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1.
Evaluating the role of terrestrial ecosystems in the global carbon cycle requires a detailed understanding of carbon exchange between vegetation, soil, and the atmosphere. Global climatic change may modify the net carbon balance of terrestrial ecosystems, causing feedbacks on atmospheric CO2 and climate. We describe a model for investigating terrestrial carbon exchange and its response to climatic variation based on the processes of plant photosynthesis, carbon allocation, litter production, and soil organic carbon decomposition. The model is used to produce geographical patterns of net primary production (NPP), carbon stocks in vegetation and soils, and the seasonal variations in net ecosystem production (NEP) under both contemporary and future climates. For contemporary climate, the estimated global NPP is 57.0 Gt C y–1, carbon stocks in vegetation and soils are 640 Gt C and 1358 Gt C, respectively, and NEP varies from –0.5 Gt C in October to 1.6 Gt C in July. For a doubled atmospheric CO2 concentration and the corresponding climate, we predict that global NPP will rise to 69.6 Gt C y–1, carbon stocks in vegetation and soils will increase by, respectively, 133 Gt C and 160 Gt C, and the seasonal amplitude of NEP will increase by 76%. A doubling of atmospheric CO2 without climate change may enhance NPP by 25% and result in a substantial increase in carbon stocks in vegetation and soils. Climate change without CO2 elevation will reduce the global NPP and soil carbon stocks, but leads to an increase in vegetation carbon because of a forest extension and NPP enhancement in the north. By combining the effects of CO2 doubling, climate change, and the consequent redistribution of vegetation, we predict a strong enhancement in NPP and carbon stocks of terrestrial ecosystems. This study simulates the possible variation in the carbon exchange at equilibrium state. We anticipate to investigate the dynamic responses in the carbon exchange to atmospheric CO2 elevation and climate change in the past and future.  相似文献   

2.
We forced a global terrestrial carbon cycle model by climate fields of 14 ocean and atmosphere general circulation models (OAGCMs) to simulate the response of terrestrial carbon pools and fluxes to climate change over the next century. These models participated in the second phase of the Coupled Model Intercomparison Project (CMIP2), where a 1% per year increase of atmospheric CO2 was prescribed. We obtain a reduction in net land uptake because of climate change ranging between 1.4 and 5.7 Gt C yr?1 at the time of atmospheric CO2 doubling. Such a reduction in terrestrial carbon sinks is largely dominated by the response of tropical ecosystems, where soil water stress occurs. The uncertainty in the simulated land carbon cycle response is the consequence of discrepancies in land temperature and precipitation changes simulated by the OAGCMs. We use a statistical approach to assess the coherence of the land carbon fluxes response to climate change. The biospheric carbon fluxes and pools changes have a coherent response in the tropics, in the Mediterranean region and in high latitudes of the Northern Hemisphere. This is because of a good coherence of soil water content change in the first two regions and of temperature change in the high latitudes of the Northern Hemisphere. Then we evaluate the carbon uptake uncertainties to the assumptions on plant productivity sensitivity to atmospheric CO2 and on decomposition rate sensitivity to temperature. We show that these uncertainties are on the same order of magnitude than the uncertainty because of climate change. Finally, we find that the OAGCMs having the largest climate sensitivities to CO2 are the ones with the largest soil drying in the tropics, and therefore with the largest reduction of carbon uptake.  相似文献   

