaf chlorophyll (chl) and nitrogen (N) concentrations, and on specific leaf area (SLA) in the 2nd and 3rd years of treatment. Continuous measurements of tau were made in addition to canopy height profiles of light and leaf characteristics.
Two years of Q(p) measurements showed an average decrease of canopy transmittance in the FACE treatment, with very small differences at canopy closure. Results were explained by an unaffected LAI in closed canopies, without a FACE-induced stimulation of relative crown depth. In agreement, leaf inclination and extinction coefficients for light were similar in control and FACE conditions. Ratios of R/FR were not significantly affected by the FACE treatment, neither were leaf characteristics, with the exception of leaf N, which allows speculation about N limitation. In general, treatment differences in canopy profiles resulted from an initial stimulation of height growth in the FACE treatment. P. x euramericana differed from P. alba and P. nigra, but species did not differ significantly in their response to the FACE treatment. By the time fast-growing high-density forest plantations have passed the exponential growth phase and reached canopy closure, the likely effects of elevated atmospheric CO2 concentration on canopy architecture and absorption of Q(p) are minor.
In a free-air CO2 enrichment (FACE) study, above-ground growth of Populus alba, Populus nigra and Populus x euramericana was followed continuously during the first rotation cycle of a short rotation culture (SRC) plantation to test possible changes in the response to elevated CO2 occurring from planting until canopy closure.
Height, stem basal area, stem volume index, branch production, and bud phenology were monitored for 3 yr. Moreover the coefficient of variation and a competition index were calculated to analyse the onset and the typology of competition.
Volume index was higher under elevated CO2 by 77%, 24% and 22%, as mean value for the three species, in the first, second and third years, respectively. The stimulating response, although univocal, differed in extent among species. Branch production was stimulated only in the first year, whereas bud phenology was unaffected.
The analysis of these results show that growth was stimulated by elevated CO2 only in the first year, although differences in volume index remained significant even in the second and third years. in the third year, under canopy closure, only competitively advantaged individuals profited by the FACE treatment.
Summary • Using the Poplar Free Air CO 2 Enrichement (PopFACE) facility we investigated the effects of elevated (CO 2 ) on the diurnal and growth cycle responses of photosyn- thesis and conductance in three poplar species. • In situ diurnal measurements of photosynthesis were made on Populus alba , P. nigra and P. × euramericana and, in parallel, in vivo maximum capacity for carboxy- lation ( V c , max ) and maximum rates of electron transport ( J max ) were determined by gas exchange measurement. • Light saturated ( A sat ) and daily integrated ( A ' ) photosynthesis increased at elevated (CO 2 ) in all species. Elevated (CO 2 ) decreased V c , max and J max for P. nigra and J max for P. × euramericana but had no effect on stomatal conductance in any of the species throughout the first growth cycle. During post-coppice re-growth, elevated (CO 2 ) did not increase A sat in P. nigra and P. × euramericana due to large decreases in V c , max and J max . •A 50% increase in (CO 2 ) under these open-air field conditions resulted in a large and sustained increase in A sat . Although there were some differences between the species, these had little effect on photosynthetic rates at the growth (CO 2 ). Never- theless the results show that even fast growing trees grown without rooting volume restriction in the open may still show some down-regulation of photosynthetic potential at elevated (CO 2 ).
The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open‐air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 ‐sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 ‐tolerant species Sugar Maple, Acer saccharum ).
The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 ‐tolerant as well as O 3 ‐sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34–36 nL L ⁻¹ during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 .
After 3 years of exposure to 560 µmol mol ⁻¹ CO 2 , the above‐ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 ‐sensitive and O 3 ‐tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.
