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Disturbance, rainfall and contrasting species responses mediated aboveground biomass response to 11 years of CO2 enrichment in a Florida scrub-oak ecosystem
Abstract
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.
... N-fixing organisms require high concentrations of iron (Fe), phosphate (P), and molybdenum (Mo), or in some instances vanadium (V) (Williams, 2002). (Smith, 1992). Responses of N-fixation to elevated CO 2 can be limited by availability of these nutrients (Niklaus et al., 1998;Jin et al., 2012). ...
... With the exception of episodic losses during disturbance, N losses from terrestrial ecosystems occur primarily as gaseous products of nitrification and denitrification (NO,N 2 O,and N 2 ) and through leaching of NO − 3 and organic N. Elevated CO 2 can alter N losses if input of labile C to the rhizosphere enhances denitrification rates (Smart et al., 1997;Robinson and Conroy, 1999;Baggs et al., 2003a, b), or by altering soil water content because of reduced evapotranspiration (Hungate et al., 1997a;Arnone and Bohlen, 1998;Robinson and Conroy, 1999). Elevated CO 2 could also reduce ammonium availability to nitrifiers, suppressing nitrification and N losses through gaseous fluxes (Hungate et al., 1997b) and NO − 3 leaching (Torbert et al., 1998). ...
... The soils at the site were acidic Spodosols (Arenic Haplohumods and Spodic Quartzipsamments). The vegetation was Florida coastal scrub oak palmetto (Dijkstra et al., 2002;Johnson et al., 2003;Seiler et al., 2009), dominated by three oaks (Quercus myrtifolia, Q. geminata, and Q. chapmanii) and several less abundant species, including saw palmetto (Seranoa repens), shiny blueberry (Vaccinium myrsinites), rusty Lyonia (Lyonia ferruginea), and tarflower (Befaria racemosa). A native vine, Elliott's milkpea (Galactia elliottii) constituted only 1 % of aboveground productivity but is important for its ability to fix nitrogen. ...
Rising atmospheric CO2 concentrations could alter the nitrogen (N) content of ecosystems by changing N inputs and N losses, but responses vary in field experiments, possibly because multiple mechanisms are at play. We measured N fixation and N losses in a subtropical oak woodland exposed to 11 yr of elevated atmospheric CO2 concentrations. We also explored the role of herbivory, carbon limitation, and competition for light and nutrients in shaping response of N fixation to elevated CO2. Elevated CO2 did not significantly alter gaseous N losses, but lower recovery and deeper distribution in the soil of a long-term 15N tracer indicated that elevated CO2 increased leaching losses. Elevated CO2 had no effect on asymbiotic N fixation, and had a transient effect on symbiotic N fixation by the dominant legume. Elevated CO2 tended to reduce soil and plant concentrations of iron, molybdenum, phosphorus, and vanadium, nutrients essential for N fixation. Competition for nutrients and herbivory likely contributed to the declining response N fixation to elevated CO2. These results indicate that positive responses of N fixation to elevated CO2 may be transient, and that chronic exposure to elevated CO2 can increase N leaching. Models that assume increased fixation or reduced N losses with elevated CO2 may overestimate future N accumulation in the biosphere.
... The response of NPP to elevated CO 2 varies, at least in part, because the availability of other resources influences the response (Field et al., 1992;de Graaff et al., 2006;Reich et al., 2006a,b;Wang et al., 2007). Elevated CO 2 can cause a sustained stimulation of NPP (Norby et al., 2005;Drake et al., 2011;Zak et al., 2011), with interannual variation caused by precipitation (Owensby et al., 1999;Smith et al., 2000;Niklaus & K orner, 2004;Seiler et al., 2009) and salinity (Rasse et al., 2005). The stimulation of NPP by elevated CO 2 can be reduced or absent when nutrient availability is chronically low (Sch appi & K orner, 1996;Dukes et al., 2005;Reich et al., 2006a,b;Reich & Hobbie, 2012), or when nutrient availability declines over time (Norby et al., 2010) as a result of progressive nutrient limitation, a theoretically inevitable influence on the response of NPP to elevated CO 2 (or to any growth enhancement), as increased growth and nutrient accumulation in organic matter reduce the nutrient supply to plants (Field, 1999;Luo et al., 2004). ...
... and Galactia elliottii Nutt. (Dijkstra et al., 2002;Johnson et al., 2003;Seiler et al., 2009). Many of the species at the site, including the three oaks, resprout from rhizomes after fire disturbance (Webber, 1935;Schmalzer & Hinkle, 1992;Guerin, 1993). ...
... Total aboveground biomass was estimated from annual surveys in which the diameter of each individual oak stem was measured in all plots (Dijkstra et al., 2002;Seiler et al., 2009). Total aboveground biomass was calculated using allometric relationships developed previously, relating the stem diameter and total mass for each of the three oak species (Seiler et al., 2009). ...
Disturbance affects most terrestrial ecosystems and has the potential to shape their responses to chronic environmental change.
Scrub‐oak vegetation regenerating from fire disturbance in subtropical F lorida was exposed to experimentally elevated carbon dioxide ( CO 2 ) concentration (+350 μl l ⁻¹ ) using open‐top chambers for 11 yr, punctuated by hurricane disturbance in year 8. Here, we report the effects of elevated CO 2 on aboveground and belowground net primary productivity ( NPP ) and nitrogen ( N ) cycling during this experiment.
The stimulation of NPP and N uptake by elevated CO 2 peaked within 2 yr after disturbance by fire and hurricane, when soil nutrient availability was high. The stimulation subsequently declined and disappeared, coincident with low soil nutrient availability and with a CO 2 ‐induced reduction in the N concentration of oak stems.
These findings show that strong growth responses to elevated CO 2 can be transient, are consistent with a progressively limited response to elevated CO 2 interrupted by disturbance, and illustrate the importance of biogeochemical responses to extreme events in modulating ecosystem responses to global environmental change.
... A large blower circulated air through each chamber at a rate of 24–30 m 3 min À1 , replacing the chamber air volume 1.3–1.6 times min À1 (Dijkstra et al., 2002). The chambers increased air temperature and vapor pressure deficit while decreasing light (Dore et al., 2003 ), microenvironmental effects that did not significantly alter growth or species composition (Seiler et al., 2009). The experiment began in May 1996 and was maintained until June 2007. ...
... The experiment began in May 1996 and was maintained until June 2007. In June–July 2007, all aboveground material was harvested from the chambers (see Seiler et al., 2009), and roots and soils were collected using multiple cores in each chamber (see Day et al., 2013). For aboveground biomass, all shoots were cut at the base of the stem, weighed immediately, and subsampled for the determination of water content and elemental analysis of leaves and stems. ...
... likely promoted N losses, accounting for our finding that elevated CO 2 reduced recovery of added tracer 15 N (Table 4). In this experiment, spanning more than a decade in a naturally occurring ecosystem, photosynthesis and aboveground plant growth exhibited strong responses to chronic exposure to elevated atmospheric CO 2 (Dijkstra et al., 2002; Seiler et al., 2009 ), leading to the increased aboveground C content reported here, as well as increased C in coarse roots (Day et al., 2013;Fig. 6 ). ...
Rising atmospheric carbon dioxide (CO2) could alter the carbon (C) and nitrogen (N) content of ecosystems, yet the magnitude of these effects are not well known. We examined C and N budgets of a subtropical woodland after 11 yr of exposure to elevated CO2.
We used open-top chambers to manipulate CO2 during regrowth after fire, and measured C, N and tracer 15N in ecosystem components throughout the experiment.
Elevated CO2 increased plant C and tended to increase plant N but did not significantly increase whole-system C or N. Elevated CO2 increased soil microbial activity and labile soil C, but more slowly cycling soil C pools tended to decline. Recovery of a long-term 15N tracer indicated that CO2 exposure increased N losses and altered N distribution, with no effect on N inputs.
Increased plant C accrual was accompanied by higher soil microbial activity and increased C losses from soil, yielding no statistically detectable effect of elevated CO2 on net ecosystem C uptake. These findings challenge the treatment of terrestrial ecosystems responses to elevated CO2 in current biogeochemical models, where the effect of elevated CO2 on ecosystem C balance is described as enhanced photosynthesis and plant growth with decomposition as a first-order response.
... Our results demonstrate that nutrient cycling is substantially altered after 11 years of exposure to elevated CO2, but the CO2 effect is element dependent [15], [42]. The strong, positive growth response of oaks to CO2
[25] led to increased pools of some elements (Na, V, Zn and Mo) in plant biomass and quantifiably lower plant available pools of most elements throughout the soil profile (Table 3). However, because there were only significant changes in the movement of some elements, it is likely that CO2 effects on element cycles are not easily generalized. ...
... The vegetation is Florida coastal scrub-oak palmetto [23], [24]. In the experimental chambers, greater than 90% of the aboveground biomass is scrub oak [25]. ...
... Calcium and V concentrations decreased, but S concentrations were higher in oak leaves in the elevated CO2 treatment. Scrub oaks at the Florida experiment showed significantly greater growth over the 11 years they were exposed to elevated CO2
[25]. The increased above ground oak biomass under elevated CO2 was high enough to consistently lead to increases in above ground plant and litter nutrient element pools irrespective of changes in element concentration (Table 1, Figure 1, Figure 2B). ...
The effects of elevated CO2 on ecosystem element stocks are equivocal, in part because cumulative effects of CO2 on element pools are difficult to detect. We conducted a complete above and belowground inventory of non-nitrogen macro- and micronutrient stocks in a subtropical woodland exposed to twice-ambient CO2 concentrations for 11 years. We analyzed a suite of nutrient elements and metals important for nutrient cycling in soils to a depth of ∼2 m, in leaves and stems of the dominant oaks, in fine and coarse roots, and in litter. In conjunction with large biomass stimulation, elevated CO2 increased oak stem stocks of Na, Mg, P, K, V, Zn and Mo, and the aboveground pool of K and S. Elevated CO2 increased root pools of most elements, except Zn. CO2-stimulation of plant Ca was larger than the decline in the extractable Ca pool in soils, whereas for other elements, increased plant uptake matched the decline in the extractable pool in soil. We conclude that elevated CO2 caused a net transfer of a subset of nutrients from soil to plants, suggesting that ecosystems with a positive plant growth response under high CO2 will likely cause mobilization of elements from soil pools to plant biomass.
... In young forests, this results in increased NPP and biomass; at least at the onset of the experiment (DeLucia et al., 1999;Norby et al., 2005). However, whereas substantial NPP and biomass increases have occurred at the onset of experiments, this NPP stimulation has persisted in some but not all forests (Oren et al., 2001;Seiler et al., 2009;McCarthy et al., 2010;Norby et al., 2010). Moreover, this response becomes less consistent as forests become older, and NPP did not increase in the only mature forest exposed to elevated CO 2 (Fig. 3;K€ orner et al., 2005;Bader et al., 2009). ...
... The responses of tree growth to elevated CO 2 are variable among species (Bazzaz, 1990;Saxe et al., 1998;Peñuelas et al., 2001;K€ orner et al., 2005;Seiler et al., 2009;Dawes et al., 2011), and differential species responses have commonly been observed in CO 2 -enrichment experiments (Table S1). For example, of the three codominant canopy tree species (Fagus sylvatica, Quercus petraea, Carpinus betulus) in the mature deciduous forest exposed to elevated Table 1) and how these are affected by experimental CO 2 fertilization, warming, and drought (increases in blue, decreases in red; color saturation scales with certainty). ...
... Climate change is also likely to impact pathways of forest recovery ( Fig. 1; Table 2), which may occur through a variety of mechanisms including altered biogeochemistry (e.g., decreased N limitation during early stages due to increased N mineralization), changing biophysical constraints (e.g., reduced frequency of years with enough precipitation to support seedling establishment), or altered community dynamics. As reviewed above, different species within the same community commonly have substantially different responses to altered CO 2 or climate (K€ orner et al., 2005;Mohan et al., 2006Mohan et al., , 2007Seiler et al., 2009;Dawes et al., 2011), and consequent changes to community structure may impact ecosystem functioning in ways that cannot be predicted based solely on characteristic physiological responses of dominant taxa (Bolker et al., 1995). For example, increased liana biomass under future climates could meaningfully reduce forest biomass (Phillips et al., 2002;Mohan et al., 2006;Ingwell et al., 2010). ...
Forest regeneration following disturbance is a key ecological process, influencing forest structure and function, species assemblages, and ecosystem-climate interactions. Climate change may alter forest recovery dynamics or even prevent recovery, triggering feedbacks to the climate system, altering regional biodiversity, and affecting the ecosystem services provided by forests. Multiple lines of evidence-including global-scale patterns in forest recovery dynamics; forest responses to experimental manipulation of CO2 , temperature, and precipitation; forest responses to the climate change that has already occurred; ecological theory; and ecosystem and earth system models-all indicate that the dynamics of forest recovery are sensitive to climate. However, synthetic understanding of how atmospheric CO2 and climate shape trajectories of forest recovery is lacking. Here, we review these separate lines of evidence, which together demonstrate that the dynamics of forest recovery are being impacted by increasing atmospheric CO2 and changing climate. Rates of forest recovery rates generally increase with CO2 , temperature, and water availability. Drought reduces growth and live biomass in forests of all ages, having a particularly strong effect on seedling recruitment and survival. Responses of individual trees and whole-forest ecosystems to CO2 and climate manipulations often vary by age, implying that forests of different ages will respond differently to climate change. Furthermore, species within a community typically exhibit differential responses to CO2 and climate, and altered community dynamics can have important consequences for ecosystem function. Age- and species-dependent responses provide a mechanism by which climate change may push some forests past critical thresholds such that they fail to recover to their previous state following disturbance. Altered dynamics of forest recovery will result in positive and negative feedbacks to climate change. Future research on this topic and corresponding improvements to earth system models will be key to understanding the future of forests and their feedbacks to the climate system. © 2013 Blackwell Publishing Ltd.
... After CO 2 addition had ended and the chambers had been removed, all aboveground vegetation was clipped to the soil surface, dried and weighed to obtain total aboveground biomass (Seiler et al., 2009). In June 2007, < 1 wk after the aboveground vegetation in experimental plots was harvested, GPR was used to image roots in all plots with a Subsurface Interface Radar (SIR-3000) and 1500-MHz antenna (Geophysical Survey Systems Inc., North Salem, NH, USA). ...
... We compared responses to CO 2 by fine roots and leaves in order to assess the relative responses to elevated CO 2 of resourceacquiring organs above-and belowground. Total leaf biomass was estimated from measurements of stem diameter and allometric relationships developed for the three co-dominant oak species, as described in Dijkstra et al. (2002) and Seiler et al. (2009). Total fine root biomass was estimated on the basis of the minirhizotron measurements and, where available, the soil cores. ...
... The CO 2 fertilization effect aboveground was significant and proportionally greater than that belowground (Figs 2, 3). However, belowground biomass, particularly coarse roots, was the major contributor to total plant biomass (84% in ambient and 78% in elevated CO 2 ), and the absolute difference between elevated and ambient biomass was greater belowground: 846 g m À2 aboveground (Seiler et al., 2009) and 1296 g m À2 belowground (Table 2, Fig. 3). The ratio of total belowground to total aboveground biomass averaged 3.9 AE 0.4 for elevated CO 2 plots, significantly less than the average of 5.5 AE 0.5 for ambient CO 2 plots (P = 0.02). ...
Uncertainty surrounds belowground plant responses to rising atmospheric CO 2 because roots are difficult to measure, requiring frequent monitoring as a result of fine root dynamics and long‐term monitoring as a result of sensitivity to resource availability.
We report belowground plant responses of a scrub‐oak ecosystem in F lorida exposed to 11 yr of elevated atmospheric CO 2 using open‐top chambers. We measured fine root production, turnover and biomass using minirhizotrons, coarse root biomass using ground‐penetrating radar and total root biomass using soil cores.
