Evaluation of terrestrial carbon cycle models for their response to climate variability and to CO2 trends

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.     

The use of CO2 flux measurements in models of the global terrestrial carbon budget

CO₂ flux measurements give access to two critical terms of the carbon budget of terrestrial ecosystems, the gross primary productivity (GPP) and the net ecosystem productivity (NEP). CO₂ fluxes measured by micrometeorological methods have spatial and temporal characteristics that make them potentially useful in modelling the global terrestrial carbon budget. The first use is in parameterizing ecosystem physiological processes. We present an example, based on parameterizing the mean light response of GPP. This parameterization can be used in diagnostic, satellite-based GPP models. A global application leads to realistic estimates of global GPP. The second use is in testing the seasonality of fluxes predicted by global models. Our example of this use tests two global GPP models. One is a diagnostic, satellite-based model, and one is a prognostic, process-based model. Despite the limitations of the models, both agree reasonably well with the measurements. The agreements and disagreements are useful in addressing the problems of available input datasets and representation of processes, in global models. Long-term CO₂ flux measurements give access to key variables of terrestrial vegetation models and therefore offer exciting perspectives.     

Interaction of ocean and biosphere in their transient responses to increasing atmospheric CO2

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.     

2020-2050年CO2施肥效应促进全球陆地生态系统碳吸收

CO₂施肥效应是全球变绿的主要原因,随着大气中CO₂浓度的持续增加,预估未来气候变化条件下,CO₂施肥效应对陆地生态系统的影响对减缓全球气候变化具有重大意义。基于未来气候情景数据和Farquhar模型,并结合生态过程模型BEPS(Boreal Ecosystem Productivity Simulator),定量化研究2020—2050年CO₂施肥效应对全球叶面积指数(LAI)和总初级生产力(GPP)的影响。研究结果显示2020—2050年,在RCP2.6、RCP4.5和RCP8.5气候情景下,CO₂施肥效应导致的LAI年际变化趋势分别为0.002、0.003和0.005 m^-2m^-2a^-1;三个气候情景下CO₂施肥效应对LAI的影响为CO₂每增加0.1%,LAI平均增加约8.1%—9.2%,由此导致GPP对应增加7.9%—14.6%;由CO₂施...     

Higher than expected CO2 fertilization inferred from leaf to global observations

Several lines of evidence point to an increase in the activity of the terrestrial biosphere over recent decades, impacting the global net land carbon sink (NLS) and its control on the growth of atmospheric carbon dioxide (c_a). Global terrestrial gross primary production (GPP)—the rate of carbon fixation by photosynthesis—is estimated to have risen by (31 ± 5)% since 1900, but the relative contributions of different putative drivers to this increase are not well known. Here we identify the rising atmospheric CO₂ concentration as the dominant driver. We reconcile leaf‐level and global atmospheric constraints on trends in modeled biospheric activity to reveal a global CO₂ fertilization effect on photosynthesis of 30% since 1900, or 47% for a doubling of c_a above the pre‐industrial level. Our historic value is nearly twice as high as current estimates (17 ± 4)% that do not use the full range of available constraints. Consequently, under a future low‐emission scenario, we project a land carbon sink (174 PgC, 2006–2099) that is 57 PgC larger than if a lower CO₂ fertilization effect comparable with current estimates is assumed. These findings suggest a larger beneficial role of the land carbon sink in modulating future excess anthropogenic CO₂ consistent with the target of the Paris Agreement to stay below 2°C warming, and underscore the importance of preserving terrestrial carbon sinks.     