The Sixth and Seventh Conference of the Parties (COP 6 and 7) at The Hague, Bonn and Marrakesh came to a final Agreement on the Kyoto Protocol, which is thus ready for ratification by the individual nations. The Agreement was only achieved by allowing countries to offset their fossil fuel emission targets (on average 95% of the 1990 emissions) by increasing biological carbon sequestration, and by trading carbon credits. Activities that would count as increasing biological carbon sequestration include afforestation and reforestation, and changes in management of agriculture and forestry. According to the Agreement reached in Marrakesh, biological carbon sequestration may reach an offset of up to 80% of the required reduction in fossil fuel emissions (4% of the 5% reduction commitment). We explain why the allowable offset rose as high during the course of the negotiations. It is highlighted that major unintended consequences may be a result of the policy as it stands in the Marrakesh Accord. Major losses of biodiversity and primary forest are expected. We present scientific concerns regarding verification, which lead to scientific doubts that the practices encouraged by the Agreement can actually increase sequestration under a full carbon accounting scheme. We explain that there is a ‘win-win’ option that would protect high carbon pools and biodiversity in an economically efficient way. But, this is not supported by the Agreement. Despite the very positive signal that most nations of the United Nations will devote major efforts towards climate protection, there remains a most urgent need to develop additional rules to avoid unintended outcomes, and to promote the ‘win-win’ options that we explain.
When atmospheric CO2 concentration increases, various consequences for plant metabolism have been suggested, such as changes in photosynthesis, photorespiration or respiration which can affect growth and carbon sequestration. In addition to long-term (indirect) effects on respiration, short-term (direct) effects of CO2 concentration on the respiration of leaves, shoots and roots are described in the literature. In most cases, respiration is reported to be inhibited by increased CO2 concentration, but the mechanism(s) are not yet understood. It has been shown previously that, when the respective technical problems and properties of a gas exchange system are fully considered, a short-term increase in CO2 (up to 4200 µmol mol−1) had no effect on respiration of Phaseolus or Populus leaves (Jahnke, Plant, Cell and Environment 24, 1139–1151, 2001). However, in the present study, large (apparent) CO2 effects were found with mature Nicotiana leaves whereas, in young leaves, the effect was absent. The experimental results clearly show that the observed direct CO2 effect on dark CO2 efflux in the mature tobacco leaves was caused by leakage of CO2 inside the leaves (and the magnitude of the effect was dependent on the size of the leakage). Nicotiana leaves are, in contrast to Phaseolus and Populus leaves (which are heterobaric), characterized by a homobaric anatomy in which intercellular air spaces are not compartmented and provide a continuous system of open pores in the lateral (paradermal) direction of the leaves. Mesophyll porosity increases with leaf development, which explains the differences between young and mature tobacco leaves. When internal leakage was experimentally restricted, the CO2 inhibition on CO2 efflux was no longer observed. It is concluded that the measured direct CO2 effect(s) on leaf CO2 efflux in the dark are artefactual, and that a true direct CO2 effect on leaf respiration does not exist.
It is a matter of debate if there is a direct (short-term) effect of elevated atmospheric CO2 concentration (Ca) on plant respiration in the dark. When Ca doubles, some authors found no (or only minor) changes in dark respiration, whereas most studies suggest a respiratory inhibition of 15–20%. The present study shows that the measurement artefacts – particularly leaks between leaf chamber gaskets and leaf surface, CO2 memory and leakage effects of gas exchange systems as well as the water vapour (‘water dilution’) effect on DCO2 measurement caused by transpiration – may result in larger errors than generally discussed. A gas exchange system that was used in three different ways – as a closed system in which Ca increased continuously from 200 to 4200 mmol (CO2) mol-1 (air) due to respiration of the enclosed leaf; as an intermittently closed system that was repeatedly closed and opened during Ca periods of either 350 or 2000 mmol mol-1, and as an open system in which Ca varied between 350 and 2000 mmol mol-1– is described. In control experiments (with an empty leaf chamber), the respective system characteristics were evaluated carefully. When all relevant system parameters were taken into account, no effects of short-term changes in CO2 on dark CO2 efflux of bean and poplar leaves were found, even when Ca increased to 4200 mmol mol-1. It is concluded that the leaf respiration of bean and poplar is not directly inhibited by elevated atmospheric CO2.