Total root biomass was greater in elevated than in ambient plots, and the absolute difference was larger than the difference aboveground. Fine root biomass fluctuated by more than a factor of two, with no unidirectional temporal trend, whereas leaf biomass accumulated monotonically. Strong increases in fine root biomass with elevated CO 2 occurred after fire and hurricane disturbance. Leaf biomass also exhibited stronger responses following hurricanes.
Responses after fire and hurricanes suggest that disturbance promotes the growth responses of plants to elevated CO 2 . Increased resource availability associated with disturbance (nutrients, water, space) may facilitate greater responses of roots to elevated CO 2 . The disappearance of responses in fine roots suggests limits on the capacity of root systems to respond to CO 2 enrichment.
... A long-term (11 years) elevated CO 2 experiment in a scrub-oak ecosystem in Florida provided a unique opportunity to study changes in soil N processes and other attributes throughout the entire soil profile ($ 300 cm) over time in the context of PNL. After 11 years of elevated CO 2 treatment, there was a $ 50% increase in aboveground plant biomass of the dominant oak, Quercus myrtifolia, into the last year of the study (Seiler et al., 2009 ). An initial increase in NPP is necessary for symptoms of PNL to appear, but stimulation of NPP is expected to decline or disappear within a few years in strongly N-limited systems (Luo et al., 2004). ...
... Sixteen octagonal open-top chambers (OTCs), 2.5 m tall, each enclosing a surface area of 9.42 m 2 , were constructed with a PVC frame and covered with rectangular panels of Mylar (Melinez 071, Courtaulds Performance Films, Martinsville, VA, USA). Aboveground biomass and stem densities were detailed elsewhere (Seiler et al., 2009). A frustum was constructed atop each chamber to reduce wind effects, which reduced the opening to 5.9 m 2 . ...
... Elevated CO 2 stimulation of biomass accumulation in this scrub-oak ecosystem was sustained over the 11 year duration of this study (Dijkstra et al., 2002; Hungate et al., 2006; Stover et al., 2007; Seiler et al., 2009), despite nutrient-poor soils that were expected to quickly cause PNL. Nutrient limitations on plant responses to elevated CO 2 in other forests have occurred in less than a decade (Oren et al., 2001; Norby et al., 2002; Ellsworth et al., 2004; Finzi et al., 2006 Finzi et al., , 2007), although they do not always entirely negate the stimulatory effects of CO 2 on NPP (e.g., Duke and ORNL) (Finzi et al., 2006Finzi et al., , 2007 Norby & Iversen, 2006). ...
A scrub-oak woodland has maintained higher aboveground biomass accumulation after 11 years of atmospheric CO2 enrichment (ambient +350 μmol CO2 mol−1), despite the expectation of strong nitrogen (N) limitation at the site. We hypothesized that changes in plant available N and exploitation of deep sources of inorganic N in soils have sustained greater growth at elevated CO2. We employed a suite of assays performed in the sixth and 11th year of a CO2 enrichment experiment designed to assess soil N dynamics and N availability in the entire soil profile. In the 11th year, we found no differences in gross N flux, but significantly greater microbial respiration (P≤0.01) at elevated CO2. Elevated CO2 lowered extractable inorganic N concentrations (P=0.096) considering the whole soil profile (0–190 cm). Conversely, potential net N mineralization, although not significant in considering the entire profile (P=0.460), tended to be greater at elevated CO2. Ion-exchange resins placed in the soil profile for approximately 1 year revealed that potential N availability at the water table was almost 3 × greater than found elsewhere in the profile, and we found direct evidence using a 15N tracer study that plants took up N from the water table. Increased microbial respiration and shorter mean residence times of inorganic N at shallower depths suggests that enhanced SOM decomposition may promote a sustained supply of inorganic N at elevated CO2. Deep soil N availability at the water table is considerable, and provides a readily available source of N for plant uptake. Increased plant growth at elevated CO2 in this ecosystem may be sustained through greater inorganic N supply from shallow soils and N uptake from deep soil.
... These surveys included an array of CO 2 augmentation techniques (open-top chambers, glasshouses, growth chambers and branch bags) and pot-and field-grown plants, many of which were juveniles (Norby et al. 1999). In some studies the effects of growth enclosure and CO 2 delivery technique on stomatal conductance can be as large as the responses to the CO 2 treatment (Seiler et al. 2009). The evidence in trees for or against temporal acclimation of stomatal conductance to elevated [CO 2 ] is also equivocal (Gunderson and Wullschleger 1994). ...
... Well defined responses of stomatal conductance, transpiration and soil water storage to elevated [CO 2 ] occurred in a scrub-oak woodland system (mainly Quercus myrtifolia Willd. and Quercus geminata Small) in an 11 year open-top chamber study during which plants recovered from fire (Seiler et al. 2009). Stomatal conductance and transpiration were reduced (Hungate et al. 2002) and water content in the upper soil layer increased. ...
Studies of responses of forest vegetation to steadily increasing atmospheric concentrations of CO 2 have focussed strongly on the potential of trees to absorb extra carbon; the effects of elevated [CO 2 ] on plant–soil water relations via decreased stomatal conductance and increased ambient temperature have received less attention, but may be significant in the long term at the ecosystem level. CO 2 augmentation experiments with young trees demonstrate small increases in aboveground carbon content, but these increases tend to diminish as trees get older. By contrast, several experiments suggest continued decreases in transpiration and increased soil water content under these conditions. In tropical forests, the major cause of increases in aboveground biomass observed in the recent past is not necessarily elevated [CO 2 ]. Undoubtedly, the potential of monitoring trees in forest dynamics plots to deduce CO 2 -specific alterations in forest structure and standing biomass will unfold in the decades to come. The comprehensive understanding of responses of forest vegetation to elevated [CO 2 ] in the Anthropocene will depend upon the inclusion of detailed measurements of soil water pools and water fluxes through the soil–plant–atmosphere continuum in future tree CO 2 augmentation experiments and forest dynamics plot studies. Additional keywords: climate change, carbon sequestration, evapotranspiration, FACE (free air CO 2 enrichment), water use efficiency.
... The scrub oak/palmetto site is located in the Merritt Island Wildlife Refuge near Cape Canaveral, Florida. The plant community is dominated by Quercus myrtifolia (76%), but Response of cellulolytic fungi to elevated atmospheric CO2 2789 also contains Quercus geminata (15%), Q. chapmannii, Serenoa repense and Lyonia ferreginea (7%) (Dijkstra et al., 2002; Brown et al., 2007; Seiler et al., 2009). The soils are Poalo and Pomello Sand. ...
... Sixteen open-top chambers were set up and fumigation with elevated CO2 (350 ppm above ambient or 2¥ ambient) began in May 1996. The octagonal chambers are 3.5 m in diameter and 2.5 m high (Dijkstra et al., 2002; Brown et al., 2007; Seiler et al., 2009). A total of six soil samples were used for the analyses presented here, three from ambient chambers and three from elevated chambers . ...
The simultaneous increase of atmospheric CO2 and nitrogen (N) deposition to terrestrial ecosystems is predicted to alter plant productivity and, consequently, to change
the amount and quality of above- and belowground carbon entering forest soils. It is not known how such changes will impact
the composition and function of soil fungal communities that play a key role in degrading complex carbon. We sequenced the
fungal cellobiohydrolase I gene (cbhI) from soil DNA and cDNA to compare the richness and composition of resident and expressed cbhI genes at a U.S. Department of Energy free air-carbon dioxide enrichment (FACE) site (NC), which had been exposed to elevated
atmospheric CO2 and/or N fertilization treatment for several years. Our results provide evidence that the richness and composition of the
cellulolytic fungi surveyed in this study were distinct in the DNA- and cDNA-based gene surveys and were dominated by Basidiomycota
that have low or no representation in public databases. The surveys did not detect differences in richness or phylum-level
composition of cbhI-containing, cellulolytic fungi that correlated with elevated CO2 or N fertilization at the time of sampling.
... Response of cellulolytic fungi to elevated atmospheric CO2 2789 also contains Quercus geminata (15%), Q. chapmannii, Serenoa repense and Lyonia ferreginea (7%) (Dijkstra et al., 2002;Brown et al., 2007;Seiler et al., 2009). The soils are Poalo and Pomello Sand. ...
... Sixteen open-top chambers were set up and fumigation with elevated CO2 (350 ppm above ambient or 2¥ ambient) began in May 1996. The octagonal chambers are 3.5 m in diameter and 2.5 m high (Dijkstra et al., 2002;Brown et al., 2007;Seiler et al., 2009). A total of six soil samples were used for the analyses presented here, three from ambient chambers and three from elevated chambers. ...
... However, most results are highly reliant on the specific genetic background, from species to individuals, and according to the specific environmental factor interacting with CO 2 . The interaction renders complex synergistic responses on a suite of functional and molecular processes that make difficult establishment of general patterns (Centritto et al., 1999;Sánchez-Gómez et al., 2017), and with a variable resultant in carbon uptake and growth according to species (Seiler et al., 2009;McCarthy et al., 2010). In the particular case of drought, the trend to closing of stomata as consequence of drought could be balanced by a compensatory increase in net photosynthesis driven by a higher source of CO 2 , at least under moderate drought (Chaves and Pereira, 1992;Tissue et al., 1995;Paudel et al., 2018). ...
Impact of drought under enriched CO2 atmosphere on ecophysiological and leaf metabolic response of the sub-mediterranean Q. pyrenaica oak was studied. Seedlings growing in climate chamber were submitted to moderate drought (WS) and well-watered (WW) under ambient ([CO2]amb =400 ppm) or CO2 enriched atmosphere ([CO2]enr =800 ppm). The moderate drought endured by seedlings brought about a decrease in leaf gas exchange. However, net photosynthesis (Anet) was highly stimulated for plants at [CO2]enr. There was a decrease of the stomatal conductance to water vapour (gwv) in response to drought, and a subtle trend to be lower under [CO2]enr. The consequence of these changes was an important increase in the intrinsic leaf water use efficiency (WUEi). The electron transport rate (ETR) was almost a 20 percent higher in plants at [CO2]enr regardless drought endured by seedlings. The ETR/Anet was lower under [CO2]enr, pointing to a high capacity to maintain sinks for the uptake of extra carbon in the atmosphere. Impact of drought on the leaf metabolome, as a whole, was more evident than that from [CO2] enrichment of the atmosphere. Changes in pool of non-structural carbohydrates were observed mainly as a consequence of water deficit including increases of fructose, glucose, and proto-quercitol. Most of the metabolites affected by drought back up to levels of non-stressed seedlings after rewetting (recovery phase). It can be concluded that carbon uptake was stimulated by [CO2]enr, even under the stomatal closure that accompanied moderate drought. In the last, there was a positive effect in intrinsic water use efficiency (WUEi), which was much more improved under [CO2]enr. Leaf metabolome was little responsible and some few metabolites changed mainly in response to drought, with little differences between [CO2] growth conditions.
... A. Koyama, et al. Geoderma 355 (2019) 113915 forests (Seiler et al., 2009;Drake et al., 2011;Zak et al., 2011) due to increased N mineralization from relatively recalcitrant SOM via priming, rather than stimulated net N input, such as N 2 fixation, or reduced N loss (Langley et al., 2009;Hofmockel et al., 2011;Phillips et al., 2011Phillips et al., , 2012. In these temperate forests, however, N limitation could have been observed if the experiments were conducted even longer, unless total ecosystem N was increased. ...
We investigated patterns and possible mechanisms in the greater soil organic carbon (SOC) and total nitrogen (N) after 10 years exposure to elevated atmospheric CO 2 concentration ([CO 2 ]) at the Nevada Desert FACE Facility, USA, reported by Evans et al. (2014). Differences in SOC under elevated and ambient [CO 2 ] depended on cover types, and most evident in soils under deciduous shrubs. Greater SOC was observed in soils close to the surface as well as 0.4 to 0.8 m suggesting elevated [CO 2 ] stimulated production of aboveground litter, root litter, and rhizodeposition. Greater total N under elevated [CO 2 ] was most evident under deciduous shrubs as well as plant interspace that had extensive coverage of biological soil crusts. Greater total N in the top soil profile under elevated [CO 2 ] was accompanied with reduced δ 15 N by 0.4‰, suggesting elevated [CO 2 ] stimulated N 2-fixation and/or reduced N loss. We conclude that, in arid ecosystems under elevated [CO 2 ], shrubs play a major role in determining C sink capacity in desert soils, and N limitation for vascular plants is unlikely to occur due to sustained net N supply.
... Exposure to elevated CO2 stimulated aboveground biomass accumulation in Florida scrub over the duration of the study. The biomass stimulation response was species specific: elevated CO2 stimulated Myrtle oak (Quercus myrtifolia) and Chapman Oak (Quercus chapmanii) but had no impact on the aboveground biomass of sand live oak (Quercus geminata) (Seiler et al. 2009;Dijkstra et al. 2002). Elevated CO2 stimulated fine root biomass following disturbance; but the effect was temporary . ...
... The higher accumulation of carbon at elevated [CO2] dilutes the plant nutrients concentrations, artificially increasing the efficiency of the plant to use these nutrients [37,38]. However, in the long-term responses, the nutrient dilution by elevation of [CO2] can be excessive and could restrict the carbon uptake (i.e. ...
In this chapter we review the main responses of plants to elevated [CO2], combining physiological and gene expression patterns of C3 and C4 crop species. CO2 fertilization pathways are contrasted with acclimation ones highlighting the importance of the sugar sensing mechanism in the long-term responses of plants to elevation of [CO2]. The disproportional amount of data about physiology and molecular studies is pointed out as a major gap to understand how plants integrate their organs when responding to high [CO2]. Genetic aspects are also reviewed underscoring the possible effects of epigenetics and the possibility of using QTL to find how genetic variability influences the responses of plants to elevated [CO2]. The lack of data on the gene-related aspects of responses is very likely to be filled by an increase in production of gene expression data due to the advent of next generation sequencing. This is put into perspective with the future need of Big Data analyses, i.e. the integration of large sets of data using systems approaches.
... Growth stimulation was sustained throughout the experiment, although there was significant variability between years resulting from variations in total annual rainfall. The absolute stimulation of above ground biomass generally declined over time, reflecting increasing environmental limitations to long-term growth response (Seiler et al. 2008). ...
... Besides N limitation, the lack of a positive CO 2 effect on plant growth in sapling grown alone may be related to soil moisture. Seiler et al. (2009) have reported that positive growth responses to CO 2 enrichment reflect soil water savings resulting from reduced stomatal conductance. However, this was not the case in our study, in which all plants were well-watered throughout the experimental period. ...
... At low elevation, positive growth responses to CO 2 enrichment in grassland are almost exhaustively explained by CO 2 -induced water savings (Morgan et al., 2004). Such stomata driven water savings under high CO 2 (rather than photosynthesis driven growth responses) had also been observed in a long-term CO 2 experiment in a water-limited scrub-oak ecosystem (Seiler et al., 2009). In contrast, our alpine glacier forefield plants, similar to the late-successional alpine grassland and the dwarf shrub heath described above, grew without water shortage and hence, water saving effects could not translate into growth responses. ...
... Curtis and Wang (1998) (2010) found that the absolute enhancements of net primary productivity of trees growing in elevated [CO 2 ] became progressively smaller as nitrogen availability decreased and were not observable when nitrogen availability was very low. In contrast, low water availability is often shown to amplify tree growth responses to elevated [CO 2 ] Seiler et al. 2009). Amplifications of CO 2 responses in water stressed conditions is caused by reduced g s and, in turn, by decreased leaf level transpiration under elevated [CO 2 ], which may lead to an increase in plant water potential and water use efficiency (Centritto et al. 2002), a delay in the onset of drought (Centritto et al. 1999c), and a conservation of soil water . ...
... Effects of water availability on forest productivity have often been expressed by plotting yield against annual or summer precipitation, or atmospheric evaporative demand (e.g. Huxman et al. 2004;Gerten et al. 2008;Seiler et al. 2009), while influences of nutrient availability were explored by comparing yield tables of a tree species for different soil or bedrock types, or by conducting fertilization trials in planted stands (e.g. Fahey et al. 1998;Sardans et al. 2004;Ste-Marie et al. 2007). ...