3.
The rapid increase in atmospheric CO2 concentrations (Ca) has resulted in extensive research efforts to understand its impact on terrestrial ecosystems, especially carbon balance. Despite these efforts, there are relatively few data comparing net ecosystem exchange of CO2 between the atmosphere and the biosphere (NEE), under both ambient and elevated Ca. Here we report data on annual sums of CO2 (NEEnet) for 19 years on a Chesapeake Bay tidal wetland for Scirpus olneyi (C3 photosynthetic pathway)‐ and Spartina patens (C4 photosynthetic pathway)‐dominated high marsh communities exposed to ambient and elevated Ca (ambient + 340 ppm). Our objectives were to (i) quantify effects of elevated Ca on seasonally integrated CO2 assimilation (NEEnet = NEEday + NEEnight, kg C m?2 y?1) for the two communities; and (ii) quantify effects of altered canopy N content on ecosystem photosynthesis and respiration. Across all years, NEEnet averaged 1.9 kg m?2 y?1 in ambient Ca and 2.5 kg m?2 y?1 in elevated Ca, for the C3‐dominated community. Similarly, elevated Ca significantly (P < 0.01) increased carbon uptake in the C4‐dominated community, as NEEnet averaged 1.5 kg m?2 y?1 in ambient Ca and 1.7 kg m?2 y?1 in elevated Ca. This resulted in an average CO2 stimulation of 32% and 13% of seasonally integrated NEEnet for the C3‐ and C4‐dominated communities, respectively. Increased NEEday was correlated with increased efficiencies of light and nitrogen use for net carbon assimilation under elevated Ca, while decreased NEEnight was associated with lower canopy nitrogen content. These results suggest that rising Ca may increase carbon assimilation in both C3‐ and C4‐dominated wetland communities. The challenge remains to identify the fate of the assimilated carbon.  相似文献   

4.
Modelling carbon balances of coastal arctic tundra under changing climate   总被引:1,自引:0,他引:1  
Rising air temperatures are believed to be hastening heterotrophic respiration (Rh) in arctic tundra ecosystems, which could lead to substantial losses of soil carbon (C). In order to improve confidence in predicting the likelihood of such loss, the comprehensive ecosystem model ecosys was first tested with carbon dioxide (CO2) fluxes measured over a tundra soil in a growth chamber under various temperatures and soil‐water contents (θ). The model was then tested with CO2 and energy fluxes measured over a coastal arctic tundra near Barrow, Alaska, under a range of weather conditions during 1998–1999. A rise in growth chamber temperature from 7 to 15 °C caused large, but commensurate, rises in respiration and CO2 fixation, and so no significant effect on net CO2 exchange was modelled or measured. An increase in growth chamber θ from field capacity to saturation caused substantial reductions in respiration but not in CO2 fixation, and so an increase in net CO2 exchange was modelled and measured. Long daylengths over the coastal tundra at Barrow caused an almost continuous C sink to be modelled and measured during most of July (2–4 g C m?2 d?1), but shortening daylengths and declining air temperatures caused a C source to be modelled and measured by early September (~1 g C m?2 d?1). At an annual time scale, the coastal tundra was modelled to be a small C sink (4 g C m?2 y?1) during 1998 when average air temperatures were 4 °C above normal, and a larger C sink (16 g C m?2 y?1) during 1999 when air temperatures were close to long‐term normals. During 100 years under rising atmospheric CO2 concentration (Ca), air temperature and precipitation driven by the IS92a emissions scenario, modelled Rh rose commensurately with net primary productivity (NPP) under both current and elevated rates of atmospheric nitrogen (N) deposition, so that changes in soil C remained small. However, methane (CH4) emissions were predicted to rise substantially in coastal tundra with IS92a‐driven climate change (from ~20 to ~40 g C m?2 y?1), causing a substantial increase in the emission of CO2 equivalents. If the rate of temperature increase hypothesized in the IS92a emissions scenario had been raised by 50%, substantial losses of soil C (~1 kg C m?2) would have been modelled after 100 years, including additional emissions of CH4.  相似文献   

5.
Whether nitrogen (N) availability will limit plant growth and removal of atmospheric CO2 by the terrestrial biosphere this century is controversial. Studies have suggested that N could progressively limit plant growth, as trees and soils accumulate N in slowly cycling biomass pools in response to increases in carbon sequestration. However, a question remains over whether longer-term (decadal to century) feedbacks between climate, CO2 and plant N uptake could emerge to reduce ecosystem-level N limitations. The symbioses between plants and microbes can help plants to acquire N from the soil or from the atmosphere via biological N2 fixation—the pathway through which N can be rapidly brought into ecosystems and thereby partially or completely alleviate N limitation on plant productivity. Here we present measurements of plant N isotope composition (δ15N) in a peat core that dates to 15,000 cal. year BP to ascertain ecosystem-level N cycling responses to rising atmospheric CO2 concentrations. We find that pre-industrial increases in global atmospheric CO2 concentrations corresponded with a decrease in the δ15N of both Sphagnum moss and Ericaceae when constrained for climatic factors. A modern experiment demonstrates that the δ15N of Sphagnum decreases with increasing N2-fixation rates. These findings suggest that plant-microbe symbioses that facilitate N acquisition are, over the long term, enhanced under rising atmospheric CO2 concentrations, highlighting an ecosystem-level feedback mechanism whereby N constraints on terrestrial carbon storage can be overcome.  相似文献   