Seventeen global models of terrestrial biogeochemistry were compared with respect to annual and seasonal fluxes of net primary productivity (NPP) for the land biosphere. The comparison, sponsored by IGBP-GAIM/DIS/GCTE, used standardized input variables wherever possible and was carried out through two international workshops and over the Internet. The models differed widely in complexity and original purpose, but could be grouped in three major categories: satellite-based models that use data from the NOAA/AVHRR sensor as their major input stream (CASA, GLO-PEM, SDBM, SIB2 and TURC), models that simulate carbon fluxes using a prescribed vegetation structure (BIOME-BGC, CARAIB 2.1, CENTURY 4.0, FBM 2.2, HRBM 3.0, KGBM, PLAI 0.2, SILVAN 2.2 and TEM 4.0), and models that simulate both vegetation structure and carbon fluxes (BIOME3, DOLY and HYBRID 3.0). The simulations resulted in a range of total NPP values (44.4–66.3 Pg C year–1), after removal of two outliers (which produced extreme results as artefacts due to the comparison). The broad global pattern of NPP and the relationship of annual NPP to the major climatic variables coincided in most areas. Differences could not be attributed to the fundamental modelling strategies, with the exception that nutrient constraints generally produced lower NPP. Regional and global NPP were sensitive to the simulation method for the water balance. Seasonal variation among models was high, both globally and locally, providing several indications for specific deficiencies in some models.
The need to assess the role of forests in the global cycling of carbon and how that role will change as the atmospheric concentration of CO 2 increases has spawned many experiments over a range of scales. Experiments using open‐top chambers have been established at many sites to test whether the short‐term responses of tree seedlings described in controlled environments would be sustained over several growing seasons under field conditions. Here we review the results of those experiments, using the framework of the interacting cycles of carbon, water and nutrients, because that is the framework of the ecosystem models that are being used to address the decades‐long response of forests.
Our analysis suggests that most of what was learned in seedling studies was qualitatively correct. The evidence from field‐grown trees suggests a continued and consistent stimulation of photosynthesis of about 60% for a 300 p.p.m. increase in [CO 2 ], and there is little evidence of the long‐term loss of sensitivity to CO 2 that was suggested by earlier experiments with tree seedlings in pots. Despite the importance of respiration to a tree's carbon budget, no strong scientific consensus has yet emerged concerning the potential direct or acclimation response of woody plant respiration to CO 2 enrichment. The relative effect of CO 2 on above‐ground dry mass was highly variable and greater than that indicated by most syntheses of seedling studies. Effects of CO 2 concentration on static measures of response are confounded with the acceleration of ontogeny observed in elevated CO 2 . The trees in these open‐top chamber experiments were in an exponential growth phase, and the large growth responses to elevated CO 2 resulted from the compound interest associated with an increasing leaf area. This effect cannot be expected to persist in a closed‐canopy forest where growth potential is constrained by a steady‐state leaf area index. A more robust and informative measure of tree growth in these experiments is the annual increment in wood mass per unit leaf area, which increased 27% in elevated CO 2 . There is no support for the conclusion from many studies of seedlings that root‐to‐shoot ratio is increased by elevated CO 2 ; the production of fine roots may be enhanced, but it is not clear that this response would persist in a forest. Foliar nitrogen concentrations were lower in CO 2 ‐enriched trees, but to a lesser extent than was indicated in seedling studies and only when expressed on a leaf mass basis. The prediction that leaf litter C/N ratio would increase was not supported in field experiments. Also contrasting with seedling studies, there is little evidence from the field studies that stomatal conductance is consistently affected by CO 2 ; however, this is a topic that demands more study.
Experiments with trees in open‐top chambers under field conditions have provided data on longer‐term, larger‐scale responses of trees to elevated CO 2 under field conditions, confirmed some of the conclusions from previous seedling studies, and challenged other conclusions. There remain important obstacles to using these experimental results to predict forest responses to rising CO 2 , but the studies are valuable nonetheless for guiding ecosystem model development and revealing the critical questions that must be addressed in new, larger‐scale CO 2 experiments.