Dominant regions and drivers of the variability of the global land carbon sink across timescales

Net biome productivity (NBP) dominates the observed large variation of atmospheric CO₂ annual increase over the last five decades. However, the dominant regions controlling inter‐annual to multi‐decadal variability of global NBP are still controversial (semi‐arid regions vs. temperate or tropical forests). By developing a theory for partitioning the variance of NBP into the contributions of net primary production (NPP) and heterotrophic respiration (R_h) at different timescales, and using both observation‐based atmospheric CO₂ inversion product and the outputs of 10 process‐based terrestrial ecosystem models forced by 110‐year observational climate, we tried to reconcile the controversy by showing that semi‐arid lands dominate the variability of global NBP at inter‐annual (<10 years) and tropical forests dominate at multi‐decadal scales (>30 years). Results further indicate that global NBP variability is dominated by the NPP component at inter‐annual timescales, and is progressively controlled by R_h with increasing timescale. Multi‐decadal NBP variations of tropical rainforests are modulated by the Pacific Decadal Oscillation (PDO) through its significant influences on both temperature and precipitation. This study calls for long‐term observations for the decadal or longer fluctuations in carbon fluxes to gain insights on the future evolution of global NBP, particularly in the tropical forests that dominate the decadal variability of land carbon uptake and are more effective for climate mitigation.     

Terrestrial fluxes of carbon in GCP carbon budgets

The Global Carbon Project (GCP) has published global carbon budgets annually since 2007 (Canadell et al. [2007], Proc Natl Acad Sci USA, 104, 18866–18870; Raupach et al. [2007], Proc Natl Acad Sci USA, 104, 10288–10293). There are many scientists involved, but the terrestrial fluxes that appear in the budgets are not well understood by ecologists and biogeochemists outside of that community. The purpose of this paper is to make the terrestrial fluxes of carbon in those budgets more accessible to a broader community. The GCP budget is composed of annual perturbations from pre‐industrial conditions, driven by addition of carbon to the system from combustion of fossil fuels and by transfers of carbon from land to the atmosphere as a result of land use. The budget includes a term for each of the major fluxes of carbon (fossil fuels, oceans, land) as well as the rate of carbon accumulation in the atmosphere. Land is represented by two terms: one resulting from direct anthropogenic effects (Land Use, Land‐Use Change, and Forestry or land management) and one resulting from indirect anthropogenic (e.g., CO₂, climate change) and natural effects. Each of these two net terrestrial fluxes of carbon, in turn, is composed of opposing gross emissions and removals (e.g., deforestation and forest regrowth). Although the GCP budgets have focused on the two net terrestrial fluxes, they have paid little attention to the gross components, which are important for a number of reasons, including understanding the potential for land management to remove CO₂ from the atmosphere and understanding the processes responsible for the sink for carbon on land. In contrast to the net fluxes of carbon, which are constrained by the global carbon budget, the gross fluxes are largely unconstrained, suggesting that there is more uncertainty than commonly believed about how terrestrial carbon emissions will respond to future fossil fuel emissions and a changing climate.     

Effects of 21st‐century climate,land use,and disturbances on ecosystem carbon balance in California

Terrestrial ecosystems are an important sink for atmospheric carbon dioxide (CO₂), sequestering ~30% of annual anthropogenic emissions and slowing the rise of atmospheric CO₂. However, the future direction and magnitude of the land sink is highly uncertain. We examined how historical and projected changes in climate, land use, and ecosystem disturbances affect the carbon balance of terrestrial ecosystems in California over the period 2001–2100. We modeled 32 unique scenarios, spanning 4 land use and 2 radiative forcing scenarios as simulated by four global climate models. Between 2001 and 2015, carbon storage in California's terrestrial ecosystems declined by ?188.4 Tg C, with a mean annual flux ranging from a source of ?89.8 Tg C/year to a sink of 60.1 Tg C/year. The large variability in the magnitude of the state's carbon source/sink was primarily attributable to interannual variability in weather and climate, which affected the rate of carbon uptake in vegetation and the rate of ecosystem respiration. Under nearly all future scenarios, carbon storage in terrestrial ecosystems was projected to decline, with an average loss of ?9.4% (?432.3 Tg C) by the year 2100 from current stocks. However, uncertainty in the magnitude of carbon loss was high, with individual scenario projections ranging from ?916.2 to 121.2 Tg C and was largely driven by differences in future climate conditions projected by climate models. Moving from a high to a low radiative forcing scenario reduced net ecosystem carbon loss by 21% and when combined with reductions in land‐use change (i.e., moving from a high to a low land‐use scenario), net carbon losses were reduced by 55% on average. However, reconciling large uncertainties associated with the effect of increasing atmospheric CO₂ is needed to better constrain models used to establish baseline conditions from which ecosystem‐based climate mitigation strategies can be evaluated.     