Precipitation as a key determinant of forest productivity influences forest ecosystems also indirectly through alteration of the nutrient status of the soil, but this interaction is not well understood. Along a steep precipitation gradient, we studied the consequences of reduced precipitation for the soil and biomass nutrient pools and dynamics in 14 mature European beech (Fagus sylvatica L.) forests on Triassic sandstone. We tested the hypotheses that lowered summer precipitation (1) is associated with less acid soils and (2) a reduced accumulation of organic matter on the forest floor, and (3) reduces nutrient supply from the soil and leads to decreasing foliar and root nutrient concentrations. Soil acidity, the amount of forest floor organic matter, and the associated organic matter N and P pools decreased to about a half from wet to dry sites; the C/P and N/P ratios, but not the C/N ratio, of forest floor organic matter were reduced as well. Net N mineralization and P and K pools in the mineral soil did not change with decreasing precipitation. Foliar P and K concentrations (beech sun leaves) increased while N remained constant, resulting in decreasing foliar N/P and N/K ratios. Estimated N resorption efficiency increased toward the dry sites. We conclude that a reduction in summer rainfall significantly reduces the soil C, N and P pools but does not result in decreasing foliar N and P contents in beech. However, the decreasing foliar N/P ratios towards the dry stands indicate that the importance of P limitation for tree growth declines with decreasing precipitation.
... The higher accumulation of carbon at elevated [CO2] dilutes the plant nutrients concentrations, artificially increasing the efficiency of the plant to use these nutrients [37,38]. However, in the long-term responses, the nutrient dilution by elevation of [CO2] can be excessive and could restrict the carbon uptake (i.e. ...
In this chapter we review the main responses of plants to elevated CO2, combining physiological and gene expression patterns of C3 and C4 crop species. CO2 fertilization pathways are contrasted with acclimation ones highlighting the importance of the sugar sensing mechanism in the long-term responses of plants to elevation of CO2. The disproportional amount of data about physiology and molecular studies is pointed out as a major gap to understand how plants integrate their organs when responding to high CO2. Genetic aspects are also reviewed underscoring the possible effects of epigenetics and the possibility of using quantitative trait loci (QTL) to find how genetic variability influences the responses of plants to elevated CO2. The lack of data on the gene-related aspects of responses is very likely to be filled by an increase in production of gene expression data due to the advent of next generation sequencing. This is put into perspective with the future need of Big Data analyses, i.e. the integration of large sets of data using systems approaches.
... At low elevation, positive growth responses to CO 2 enrichment in grassland are almost exhaustively explained by CO 2 -induced water savings (Morgan et al., 2004). Such stomata driven water savings under high CO 2 (rather than photosynthesis driven growth responses) had also been observed in a long-term CO 2 experiment in a water-limited scrub-oak ecosystem (Seiler et al., 2009). In contrast, our alpine glacier forefield plants, similar to the late-successional alpine grassland and the dwarf shrub heath described above, grew without water shortage and hence, water saving effects could not translate into growth responses. ...
Since 1850, glaciers in the European Alps have lost around 40% of their original area, releasing bare forefields, which are colonized by alpine pioneer species, setting the scene for later successional stages. These expanding pioneer communities are likely less restricted by resources and competition than late-successional systems. We thus hypothesized that rising atmospheric CO2 concentration will enhance plant growth in these high-elevation communities. Nine characteristic, perennial glacier forefield species were assembled in microcosms and grown at a nearby experimental site in the Swiss Alps (2440 m a.s.l.). The communities were exposed to an elevated CO2 concentration of 580 ppm by free-air CO2 enrichment for three seasons. Four study species were additionally grown in isolation in containers, half of which received a low dose of mineral fertilizer (25 kg N ha-1 a-1) in order to explore a potential nutrient limitation of the CO2 response. Responses of growth dynamics and peak season biomass of the two graminoid species, four forbs and three cushion forming species were analysed by repeated nondestructive assessments and a final biomass harvest. After three seasons, none of the species were stimulated by elevated CO2, irrespective of mineral nutrient addition, which by itself enhanced growth in the fertilized plants by +34% on average. Increased CO2 concentration did not affect total (above- plus belowground) biomass but reduced aboveground biomass by −35% across all species, even in the fast growing ones. This reduced aboveground biomass was associated with higher biomass partitioning to roots. Foliar nonstructural carbohydrate concentration increased and nitrogen concentration in leaves decreased under elevated CO2. We observed downward adjustment of photosynthetic capacity by on average −26% under long-term exposure to 580 ppm CO2 (assessed in graminoids only). Our results indicate that glacier forefield pioneers, growing under harsh climatic conditions are not carbon limited at current atmospheric CO2 concentration.
... While community shifts may take decades to occur, direct physiological responses can occur within seconds but may diminish through time because of acclimation Dukes, 2013, Isbell et al., 2013). For instance, the shortterm physiological response and ecosystem productivity response to CO 2 often diminish in the longer term (Seiler et al., 2009, Lee et al., 2011. Likewise, warming of soil causes a short-term increase in respiration that wanes over time (Bradford et al., 2008). ...
While short-term plant responses to global change are driven by physiological mechanisms, which are represented relatively well by models, long-term ecosystem responses to global change may be determined by shifts in plant community structure resulting from other ecological phenomena such as interspecific interactions, which are represented poorly by models. In single-factor scenarios, plant communities often adjust to increase ecosystem response to that factor. For instance, some early global change experiments showed that elevated CO2 favours plants that respond strongly to elevated CO2, generally amplifying the response of ecosystem productivity to elevated CO2, creating a positive community feedback. However, most ecosystems are subject to multiple drivers of change, which can complicate the community feedback effect in ways that are more difficult to generalize. Recent studies have shown that (i) shifts in plant community structure cannot be reliably predicted from short-term plant physiological response to global change and (ii) that the ecosystem response to multi-factored change is commonly less than the sum of its parts. Here, we survey results from long-term field manipulations to examine the role community shifts may play in explaining these common findings. We use a simple model to examine the potential importance of community shifts in governing ecosystem response. Empirical evidence and the model demonstrate that with multi-factored change, the ecosystem response depends on community feedbacks, and that the magnitude of ecosystem response will depend on the relationship between plant response to one factor and plant response to another factor. Tradeoffs in the ability of plants to respond positively to, or to tolerate, different global change drivers may underlie generalizable patterns of covariance in responses to different drivers of change across plant taxa. Mechanistic understanding of these patterns will help predict the community feedbacks that determine long-term ecosystem responses.
... An increase in atmospheric CO 2 concentration has been widely reported to affect important physiological processes, such as CO 2 exchange, stomatal conductance, transpiration, growth and anatomy ( Pritchard et al. 1997;Tissue et al. 1997;Stitt and Krapp 1999;Davey et al. 2004;Katul et al. 2009;Seiler et al. 2009;Ghannoum et al. 2010). The initial and direct responses of plants to elevated CO 2 occur at leaf level where stomata act as regulating openings for gas exchange of CO 2 , H 2 O and O 2 between leaves and atmosphere ( Pearson et al. 1995;Hetherington and Woodward 2003;Brodribb et al. 2009). ...
Stomatal number and stomatal conductance are important structural and functional parameters for the assessment of carbon assimilation and water use under elevated CO2. We studied stomatal density, number of stomatal rows and stomatal conductance of Pinus
sylvestriformis and P.
koraiensis needles exposed to elevated CO2 (500 μmol mol−1 CO2) in open-top chambers for 10 years (1999–2009). Elevated CO2 increased stomatal density on P. sylvestriformis by 10.8 % (13.5 % on abaxial surface and 8.0 % on adaxial surface) and the number of stomatal rows on P. koraiensis by 7.9 % (5.0 % in 1-year-old needles and 10.7 % in current-year needles). Increased stomatal density for P. sylvestriformis and number of stomatal rows for P. koraiensis indicate that elevated CO2 increases stomatal number in both tree species. Needle age significantly influenced stomatal density and number of stomatal rows in P.
koraiensis but not in P.
sylvestriformis. For both species, elevated CO2 did not significantly affect stomatal conductance but increased water use efficiency. The increase in stomatal number is not accompanied by significant changes in stomatal conductance at elevated CO2 for both tree species suggesting that there may be no direct relationship between stomatal conductance and stomatal numbers.
... Beginning in the 1990s, outdoor experiments were conducted in which elevated CO 2 concentrations were maintained over an experimental plot by continuously releasing CO 2 from storage canisters. These are referred to as Free Air Concentration Enhancement or FACE experiments, some of which (at various sites in the USA, Europe, and New Zealand) have now been carried out for more than a decade (e.g., Seiler et al., 2009;Norby et al., 2010;van Kessel et al., 2006;Rütting et al., 2010). These studies confirm that higher CO 2 stimulates increased photosynthesis and storage of carbon, but saturation of the photosynthesis response occurs at a CO 2 concentration of around 500e600 ppmv, which is much lower than expected based on leaf-level physiology (Canadell et al., 2007). ...
The term “climate sensitivity” refers to the ratio of the steady-state increase in the global and annual mean surface air temperature to the global and annual mean radiative forcing. It is standard practice to include only the fast feedback processes, including changes in water vapor, in the calculation of climate sensitivity, but to exclude possible induced changes in the concentrations of other GHGs. Changes in climate in response to changes in GHG concentrations and other driving factors can be computed using relatively simple climate models in which the climate sensitivity is prescribed and the radiative forcing is computed from the concentrations of individual GHGs using simple formulae based on the results of detailed calculations. They can also be computed using 3D atmospheric general circulation models (AGCMs) coupled to a slab that represents the surface layer (mixed layer) of the ocean only. Alternatively, they can be computed using coupled 3D atmospheric and oceanic general circulation models (AOGCMs). In simple models, one can simply add up the individual radiative forcings to get the total global mean radiative forcing, and then apply the climate sensitivity obtained for a CO2 doubling in simulating the global mean temperature response. AML models and AOGCMs, on the other hand, have the flexibility to respond in separate ways to different forcing mechanisms.
... (d) Biophysical response of tropical forest species to increasing carbon dioxide Elevated atmospheric CO 2 generally stimulates plant growth directly through increased photosynthesis and indirectly through increased water-use efficiency [75]. Global vegetation models predict large increases in Amazonian forest biomass with increasing CO 2 because this biophysical response is applied as a constant, and the simulated vegetation does not acclimate to this forcing [76]. In most climate/vegetation models, this biophysical effect compensates for any climate-change-driven loss of biomass [77,78]. ...
A mosaic of protected areas, including indigenous lands, sustainable-use production forests and reserves and strictly protected forests is the cornerstone of conservation in the Amazon, with almost 50 per cent of the region now protected. However, recent research indicates that isolation from direct deforestation or degradation may not be sufficient to maintain the ecological integrity of Amazon forests over the next several decades. Large-scale changes in fire and drought regimes occurring as a result of deforestation and greenhouse gas increases may result in forest degradation, regardless of protected status. How severe or widespread these feedbacks will be is uncertain, but the arc of deforestation in south-southeastern Amazonia appears to be particularly vulnerable owing to high current deforestation rates and ecological sensitivity to climate change. Maintaining forest ecosystem integrity may require significant strengthening of forest conservation on private property, which can in part be accomplished by leveraging existing policy mechanisms.
... The unexpected outcome of these long-term experiments is the variety of ecosystem responses to elevated CO 2 . Indeed, the response of primary productivity to elevated CO 2 spans a gradient, from experiments in which (1) there is no response to elevated CO 2 alone (Korner et al. 2005 ) to experiments in which (2) the response is initially high but then declines to levels observed under ambient CO 2 (Reich et al. 2006; Seiler et al. 2009), or (3) the response is sustained through time (Finzi et al. 2006). Increases in primary production associated with high CO 2 concentrations require sufficient availability of other resources, and much of the variation in net primary productivity in response to CO 2 enrichment can be tied to nutrient limitation (Finzi et al. 2001; Reich et al. 2006). ...
The understanding of biogeochemical cycles has benefited from technological advances facilitating new kinds of measurements and observations. Satellite-borne ocean-color sensors that assess the physiological status of phytoplankton have led to improved estimates of oceanic productivity, as have micrometeorological approaches measuring terrestrial photosynthesis and respiration. The advent of satellites fitted with synthetic aperture radar (a specialized sensor used to determine inundation extent and vegetation types in wetlands) has revealed large fluxes of carbon dioxide and methane from these areas. Advances in the measurement of chemical constituents and turbulence have allowed the detection of high-resolution coupling between physical and biogeochemical processes. Genomics and proteomics - the study of genes and of an organism's complement of proteins, respectively - have revolutionized our understanding of the types of cells present in the environment and their ability to transform elements by allowing direct assessment of gene and protein sequences.
... of the earth's future climate state are highly sensitive to the C-cycle response of ecosystems to rising concentrations of atmospheric CO 2 (Friedlingstein et al. 2006; Meehl et al. 2007). Ecosystem responses to experimental increases in atmospheric CO 2 concentrations vary widely, from ecosystems in which low soil-N availability precludes an enhancement of net primary productivity (NPP) in response to elevated CO 2 (Oren et al. 2001; Menge & Field 2007) to experiments that, in the absence of N fertilization, show only a transient response of NPP to elevated CO 2 (Reich et al. 2006; Seiler et al. 2009; Norby et al. 2010), to demonstrably N limited ecosystems where the enhancement in NPP is sustained through time (Langley et al. 2009; McCarthy et al. 2010). To understand why ecosystems respond differently to elevated CO 2 , it is necessary to understand the mechanistic connection between plant physiological responses to elevated CO 2 and their attending effects on nutrient availability and uptake. ...
Ecology Letters (2011) 14: 349–357AbstractThe earth’s future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO2. Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C-cascade through the root-microbe-soil system; increases in the flux of C belowground under elevated CO2 stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO2 as a result of increases in canopy N content and higher photosynthetic N-use efficiency. The ecosystem-level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.
... While enhancement of soil water availability has been found to increase plant biomass under elevated CO 2 (especially C 4 plants; Morgan et al., 2004;Seiler et al., 2009;Owensby et al., 1993;Jackson et al., 1994;Chiariello & Field, 1996;Morgan et al., 2001;Nelson et al., 2004;Leakey et al., 2009), it is not clear if SCF is affected similarly. The higher water use efficiency of plants under elevated vs. ambient CO 2 suggests that the response of SCF to elevated CO 2 should be greater during dry vs. wet periods (Owensby et al., 1999). ...
It is not clear whether the consistent positive effect of elevated CO2 on soil respiration (soil carbon flux, SCF) results from increased plant and microbial activity due to (i) greater C availability through CO2-induced increases in C inputs or (ii) enhanced soil moisture via CO2-induced declines in stomatal conductance and plant water use. Global changes such as biodiversity loss or nitrogen (N) deposition may also affect these drivers, interacting with CO2 to affect SCF. To determine the effects of these factors on SCF and elucidate the mechanism(s) behind the effect of elevated CO2 on SCF, we measured SCF and soil moisture throughout a growing season in the Biodiversity, CO2, and N (BioCON) experiment. Increasing diversity and N caused small declines in soil moisture. Diversity had inconsistent small effects on SCF through its effects on abiotic conditions, while N had a small positive effect that was unrelated to soil moisture. Elevated CO2 had large consistent effects, increasing soil moisture by 26% and SCF by 45%. However, CO2-induced changes in soil moisture were weak drivers of SCF: CO2 effects on SCF and soil moisture were uncorrelated, CO2 effect size did not change with soil moisture, within-day CO2 effects via soil moisture were neutral or weakly negative, and the estimated effect of increased C availability was 14 times larger than that of increased soil moisture. Combined with previous BioCON results indicating elevated CO2 increases C availability to plants and microbes, our results suggest that increased SCF is driven by CO2-induced increases in substrate availability. Our results provide further support for increased rates of belowground C cycling at elevated CO2 and evidence that, unlike the response of productivity to elevated CO2 in BioCON, the response of SCF is not strongly N limited. Thus, N limited grasslands are unlikely to act as a N sink under elevated CO2.