The leaf model of C3 photosynthesis of Farquhar, von Caemmerer & Berry (Planta 149, 78–90, 1980) provides the basis for scaling carbon exchange from leaf to canopy and Earth-System models, and is widely used to project biosphere responses to global change. This scaling requires using the leaf model over a wider temperature range than that for which the model was originally parameterized. The leaf model assumes that photosynthetic CO2 uptake within a leaf is either limited by the rate of ribulose-1,5-bisphosphate (RuBP) regeneration or the activity of RuBP carboxylase-oxygenase (Rubisco). Previously we reported a re-parameterization of the temperature responses of Rubisco activity that proved robust when applied to a range of species. Herein this is extended to re-parameterizing the response of RuBP-limited photosynthesis to temperature. RuBP-limited photosynthesis is assumed to depend on the whole chain electron transport rate, which is described as a three-parameter non-rectangular hyperbolic function of photon flux. Herein these three parameters are determined from simultaneous measurement of chlorophyll fluorescence and CO2 exchange of tobacco leaves, at temperatures from 10 to 40 °C. All varied significantly with temperature and were modified further with variation in growth temperature from 15 to 35 °C. These parameters closely predicted the response of RuBP-limited photosynthesis to temperature measured in both lemon and poplar and showed a significant improvement over predictions based on earlier parameterizations. We provide the necessary equations for use of the model of Farquhar et al. (1980) with our newly derived temperature functions for predicting both Rubisco- and RuBP-limited photosynthesis.
We have developed a model to simulate the dynamics of C, N, P, and S in cultivated and uncultivated grassland soils. The model uses a monthly time step and can simulate the dynamics of soil organic matter over long time periods (100 to 10,000 years). It was used to simulate the impact of cultivation (100 years) on soil organic matter dynamics, nutrient mineralization, and plant production and to simulate soil formation during a 10,000 year run. The model was validated by comparing the simulated impact of cultivation on soil organic matter C, N, P, and S dynamics with observed data from sites in the northern Great Plains. The model correctly predicted that N and P are the primary limiting nutrients for plant production and simulated the response of the system to inorganic N, P, and S fertilizer. Simulation results indicate that controlling the C:P and C:S ratios of soil organic matter fractions as functions of the labile P and S levels respectively, allows the model to correctly simulate the observed changes in C:P and C:S ratios in the soil and to simulate the impact of varying the labile P and S levels on soil P and S net mineralization rates.
Quantitative integration of the literature on the effect of elevated CO2 on woody plants is important to aid our understanding of forest health in coming decades and to better predict terrestrial
feedbacks on the global carbon cycle. We used meta-analytic methods to summarize and interpret more than 500 reports of effects
of elevated CO2 on woody plant biomass accumulation and partitioning, gas exchange, and leaf nitrogen and starch content. The CO2 effect size metric we used was the log-transformed ratio of elevated compared to ambient response means weighted by the inverse
of the variance of the log ratio. Variation in effect size among studies was partitioned according to the presence of interacting
stress factors, length of CO2 exposure, functional group status, pot size, and type of CO2 exposure facility. Both total biomass (W
T) and net CO2 assimilation (A) increased significantly at about twice ambient CO2, regardless of growth conditions. Low soil nutrient availability reduced the CO2 stimulation of W
T by half, from +31% under optimal conditions to +16%, while low light increased the response to +52%. We found no significant
shifts in biomass allocation under high CO2. Interacting stress factors had no effect on the magnitude of responses of A to CO2, although plants grown in growth chambers had significantly lower responses (+19%) than those grown in greenhouses or in
open-top chambers (+54%). We found no consistent evidence for photosynthetic acclimation to CO2 enrichment except in trees grown in pots <0.5 l (−36%) and no significant CO2 effect on stomatal conductance. Both leaf dark respiration and leaf nitrogen were significantly reduced under elevated CO2 (−18% and −16% respectively, data expressed on a leaf mass basis), while leaf starch content increased significantly except
in low nutrient grown gymnosperms. Our results provide robust, statistically defensible estimates of elevated CO2 effect sizes against which new results may be compared or for use in forest and climate model parameterization.