6.
The interest in national terrestrial ecosystem carbon budgets has been increasing because the Kyoto Protocol has included some terrestrial carbon sinks in a legally binding framework for controlling greenhouse gases emissions. Accurate quantification of the terrestrial carbon sink must account the interannual variations associated with climate variability and change. This study used a process‐based biogeochemical model and a remote sensing‐based production efficiency model to estimate the variations in net primary production (NPP), soil heterotrophic respiration (HR), and net ecosystem production (NEP) caused by climate variability and atmospheric CO2 increases in China during the period 1981–2000. The results show that China's terrestrial NPP varied between 2.86 and 3.37 Gt C yr?1 with a growth rate of 0.32% year?1 and HR varied between 2.89 and 3.21 Gt C yr?1 with a growth rate of 0.40% year?1 in the period 1981–1998. Whereas the increases in HR were related mainly to warming, the increases in NPP were attributed to increases in precipitation and atmospheric CO2. Net ecosystem production (NEP) varied between ?0.32 and 0.25 Gt C yr?1 with a mean value of 0.07 Gt C yr?1, leading to carbon accumulation of 0.79 Gt in vegetation and 0.43 Gt in soils during the period. To the interannual variations in NEP changes in NPP contributed more than HR in arid northern China but less in moist southern China. NEP had no a statistically significant trend, but the mean annual NEP for the 1990s was lower than for the 1980s as the increases in NEP in southern China were offset by the decreases in northern China. These estimates indicate that China's terrestrial ecosystems were taking up carbon but the capacity was undermined by the ongoing climate change. The estimated NEP related to climate variation and atmospheric CO2 increases may account for from 40 to 80% to the total terrestrial carbon sink in China.  相似文献   

7.
The net balance of greenhouse gas (GHG) exchanges between terrestrial ecosystems and the atmosphere under elevated atmospheric carbon dioxide (CO2) remains poorly understood. Here, we synthesise 1655 measurements from 169 published studies to assess GHGs budget of terrestrial ecosystems under elevated CO2. We show that elevated CO2 significantly stimulates plant C pool (NPP) by 20%, soil CO2 fluxes by 24%, and methane (CH4) fluxes by 34% from rice paddies and by 12% from natural wetlands, while it slightly decreases CH4 uptake of upland soils by 3.8%. Elevated CO2 causes insignificant increases in soil nitrous oxide (N2O) fluxes (4.6%), soil organic C (4.3%) and N (3.6%) pools. The elevated CO2‐induced increase in GHG emissions may decline with CO2 enrichment levels. An elevated CO2‐induced rise in soil CH4 and N2O emissions (2.76 Pg CO2‐equivalent year?1) could negate soil C enrichment (2.42 Pg CO2 year?1) or reduce mitigation potential of terrestrial net ecosystem production by as much as 69% (NEP, 3.99 Pg CO2 year?1) under elevated CO2. Our analysis highlights that the capacity of terrestrial ecosystems to act as a sink to slow climate warming under elevated CO2 might have been largely offset by its induced increases in soil GHGs source strength.  相似文献   

8.
The terrestrial carbon cycle plays a critical role in determining levels of atmospheric CO2 that result from anthropogenic carbon emissions. Elevated atmospheric CO2 is thought to stimulate terrestrial carbon uptake, through the process of CO2 fertilization of vegetation productivity. This negative carbon cycle feedback results in reduced atmospheric CO2 growth, and has likely accounted for a substantial portion of the historical terrestrial carbon sink. However, the future strength of CO2 fertilization in response to continued carbon emissions and atmospheric CO2 rise is highly uncertain. In this paper, the ramifications of CO2 fertilization in simulations of future climate change are explored, using an intermediate complexity coupled climate–carbon model. It is shown that the absence of future CO2 fertilization results in substantially higher future CO2 levels in the atmosphere, as this removes the dominant contributor to future terrestrial carbon uptake in the model. As a result, climate changes are larger, though the radiative effect of higher CO2 on surface temperatures in the model is offset by about 30% due to reduced positive dynamic vegetation feedbacks; that is, the removal of CO2 fertilization results in less vegetation expansion in the model, which would otherwise constitute an important positive surface albedo‐temperature feedback. However, the effect of larger climate changes has other important implications for the carbon cycle – notably to further weaken remaining carbon sinks in the model. As a result, positive climate–carbon cycle feedbacks are larger when CO2 fertilization is absent. This creates an interesting synergism of terrestrial carbon cycle feedbacks, whereby positive (climate–carbon cycle) feedbacks are amplified when a negative (CO2 fertilization) feedback is removed.  相似文献   