... Similarly, McCarthy et al. (2010) found that the absolute enhancements of net primary productivity of trees growing in elevated [CO 2 ] became progressively smaller as nitrogen availability decreased and were not observable when nitrogen availability was very low. In contrast, low water availability is often shown to amplify tree growth responses to elevated [CO 2 ] (Wullschleger et al. 2002; Seiler et al. 2009 ). Amplifications of CO 2 responses in water stressed conditions is caused by reduced g s and, in turn, by decreased leaf level transpiration under elevated [CO 2 ], which may lead to an increase in plant water potential and water use efficiency (Centritto et al. 2002), a delay in the onset of drought (Centritto et al. 1999c), and a conservation of soil water (Wullschleger et al. 2002). ...
This chapter reviews the various interactions between tree processes and the environment in the context of observed and expected
environmental changes. The chapter begins with the influences of the ubiquitous atmospheric increases in CO2 concentration on leaf photosynthesis and respiration, followed by the expected influences on tree processes. Influences of
increasing incidence of drought, increased temperatures and extreme events are then discussed with respect to leaf and tree
level processes. Specific attention is given to hydraulic architecture, tree growth and water use efficiency, and species
differences in water relations and canopy structure across Europe. The chapter ends with a brief review of canopy-atmosphere
interaction and forest influences on climate.
... Boron is also involved in hemi-cellulose and structural protein formation, and its concentration will likely be related to how plant N concentration is affected by elevated CO 2 (Taiz and Zeiger 2002). Although S is also organically bound, soils generally have S in excess of plant needs (Schlesinger 1997), and luxury uptake could lead to increased S concentrations under high CO 2 if other essential nutrients are limited (Sterner and Elser 2002). Effects of CO 2 enrichment on Ca, Cu, K, Fe, Mg, Mn, and Zn concentrations are likely influenced by rhizosphere conditions. ...
Elevated CO2 is expected to lower plant nutrient concentrations via carbohydrate dilution and increased nutrient use efficiency. Elevated CO2 consistently lowers plant foliar nitrogen, but there is no consensus on CO2 effects across the range of plant nutrients. We used meta-analysis to quantify elevated CO2 effects on leaf, stem, root, and seed concentrations of B, Ca, Cu, Fe, K, Mg, Mn, P, S, and Zn among four plant functional groups and two levels of N fertilization. CO2 effects on plant nutrient concentration depended on the nutrient, plant group, tissue, and N status. CO2 reduced B, Cu, Fe, and Mg, but increased Mn concentration in the leaves of N-2 fixers. Elevated CO2 increased Cu, Fe, and Zn, but lowered Mn concentration in grass leaves. Tree leaf responses were strongly related to N status: CO2 significantly decreased Cu, Fe, Mg, and S at high N, but only Fe at low N. Elevated CO2 decreased Mg and Zn in crop leaves grown with high N, and Mn at low N. Nutrient concentrations in crop roots were not affected by CO2 enrichment, but CO2 decreased Ca, K, Mg and P in tree roots. Crop seeds had lower S under elevated CO2. We also tested the validity of a "dilution model." CO2 reduced the concentration of plant nutrients 6.6% across nutrients and plant groups, but the reduction is less than expected (18.4%) from carbohydrate accumulation alone. We found that elevated CO2 impacts plant nutrient status differently among the nutrient elements, plant functional groups, and among plant tissues. Our synthesis suggests that differences between plant groups and plant organs, N status, and differences in nutrient chemistry in soils preclude a universal hypothesis strictly related to carbohydrate dilution regarding plant nutrient response to elevated CO2.
Theory and experimentation play important roles in estimating the biological effects of climate change, especially the direct effects of CO2 on plants and ecosystems. Theory suggests possible direct effects of increasing atmospheric CO2 concentrations that have been increasingly tested in laboratory and field settings. Experimentation informs understanding of complex processes during warming and of the interactions of warming, changes in precipitation, and direct influences of CO2. Simple enhancement effects seen in theory and laboratory settings have been shown to be reduced or complicated in more realistic multi-species settings and Free Air CO2 Enhancement (FACE) experiments.
Elevated atmospheric CO2 concentration (eCO2) commonly stimulates net leaf assimilation, decreases stomatal conductance and has no clear effect on leaf respiration. However, effects of eCO2 on whole-tree functioning and its seasonal dynamics remain far more uncertain. To evaluate temporal and spatial variability in eCO2 effects, one-year-old European aspen trees were grown in two treatment chambers under ambient (aCO2, 400 ppm) and elevated (eCO2, 700 ppm) CO2 concentrations during an early (spring 2019) and late (autumn 2018) seasonal experiment (ESE and LSE, respectively). Leaf (net carbon assimilation, stomatal conductance and leaf respiration) and whole-tree (stem growth, sap flow and stem CO2 efflux) responses to eCO2 were measured. Under eCO2, carbon assimilation was stimulated during the early (1.63-fold) and late (1.26-fold) seasonal experiments. Stimulation of carbon assimilation changed over time with largest increases observed in spring when stem volumetric growth was highest, followed by late season down-regulation, when stem volumetric growth ceased. The neutral eCO2 effect on stomatal conductance and leaf respiration measured at leaf level paralleled the unresponsive canopy conductance (derived from sap flow measurements) and stem CO2 efflux measured at tree level. Our results highlight that seasonality in carbon demand for tree growth substantially affects the magnitude of the response to eCO2 at both leaf and whole-tree level.
At leaf level, elevated atmospheric CO2 concentration (eCO2) results in stimulation of carbon net assimilation and reduction of stomatal conductance. However, a comprehensive understanding of the impact of eCO2 at larger temporal (seasonal and annual) and spatial (from leaf to whole‐tree) scales is still lacking. Here, we review overall trends, magnitude, and drivers of dynamic tree responses to eCO2, including carbon and water relations at the leaf and the whole‐tree level. Spring and early season leaf responses are most susceptible to eCO2, and is followed by a down‐regulation towards the onset of autumn. At the whole‐tree level, CO2 fertilization causes consistent biomass increments in young seedlings only, whereas mature trees show a variable response. Elevated CO2‐induced reductions in leaf stomatal conductance do not systematically translate into limitation of whole‐tree transpiration due to the unpredictable response of canopy area. Reduction in the end‐of‐season carbon sink demand and water‐limiting strategies are considered the main drivers of seasonal tree responses to eCO2. These large temporal and spatial variabilities in tree responses to eCO2 highlight the risk of predicting tree behavior to eCO2 based on single leaf level point measurements as they only reveal snapshots of the dynamic responses to eCO2.
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Interactions between the carbon (C) and nitrogen (N) cycles can impact on the sensitivity of terrestrial C storage to elevated atmospheric carbon dioxide (CO2) concentrations (eCO2). However, the underlying mechanisms associated with CN interactions that influence terrestrial ecosystem C sequestration (Cseq) remains unclear. Here, we quantitatively analyzed published C and N responses to experimentally eCO2 using a meta-analysis approach. We determined the relative importance of three principal mechanisms (changes in the total ecosystem N amount, redistribution of N between plant and soil pools, and flexibility of the C:N ratio) that contribute to increases in ecosystem C storage in response to eCO2. Our results showed that eCO2 increased C and N accumulation, resulted in higher C:N ratios in plant, litter, and soil pools and induced a net shift of N from soils to vegetation. These three mechanisms largely explained the increment of ecosystem Cseq under eCO2, although the relative contributions differed across ecosystem types, with changes in the C:N ratio contributing 50% of the increment in forests Cseq, while the total N change contributed 60% of the increment in grassland Cseq. In terms of temporal variation in the relative importance of each of these three mechanisms to ecosystem Cseq: changes in the C:N ratio was the most important mechanism during the early years (~5 years) of eCO2 treatment, whilst the contribution to ecosystem Cseq by N redistribution remained rather small, and the contribution by total N change did not show a clear temporal pattern. This study highlights the differential contributions of the three mechanisms to Cseq, which may offer important implications for future predictions of the C cycle in terrestrial ecosystems subjected to global change.
Crop production in Africa is vulnerable to changing climatic conditions which needs to adapt for increased yields, sustenance of livelihoods and achievement of Sustainable Development Goals (SDGs). Bambara groundnut is an underutilised and nitrogen-fixing crop grown by smallholder farmers in Africa. However, beyond cultural value, the drought tolerance is the main trait which bambara groundnut exhibits with reasonable yield which is possibly the main reason why it is still being maintained by local populations whose livelihood are threatened by climate change. Also, the impact of plant diseases is one of the important factors affecting agricultural productivity and a major constraint which may be further aggravated by climate change. The current review investigates the nutritional components, agronomic practices, associated diseases of Bambara groundnut under a changing climatic condition.
When estimating tree-level biomass and carbon, it is common practice to develop generalized models across numerous species and large spatial extents. However, sampling efforts are generally incomplete and trees are not randomly selected. In this analysis, of the more than 1,000 biomass-related articles that were reviewed, trees were destructively sampled in over 300 studies to estimate biomass in the United States. Studies were summarized and past sampling efforts were explored to illuminate where the largest data gaps occurred in terms of tree components sampled, tree size, tree form, tree species, and location. The most prominent gaps were in large trees, particularly in Douglas-fir trees in the Pacific Northwest. In addition, tree roots were notably undersampled. Lastly, trees of poor or unusual form and low vigor were often not sampled, and this may introduce a systematic bias if not dealt with appropriately. More than 200 species did not have a biomass model or a single data point. The gaps presented here can be viewed as suggestions for future destructive sampling efforts, but the magnitude of a gap for a given model will ultimately depend on the selected modeling framework and the user's objectives.
Differences in the biogeochemistry of nitrogen (N) and phosphorus (P) lead to differential losses and inputs during and over time after fire such that fire may affect nutrient limitation of primary productivity. We conducted a nutrient addition experiment in scrubby flatwoods, a Florida scrub community type, to test the hypothesis that nutrient limitation of primary productivity shifts from N limitation in recently burned sites to P limitation in longer unburned sites. We added three levels of N, P, and N and P together to sites 6 weeks, 8 years, and 20 years postfire and assessed the effects of nutrient addition on above- and belowground productivity and nutrient concentrations. At the community level, nutrient addition did not affect aboveground biomass, but root productivity increased with high N + P addition in sites 8 and 20 years after fire. At the species level, N addition increased leaf biomass of saw palmetto (Serenoa repens) in sites 6 weeks and 20 years postfire, while P addition increased foliar %P and apical shoot growth of scrub oak (Quercus inopina) in sites 8 and 20 years postfire, respectively. Contrary to our hypothesis, nutrient limitation does not appear to shift with time after fire; recently burned sites show little evidence of nutrient limitation, while increased belowground productivity indicates that scrubby flatwoods are co-limited by N and P at intermediate and longer times after fire.
Theory and experimentation play important roles in estimating the biological effects of climate change, especially the direct effects of CO2 on plants and ecosystems. Theory suggests possible direct effects of increasing atmospheric CO2 concentrations that have been increasingly tested in laboratory and field settings. Experimentation informs understanding of complex processes during warming and of the interactions of warming, changes in precipitation, and direct influences of CO2. Simple enhancement effects seen in theory and laboratory settings have been shown to be reduced or complicated in more realistic multi-species settings and Free Air CO2 Enhancement (FACE) experiments.
As an important product of Moderate Resolution Imaging Spectroradiometer (MODIS), MOD17A2 provides dramatic improvements in our ability to accurately and continuously monitor global terrestrial primary production, which is also significant in effort to advance scientific research and eco-environmental management. Over the past decades, forests have moderated climate change by sequestrating about one-quarter of the carbon emitted by human activities through fossil fuels burning and land use/land cover change. Thus, the carbon uptake by forests reduces the rate at which carbon accumulates in the atmosphere. However, the sensitivity of near real-time MODIS gross primary productivity (GPP) product is directly constrained by uncertainties in the modeling process, especially in complicated forest ecosystems. Although there have been plenty of studies to verify MODIS GPP with ground-based measurements using the eddy covariance (EC) technique, few have comprehensively validated the performance of MODIS estimates (Collection 5) across diverse forest types. Therefore, the present study examined the degree of correspondence between MODIS-derived GPP and EC-measured GPP at seasonal and interannual time scales for the main forest ecosystems, including evergreen broadleaf forest (EBF), evergreen needleleaf forest (ENF), deciduous broadleaf forest (DBF), and mixed forest (MF) relying on 16 flux towers with a total of 68 site-year datasets. Overall, site-specific evaluation of multi-year mean annual GPP estimates indicates that the current MOD17A2 product works highly effectively for MF and DBF, moderately effectively for ENF, and ineffectively for EBF. Except for tropical forest, MODIS estimates could capture the broad trends of GPP at 8-day time scale for all other sites surveyed. On the annual time scale, the best performance was observed in MF, followed by ENF, DBF, and EBF. Trend analyses also revealed the poor performance of MODIS GPP product in EBF and DBF. Thus, improvements in the sensitivity of MOD17A2 to forest productivity require continued efforts. © 2015, Science Press, Northeast Institute of Geography and Agricultural Ecology, CAS and Springer-Verlag Berlin Heidelberg.
Accurate and continuous monitoring of forest production is critical for quantifying the dynamics of regional-to-global carbon cycles. MOD17A2 provides high frequency observations of terrestrial gross primary productivity (GPP) and is widely used to evaluate the spatiotemporal variability and responses to changing climate. However, the effectiveness of the Moderate Resolution Imaging Spectroradiometer (MODIS) in measuring GPP is directly constrained by the large uncertainties in the modeling process, specifically for complicated and extensive forest ecosystems. Although there have been plenty of studies to verify the MODIS GPP product with ground-based measurements covering a range of biome types, few have comprehensively validated the performance of MODIS estimates (C5.5) for diverse forests. Thus, this study examined the degree of correspondence between the MODIS-derived GPP and the EC-measured GPP at seasonal and interannual time scales for the main forest ecosystems, encompassing evergreen broadleaf forest (EBF), evergreen needleleaf forest (ENF), deciduous broadleaf forest (DBF), and mixed forest (MF) relying on 16 flux towers with a total dataset of 68 site-years. Overall, the site-specific evaluation of multi-year mean annual GPP estimates indicates that the current MODIS product works more significantly for DBF and MF, less for ENF, and least for EBF. Except for the tropical forest, MODIS estimates could capture the broad trends of GPP at an 8-day time scale for the other sites. At the seasonal time scale, the highest performance was observed in ENF, followed by MF and DBF, and the least performance was observed in EBF. Trend analyses also revealed the weak performance in EBF and DBF. This study suggested that current MODIS GPP estimates still need to improve the quality of different upstream inputs in addition to the algorithm for accurately quantifying forest production.