Plants grown at elevated pCO(2) often fail to sustain the initial stimulation of net CO2 uptake rate (A). This reduced, acclimated, stimulation of A often occurs concomitantly with a reduction in the maximum carboxylation velocity (V-c,V-max) of Rubisco. To investigate this relationship we used the Farquhar model of C-3 photosynthesis to predict the minimum V-c,V-max capable of supporting the acclimated stimulation in A observed at elevated pCO(2). For a wide range of species grown at elevated pCO(2) under contrasting conditions we found a strong correlation between observed and predicted values of V-c,V-max. This exercise mechanistically and quantitatively demonstrated that the observed acclimated stimulation of A and the simultaneous decrease in V-c,V-max observed at elevated pCO(2) is mechanistically consistent. With the exception of plants grown at a high elevated pCO(2) (> 90 Pa), which show evidence of an excess investment in Rubisco, the failure to maintain the initial stimulation of A is almost entirely attributable to the decrease in V-c,V-max and investment in Rubisco is coupled to requirements.
Measurements of total and diffuse, solar and visible (photosynthetically active, PAR) irradiance were made during portions of the summer months of 1979, 1980, and 1981 near Scottsbluff, NE. The objective of this experiment was to obtain equations to predict direct beam and diffuse irradiance in both the visible (PAR) and near-infrared wave bands from measurements of only the total incoming solar radiation. Relationships for potential values of direct and diffuse, visible and near-infrared radiation were established from “clear day” experimental data. Estimates of total visible (PAR) and near-infrared radiation were made by partitioning the measurement of solar radiation using estimates of potential visible and potential near-infrared radiation. Estimates of direct components in the visible and near-infrared were made with relationships developed from the experimental data. There was good agreement between predicted and measured values of the various components of solar radiation. In addition, a correction term for the measurement of diffuse solar radiation with a silicon cell pyranometer was developed.
There have been many experimental studies to evaluate the response of vegetation to the effect of increases in the partial pressure of carbon dioxide in the atmosphere (p CO2) that are expected to occur during the next century. This knowledge is important for the future protection of food supplies, for understanding changes in natural ecosystems and for quantifying the role of terrestrial plants in regulating the rate of change of p CO2 and resulting changes in the global climate. Most of our knowledge about these effects has derived from experimental studies that have used open-top or closed chambers. These methods are subject to “chamber effects” caused by differences in energy balance and water relations that may significantly modify the response of vegetation to elevated p CO2. The small plot sizes imposed by these techniques add other limitations both to interpretation of results and scope of investigations. Free-air carbon dioxide enrichment (FACE) provides an experimental technique for studying the effects of elevated p CO2 on vegetation and other ecosystem components in large unenclosed plots (>20 m diameter). FACE avoids many modifications to the microclimate imposed by chamber methods and therefore provides some of the most reliable estimates of plant response to elevated p CO2. Control of p CO2 in large-scale FACE experiments has now been developed to an extent where performance is similar to that achieved with sophisticated closed-chamber facilities. Experience has shown that, when FACE facilities are fully utilised, the cost per unit of usable ground area enriched with CO2, is significantly lower than alternative methods. The large scale of FACE plots can support a range of integrated studies on the same material, thereby achieving a more complete analysis than has been possible with other methods of elevating p CO2. This review considers the technical aspects of FACE methodology, outlines the major FACE experiments and summarises the advances in understanding of p CO2 effects on ecosystems that it has allowed.Published data on large-scale FACE experiments with adequate plot replication are limited to experiments on four crop/vegetation types at three locations. FACE has been used for experiments on two crops, cotton and wheat, at Maricopa, Arizona, and on grassland species, principally ryegrass and clover, at Eschikon, Switzerland. The method has also been adapted for the first study of mature forest trees, loblolly pine at Duke Forest, North Carolina. A number of other large-scale FACE experiments are in progress and the method has been adapted for use in much smaller experimental plots.The results of the major FACE experiments represent important advances over understanding obtained from previous p CO2 treatment