9.
We tested the hypothesis that CO2 supersaturation along the aquatic conduit over Sweden can be explained by processes other than aquatic respiration. A first generalized‐additive model (GAM) analysis evaluating the relationships between single water chemistry variables and pCO2 in lakes and streams revealed that water chemistry variables typical for groundwater input, e.g., dissolved silicate (DSi) and Mg2+ had explanatory power similar to total organic carbon (TOC). Further GAM analyses on various lake size classes and stream orders corroborated the slightly higher explanatory power for DSi in lakes and Mg2+ for streams compared with TOC. Both DSi and TOC explained 22–46% of the pCO2 variability in various lake classes (0.01–>100 km2) and Mg2+ and TOC explained 11–41% of the pCO2 variability in the various stream orders. This suggests that aquatic pCO2 has a strong groundwater signature. Terrestrial respiration is a significant source of the observed supersaturation and we may assume that both terrestrial respiration and aquatic respiration contributed equally to pCO2 efflux. pCO2 and TOC concentrations decreased with lake size suggesting that the longer water residence time allow greater equilibration of CO2 with the atmosphere and in‐lake mineralization of TOC. For streams, we observed a decreasing trend in pCO2 with stream orders between 3 and 6. We calculated the total CO2 efflux from all Swedish lakes and streams to be 2.58 Tg C yr?1. Our analyses also demonstrated that 0.70 Tg C yr?1 are exported to the ocean by Swedish watersheds as HCO3? and CO32? of which about 0.56 Tg C yr?1 is also a residual from terrestrial respiration and constitute a long‐term sink for atmospheric CO2. Taking all dissolved inorganic carbon (DIC) fluxes along the aquatic conduit into account will lower the estimated net ecosystem C exchange (NEE) by 2.02 Tg C yr?1, which corresponds to 10% of the NEE in Sweden.  相似文献   

10.
The purpose of this study was to evaluate 10 process‐based terrestrial biosphere models that were used for the IPCC fifth Assessment Report. The simulated gross primary productivity (GPP) is compared with flux‐tower‐based estimates by Jung et al. [Journal of Geophysical Research 116 (2011) G00J07] (JU11). The net primary productivity (NPP) apparent sensitivity to climate variability and atmospheric CO2 trends is diagnosed from each model output, using statistical functions. The temperature sensitivity is compared against ecosystem field warming experiments results. The CO2 sensitivity of NPP is compared to the results from four Free‐Air CO2 Enrichment (FACE) experiments. The simulated global net biome productivity (NBP) is compared with the residual land sink (RLS) of the global carbon budget from Friedlingstein et al. [Nature Geoscience 3 (2010) 811] (FR10). We found that models produce a higher GPP (133 ± 15 Pg C yr?1) than JU11 (118 ± 6 Pg C yr?1). In response to rising atmospheric CO2 concentration, modeled NPP increases on average by 16% (5–20%) per 100 ppm, a slightly larger apparent sensitivity of NPP to CO2 than that measured at the FACE experiment locations (13% per 100 ppm). Global NBP differs markedly among individual models, although the mean value of 2.0 ± 0.8 Pg C yr?1 is remarkably close to the mean value of RLS (2.1 ± 1.2 Pg C yr?1). The interannual variability in modeled NBP is significantly correlated with that of RLS for the period 1980–2009. Both model‐to‐model and interannual variation in model GPP is larger than that in model NBP due to the strong coupling causing a positive correlation between ecosystem respiration and GPP in the model. The average linear regression slope of global NBP vs. temperature across the 10 models is ?3.0 ± 1.5 Pg C yr?1 °C?1, within the uncertainty of what derived from RLS (?3.9 ± 1.1 Pg C yr?1 °C?1). However, 9 of 10 models overestimate the regression slope of NBP vs. precipitation, compared with the slope of the observed RLS vs. precipitation. With most models lacking processes that control GPP and NBP in addition to CO2 and climate, the agreement between modeled and observation‐based GPP and NBP can be fortuitous. Carbon–nitrogen interactions (only separable in one model) significantly influence the simulated response of carbon cycle to temperature and atmospheric CO2 concentration, suggesting that nutrients limitations should be included in the next generation of terrestrial biosphere models.  相似文献   