Historically, tree biomass at large scales has been estimated by applying dimensional analysis techniques and field measurements such as diameter at breast height (dbh) in allometric regression equations. Equations often have been developed using differing methods and applied only to certain species or isolated areas. We previously had compiled and combined (in meta-analysis) available diameter-based allometric regression equations for estimating total aboveground and component dry-weight biomass for US trees. This had resulted in a set of 10 consistent, national-scale aboveground biomass regression equations for US species, as well as equations for predicting biomass of tree components as proportions of total aboveground biomass. In this update of our published equation database and refinement of our model, we developed equations based on allometric scaling theory, using taxonomic groupings and wood specific gravity as surrogates for scaling parameters that we could not estimate. The new approach resulted in 35 theoretically based generalized equations (13 conifer, 18 hardwood, 4 woodland), compared with the previous empirically grouped 10. For trees from USDA Forest Inventory and Analysis Program (FIA) plots, with forest types grouped into conifers and hardwoods, previous and updated equations produced nearly identical estimates that predicted ∼20 per cent higher biomass than FIA estimates. Differences were observed between previous and updated equation estimates when comparisons were made using individual FIA forest types. © 2013 Institute of Chartered Foresters, All rights reserved. For Permissions, please e-mail: [email protected]
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Studies of responses of forest vegetation to steadily increasing atmospheric concentrations of CO2 have focussed strongly on the potential of trees to absorb extra carbon; the effects of elevated [CO2] on plant-soil water relations via decreased stomatal conductance and increased ambient temperature have received less attention, but may be significant in the long term at the ecosystem level. CO2 augmentation experiments with young trees demonstrate small increases in aboveground carbon content, but these increases tend to diminish as trees get older. By contrast, several experiments suggest continued decreases in transpiration and increased soil water content under these conditions. In tropical forests, the major cause of increases in aboveground biomass observed in the recent past is not necessarily elevated [CO2]. Undoubtedly, the potential of monitoring trees in forest dynamics plots to deduce CO2-specific alterations in forest structure and standing biomass will unfold in the decades to come. The comprehensive understanding of responses of forest vegetation to elevated [CO2] in the Anthropocene will depend upon the inclusion of detailed measurements of soil water pools and water fluxes through the soil-plant-atmosphere continuum in future tree CO2 augmentation experiments and forest dynamics plot studies.
Precipitation as a key determinant of forest productivity influences
forest ecosystems also indirectly through alteration of the nutrient
status of the soil, but this interaction is not well understood. Along a
steep precipitation gradient (from 970 to 520 mm yr-1 over
150 km distance), we studied the consequences of reduced precipitation
for the soil and biomass nutrient pools and dynamics in 14 mature
European beech (Fagus sylvatica L.) forests on uniform geological
substrate. We tested the hypotheses that lowered summer precipitation
(1) is associated with less acid soils and a reduced accumulation of
organic matter on the forest floor, and (2) reduces nutrient supply from
the soil and leads to decreasing foliar and root nutrient
concentrations. Soil acidity, the amount of forest floor organic matter,
and the associated organic matter N and P pools decreased to about a
half from wet to dry sites; the C/P and N/P ratios, but not the C/N
ratio, of forest floor organic matter decreased. Net N mineralization
(and nitrification) rate and the available P and K pools in the mineral
soil did not change with decreasing precipitation. Foliar P and K
concentrations (beech sun leaves) increased while N remained constant,
resulting in decreasing foliar N/P and N/K ratios. N resorption
efficiency increased toward the dry sites. We conclude that a reduction
in summer rainfall significantly reduces the soil C, N and P pools but
does not result in decreasing foliar N and P contents in beech. However,
more effective tree-internal N cycling and the decreasing foliar N/P
ratio towards the dry stands indicate that tree growth may increasingly
be limited by N and not by P with decreasing precipitation.
The effects of elevated CO on ecosystem element stocks are equivocal, in part because cumulative effects of CO on element pools are difficult to detect. We conducted a complete above and belowground inventory of non-nitrogen macro-and micronutrient stocks in a subtropical woodland exposed to twice-ambient CO concentrations for 11 years. We analyzed a suite of nutrient elements and metals important for nutrient cycling in soils to a depth of ~2 m, in leaves and stems of the dominant oaks, in fine and coarse roots, and in litter. In conjunction with large biomass stimulation, elevated CO increased oak stem stocks of Na, Mg, P, K, V, Zn and Mo, and the aboveground pool of K and S. Elevated CO increased root pools of most elements, except Zn. CO -stimulation of plant Ca was larger than the decline in the extractable Ca pool in soils, whereas for other elements, increased plant uptake matched the decline in the extractable pool in soil. We conclude that elevated CO caused a net transfer of a subset of nutrients from soil to plants, suggesting that ecosystems with a positive plant growth response under high CO will likely cause mobilization of elements from soil pools to plant biomass. This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.
Increasing atmospheric CO 2 concentrations alter leaf physiology, with effects that cascade to communities and ecosystems. Yet, responses over cycles of disturbance and recovery are not well known, because most experiments span limited ecological time. We examined the effects of CO 2 on root growth, herbivory and arthropod biodiversity in a woodland from 1996 to 2006, and the legacy of CO 2 enrichment on these processes during the year after the CO 2 treatment ceased.
We used minirhizotrons to study root growth, leaf censuses to study herbivory and pitfall traps to determine the effects of elevated CO 2 on arthropod biodiversity.
Elevated CO 2 increased fine root biomass, but decreased foliar nitrogen and herbivory on all plant species. Insect biodiversity was unchanged in elevated CO 2 . Legacy effects of elevated CO 2 disappeared quickly as fine root growth, foliar nitrogen and herbivory levels recovered in the next growing season following the cessation of elevated CO 2 .
Although the effects of elevated CO 2 cascade through plants to herbivores, they do not reach other trophic levels, and biodiversity remains unchanged. The legacy of 10 yr of elevated CO 2 on plant–herbivore interactions in this system appear to be minimal, indicating that the effects of elevated CO 2 may not accumulate over cycles of disturbance and recovery.
We evaluated the impacts of elevated CO(2) in a treeline ecosystem in the Swiss Alps in a 9-year free-air CO(2) enrichment (FACE) study. We present new data and synthesize plant and soil results from the entire experimental period. Light-saturated photosynthesis (A (max)) of ca. 35-year-old Larix decidua and Pinus uncinata was stimulated by elevated CO(2) throughout the experiment. Slight down-regulation of photosynthesis in Pinus was consistent with starch accumulation in needle tissue. Above-ground growth responses differed between tree species, with a 33 % mean annual stimulation in Larix but no response in Pinus. Species-specific CO(2) responses also occurred for abundant dwarf shrub species in the understorey, where Vaccinium myrtillus showed a sustained shoot growth enhancement (+11 %) that was not apparent for Vaccinium gaultherioides or Empetrum hermaphroditum. Below ground, CO(2) enrichment did not stimulate fine root or mycorrhizal mycelium growth, but increased CO(2) effluxes from the soil (+24 %) indicated that enhanced C assimilation was partially offset by greater respiratory losses. The dissolved organic C (DOC) concentration in soil solutions was consistently higher under elevated CO(2) (+14 %), suggesting accelerated soil organic matter turnover. CO(2) enrichment hardly affected the C-N balance in plants and soil, with unaltered soil total or mineral N concentrations and little impact on plant leaf N concentration or the stable N isotope ratio. Sustained differences in plant species growth responses suggest future shifts in species composition with atmospheric change. Consistently increased C fixation, soil respiration and DOC production over 9 years of CO(2) enrichment provide clear evidence for accelerated C cycling with no apparent consequences on the N cycle in this treeline ecosystem.
1. This study uses pitfall traps and sticky traps to examine the effects of elevated CO2 on the densities of insect herbivores, insectivores, and detritivores.
2. Pitfall trapping for the last 3 years of 11 years of continuously elevated CO2 revealed increases of insect herbivore species such as Thysanoptera (thrips), Hemiptera, and Lepidoptera, but no effects on insectivores such as spiders, parasitic wasps, and ants; or on detritivores such as Diptera (flies), Psocoptera (book lice), Blattodea (cockroaches), Collembola (spring tails), Orthoptera (crickets), and Coleoptera (beetles).
3. As the bottom-up effects of elevated CO2 are transmitted through plants to herbivores, they do not appear to reach insect natural enemies or decomposers.
Abstract The effects of elevated CO2 on plant growth and insect herbivory have been frequently investigated over the past 20 years. Most studies have shown an increase in plant growth, a decrease in plant nitrogen concentration, an increase in plant secondary metabolites and a decrease in herbivory. However, such studies have generally overlooked the fact that increases in plant production could cause increases of herbivores per unit area of habitat. Our study investigated leaf production, herbivory levels and herbivore abundance per unit area of leaf litter in a scrub-oak system at Kennedy Space Center, Florida, under conditions of ambient and elevated CO2, over an 11-year period, from 1996 to 2007. In every year, herbivory, that is leafminer and leaftier abundance per 200 leaves, was lower under elevated CO2 than ambient CO2 for each of three species of oaks, Quercus myrtifolia, Quercus chapmanii and Quercus geminata. However, leaf litter production per 0.1143 m2 was greater under elevated CO2 than ambient CO2 for Q. myrtifolia and Q. chapmanii, and this difference increased over the 11 years of the study. Leaf production of Q. geminata under elevated CO2 did not increase. Leafminer densities per 0.1143 m2 of litterfall for Q. myrtifolia and Q. chapmanii were initially lower under elevated CO2. However, shortly after canopy closure in 2001, leafminer densities per 0.1143 m2 of litter fall became higher under elevated CO2 and remained higher for the remainder of the experiment. Leaftier densities per 0.1143 m2 were also higher under elevated CO2 for Q. myrtifolia and Q. chapmanii over the last 6 years of the experiment. There were no differences in leafminer or leaftier densities per 0.1143 m2 of litter for Q. geminata. These results show three phenomena. First, they show that elevated CO2 decreases herbivory on all oak species in the Florida scrub-oak system. Second, despite lower numbers of herbivores per 200 leaves in elevated CO2, increased leaf production resulted in higher herbivore densities per unit area of leaf litter for two oak species. Third, they corroborate other studies which suggest that the effects of elevated CO2 on herbivores are species specific, meaning they depend on the particular plant species involved. Two oak species showed increases in leaf production and herbivore densities per 0.1143 m2 in elevated CO2 over time while another oak species did not. Our results point to a future world of elevated CO2 where, despite lower plant herbivory, some insect herbivores may become more common.
The changes in light availability in the understory of a subtropical wet forest (Luquillo Experimental Forest, Puerto Rico) were monitored after the passage of Hurricane Hugo on 18 September 1989. Gallium arsenide phosphide sensors were placed 1 m apart along a 32 m transect. Data were collected for periods of 7-10 d in October and December 1989, and in March, July, and November 1990. Daily histograms were generated for observations of photosynthetic photon flux density (PPFD) taken every two seconds. Mean total daily PPFD was calculated for each sensor in each data set. During the 14 mo after the passage of the hurricane, the PPFD showed a highly skewed distribution with most values < 200-mu-mol m-2 s-1. The maximum spatial heterogeneity was observed in July 1990 because of the shading of some sensors by the growing pioneer vegetation. Median values of total daily PPFD for ten months after the hurricane ranged from 7.7 to 10.8 mol m-2 d-1, which is similar to values previously observed for large (> 400 m2) treefall gaps. Median total daily PPFD fell to 0.8 mol m-2 d-1 in November 1990 because of almost complete coverage of the transect by a canopy of Cecropia schreheriana Miq. ex. C. peltata. An analysis of semivariance was used to discern patterns of autocorrelation in total daily PPFD along the transect. Through March 1990 patches of high and low light separated by distances of 10-12 m were detected. By July 1990 the patchiness was replaced by a pattern that showed no autocorrelation at distances of 1 m or greater.
We sampled aboveground biomass in four stands of oak-saw palmetto scrub vegetation that were 2, 4, 8, and 25 years since the previous fire by harvesting 1 m 2 plots. Biomass samples were analyzed for major nutrients. We sampled and analyzed soils from the 0-15 em and 15-30 em layers. Stands were dominated by Quercus myrtifolia, Q. geminata, Q. chapmanii, Serenoa repens, and ericaceous shrubs. Live aboveground biomass (excluding saw palmetto rhizomes) increased with time since fire. Litter biomass increased for eight years after fire. Standing dead biomass was an important component of above ground biomass throughout the time sequence. Aboveground saw palmetto rhizomes were a major biomass category. Nutrient concentrations in live aboveground biomass did not appear to change with time since fire and were similar to those in other shrublands. Biomass pools of major nutrients frequently equaled or exceeded those in the soil, but wetter sites had more organic matter and nutrients in the soil. Atmospheric deposition of N, P, Ca, Mg, and K was low compared to biomass pools. Retention of nutrients in soils and regrowing vegetation after fire may be important to the persistence of scrub on low nutrient soils.
Hurricane Iniki damaged a forest in which we had previously studied nutrient limitation to productivity. We had measured the response of aboveground net primary productivity (ANPP) to fertilizer applications and had found phosphorus to be limiting. Reductions of leaf area index (LAI) after the hurricane's passage ranged from 3% to 59%, were correlated with prehurricane LAI, and were greatest in P-amended treatments (+P). LAI recovered to near prehurricane levels by 9 mo after passage, and rates of recovery were unaffected by treatment. Mortality of fine roots ranged from 35 to 48% following the hurricane and recovered in 2 yr. Stem damage was largely branch removal, but some stems were partially uprooted or decapitated. Large trees were damaged with greater frequency than small trees, and severity of damage increased in +P treatments. Fine litterfall caused by the storm was 1.4 times the annual input, and nutrient transfers to the forest floor approximated that of a typical year. Stem diameter increment and aboveground net primary productivity (ANPP) declined but returned to prehurricane values 2 yr later in +P treatments while remaining low in -P treatments (i.e., those without P supplementation). Rates of recovery to prehurricane stem growth and ANPP were greater in +P treatments and were accompanied by a much greater ANPP per unit leaf area (E). The results support hypotheses that ecosystem resistance and resilience are inversely related and that resistance decreases and resilience increases as supply rates of limiting resources increase. However, they also suggest that structural and functional components of resistance and resilience should be considered separately.
Interactive effects of increases in atmospheric CO2 and reductions in plant species diversity were investigated in planted calcareous grassland communities in northwestern Switzerland. The experimental communities were composed of 5, 12, and 31 species assembled from the native species pool. The study aimed at testing whether the CO2 responses of ecosystems change when specific sets of species are lost from plant communities. Species were selected so that the proportion of grasses, legumes, and non-legume forb individuals remained constant across levels of diversity. The most diverse plant community had approximately the same diversity as the surrounding grassland, and species occurring in less diverse Communities were subsets of the species in the more diverse communities. The factorial atmospheric-CO2 treatment was applied using 50-cm-tall, open-bottom, open-top wind screens. Plant community-level responses and the responses of the individual species were assessed over a period of five. years. A significant positive correlation between plant community diversity and biomass was detected, but this effect was not present on all dates. Significant effects of elevated CO2 on community biomass were only found in the first years of treatment. CO2 effects were largest in the communities with the highest number of plant species and were primarily due to the presence of responsive species not present in the less diverse communities. The time dependency of community responses to elevated CO2 and species diversity was related to shifts in community structure of the experimental plots. Community responses at the beginning of the experiment were dominated by the response of species with a less competitive/stress-tolerant life history. These species were successively lost from experimental plots as the experiment proceeded, and the observed community-level effects became smaller. Changes in species composition over the experimental duration were affected by elevated CO2 in the way that species loss was reduced (i.e., coexistence of species performing well at the beginning and at the end of the five-year period increased) and the way that community evenness was increased (i.e.,dominance was reduced). Based on these results our main conclusions are that (1) community-level responses to CO2 enrichment depend on the species Present; (2) the positive correlation between productivity and species numbers was caused by different species at the beginning and at the end of the experiment; (3) therefore, a large, redundant species pool is important in assuring high productivity under altering environmental conditions; (4) elevated CO2 has the potential to substantially alter the structure of grassland communities, even if community productivity does not increase; and (5) a short-term effect of elevated CO2 may be misleading when attempting to predict longer-term effects.
Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmospheric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems. Atmospheric carbon dioxide and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s. This recent sink can be largely attributed to northern extratropical areas, and is roughly split between North America and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrowth on abandoned agricultural land and fire prevention, in addition to responses to environmental changes, such as longer growing seasons, and fertilization by carbon dioxide and nitrogen. Nevertheless, there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes.
Changes in Earth's surface temperatures caused by anthropogenic emissions of greenhouse gases are expected to affect global
and regional precipitation regimes. Interactions between changing precipitation regimes and other aspects of global change
are likely to affect natural and managed terrestrial ecosystems as well as human society. Although much recent research has
focused on assessing the responses of terrestrial ecosystems to rising carbon dioxide or temperature, relatively little research
has focused on understanding how ecosystems respond to changes in precipitation regimes. Here we review predicted changes
in global and regional precipitation regimes, outline the consequences of precipitation change for natural ecosystems and
human activities, and discuss approaches to improving understanding of ecosystem responses to changing precipitation. Further,
we introduce the Precipitation and Ecosystem Change Research Network (PrecipNet), a new interdisciplinary research network
assembled to encourage and foster communication and collaboration across research groups with common interests in the impacts
of global change on precipitation regimes, ecosystem structure and function, and the human enterprise.