methods. Most significant in terms of the global climate and atmosphere system are the clear observations with cotton and wheat crops that elevated p CO2 increases the ratio of sensible: latent heat transfer and causes daytime warming of the surface vegetation. This results from decreased water use and loss, and has been evident at a range of scales. The scale of FACE plots has allowed quantitative and detailed studies of the dynamics of below-ground production and C accumulation in a range of systems, and all have shown surprisingly large increases. Of particular note are the increases observed in grassland grown with low N, where there was no response of the above-ground biomass, but an increased rate of turnover of leaves and input of surface litter. FACE has allowed cultivation of crops at a scale appropriate to agronomic trials and shown statistically significant increases in the yields of wheat, cotton and pasture crops, although some of these increases are less than suggested by chamber experiments. Against expectations, the FACE experiments at Maricopa have shown a greater relative increase in yield in crops grown under water shortage than in water-sufficient crops. Acclimatory loss of photosynthetic capacity has been widely anticipated to offset the increase in photosynthesis that follows initial transfer of vegetation to elevated p CO2. None of the FACE experiments provides any evidence of such a loss; however, changes which will allow a re-optimisation of N distribution within plants have been reported.FACE methods have now been demonstrated to be feasible and effective within a range of crops and vegetation types. The information from past experiments has greatly improved our understanding of the impacts of global atmospheric change on terrestrial ecosystems.
A central question concerning the response of terrestrial ecosystems to a changing atmosphere is whether increased uptake of carbon in response to increasing atmospheric carbon dioxide concentration results in greater plant biomass and carbon storage or, alternatively, faster cycling of C through the ecosystem. Net primary productivity (NPP) of a closed-canopy Liquidambar styraciflua (sweetgum) forest stand was assessed for three years in a free-air CO2-enrichment (FACE) experiment. NPP increased 21% in stands exposed to elevated CO2, and there was no loss of response over time. Wood increment increased significantly during the first year of exposure, but subsequently most of the extra C was allocated to production of leaves and fine roots. These pools turn over more rapidly than wood, thereby reducing the potential of the forest stand to sequester additional C in response to atmospheric CO2 enrichment. Hence, while this experiment provides the first evidence that CO2 enrichment can increase productivity in a closed-canopy deciduous forest, the implicatfons of this result must be tempered because the increase in productivity resulted in faster cycling of C through the system rather than increased C storage in wood. The fate of the additional C entering the soil system and the environmental interactions that influence allocation need further investigation.
Predicting the environmental responses of leaf photosynthesis is central to many models of changes in the future global carbon cycle and terrestrial biosphere. The steady state biochemical model of C3 photosynthesis of Farquhar et al. (Planta 149, 78–90, 1980) provides a basis for these larger scale predictions; but a weakness in the application of the model as currently parameterized is the inability to accurately predict carbon assimilation at the range of temperatures over which significant photosynthesis occurs in the natural environment. The temperature functions used in this model have been based on in vitro measurements made over a limited temperature range and require several assumptions of in vivo conditions. Since photosynthetic rates are often Rubisco-limited (ribulose, 1-5 bisphosphate carboxylase/oxygenase) under natural steady-state conditions, inaccuracies in the functions predicting Rubisco kinetic properties at different temperatures may cause significant error. In this study, transgenic tobacco containing only 10% normal levels of Rubisco were used to measure Rubisco-limited photosynthesis over a large range of CO2 concentrations. From the responses of the rate of CO2 assimilation at a wide range of temperatures, and CO2 and O2 concentrations, the temperature functions of Rubisco kinetic properties were estimated in vivo. These differed substantially from previously published functions. These new functions were then used to predict photosynthesis in lemon and found to faithfully mimic the observed pattern of temperature response. There was also a close correspondence with published C3 photosynthesis temperature responses. The results represent an improved ability to model leaf photosynthesis over a wide range of temperatures (10–40 °C) necessary for predicting carbon uptake by terrestrial C3 systems.