11.
12.
Elevated atmospheric CO2 concentration may result in increased below‐ground carbon allocation by trees, thereby altering soil carbon cycling. Seasonal estimates of soil surface carbon flux were made to determine whether carbon losses from Pinus radiata trees growing at elevated CO2 concentration were higher than those at ambient CO2 concentration, and whether this was related to increased fine root growth. Monthly soil surface carbon flux density (f) measurements were made on plots with trees growing at ambient (350) and elevated (650 μmol mol?1) CO2 concentration in large open‐top chambers. Prior to planting the soil carbon concentration (0.1%) and f (0.28 μmol m?2 s?1 at 15 °C) were low. A function describing the radial pattern of f with distance from tree stems was used to estimate the annual carbon flux from tree plots. Seasonal estimates of fine root production were made from minirhizotrons and the radial distribution of roots compared with radial measurements of f. A one‐dimensional gas diffusion model was used to estimate f from soil CO2 concentrations at four depths. For the second year of growth, the annual carbon flux from the plots was 1671 g y?1 and 1895 g y?1 at ambient and elevated CO2 concentrations, respectively, although this was not a significant difference. Higher f at elevated CO2 concentration was largely explained by increased fine root biomass. Fine root biomass and stem production were both positively related to f. Both root length density and f declined exponentially with distance from the stem, and had similar length scales. Diurnal changes in f were largely explained by changes in soil temperature at a depth of 0.05 m. Ignoring the change of f with increasing distance from tree stems when scaling to a unit ground area basis from measurements with individual trees could result in under‐ or overestimates of soil‐surface carbon fluxes, especially in young stands when fine roots are unevenly distributed.  相似文献   

13.
A model of the interacting global carbon and nitrogen cycles (CQUESTN) is developed to explore the possible history of C-sequestration into the terrestrial biosphere in response to the global increases (past and possible future) in atmospheric CO2 concentration, temperature and N-deposition. The model is based on published estimates of pre-industrial C and N pools and fluxes into vegetation, litter and soil compartments. It was found necessary to assign low estimates of N pools and fluxes to be compatible with the more firmly established C-cycle data. Net primary production was made responsive to phytomass N level, and to CO2 and temperature deviation from preindustrial values with sensitivities covering the ranges in the literature. Biological N-fixation could be made either unresponsive to soil C:N ratio, or could act to tend to restore the preindustrial C:N of humus with different N-fixation intensities. As for all such simulation models, uncertainties in both data and functional relationships render it more useful for qualitative evaluation than for quantitative prediction.With the N-fixation response turned off, the historic CO2 increase led to standard-model sequestration into terrestrial ecosystems in 1995AD of 1.8 Gt C yr–1. With N-fixation restoring humus C:N strongly, C sequestration was 3 Gt yr–1 in 1995. In both cases C:N of phytomass and litter increased with time and these increases were plausible when compared with experimental data on CO2 effects. The temperature increase also caused net C sequestration in the model biosphere because decrease in soil organic matter was more than offset by the increase in phytomass deriving from the extra N mineralised. For temperature increase to reduce system C pool size, the biosphere leakiness to N would have to increase substantially with temperature. Assuming a constant N-loss coefficient, the historic temperature increase alone caused standard-model net C sequestration to be about 0.6 Gt C in 1995. Given the disparity of plant and microbial C:N, the modelled impact of anthropogenic N-deposition on C-sequestration depends substantially on whether the deposited N is initially taken up by plants or by soil microorganisms. Assuming the latter, standard-model net sequestration in 1995 was 0.2 Gt C in 1995 from the N-deposition effect alone. Combining the effects of the historic courses of CO2, temperature and N-deposition, the standard-model gave C-sequestration of 3.5 Gt in 1995. This involved an assumed weak response of biological N-fixation to the increased carbon status of the ecosystem. For N-fixation to track ecosystem C-fixation in the long term however, more phosphorus must enter the biological cycle. New experimental evidence shows that plants in elevated CO2 have the capacity to mobilize more phosphorus from so-called unavailable sources using mechanisms involving exudation of organic acids and phosphatases.  相似文献   