The effects of land use and climate change on the carbon cycle of Europe over the past 500 years

The long residence time of carbon in forests and soils means that both the current state and future behavior of the terrestrial biosphere are influenced by past variability in climate and anthropogenic land use. Over the last half‐millennium, European terrestrial ecosystems were affected by the cool temperatures of the Little Ice Age, rising CO₂ concentrations, and human induced deforestation and land abandonment. To quantify the importance of these processes, we performed a series of simulations with the LPJ dynamic vegetation model driven by reconstructed climate, land use, and CO₂ concentrations. Although land use change was the major control on the carbon inventory of Europe over the last 500 years, the current state of the terrestrial biosphere is largely controlled by land use change during the past century. Between 1500 and 2000, climate variability led to temporary sequestration events of up to 3 Pg, whereas increasing atmospheric CO₂ concentrations during the 20th century led to an increase in carbon storage of up to 15 Pg. Anthropogenic land use caused between 25 Pg of carbon emissions and 5 Pg of uptake over the same time period, depending on the historical and spatial pattern of past land use and the timing of the reversal from deforestation to afforestation during the last two centuries. None of the currently existing anthropogenic land use change datasets adequately capture the timing of the forest transition in most European countries as recorded in historical observations. Despite considerable uncertainty, our scenarios indicate that with limited management, extant European forests have the potential to absorb between 5 and 12 Pg of carbon at the present day.     

The European carbon balance. Part 4: integration of carbon and other trace‐gas fluxes

Overviewing the European carbon (C), greenhouse gas (GHG), and non‐GHG fluxes, gross primary productivity (GPP) is about 9.3 Pg yr^?1, and fossil fuel imports are 1.6 Pg yr^?1. GPP is about 1.25% of solar radiation, containing about 360 × 10¹⁸ J energy – five times the energy content of annual fossil fuel use. Net primary production (NPP) is 50%, terrestrial net biome productivity, NBP, 3%, and the net GHG balance, NGB, 0.3% of GPP. Human harvest uses 20% of NPP or 10% of GPP, or alternatively 1‰ of solar radiation after accounting for the inherent cost of agriculture and forestry, for production of pesticides and fertilizer, the return of organic fertilizer, and for the C equivalent cost of GHG emissions. C equivalents are defined on a global warming potential with a 100‐year time horizon. The equivalent of about 2.4% of the mineral fertilizer input is emitted as N₂O. Agricultural emissions to the atmosphere are about 40% of total methane, 60% of total NO‐N, 70% of total N₂O‐N, and 95% of total NH₃‐N emissions of Europe. European soils are a net C sink (114 Tg yr^?1), but considering the emissions of GHGs, soils are a source of about 26 Tg CO₂ C‐equivalent yr^?1. Forest, grassland and sediment C sinks are offset by GHG emissions from croplands, peatlands and inland waters. Non‐GHGs (NH₃, NOx) interact significantly with the GHG and the C cycle through ammonium nitrate aerosols and dry deposition. Wet deposition of nitrogen (N) supports about 50% of forest timber growth. Land use change is regionally important. The absolute flux values total about 50 Tg C yr^?1. Nevertheless, for the European trace‐gas balance, land‐use intensity is more important than land‐use change. This study shows that emissions of GHGs and non‐GHGs significantly distort the C cycle and eliminate apparent C sinks.     