Summary • The effect of elevated CO2 on species’ performance was investigated in communities composed of five annual weeds that are characteristic of early old field succession in central Europe: Centaurea cyanus L., Matricaria chamomilla L., Silene noctiflora L., Papaver rhoeas L. and Legousia speculum-veneris (L.) Chaix. • The experiment was based on a simplex design, repeated at two overall levels of initial stand density, to give a wide range of five-species communities across which the initial composition and species abundance varied systematically. • A multivariate method, based on analysing the differences in relative growth rates (RGRD) between pairs of species, was extended for use with more than two species, in order to assess the relative importance of various determinants of change in stand biomass composition. • On average, Centaurea (54.6% of final yield) gave the highest yield, followed by Matricaria (22.9%), Silene (16.9%), Legousia (3.1%) and Papaver (2.7%). • The major determinants of change in community structure were species identity and CO2 level. Elevated CO2 significantly changed community composition towards the previously more poorly performing species Silene, Legousia and Papaver. • Despite strong effects of intra- and interspecific competition on individual species performance, species’ initial abundance had relatively little impact on the change in community composition. Most cases where such effects were significant involved Silene: performance of Papaver was poorer in communities with higher initial presence of Silene and higher initial abundances of Centaurea and Matricaria always facilitated performance of Silene. • These new methods proved a powerful system for identifying the biotic and abiotic factors determining change in biomass composition in multispecies communities. Journal of Ecology (2005) doi: 10.1111/j.1365-2745.2005.00999.x
Forests exchange large amounts of CO 2 with the atmosphere and can influence and be influenced by atmospheric CO 2 . There has been a recent proliferation of literature on the effects of atmospheric CO 2 on forest trees. More than 300 studies of trees on five different continents have been published in the last five years. These include an increasing number of field studies with a long‐term focus and involving CO 2 ×stress or environment interactions. The recent data on long‐term effects of elevated atmospheric CO 2 on trees indicate a potential for a persistent enhancement of tree growth for several years, although the only relevant long‐term datasets currently available are for juvenile trees.
The current literature indicates a significantly larger average long‐term biomass increment under elevated CO 2 for conifers (130%) than for deciduous trees (49%) in studies not involving stress components. However, stimulation of photosynthesis by elevated CO 2 in long‐term studies was similar for conifers (62%) and deciduous trees (53%). Recent studies indicate that elevated CO 2 causes a more persistent stimulation of biomass increment and photosynthesis than previously expected. Results of seedling studies, however, might not be applicable to other stages of tree development because of complications of age‐dependent and size‐dependent shifts in physiology and carbon allocation, which are accelerated by elevated CO 2 . In addition, there are many possible avenues to down‐regulation, making the predicted canopy CO 2 exchange and growth of mature trees and forests in a CO 2 ‐rich atmosphere uncertain. Although, physiological down‐regulation of photosynthetic rates has been documented in field situations, it is rarely large enough to offset entirely photosynthetic gains in elevated CO 2 . A persistent growth stimulation of individual mature trees has been demonstrated although this effect is more uncertain in trees in natural stands.
Resource interactions can both constrain tree responses to elevated CO 2 and be altered by them. Although drought can reduce gas‐exchange rates and offset the benefits of elevated CO 2 , even in well watered trees, stomatal conductance is remarkably less responsive to elevated CO 2 than in herbaceous species. Stomata of a number of tree species have been demonstrated to be unresponsive to elevated CO 2 . We conclude that positive effects of CO 2 on leaf area can be at least as important in determining canopy transpiration as negative, direct effects of CO 2 on stomatal aperture. With respect to nutrition, elevated CO 2 has the potential to alter tree–soil interactions that might influence future changes in ecosystem productivity. There is continued evidence that in most cases nutrient limitations diminish growth and photosynthetic responses to elevated CO 2 at least to some degree, and that elevated CO 2 can accelerate the appearance of nutrient limitations with increasing time of treatment. In many studies, tree biomass responses to CO 2 are artefacts in the sense that they are merely responses to CO 2 ‐induced changes in internal nutritional status of the tree.
There are numerous interactions between CO 2 and factors of the biotic and abiotic environment. The importance of increasing atmospheric CO 2 concentrations for productivity is likely to be overestimated if these are not taken into account. Many interactions, however, are simply additive rather than synergistic or antagonistic. This appears to hold true for many parameters under elevated CO 2 in combination with temperature, elevated O 3 , and other atmospheric pollutants. However, there is currently little evidence that elevated CO 2 will counteract O 3 damage. When the foliage content of C, mineral nutrients and secondary metabolites is altered by elevated CO 2 , tree×insect interactions are modified. In most trees, mycorrhizal interactions might be less important for direct effects of CO 2 than for alleviating general nutrient deficiencies.
Since many responses to elevated CO 2 and their interactions with stress show considerable variability among species/genotypes, one principal research need is for comparative studies of a large variety of woody species and ecosystems under realistic conditions. We still need more long‐term experiments on mature trees and stands to address critical scaling issues likely to advance our understanding of responses to elevated CO 2 at different stages of forest development and their interactions with climate and environment. The only tools available at present for coping with the consequences of rising CO 2 are management of resources and selection of genotypes suitable for the future climate and environment.
CONTENTS
Summary
396
I.
Introduction
396
II.
Influence of experimental approach
398
III.
Growth and development
403
IV.
Impact of water relations
405
V.
Photosynthesis and respiration
410
VI.
Foliar chemical composition
415
VII.
Interaction with mineral nutrition
417
VIII.
Interactions with temperature
419
IX.
Interactions with air pollution, u.v.‐B and salt
420
X.
Biotic interactions
422
XI.
The Ecosystem
423
XII.
Future research needs and unresolved questions
429
Acknowledgements
429
References
430
Photosynthesis is commonly stimulated in grasslands with experimental increases in atmospheric CO2 concentration ([CO2]), a physiological response that could significantly alter the future carbon cycle if it persists in the long term. Yet an acclimation of photosynthetic capacity suggested by theoretical models and short-term experiments could completely remove this effect of CO2. Perennial ryegrass (Lolium perenne L. cv. Bastion) was grown under an elevated [CO2] of 600 µmol mol−1 for 10 years using Free Air CO2Enrichment (FACE), with two contrasting nitrogen levels and abrupt changes in the source : sink ratio following periodic harvests. More than 3000 measurements characterized the response of leaf photosynthesis and stomatal conductance to elevated [CO2] across each growing season for the duration of the experiment. Over the 10 years as a whole, growth at elevated [CO2] resulted in a 43% higher rate of light-saturated leaf photosynthesis and a 36% increase in daily integral of leaf CO2 uptake. Photosynthetic stimulation was maintained despite a 30% decrease in stomatal conductance and significant decreases in both the apparent, maximum carboxylation velocity (Vc,max) and the maximum rate of electron transport (Jmax). Immediately prior to the periodic (every 4–8 weeks) cuts of the L. perenne stands, Vc,max and Jmax, were significantly lower in elevated than in ambient [CO2] in the low-nitrogen treatment. This difference was smaller after the cut, suggesting a dependence upon the balance between the sources and sinks for carbon. In contrast with theoretical expectations and the results of shorter duration experiments, the present results provide no significant change in photosynthetic stimulation across a 10-year period, nor greater acclimation in Vc,max and Jmax in the later years in either nitrogen treatment.
The microclimate in facilities for studying effects of elevated CO 2 on crops differs from ambient conditions. Open‐top chambers (OTCs) increase temperature by 1–3 °C. If temperature and CO 2 interact in their effect on crops, this would limit the value of OTC experiments. Furthermore, interaction of CO 2 and temperature deserves study because increases in atmospheric CO 2 concentration are expected to cause global warming.
This paper describes two experiments in which a recently developed cooling system for OTCs was used to analyse the effects of temperature on photosynthesis, growth and yield of spring wheat ( Triticum aestivum L., cv. Minaret). Two levels of CO 2 were used (350 and 700 ppm), and two levels of temperature, with cooled OTCs being 1.6–2.4 °C colder than noncooled OTCs.
Photosynthetic rates were increased by elevated CO 2 , but no effect of temperature was found. Cross‐switching CO 2 concentrations as well as determination of A–C i curves showed that plant photosynthetic capacity after anthesis acclimated to elevated CO 2 . The acclimation may be related to the effects of CO 2 on tissue composition: elevated CO 2 decreased leaf nitrogen concentrations and increased sugar content. Calculations of the seasonal mean crop light‐use efficiency (LUE) were consistent with the photosynthesis data in that CO 2 increased LUE by 20% on average whereas temperature had no effect. Both elevating CO 2 and cooling increased grain yield, by an average of 11% and 23%, respectively. CO 2 and temperature stimulated yield via different mechanisms: CO 2 increased photosynthetic rate, but decreased crop light interception capacity (LAI), whereas cooling increased grain yield by increasing LAI and extending the growing season with 10 days. The effects of CO 2 and temperature were not additive: the CO 2 effect was about doubled in the noncooled open‐top chambers. In most cases, effects on yield were mediated through increased grain density rather than increased individual grain weights.
The higher growth response to elevated CO 2 in noncooled vs. cooled OTCs shows that a cooling system may remove a bias towards overestimating crop growth response to CO 2 in open‐top chambers.
This review first summarizes the numerous studies that have described the interaction between the nitrogen supply and the response of photosynthesis, metabolism and growth to elevated [CO2]. The initial stimulation of photosynthesis in elevated [CO2] is often followed by a decline of photosynthesis, that is typically accompanied by a decrease of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), an accumulation of carbohydrate especially starch, and a decrease of the nitrogen concentration in the plant. These changes are particularly marked when the nitrogen supply is low, whereas when the nitrogen supply is adequate there is no acclimation of photosynthesis, no major decrease in the internal concentration of nitrogen or the levels of nitrogen metabolites, and growth is stimulated markedly. Second, emerging evidence is discussed that signals derived from nitrate and nitrogen metabolites such as glutamine act to regulate the expression of genes involved in nitrate and ammonium uptake and assimilation, organic acid synthesis and starch accumulation, to modulate the sugar-mediated repression of the expression of genes involved in photosynthesis, and to modulate whole plant events including shoot–root allocation, root architecture and flowering. Third, increased rates of growth in elevated [CO2] will require higher rates of inorganic nitrogen uptake and assimilation. Recent evidence is discussed that an increased supply of sugars can increase the rates of nitrate and ammonium uptake and assimilation, the synthesis of organic acid acceptors, and the synthesis of amino acids. Fourth, interpretation of experiments in elevated [CO2] requires that the nitrogen status of the plants is monitored. The suitability of different criteria to assess the plant nitrogen status is critically discussed. Finally the review returns to experiments with elevated [CO2] and discusses the following topics: is, and if so how, are nitrate and ammonium uptake and metabolism stimulated in elevated [CO2], and does the result depend on the nitrogen supply? Is acclimation of photosynthesis the result of sugar-mediated repression of gene expression, end-product feedback of photosynthesis, nitrogen-induced senescence, or ontogenetic drift? Is the accumulation of starch a passive response to increased carbohydrate formation, or is it triggered by changes in the nutrient status? How do changes in sugar production and inorganic nitrogen assimilation interact in different conditions and at different stages of the life history to determine the response of whole plant growth and allocation to elevated [CO2]?
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 self-thinning rule describes plant mortality because of competition in crowded even-aged stands. The rule is best understood with respect to a graph of log biomass (log B) per unit area vs. log density (log N) of survivors, known as the B–N diagram. The rule has three notable features—(1) mortality is a function only of biomass accumulation, (2) because mortality is driven by the rate of accumulation of biomass, mortality is slower when conditions for growth are worse, and (3) the thinning line has a slope of about –½ for most studied species under most conditions. Two main effects operate in developing distributions in even-aged stands. First, large plants suppress small plants. The result is a “hierarchy of dominance and suppression” in which the smaller plants are at an accumulating disadvantage and finally die. Second, the mortality of smaller individuals truncates the distribution from the left.
Small open top chambers (0.8 m x 1.0 m) were developed to maintain elevated CO2 concentrations in three plant communities in a brackish marsh ecosystem. Mean annual CO2 concentrations were 350 ± 22 μl l-1 in chambers which received no added CO2 and 686 ± 30 μl l-1 in chambers with elevated CO2 concentrations. Light quality was not affected in the photosynthetically active wavelengths but the chamber reduced light quantity by 10%. Night-time air temperatures inside the chamber (T(i)) averaged 2°C above air temperature outside the chamber (T(o)) due to heating from the air blowers. Air temperature profiles through the plant canopy and boundary layer showed that daytime temperature differences (T(i) - T(o)) were greater than night-time differences and this day/night difference also depended on the plant community. Effects of the chamber on the micro-environment of the plant communities resulted in a significant growth enhancement in the plant community dominated by the C3 sedge Scirpus olneyi Grey but not in the other two communities.
On 18 September 1989 Hurricane Hugo defoliated large forested areas of northeastern Puerto Rico. In two severely damaged subtropical wet forest sites, a mean of 1006-1083 g/m$^2$, or 419-451 times the mean daily input of fine litter (leaves, small wood, and miscellaneous debris) was deposited on the forest floor. An additional 928 g/m$^2$ of litter was suspended above the ground. A lower montane rain forest site received 682 times the mean daily fine litterfall. The concentrations of N and P in the hurricane leaf litter ranged from 1.1 to 1.5 and 1.7 to 3.3 times the concentrations of N and P in normal leaffall, respectively. In subtropical wet forest, fine litterfall from the hurricane contained 1.3 and 1.5-2.4 times the mean annual litterfall inputs of N and P, respectively. These sudden high nutrient inputs apparently altered nutrient cycling.
Abstract For two species of oak, we determined whether increasing atmospheric CO2 concentration (Ca) would decrease leaf mitochondrial respiration (R) directly, or indirectly owing to their growth in elevated Ca, or both. In particular, we tested whether acclimatory decreases in leaf-Rubisco content in elevated Ca would decrease R associated with its maintenance. This hypothesis was tested in summer 2000 on sun and shade leaves of Quercus myrtifolia Willd. and Quercus geminata Small. We also measured R on five occasions between summer 1999 and 2000 on leaves of Q. myrtifolia. The oaks were grown in the field for 4 years, in either current ambient or elevated (current ambient + 350 µmol mol−1) Ca, in open-top chambers (OTCs). For Q. myrtifolia, an increase in Ca from 360 to 710 µmol mol−1 had no direct effect on R at any time during the year. In April 1999, R in young Q. myrtifolia leaves was significantly higher in elevated Ca—the only evidence for an indirect effect of growth in elevated Ca. Leaf R was significantly correlated with leaf nitrogen (N) concentration for the sun and shade leaves of both the species of oak. Acclimation of photosynthesis in elevated Ca significantly reduced maximum RuBP-saturated carboxylation capacity (Vc max) for both the sun and shade leaves of only Q. geminata. However, we estimated that only 11–12% of total leaf N was invested in Rubisco; consequently, acclimation in this plant resulted in a small effect on N and an insignificant effect on R. In this study measurements of respiration and photosynthesis were made on material removed from the field; this procedure had no effect on gas exchange properties. The findings of this study were applicable to R expressed either per unit leaf area or unit dry weight, and did not support the hypothesis that elevated Ca decreases R directly, or indirectly owing to acclimatory decreases in Rubisco content.