14.
Net grassland carbon flux over a subambient to superambient CO2 gradient   总被引:2,自引:0,他引:2  
Increasing atmospheric CO2 concentrations may have a profound effect on the structure and function of plant communities. A previously grazed, central Texas grassland was exposed to a 200‐µmol mol?1 to 550 µmol mol?1 CO2 gradient from March to mid‐December in 1998 and 1999 using two, 60‐m long, polyethylene‐ covered chambers built directly onto the site. One chamber was operated at subambient CO2 concentrations (200–360 µmol mol?1 daytime) and the other was regulated at superambient concentrations (360–550 µmol mol?1). Continuous CO2 gradients were maintained in each chamber by photosynthesis during the day and respiration at night. Net ecosystem CO2 flux and end‐of‐year biomass were measured in each of 10, 5‐m long sections in each chamber. Net CO2 fluxes were maximal in late May (c. day 150) in 1998 and in late August in 1999 (c. day 240). In both years, fluxes were near zero and similar in both chambers at the beginning and end of the growing season. Average daily CO2 flux in 1998 was 13 g CO2 m?2 day?1 in the subambient chamber and 20 g CO2 m?2 day?1 in the superambient chamber; comparable averages were 15 and 26 g CO2 m?2 day?1 in 1999. Flux was positively and linearly correlated with end‐of‐year above‐ground biomass but flux was not linearly correlated with CO2 concentration; a finding likely to be explained by inherent differences in vegetation. Because C3 plants were the dominant functional group, we adjusted average daily flux in each section by dividing the flux by the average percentage C3 cover. Adjusted fluxes were better correlated with CO2 concentration, although scatter remained. Our results indicate that after accounting for vegetation differences, CO2 flux increased linearly with CO2 concentration. This trend was more evident at subambient than superambient CO2 concentrations.  相似文献   

15.
Atmospheric measurements and land‐based inventories imply that terrestrial ecosystems in the northern hemisphere are taking up significant amounts of anthropogenic cabon dioxide (CO2) emissions; however, there is considerable disagreement about the causes of this uptake, and its expected future trajectory. In this paper, we use the ecosystem demography (ED) model to quantify the contributions of disturbance history, CO2 fertilization and climate variability to the past, current, and future terrestrial carbon fluxes in the Eastern United States. The simulations indicate that forest regrowth following agricultural abandonment accounts for uptake of 0.11 Pg C yr?1 in the 1980s and 0.15 Pg C yr?1 in the 1990s, and regrowth following forest harvesting accounts for an additional 0.1 Pg C yr?1 of uptake during both these decades. The addition of CO2 fertilization into the model simulations increases carbon uptake rates to 0.38 Pg C yr?1 in the 1980s and 0.47 Pg C yr?1 in the 1990s. Comparisons of predicted aboveground carbon uptake to regional‐scale forest inventory measurements indicate that the model's predictions in the absence of CO2 fertilization are 14% lower than observed, while in the presence of CO2 fertilization, predicted uptake rates are 28% larger than observed. Comparable results are obtained from comparisons of predicted total Net Ecosystem Productivity to the carbon fluxes observed at the Harvard Forest flux tower site and in model simulations free‐air CO2 enrichment (FACE) experiments. These results imply that disturbance history is the principal mechanism responsible for current carbon uptake in the Eastern United States, and that conventional biogeochemical formulations of plant growth overestimate the response of plants to rising CO2 levels. Model projections out to 2100 imply that the carbon uptake arising from forest regrowth will increasingly be dominated by forest regrowth following harvesting. Consequently, actual carbon storage declines to near zero by the end of the 21st century as the forest regrowth that has occurred since agricultural abandonment comes into equilibrium with the landscape's new disturbance regime. Incorporating interannual climate variability into the model simulations gives rise to large interannual variation in regional carbon fluxes, indicating that long‐term measurements are necessary to detect the signature of processes that give rise to long‐term uptake and storage.  相似文献   