CO2 balance of boreal, temperate, and tropical forests derived from a global database

Terrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome‐specific carbon budgets; to re‐examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO₂ balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 °C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO₂ balance required the introduction of substantial biome‐specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non‐CO₂ carbon fluxes are not presently being adequately accounted for.     

Ensemble projections elucidate effects of uncertainty in terrestrial nitrogen limitation on future carbon uptake

The magnitude of the nitrogen (N) limitation of terrestrial carbon (C) storage over the 21st century is highly uncertain because of the complex interactions between the terrestrial C and N cycles. We use an ensemble approach to quantify and attribute process‐level uncertainty in C‐cycle projections by analysing a 30‐member ensemble representing published alternative representations of key N cycle processes (stoichiometry, biological nitrogen fixation (BNF) and ecosystem N losses) within the framework of one terrestrial biosphere model. Despite large differences in the simulated present‐day N cycle, primarily affecting simulated productivity north of 40°N, ensemble members generally conform with global C‐cycle benchmarks for present‐day conditions. Ensemble projections for two representative concentration pathways (RCP 2.6 and RCP 8.5) show that the increase in land C storage due to CO₂ fertilization is reduced by 24 ± 15% due to N constraints, whereas terrestrial C losses associated with climate change are attenuated by 19 ± 20%. As a result, N cycling reduces projected land C uptake for the years 2006–2099 by 19% (37% decrease to 3% increase) for RCP 2.6, and by 21% (40% decrease to 9% increase) for RCP 8.5. Most of the ensemble spread results from uncertainty in temperate and boreal forests, and is dominated by uncertainty in BNF (10% decrease to 50% increase for RCP 2.6, 5% decrease to 100% increase for RCP 8.5). However, choices about the flexibility of ecosystem C:N ratios and processes controlling ecosystem N losses regionally also play important roles. The findings of this study demonstrate clearly the need for an ensemble approach to quantify likely future terrestrial C–N cycle trajectories. Present‐day C‐cycle observations only weakly constrain the future ensemble spread, highlighting the need for better observational constraints on large‐scale N cycling, and N cycle process responses to global change.     

Enhanced terrestrial carbon uptake in the Northern High Latitudes in the 21st century from the Coupled Carbon Cycle Climate Model Intercomparison Project model projections

The ongoing and projected warming in the northern high latitudes (NHL; poleward of 60 °N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO₂ concentration stimulate vegetation productivity, taking up CO₂. On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 ± 0.3 Pg C yr^?1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 ± 20 Pg C over 1901 levels, of which 17 ± 8 Pg C comes from vegetation (43%) and 21 ± 16 Pg C from the soil (8%). Both CO₂ fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO₂ fertilization effect, permafrost, and the role of soil moisture. Unlike the CO₂ fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid‐latitudes. These effects are further enhanced as a result of positive carbon cycle–climate feedbacks due to additional CO₂ and warming.     

Pan-Arctic soil moisture control on tundra carbon sequestration and plant productivity

Long-term atmospheric CO₂ concentration records have suggested a reduction in the positive effect of warming on high-latitude carbon uptake since the 1990s. A variety of mechanisms have been proposed to explain the reduced net carbon sink of northern ecosystems with increased air temperature, including water stress on vegetation and increased respiration over recent decades. However, the lack of consistent long-term carbon flux and in situ soil moisture data has severely limited our ability to identify the mechanisms responsible for the recent reduced carbon sink strength. In this study, we used a record of nearly 100 site-years of eddy covariance data from 11 continuous permafrost tundra sites distributed across the circumpolar Arctic to test the temperature (expressed as growing degree days, GDD) responses of gross primary production (GPP), net ecosystem exchange (NEE), and ecosystem respiration (ER) at different periods of the summer (early, peak, and late summer) including dominant tundra vegetation classes (graminoids and mosses, and shrubs). We further tested GPP, NEE, and ER relationships with soil moisture and vapor pressure deficit to identify potential moisture limitations on plant productivity and net carbon exchange. Our results show a decrease in GPP with rising GDD during the peak summer (July) for both vegetation classes, and a significant relationship between the peak summer GPP and soil moisture after statistically controlling for GDD in a partial correlation analysis. These results suggest that tundra ecosystems might not benefit from increased temperature as much as suggested by several terrestrial biosphere models, if decreased soil moisture limits the peak summer plant productivity, reducing the ability of these ecosystems to sequester carbon during the summer.     