Abstract It has been suggested that desert vegetation will show the strongest response to rising atmospheric carbon dioxide due to strong water limitations in these systems that may be ameliorated by both photosynthetic enhancements and reductions in stomatal conductance. Here, we report the long-term effect of 55 Pa atmospheric CO2 on photosynthesis and stomatal conductance for three Mojave Desert shrubs of differing leaf phenology (Ambrosia dumosa—drought-deciduous, Krameria erecta—winter-deciduous, Larrea tridentata—evergreen). The shrubs were growing in an undisturbed ecosystem fumigated using FACE technology and were measured over a four-year period that included both above and below-average precipitation. Daily integrated photosynthesis (Aday) was significantly enhanced by elevated CO2 for all three species, although Krameria erecta showed the greatest enhancements (63% vs. 32% for the other species) enhancements were constant throughout the entire measurement period. Only one species, Larrea tridentata, decreased stomatal conductance by 25–50% in response to elevated CO2, and then only at the onset of the summer dry season and following late summer convective precipitation. Similarly, reductions in the maximum carboxylation rate of Rubisco were limited to Larrea during spring. These results suggest that the elevated CO2 response of desert vegetation is a function of complex interactions between species functional types and prevailing environmental conditions. Elevated CO2 did not extend the active growing season into the summer dry season because of overall negligible stomatal conductance responses that did not result in significant water conservation. Overall, we expect the greatest response of desert vegetation during years with above-average precipitation when the active growing season is not limited to ∼2 months and, consequently, the effects of increased photosynthesis can accumulate over a biologically significant time period.
The structure, development, and response to fire of sand-live oak (Quercus geminata Small) and myrtle oak (Q. myrtifolia Willd.) colonies (domes) were examined in a longleaf pine-wiregrass (Pinus palustris Mill.-Aristida stricta Michaux.) community in Ocala National Forest, Florida. Dome areas ranged from 30 m2 to 1000 m2 and domes achieved heights of up to 10 m. The oak clumps excavated (<2 m tall) were clonal with roots and rhizomes concentrated in the upper 50 cm of soil. In the 3-4 year old clumps excavated (above-ground age), 2/3 of the biomass was located below-ground and 1/3 was above-ground. The proportion of above-ground stems that survived a prescribed burn increased markedly for stems >2 m tall with basal diameters >2.0 cm. Stems averaging >2 m tall in domes >200 m2 in area survived better than those in smaller domes. Oak domes <2 m tall burned back to ground level but resprouted readily from underground rhizomes. Height appeared to be the most important factor influencing oak dome persistence in pyrogenic pinelands, with domes >2 m tall having a high probability of above-ground stem survival and domes >4.5 m tall being nearly fire resistant. Height is positively correlated with dome age and therefore to the period of time needed without fire for tall dome (>4.5 m tall) development. Tall domes present within the pinelands are probably a direct result of approximately 20 years of fire suppression prior to the initiation of regular prescribed burning in the 1950's. Seedling establishment was apparently low in years when fire occurred and the year after but increased in the second, third, and fourth years following burning. Dome distribution, abundance, and height are thus dependent on fire frequency, intensity, and land-use history.
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
442
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.
ABSTRACT Native scrub-oak communities in Florida were exposed for three seasons in open top chambers to present atmospheric [CO2] (approx. 350 μmol mol−1) and to high [CO2] (increased by 350 μmol mol−1). Stomatal and photosynthetic acclimation to high [CO2] of the dominant species Quercus myrtifolia was examined by leaf gas exchange of excised shoots. Stomatal conductance (gs) was approximately 40% lower in the high- compared to low-[CO2]-grown plants when measured at their respective growth concentrations. Reciprocal measurements of gs in both high- and low-[CO2]-grown plants showed that there was negative acclimation in the high-[CO2]-grown plants (9–16% reduction in gs when measured at 700 μmol mol−1), but these were small compared to those for net CO2 assimilation rate (A, 21–36%). Stomatal acclimation was more clearly evident in the curve of stomatal response to intercellular [CO2] (ci) which showed a reduction in stomatal sensitivity at low ci in the high-[CO2]-grown plants. Stomatal density showed no change in response to growth in high growth [CO2]. Long-term stomatal and photosynthetic acclimation to growth in high [CO2] did not markedly change the 2·5- to 3-fold increase in gas-exchange-derived water use efficiency caused by high [CO2].
We review experimental studies to evaluate how the nitrogen cycle influences the response of forest net primary production (NPP) to elevated CO2. The studies in our survey report that at the tissue level, elevated CO2 reduces leaf nitrogen concentration an average 21%, but that it has a smaller effect on nitrogen concentrations in stems and fine roots. In contrast, higher soil nitrogen availability generally increases leaf nitrogen concentration. Among studies that manipulate both soil nitrogen availability and atmospheric C02, photosynthetic response depends on a linear relationship with the response of leaf nitrogen concentration and the amount of change in atmospheric CO2 concentration. Although elevated CO2 often results in reduced tissue respiration rate per unit biomass, the link to changes in tissue nitrogen concentration is not well studied. 1The US government has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper.
Elevated atmospheric carbon dioxide concentrations ([CO2]) generally increase plant photosynthesis in C3 species, but not in C4 species, and reduce stomatal conductance in both C3 and C4 plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO2, particularly in C3 species. While these responses are well documented in many species, implications for plant growth and nutrient cycling in native ecosystems are not clear. Here we present data on 18 years of measurement of above and belowground biomass, tissue [N] and total standing crop of N for a Scirpus olneyi-dominated (C3 sedge) community, a Spartina patens-dominated (C4 grass) community and a C3–C4-mixed species community exposed to ambient and elevated (ambient +340 ppm) atmospheric [CO2] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO2] in the S. olneyi-dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO2] was found for total biomass in the C4-dominated community. We found a significant decline in C4 biomass (correlated with rising sea level) and a concomitant increase in C3 biomass in the mixed community. This shift from C4 to C3 was accelerated by the elevated [CO2] treatment. The elevated [CO2] stimulation of total biomass accumulation was greatest during rainy, low salinity years: the average increase above the ambient treatment during the three wettest years (1994, 1996, 2003) was 2.9 t ha−1 but in the three driest years (1995, 1999, 2002), it was 1.2 t ha−1. Elevated [CO2] depressed tissue [N] in both species, but especially in the S. olneyi where the relative depression was positively correlated with salinity and negatively related with the relative enhancement of total biomass production. Thus, the greatest amount of carbon was added to the S. olneyi-dominated community during years when shoot [N] was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated [CO2] stimulation of carbon accumulation in this ecosystem.
Future climate change is projected to include a strong likelihood of continued increases in atmospheric carbon dioxide concentration ([CO2]) and possible shifts in precipitation patterns. Due mainly to uncertainties in the timing and amounts of monsoonal rainfall, drought is common in rainfed rice production systems. The objectives of this study were to quantify the effects and possible interactions of [CO2] and drought stress on rice (Oryza sativa, L.) photosynthesis, evapotranspiration and water-use efficiency. Rice (cv. IR-72) was grown to maturity in eight naturally sunlit, plant growth chambers in atmospheric carbon dioxide concentrations [CO2] of 350 and 700 μmol CO2 mol–1 air. In both [CO2], water management treatments included continuously flooded controls, flood water removed and drought stress imposed at panicle initiation, anthesis, and both panicle initiation and anthesis. Potential acclimation of rice photosynthesis to long-term [CO2] growth treatments of 350 and 700 μmol mol–1 was tested by comparing canopy photosynthesis rates across short-term [CO2] ranging from 160 to 1000 μmol mol–1. These tests showed essentially no acclimation response with photosynthetic rate being a function of current short-term [CO2] rather than long-term [CO2] growth treatment. In both long-term [CO2] treatments, photosynthetic rate saturated with respect to [CO2] near 510 μmol mol–1. Carbon dioxide enrichment significantly increased both canopy net photosynthetic rate (21–27%) and water-use efficiency while reducing evapotranspiration by about 10%. This water saving under [CO2] enrichment allowed photosynthesis to continue for about one to two days longer during drought in the enriched compared with the ambient [CO2] control treatments.
Increased atmospheric CO2 concentration (Ca) produces a short-term stimulation of photosynthesis and plant growth across terrestrial ecosystems. However, the long-term response remains uncertain and is thought to depend on environmental constraints. In the longest experiment on natural ecosystem response to elevated Ca, we measured the shoot-density, biomass and net CO2 exchange (NEE) responses to elevated Ca from 1987 to 2003 in a Scirpus olneyi wetland sedge community of the Chesapeake Bay, MD, USA. Measurements were conducted in five replicated open-top chambers per CO2 treatment (ambient and elevated). In addition, unchambered control plots were monitored for shoot density. Responses of daytime NEE, Scirpus plant biomass and shoot density to elevated Ca were positive for any single year of the 17-year period of study. Daytime NEE stimulation by elevated Ca rapidly dropped from 80% at the onset of the experiment to a long-term stimulation average of about 35%. Shoot-density stimulation by elevated Ca increased linearly with duration of exposure (r2=0.89), exceeding 120% after 17 years. Although of lesser magnitude, the shoot biomass response to elevated Ca was similar to that of the shoot density. Daytime NEE response to elevated Ca was not explained by the duration of exposure, but negatively correlated with salinity of the marsh, indicating that this elevated-Ca response was decreased by water-related stress. By contrast, circumstantial evidence suggested that salinity stress increased the stimulation of shoot density by elevated Ca, which highlights the complexity of the interaction between water-related stresses and plant community responses to elevated Ca. Notwithstanding the effects of salinity stress, we believe that the most important finding of the present research is that a species response to elevated Ca can continually increase when this species is under stress and declining in its natural environment. This is particularly important because climate changes associated with elevated Ca are likely to increase environmental stresses on numerous species and modify their present distribution. Our results point to an increased resilience to change under elevated Ca when plants are exposed to adverse environmental conditions.
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 possible responses of ecosystem processes to rising atmospheric CO2 concentration and climate change are illustrated using six dynamic global vegetation models that explicitly represent the interactions of ecosystem carbon and water exchanges with vegetation dynamics. The models are driven by the IPCC IS92a scenario of rising CO2 (Wigley et al. 1991), and by climate changes resulting from effective CO2 concentrations corresponding to IS92a, simulated by the coupled ocean atmosphere model HadCM2-SUL. Simulations with changing CO2 alone show a widely distributed terrestrial carbon sink of 1.4–3.8 Pg C y−1 during the 1990s, rising to 3.7–8.6 Pg C y−1 a century later. Simulations including climate change show a reduced sink both today (0.6–3.0 Pg C y−1) and a century later (0.3–6.6 Pg C y−1) as a result of the impacts of climate change on NEP of tropical and southern hemisphere ecosystems. In all models, the rate of increase of NEP begins to level off around 2030 as a consequence of the ‘diminishing return’ of physiological CO2 effects at high CO2 concentrations. Four out of the six models show a further, climate-induced decline in NEP resulting from increased heterotrophic respiration and declining tropical NPP after 2050. Changes in vegetation structure influence the magnitude and spatial pattern of the carbon sink and, in combination with changing climate, also freshwater availability (runoff). It is shown that these changes, once set in motion, would continue to evolve for at least a century even if atmospheric CO2 concentration and climate could be instantaneously stabilized. The results should be considered illustrative in the sense that the choice of CO2 concentration scenario was arbitrary and only one climate model scenario was used. However, the results serve to indicate a range of possible biospheric responses to CO2 and climate change. They reveal major uncertainties about the response of NEP to climate change resulting, primarily, from differences in the way that modelled global NPP responds to a changing climate. The simulations illustrate, however, that the magnitude of possible biospheric influences on the carbon balance requires that this factor is taken into account for future scenarios of atmospheric CO2 and climate change.
AGPase, ADP glucose pyrophosphorylase
GS, glutamine synthetase
GOGAT, glutamate : oxoglutarate amino transferase
NADP‐ICDH, NADP‐dependent isocitrate dehydrogenase
NR, nitrate reductase
OPPP, oxidative pentose phosphate pathway
3PGA, glycerate‐3‐phosphate
PEPCase, phosphoenolpyruvate carboxylase
Rubisco, ribulose‐1,5‐bisphosphate carboxylase/oxygenase
SPS, sucrose phosphate‐synthase
This review first summarizes the numerous studies that have described the interaction between the nitrogen supply and the response of photosynthesis, metabolism and growth to elevated [CO 2 ]. The initial stimulation of photosynthesis in elevated [CO 2 ] is often followed by a decline of photosynthesis, that is typically accompanied by a decrease of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), an accumulation of carbohydrate especially starch, and a decrease of the nitrogen concentration in the plant. These changes are particularly marked when the nitrogen supply is low, whereas when the nitrogen supply is adequate there is no acclimation of photosynthesis, no major decrease in the internal concentration of nitrogen or the levels of nitrogen metabolites, and growth is stimulated markedly. Second, emerging evidence is discussed that signals derived from nitrate and nitrogen metabolites such as glutamine act to regulate the expression of genes involved in nitrate and ammonium uptake and assimilation, organic acid synthesis and starch accumulation, to modulate the sugar‐mediated repression of the expression of genes involved in photosynthesis, and to modulate whole plant events including shoot–root allocation, root architecture and flowering. Third, increased rates of growth in elevated [CO 2 ] will require higher rates of inorganic nitrogen uptake and assimilation. Recent evidence is discussed that an increased supply of sugars can increase the rates of nitrate and ammonium uptake and assimilation, the synthesis of organic acid acceptors, and the synthesis of amino acids. Fourth, interpretation of experiments in elevated [CO 2 ] requires that the nitrogen status of the plants is monitored. The suitability of different criteria to assess the plant nitrogen status is critically discussed. Finally the review returns to experiments with elevated [CO 2 ] and discusses the following topics: is, and if so how, are nitrate and ammonium uptake and metabolism stimulated in elevated [CO 2 ], and does the result depend on the nitrogen supply? Is acclimation of photosynthesis the result of sugar‐mediated repression of gene expression, end‐product feedback of photosynthesis, nitrogen‐induced senescence, or ontogenetic drift? Is the accumulation of starch a passive response to increased carbohydrate formation, or is it triggered by changes in the nutrient status? How do changes in sugar production and inorganic nitrogen assimilation interact in different conditions and at different stages of the life history to determine the response of whole plant growth and allocation to elevated [CO 2 ]?
1 In water-limited environments, the availability of water and nutrients to plants depends on environmental conditions, sizes and shapes of their root systems, and root competition. The goal of this study was to predict root system sizes and shapes for different plant growth forms using data on above-ground plant sizes, climate and soil texture. 2 A new data set of > 1300 records of root system sizes for individual plants was collected from the literature for deserts, scrublands, grasslands and savannas with ≤ 1000 mm mean annual precipitation (MAP). Maximum rooting depths, maximum lateral root spreads and their ratios were measured. 3 Root system sizes differed among growth forms and increased with above-ground size: annuals < perennial forbs = grasses < semi-shrubs < shrubs < trees. Stem succulents were as shallowly rooted as annuals but had lateral root spreads similar to shrubs. 4 Absolute rooting depths increased with MAP in all growth forms except shrubs and trees, but were not strongly related to potential evapotranspiration (PET). Except in trees, root systems tended to be shallower and wider in dry and hot climates and deeper and narrower in cold and wet climates. Shrubs were more shallowly rooted under climates with summer than winter precipitation regimes. 5 Relative to above-ground plant sizes, root system sizes decreased with increasing PET for all growth forms, but decreased with increasing MAP only for herbaceous plants. Thus relative rooting depths tended to increase with aridity, although absolute rooting depths decreased with aridity. 6 Using an independent data set of 20 test locations, rooting depths were predicted from MAP using regression models for three broad growth forms. The models suc-ceeded in explaining 62% of the observed variance in median rooting depths. 7 Based on the data analysed here, Walter's two-layer model of soil depth partitioning between woody and herbaceous plants appears to be most appropriate in drier regimes (< 500 mm MAP) and in systems with substantial winter precipitation.
Canopy transpiration rates, as a major component of forest hydrologic budgets, are reported for 12‐yr‐old sweetgum ( Liquidambar styraciflua ) trees growing in a free‐air CO 2 enrichment (FACE) study in eastern Tennessee, USA.