16.
Goudriaan  J. 《Plant Ecology》1993,(1):329-337
Increasing atmospheric CO2 induces a net uptake of carbon in the ocean by a shift in chemical equilibrium in seawater, and in the terrestrial biosphere by a stimulated photosynthesis and productivity. The fractions absorbed in both biosphere and ocean decline with increasing dynamics of the release rate of CO2 into the atmosphere. However, the relative portion of ocean absorption descends much faster with annual growth rate of CO2 release than biospheric absorption does, due to a difference in dynamics. The equilibrium absorption capacity of the biosphere is estimated to be only one quarter of that of the ocean, but the current sink size of the biosphere is about half of that of the ocean.Apart from CO2-stimulated carbon fixation, the biosphere releases CO2 as a result of land use changes, in particular after deforestation. Both of these fluxes are of the order of 1–1.5 Pg of carbon per year. The CO2-fertilization effect and regrowth together have turned the terrestrial biosphere as a whole from a source into a sink.  相似文献   

17.
The high-latitude terrestrial carbon sink: a model analysis   总被引:7,自引:1,他引:6  
A dynamic, global vegetation model, hybrid v4.1 ( Friend et al. 1997 ), was driven by transient climate output from the UK Hadley Centre GCM (HadCM2) with the IS92a scenario of increasing atmospheric CO2 equivalent, sulphate aerosols and predicted patterns of atmospheric N deposition. Changes in areas of vegetation types and carbon storage in biomass and soils were predicted for areas north of 50°N from 1860 to 2100. Hybrid is a combined biogeochemical, biophysical and biogeographical model of natural, potential ecosystems. The effect of periodic boreal forest fires was assessed by adding a simple stochastic fire model. Hybrid represents plant physiological and soil processes regulating the carbon, water and N cycles and competition between individuals of parameterized generalized plant types. The latter were combined to represent tundra, temperate grassland, temperate/mixed forest and coniferous forest. The model simulated the current areas and estimated carbon stocks in the four vegetation types. It was predicted that land areas above 50°N (about 23% of the vegetated global land area) are currently accumulating about 0.4 PgC y?1 (about 30% of the estimated global terrestrial sink) and that this sink could grow to 0.8–1.0 PgC y?1 by the second half of the next century and persist undiminished until 2100. This sink was due mainly to an increase in forest productivity and biomass in response to increasing atmospheric CO2, temperature and N deposition, and includes an estimate of the effect of boreal forest fire, which was estimated to diminish the sink approximately by the amount of carbon emitted to the atmosphere during fires. Averaged over the region, N deposition contributed about 18% to the sink by the 2080 s. As expected, climate change (temperature, precipitation, solar radiation and saturation pressure deficit) and N deposition without increasing atmospheric CO2 produced a carbon source. Forest areas expanded both south and north, halving the current tundra area by 2100. This expansion contributed about 30% to the sink by the 2090 s. Tundra areas which were not invaded by forest fluctuated from sink to source. It was concluded that a high latitude carbon sink exists at present and, even assuming little effect of N deposition, no forest expansion and continued boreal forest fires, the sink is likely to persist at its current level for a century.  相似文献   