Direct and indirect climate change effects on carbon dioxide fluxes in a thawing boreal forest–wetland landscape

In the sporadic permafrost zone of northwestern Canada, boreal forest carbon dioxide (CO₂) fluxes will be altered directly by climate change through changing meteorological forcing and indirectly through changes in landscape functioning associated with thaw‐induced collapse‐scar bog (‘wetland’) expansion. However, their combined effect on landscape‐scale net ecosystem CO₂ exchange (NEE_LAND), resulting from changing gross primary productivity (GPP) and ecosystem respiration (ER), remains unknown. Here, we quantify indirect land cover change impacts on NEE_LAND and direct climate change impacts on modeled temperature‐ and light‐limited NEE_LAND of a boreal forest–wetland landscape. Using nested eddy covariance flux towers, we find both GPP and ER to be larger at the landscape compared to the wetland level. However, annual NEE_LAND (?20 g C m^?2) and wetland NEE (?24 g C m^?2) were similar, suggesting negligible wetland expansion effects on NEE_LAND. In contrast, we find non‐negligible direct climate change impacts when modeling NEE_LAND using projected air temperature and incoming shortwave radiation. At the end of the 21st century, modeled GPP mainly increases in spring and fall due to reduced temperature limitation, but becomes more frequently light‐limited in fall. In a warmer climate, ER increases year‐round in the absence of moisture stress resulting in net CO₂ uptake increases in the shoulder seasons and decreases during the summer. Annually, landscape net CO₂ uptake is projected to decline by 25 ± 14 g C m^?2 for a moderate and 103 ± 38 g C m^?2 for a high warming scenario, potentially reversing recently observed positive net CO₂ uptake trends across the boreal biome. Thus, even without moisture stress, net CO₂ uptake of boreal forest–wetland landscapes may decline, and ultimately, these landscapes may turn into net CO₂ sources under continued anthropogenic CO₂ emissions. We conclude that NEE_LAND changes are more likely to be driven by direct climate change rather than by indirect land cover change impacts.     