The compensated heat‐pulse technique was used to measure rates of sap velocity, and stand transpiration was estimated as a function of measured sap velocity, total stand sapwood area and the fraction of sapwood functional in water transport.
Sap velocity averaged 13% less for trees in elevated compared with ambient CO 2 concentration. Stand transpiration reached 5.6 and 4.4 mm d ⁻¹ for the ambient and elevated CO 2 treatments, respectively. Stratification of the data showed that significant differences in stand transpiration were observed between ambient and elevated CO 2 only at mean daily radiation levels > 400 J m ⁻² s ⁻¹ and at vapor pressure deficits > 1.0 kPa.
These data illustrate that while elevated CO 2 may reduce canopy transpiration, the apparent dependency of such an effect on prevailing weather makes detecting a CO 2 ‐induced impact on forest water use difficult.
Summary • Rising atmospheric CO 2 concentrations are likely to have direct effects on terres- trial ecosystems. Here, we describe effects of elevated concentrations of CO 2 on an understory plant community in terms of production and community composition. • In 2001 and 2002 total and species-specific above-ground net primary produc- tivity (ANPP) were estimated by harvesting above-ground biomass within an under- story community receiving ambient (CO 2 ) and elevated (CO 2 ) at Oak Ridge National Laboratory's free-air carbon dioxide enrichment (FACE) facility. • During a wet year, community composition differed between plots receiving ambient (CO 2 ) and elevated (CO 2 ), but total ANPP did not differ. By contrast, during a drier year, community composition did not differ, but total ANPP was greater in elevated than ambient (CO 2 ) plots. These patterns were driven by the response of two codo- minant species, Lonicera japonica and Microstegium vimineum , both considered invasive species in the south-eastern United States. The ANPP of L. japonica was consistently greater under elevated (CO 2 ), whereas the response of M. vimineum to CO 2 enrichment differed between years and mediated total community response. • These data suggest that community and species responses to a future, CO 2 - enriched atmosphere may be mediated by other environmental factors and will depend on individual species responses.
Atmospheric CO2 concentration is rising and it has been suggested that a portion of the additional carbon is being sequestered in terrestrial vegetation and much of that in below-ground structures. The objective of the present study was to quantify the effects of elevated atmospheric CO2 on fine root length and distribution with depth with minirhizotrons in an open-top chamber experiment in an oak-palmetto scrub ecosystem at Kennedy Space Centre, Florida, USA. Observations were made five times over a period of one and a half years in three ambient chambers (350 p.p.m. CO2), three CO2 enriched chambers (700 p.p.m. CO2), and three unchambered plots. Greater root length densities were produced in the elevated CO2 chambers (14.2 mm cm−2) compared to the ambient chambers (8.7 mm cm−2). More roots may presumably lead to more efficient acquisition of resources. Fine root abundance varied significantly with soil depth, and there appeared to be enhanced proliferation of fine roots near the surface (0–12 cm) and at greater depth (49–61 cm) in the elevated CO2 chambers. The vertical root distribution pattern may be a response to availability of nutrients and water. More studies are needed to determine if increased root length under CO2 enriched conditions actually results in greater sequestering of carbon below ground.
To determine the long-term impact of elevated CO2 on primary production of native tallgrass prairie, we compared the responses of tallgrass prairie at ambient and twice-ambient atmospheric CO2 levels over an 8-year period. Plots in open-top chambers (4.5 m diameter) were exposed continuously (24 h) to ambient and elevated CO2 from early April to late October each year. Unchambered plots were monitored also. Above-ground peak biomass was determined by clipping each year in early August, and root growth was estimated by harvesting roots from root ingrowth bags. Plant community composition was censused each year in early June. In the last 2 years of the study, subplots were clipped on 1 June or 1 July, and regrowth was harvested on 1 October. Volumetric soil water content of the 0–100 cm soil layer was determined using neutron scattering, and was generally higher in elevated CO2 plots than ambient. Peak above-ground biomass was greater on elevated CO2 plots than ambient CO2 plots with or without chambers during years with significant plant water stress. Above-ground regrowth biomass was greater under elevated CO2 than under ambient CO2 in a year with late-season water stress, but did not differ in a wetter year. Root ingrowth biomass was also greater in elevated CO2 plots than ambient CO2 plots when water stress occurred during the growing season. The basal cover and relative amount of warm-season perennial grasses (C4) in the stand changed little during the 8-year period, but basal cover and relative amount of cool-season perennial grasses (C3) in the stand declined in the elevated CO2 plots and in ambient CO2 plots with chambers. Forbs (C3) and members of the Cyperaceae (C3) increased in basal cover and relative amount in the stand at elevated compared to ambient CO2. Greater biomass production under elevated CO2 in C4-dominated grasslands may lead to a greater carbon sequestration by those ecosystems and reduce peak atmospheric CO2 concentrations in the future.
We report changes in nitrogen cycling in Florida scrub oak in response to elevated atmospheric CO2 during the first 14 months of experimental treatment. Elevated CO2 stimulated above-ground growth, nitrogen mass, and root nodule production of the nitrogen-fixing vine, Galactia elliottii Nuttall. During this period, elevated CO2 reduced rates of gross nitrogen mineralization in soil, and resulted in lower recovery of nitrate on resin lysimeters. Elevated CO2 did not alter nitrogen in the soil microbial biomass, but increased the specific rate of ammonium immobilization (NH4+ immobilized per unit microbial N) measured over a 24-h period. Increased carbon input to soil through greater root growth combined with a decrease in the quality of that carbon in elevated CO2 best explains these changes.
These results demonstrate that atmospheric CO2 concentration influences both the internal cycling of nitrogen (mineralization, immobilization, and nitrification) as well as the processes that regulate total ecosystem nitrogen mass (nitrogen fixation and nitrate leaching) in Florida coastal scrub oak. If these changes in nitrogen cycling are sustained, they could cause long-term feedbacks to the growth responses of plants to elevated CO2. Greater nitrogen fixation and reduced leaching could stimulate nitrogen-limited plant growth by increasing the mass of labile nitrogen in the ecosystem. By contrast, reduced nitrogen mineralization and increased immobilization will restrict the supply rate of plant-available nitrogen, potentially reducing plant growth. Thus, the net feedback to plant growth will depend on the balance of these effects through time.
The effects of elevated atmospheric CO2 on fine root decomposition over a 828-day period were investigated using open top chambers with both ambient and elevated (700 ppm) CO2 treatments in an oak–palmetto scrub ecosystem at Kennedy Space Center, Florida. Carbon dioxide enrichment of the chambers began 15 May 1996. The experiment included roots grown in ambient and elevated carbon dioxide. Vertical litterbags installed in September 1996 in each elevated and ambient chamber incubated from December 1996 to December 1998 showed no significant treatment effect on fine root or rhizome mass loss. Initial fine root percentage mass loss varied from 10.3% to 13.5% after three months; 55.5% to 38.3% of original mass had been lost after 828 days. A period of nitrogen immobilization occurred in both fine roots and rhizomes in the elevated CO2 incubation, which is a potential mechanism for nitrogen conservation for this system in an elevated CO2 world.
Elevated atmospheric carbon dioxide (CO2) often stimulates the growth of fine roots, yet there are few reports of responses of intact root systems to long-term CO2 exposure. We investigated the effects of elevated CO2 on fine root growth using open top chambers in a scrub oak ecosystem at Kennedy Space Center, Florida for more than 7 years. CO2 enrichment began immediately after a controlled burn, which simulated the natural disturbance that occurs in this system every 10–15 years. We hypothesized that (1) root abundance would increase in both treatments as the system recovered from fire; (2) elevated CO2 would stimulate root growth; and (3) elevated CO2 would alter root distribution. Minirhizotron tubes were used to measure fine root length density (mm cm−2) every three months. During the first 2 years after fire recovery, fine root abundance increased in all treatments and elevated CO2 significantly enhanced root abundance, causing a maximum stimulation of 181% after 20 months. The CO2 stimulation was initially more pronounced in the top 10 cm and 38–49 cm below the soil surface. However, these responses completely disappeared during the third year of experimental treatment: elevated CO2 had no effect on root abundance or on the depth distribution of fine roots during years 3–7. The results suggest that, within a few years following fire, fine roots in this scrub oak ecosystem reach closure, defined here as a dynamic equilibrium between production and mortality. These results further suggest that elevated CO2 hastens root closure but does not affect maximum root abundance. Limitation of fine root growth by belowground resources – particularly nutrients in this nutrient-poor soil – may explain the transient response to elevated CO2.
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.
In a field microcosm experiment, species-specific responses of aboveground biomass of two California annual grassland communities to elevated CO2 and nutrient availability were investigated. One community grows on shallow, nutrient-poor serpentine-derived soil whereas the other occurs on deeper, modestly fertile sandstone/greenstone-derived substrate. In most species, CO2 effects did not appear until late in the growing season, probably because the elevated CO2 increased water-use-efficiency easing, the onset of the summer drought. Responses of aboveground biomass to elevated CO2 differed depending on nutrient availability. Similarly, biomass responses to nutrient treatments differed depending on the CO2 status. For the majority of the species, production increased most under elevated CO2 with added nutrients (N,P,K, and micro nutrients). Some species were losers under conditions that increased overall community production, including Bromus hordeaceus in the serpentine community (negative biomass response under elevated CO2) and Lotus wrangelianus in both communities (negative biomass response with added nitrogen). Treatment and competitive effects on species-specific biomass varied in both magnitude and direction, especially in the serpentine community, significantly affecting community structure. Individual resource environments are likely to be affected by neighbouring plants, and these competitive interactions complicate predictions of species' responses to elevated CO2.
The effect of elevated atmospheric CO2 concentration (Ca) on the aboveground biomass of three oak species, Quercus myrtifolia, Q. geminata, and Q. chapmanii, was estimated nondestructively using allometric relationships between stem diameter and aboveground biomass after four years of experimental treatment in a naturally fire-regenerated scrub-oak ecosystem. After burning a stand of scrub-oak vegetation, re-growing plants were exposed to either current ambient (379 µL L−1 CO2) or elevated (704 µL L−1 CO2) Ca in 16 open-top chambers over a four-year period, and measurements of stem diameter were carried out annually on all oak shoots within each chamber. Elevated Ca significantly increased aboveground biomass, expressed either per unit ground area or per shoot; elevated Ca had no effect on shoot density. The relative effect of elevated Ca on aboveground biomass increased each year of the study from 44% (May 96–Jan 97), to 55% (Jan 97–Jan 98), 66% (Jan 98–Jan 99), and 75% (Jan 99–Jan 00). The effect of elevated Ca was species specific: elevated Ca significantly increased aboveground biomass of the dominant species, Q. myrtifolia, and tended to increase aboveground biomass of Q. chapmanii, but had no effect on aboveground biomass of the subdominant, Q. geminata. These results show that rising atmospheric CO2 has the potential to stimulate aboveground biomass production in ecosystems dominated by woody species, and that species-specific growth responses could, in the long term, alter the composition of the scrub-oak community.
We report the results of a 2-year study of effects of the elevated (current ambient plus 350 μmol CO2 mol−1) atmospheric CO2 concentration (Ca) on net ecosystem CO2 exchange (NEE) of a scrub–oak ecosystem. The measurements were made in open-top chambers (OTCs) modified to function as open gas-exchange systems. The OTCs enclosed samples of the ecosystem (ca. 10 m2 surface area) that had regenerated after a fire, 5 years before, in either current ambient or elevated Ca. Throughout the study, elevated Ca increased maximum NEE (NEEmax) and the apparent quantum yield of the NEE (φNEE) during the photoperiod. The magnitude of the stimulation of NEEmax, expressed per unit ground area, was seasonal, rising from 50% in the winter to 180% in the summer. The key to this stimulation was effects of elevated Ca, and their interaction with the seasonal changes in the environment, on ecosystem leaf area index, photosynthesis and respiration. The separation of these factors was difficult. When expressed per unit leaf area the stimulation of the NEEmax ranged from 7% to 60%, with the increase being dependent on increasing soil water content (Wsoil). At night, the CO2 effluxes from the ecosystem (NEEnight) were on an average 39% higher in elevated Ca. However, the increase varied between 6% and 64%, and had no clear seasonality. The partitioning of NEEnight into its belowground (Rbelow) and aboveground (Rabove) components was carried out in the winter only. A 35% and 27% stimulation of NEEnight in December 1999 and 2000, respectively, was largely due to a 26% and 28% stimulation of Rbelow in the respective periods, because Rbelow constituted ca. 87% of NEEnight. The 37% and 42% stimulation of Rabove in December 1999 and 2000, respectively, was less than the 65% and 80% stimulation of the aboveground biomass by elevated Ca at these times. An increase in the relative amount of the aboveground biomass in woody tissue, combined with a decrease in the specific rate of stem respiration of the dominant species Quercus myrtifolia in elevated Ca, was responsible for this effect. Throughout this study, elevated Ca had a greater effect on carbon uptake than on carbon loss, in terms of both the absolute flux and relative stimulation. Consequently, for this scrub–oak ecosystem carbon sequestration was greater in the elevated Ca during this 2-year study period.
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.
ContentsSummary 1I. Introduction 2II. Early assessments of [CO2] responses in natural ecosystems 2III. Global network of FACE sites 4IV. Assimilation and leaf N-content 5V. Primary productivity 13VI. Response of plant functional types 20VII. Conclusions 23Acknowledgements 24References 24SummaryResults from 16 free-air CO2 enrichment (FACE) sites representing four different global vegetation types indicate that only some early predictions of the effects of increasing CO2 concentration (elevated [CO2]) on plant and ecosystem processes are well supported. Predictions for leaf CO2 assimilation (Anet) generally fit our understanding of limitations to photosynthesis, and the FACE experiments indicate concurrent enhancement of photosynthesis and of partial downregulation. In addition, most herbaceous species had reduced leaf nitrogen (N)-content under elevated [CO2] and thus only a modest enhancement of Anet, whereas most woody species had little change in leaf N with elevated [CO2] but a larger enhancement of Anet. Early predictions for primary production are more mixed. Predictions that enhancement of productivity would be greater in drier ecosystems or in drier years has only limited support. Furthermore, differences in productivity enhancements among six plant functional types were not significant. By contrast, increases in productivity enhancements with increased N availability are well supported by the FACE results. Thus, neither a resource-based conceptual model nor a plant functional type conceptual model is exclusively supported by FACE results, but rather both species identity and resource availability are important factors influencing the response of ecosystems to elevated [CO2].
Over a large part of the photoperiod, light energy absorbed by upper canopy leaves saturates photosynthesis and exceeds the energetic requirements for light-saturated linear electron flow through photosystem II (JPSII), so that photoinhibition results. From a theoretical consideration of the response of light-saturated photosynthesis to elevated atmospheric CO2 partial pressure (pCO2) it may be predicted that, where light-saturated photosynthesis is Rubisco-limited, an increase in pCO2 will stimulate JPSII. Therefore, the proportion of absorbed quanta dissipated photochemically will increase and the potential for photoinhibition of photosynthesis will decrease. This was tested by measuring modulated chlorophyll a fluorescence from Quercus myrtifolia Willd. growing in the field in open-top chambers, at either current ambient or elevated (ambient + 35 Pa) pCO2 on Merritt Island, Florida, USA. During spring and summer, light-saturated photosynthesis at current ambient pCO2 was Rubisco-limited. Consistent with theoretical prediction, JPSII was increased and photoinhibition decreased by elevated pCO2 in spring. In the summer, when growth had largely ceased, an acclimatory decrease in the maximum Ribulose 1,5 bisphosphate saturated carboxylation capacity (Vc max) removed the stimulation of JPSII seen in the spring, and photoinhibition was increased in elevated pCO2. It is concluded that, for Q. myrtifolia growing in the field, the effects of elevated pCO2 on JPSII and photoinhibition will reflect seasonal differences in photosynthetic acclimation to elevated pCO2 in a predictable manner.