18.
Annual carbon budgets of ecosystems are central to our understanding of the biotic control of atmospheric composition, but they are not available under elevated CO2 for most vegetation types. Using gas exchange techniques, we assessed carbon fluxes of four early successional Mediterranean model communities, consisting of grasses, legumes and composites. The assemblages were grown on the same monoliths for three consecutive years in greenhouses tracking field conditions except for CO2 maintained at ambient (370 μmol mol?1) or elevated (700 μmol mol?1) concentration. During the third year of study, CO2 enrichment consistently shifted the annual carbon balance towards lower efflux, with displacements between 4.3 and 26.2 mol m?2 y?1 (one assemblage became a net CO2 sink, another just reached equilibrium, and the remaining two remained as a CO2 source). At least 50% of the shift under elevated CO2 originated from a decrease in belowground respiration. This indicates that, during this year, CO2 enrichment did not predominantly enhance C‐cycling, but on the contrary inhibited root respiration or microbial C‐utilization. Although elevated‐CO2‐grown systems acted as a net CO2 sink during a longer period of the year (4–7 months) compared with ambient‐CO2‐grown systems (3–3.5 months), gross canopy photosynthesis was modified only to a limited extent (between ?5.9 and + 14.8%). Interaction between the carbon and the water cycle was apparently responsible for this weak stimulation. In particular, reduced evapotranspiration under elevated CO2 coincided with inhibited canopy photosynthesis in early spring, most likely resulting from water saturation of the soil. In addition, only the earliest‐planted assemblages had an increased gross canopy photosynthesis during late autumn and early winter. This suggests that a longer summer drought, by delaying the establishment of such an annual type of vegetation, would reduce the positive impact of elevated CO2 on productivity. Water regime appears to strongly govern the influence of CO2 on the carbon fluxes in Mediterranean ecosystems with annual herbaceous vegetation.  相似文献   

19.
We develop an approach for estimating net ecosystem exchange (NEE) using inventory‐based information over North America (NA) for a recent 7‐year period (ca. 2000–2006). The approach notably retains information on the spatial distribution of NEE, or the vertical exchange between land and atmosphere of all non‐fossil fuel sources and sinks of CO2, while accounting for lateral transfers of forest and crop products as well as their eventual emissions. The total NEE estimate of a ?327 ± 252 TgC yr?1 sink for NA was driven primarily by CO2 uptake in the Forest Lands sector (?248 TgC yr?1), largely in the Northwest and Southeast regions of the US, and in the Crop Lands sector (?297 TgC yr?1), predominantly in the Midwest US states. These sinks are counteracted by the carbon source estimated for the Other Lands sector (+218 TgC yr?1), where much of the forest and crop products are assumed to be returned to the atmosphere (through livestock and human consumption). The ecosystems of Mexico are estimated to be a small net source (+18 TgC yr?1) due to land use change between 1993 and 2002. We compare these inventory‐based estimates with results from a suite of terrestrial biosphere and atmospheric inversion models, where the mean continental‐scale NEE estimate for each ensemble is ?511 TgC yr?1 and ?931 TgC yr?1, respectively. In the modeling approaches, all sectors, including Other Lands, were generally estimated to be a carbon sink, driven in part by assumed CO2 fertilization and/or lack of consideration of carbon sources from disturbances and product emissions. Additional fluxes not measured by the inventories, although highly uncertain, could add an additional ?239 TgC yr?1 to the inventory‐based NA sink estimate, thus suggesting some convergence with the modeling approaches.  相似文献   

20.
Tropical forests absorb large amounts of atmospheric CO2 through photosynthesis but elevated temperatures suppress this absorption and promote monoterpene emissions. Using 13CO2 labeling, here we show that monoterpene emissions from tropical leaves derive from recent photosynthesis and demonstrate distinct temperature optima for five groups (Groups 1–5), potentially corresponding to different enzymatic temperature‐dependent reaction mechanisms within β‐ocimene synthases. As diurnal and seasonal leaf temperatures increased during the Amazonian 2015 El Niño event, leaf and landscape monoterpene emissions showed strong linear enrichments of β‐ocimenes (+4.4% °C?1) at the expense of other monoterpene isomers. The observed inverse temperature response of α‐pinene (?0.8% °C?1), typically assumed to be the dominant monoterpene with moderate reactivity, was not accurately simulated by current global emission models. Given that β‐ocimenes are highly reactive with respect to both atmospheric and biological oxidants, the results suggest that highly reactive β‐ocimenes may play important roles in the thermotolerance of photosynthesis by functioning as effective antioxidants within plants and as efficient atmospheric precursors of secondary organic aerosols. Thus, monoterpene composition may represent a new sensitive ‘thermometer’ of leaf oxidative stress and atmospheric reactivity, and therefore a new tool in future studies of warming impacts on tropical biosphere‐atmosphere carbon‐cycle feedbacks.  相似文献   

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