Terrestrial ecosystems and the carbon cycle

The terrestrial biosphere plays an important role in the global carbon cycle. In the 1994 Intergovernmental Panel Assessment on Climate Change (IPCC), an effort was made to improve the quantification of terrestrial exchanges and potential feedbacks from climate, changing CO₂, and other factors; this paper presents the key results from that assessment, together with expanded discussion. The carbon cycle is the fluxes of carbon among four main reservoirs: fossil carbon, the atmosphere, the oceans, and the terrestrial biosphere. Emissions of fossil carbon during the 1980s averaged 5.5 Gt y^?1. During the same period, the atmosphere gained 3.2 Gt C y^?1 and the oceans are believed to have absorbed 2.0 Gt C y^?1. The regrowing forests of the Northern Hemisphere may have absorbed 0.5 Gt C y^?1 during this period. Meanwhile, tropical deforestation is thought to have released an average 1.6 Gt C y^?1 over the 1980s. While the fluxes among the four pools should balance, the average 198Ds values lead to a ‘missing sink’ of 1.4 Gt C y^?1 Several processes, including forest regrowth, CO₂ fertilization of plant growth (c. 1.0 Gt C y^?1), N deposition (c. 0.6 Gt C y^?1), and their interactions, may account for the budget imbalance. However, it remains difficult to quantify the influences of these separate but interactive processes. Uncertainties in the individual numbers are large, and are themselves poorly quantified. This paper presents detail beyond the IPCC assessment on procedures used to approximate the flux uncertainties. Lack of knowledge about positive and negative feedbacks from the biosphere is a major limiting factor to credible simulations of future atmospheric CO₂ concentrations. Analyses of the atmospheric gradients of CO₂ and ¹³ CO₂ concentrations provide increasingly strong evidence for terrestrial sinks, potentially distributed between Northern Hemisphere and tropical regions, but conclusive detection in direct biomass and soil measurements remains elusive. Current regional-to-global terrestrial ecosystem models with coupled carbon and nitrogen cycles represent the effects of CO₂ fertilization differently, but all suggest longterm responses to CO₂ that are substantially smaller than potential leaf- or laboratory whole plant-level responses. Analyses of emissions and biogeochemical fluxes consistent with eventual stabilization of atmospheric CO₂ concentrations are sensitive to the way in which biospheric feedbacks are modeled by c. 15%. Decisions about land use can have effects of 100s of Gt C over the next few centuries, with similarly significant effects on the atmosphere. Critical areas for future research are continued measurements and analyses of atmospheric data (CO₂ and ¹³CO₂) to serve as large-scale constraints, process studies of the scaling from the photosynthetic response to CO₂ to whole-ecosystem carbon storage, and rigorous quantification of the effects of changing land use on carbon storage.     

The fate of Amazonian ecosystems over the coming century arising from changes in climate,atmospheric CO2, and land use

There is considerable interest in understanding the fate of the Amazon over the coming century in the face of climate change, rising atmospheric CO₂ levels, ongoing land transformation, and changing fire regimes within the region. In this analysis, we explore the fate of Amazonian ecosystems under the combined impact of these four environmental forcings using three terrestrial biosphere models (ED2, IBIS, and JULES) forced by three bias‐corrected IPCC AR4 climate projections (PCM1, CCSM3, and HadCM3) under two land‐use change scenarios. We assess the relative roles of climate change, CO₂ fertilization, land‐use change, and fire in driving the projected changes in Amazonian biomass and forest extent. Our results indicate that the impacts of climate change are primarily determined by the direction and severity of projected changes in regional precipitation: under the driest climate projection, climate change alone is predicted to reduce Amazonian forest cover by an average of 14%. However, the models predict that CO₂ fertilization will enhance vegetation productivity and alleviate climate‐induced increases in plant water stress, and, as a result, sustain high biomass forests, even under the driest climate scenario. Land‐use change and climate‐driven changes in fire frequency are predicted to cause additional aboveground biomass loss and reductions in forest extent. The relative impact of land use and fire dynamics compared to climate and CO₂ impacts varies considerably, depending on both the climate and land‐use scenario, and on the terrestrial biosphere model used, highlighting the importance of improved quantitative understanding of all four factors – climate change, CO₂ fertilization effects, fire, and land use – to the fate of the Amazon over the coming century.     

Is carbon within the global terrestrial biosphere becoming more oxidized? Implications for trends in atmospheric O2

Measurements of atmospheric O₂ and CO₂ concentrations serve as a widely used means to partition global land and ocean carbon sinks. Interpretation of these measurements has assumed that the terrestrial biosphere contributes to changing O₂ levels by either expanding or contracting in size, and thus serving as either a carbon sink or source (and conversely as either an oxygen source or sink). Here, we show how changes in atmospheric O₂ can also occur if carbon within the terrestrial biosphere becomes more reduced or more oxidized, even with a constant carbon pool. At a global scale, we hypothesize that increasing levels of disturbance within many biomes has favored plant functional types with lower oxidative ratios and that this has caused carbon within the terrestrial biosphere to become increasingly more oxidized over a period of decades. Accounting for this mechanism in the global atmospheric O₂ budget may require a small increase in the size of the land carbon sink. In a scenario based on the Carnegie–Ames–Stanford Approach model, a cumulative decrease in the oxidative ratio of net primary production (NPP) (moles of O₂ produced per mole of CO₂ fixed in NPP) by 0.01 over a period of 100 years would create an O₂ disequilibrium of 0.0017 and require an increased land carbon sink of 0.1 Pg C yr⁻¹ to balance global atmospheric O₂ and CO₂ budgets. At present, however, it is challenging to directly measure the oxidative ratio of terrestrial ecosystem exchange and even more difficult to detect a disequilibrium caused by a changing oxidative ratio of NPP. Information on plant and soil chemical composition complement gas exchange approaches for measuring the oxidative ratio, particularly for understanding how this quantity may respond to various global change processes over annual to decadal timescales.     

The effect of a dry spring on seasonal carbon allocation and vegetation dynamics in a poplar bioenergy plantation

In this study the seasonal variation in carbon, water and energy fluxes as well as in net primary productivity (NPP) of different tree components is presented for a 2‐year‐old poplar (Populus spp.) plantation. A thorough ecophysiological study was performed at ecosystem scale, at tree and at leaf level, in this high‐density bioenergy plantation. Seasonal variation in NPP and fluxes was analysed in relation to meteorological parameters at the field site. The growing season length in terms of carbon uptake was controlled by leaf area development until the maximum leaf area index (LAI_max) was reached. Afterwards, a shift to belowground carbon allocation was observed. A dry period in spring caused a reduced leaf area production as well as a decrease in net ecosystem exchange and gross primary production (GPP) due to stomatal closure. Water use efficiency and fine root growth increased in response to limiting soil water availability in the root zone. When soil water availability was not limiting, GPP was controlled by a decrease in solar radiation and air temperature. The results of this study indicate that the productivity of recently established bioenergy plantations with fast‐growing trees is very sensitive to drought. The interaction between soil water availability and factors controlling ecosystem GPP is crucial in assessing the CO₂ mitigation potential under future climate conditions.     

基于BIOME-BGC模型的秦岭北坡太白红杉林碳源/汇动态和趋势研究

为了解秦岭北坡太白红杉(Larix chinensis)的碳源/汇动态,运用BIOME-BGC模型模拟了1959-2016年太白红杉生产力、碳储量和碳利用效率(CUE),并利用气候情景设定方法预测碳源/汇功能的未来趋势。结果表明,58年间太白红杉的平均净初级生产力(NPP)、初级生产力(GPP)和净生态系统生产力(NEP)分别为328.59、501.56和31.42 g C m–2a–1,平均碳储量为35.38 kg C m–2a–1,平均CUE为0.65;除1960-1961、1969-1970、1997-1999年为"碳源"年外,绝大多数年份为"碳汇"年,年内呈现"碳源-碳汇-碳源"的变化特征,碳储量总体增加,潜在固碳能力较为稳定。GPP、NPP、碳储量的正向作用排序为气温上升CO_2浓度增加,NEP的正向作用排序反之,降水增加对生产力和碳储量增加起反作用,气温升高对CUE起反作用;气温和CO_2浓度是北坡太白红杉生长的限制因子,气温的限制性强于CO_2浓度,未来气温或CO_2浓度升高有利于碳汇功能发挥,降水增加减弱碳汇效果。RCP4.5、RCP8.5情景下太白红杉生产力和碳储量在21世纪呈上升趋势,RCP8.5上升幅度略大于RCP4.5,潜在固碳能力仍较强;1-3月和10-12月为"碳源"月,5-9月为"碳汇"月。这揭示了气候变化背景下气温、降水和CO_2浓度对太白红杉碳源/汇的影响方式,气温和CO_2浓度上升是碳汇的促进因素,降水增加为阻碍因素。