ABSTRACT: The purpose of this study is to evaluate 10 process-based terrestrial biosphere models that were used for the IPCC 5(th) Assessment Report. The simulated gross primary productivity (GPP) is compared with flux-tower based estimates by Jung et al. (2011) (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. (2010) (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, modelled 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 of modelled 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) ). Yet, 9 of 10 models overestimate the regression slope of NBP vs. precipitation, compared to 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 modelled 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. © 2013 Blackwell Publishing Ltd.
Global Change Biology 03/2013; · 6.86 Impact Factor
ABSTRACT: Changes in climate and land use, caused by socio-economic changes, greenhouse gas emissions, agricultural policies and other
factors, are known to affect both natural and managed ecosystems, and will likely impact on the European terrestrial carbon
balance during the coming decades. This study presents a comprehensive European Union wide (EU15 plus Norway and Switzerland,
EU*) assessment of potential future changes in terrestrial carbon storage considering these effects based on four illustrative
IPCC-SRES storylines (A1FI, A2, B1, B2). A process-based land vegetation model (LPJ-DGVM), adapted to include a generic representation
of managed ecosystems, is forced with changing fields of land-use patterns from 1901 to 2100 to assess the effect of land-use
and cover changes on the terrestrial carbon balance of Europe. The uncertainty in the future carbon balance associated with
the choice of a climate change scenario is assessed by forcing LPJ-DGVM with output from four different climate models (GCMs:
CGCM2, CSIRO2, HadCM3, PCM2) for the same SRES storyline. Decrease in agricultural areas and afforestation leads to simulated
carbon sequestration for all land-use change scenarios with an average net uptake of 17–38Tg C/year between 1990 and 2100,
corresponding to 1.9–2.9% of the EU*s CO2 emissions over the same period. Soil carbon losses resulting from climate warming reduce or even offset carbon sequestration
resulting from growth enhancement induced by climate change and increasing atmospheric CO2 concentrations in the second half of the twenty-first century. Differences in future climate change projections among GCMs
are the main cause for uncertainty in the cumulative European terrestrial carbon uptake of 4.4–10.1 Pg C between 1990 and
Ecosystems 04/2012; 10(3):380-401. · 3.49 Impact Factor
ABSTRACT: This study investigates commonalities and differences in projected land biosphere carbon storage among climate change projections
derived from one emission scenario by five different general circulation models (GCMs). Carbon storage is studied using a
global biogeochemical process model of vegetation and soil that includes dynamic treatment of changes in vegetation composition,
a recently enhanced version of the Lund-Potsdam-Jena Dynamic Global Vegetation Model (LPJ-DGVM). Uncertainty in future terrestrial
carbon storage due to differences in the climate projections is large. Changes by the end of the century range from −106 to
+201 PgC, thus, even the sign of the response whether source or sink, is uncertain. Three out of five climate projections
produce a land carbon source by the year 2100, one is approximately neutral and one a sink. A regional breakdown shows some
robust qualitative features. Large areas of the boreal forest are shown as a future CO2 source, while a sink appears in the arctic. The sign of the response in tropical and sub-tropical ecosystems differs among
models, due to the large variations in simulated precipitation patterns. The largest uncertainty is in the response of tropical
rainforests of South America and Central Africa.
Climatic Change 04/2012; 74(1):97-122. · 3.38 Impact Factor
ABSTRACT: Unknowns in future global warming are usually assumed to arise from uncertainties either in the amount of anthropogenic greenhouse gas emissions or in the sensitivity of the climate to changes in greenhouse gas concentrations. Characterizing the additional uncertainty in relating CO2 emissions to atmospheric concentrations has relied on either a small number of complex models with diversity in process representations, or simple models. To date, these models indicate that the relevant carbon cycle uncertainties are smaller than the uncertainties in physical climate feedbacks and emissions. Here, for a single emissions scenario, we use a full coupled climate–carbon cycle model and a systematic method to explore uncertainties in the land carbon cycle feedback. We find a plausible range of climate–carbon cycle feedbacks significantly larger than previously estimated. Indeed the range of CO2 concentrations arising from our single emissions scenario is greater than that previously estimated across the full range of IPCC SRES emissions scenarios with carbon cycle uncertainties ignored. The sensitivity of photosynthetic metabolism to temperature emerges as the most important uncertainty. This highlights an aspect of current land carbon modelling where there are open questions about the potential role of plant acclimation to increasing temperatures. There is an urgent need for better understanding of plant photosynthetic responses to high temperature, as these responses are shown here to be key contributors to the magnitude of future change.
Environmental Research Letters 04/2012; 7(2):024002. · 3.63 Impact Factor
Ecology and Evolution. 02/2012; 2(3):593 - 614.
ABSTRACT: The rate of above-ground woody biomass production, W(P), in some western Amazon forests exceeds those in the east by a factor of 2 or more. Underlying causes may include climate, soil nutrient limitations and species composition. In this modelling paper, we explore the implications of allowing key nutrients such as N and P to constrain the photosynthesis of Amazon forests, and also we examine the relationship between modelled rates of photosynthesis and the observed gradients in W(P). We use a model with current understanding of the underpinning biochemical processes as affected by nutrient availability to assess: (i) the degree to which observed spatial variations in foliar [N] and [P] across Amazonia affect stand-level photosynthesis; and (ii) how these variations in forest photosynthetic carbon acquisition relate to the observed geographical patterns of stem growth across the Amazon Basin. We find nutrient availability to exert a strong effect on photosynthetic carbon gain across the Basin and to be a likely important contributor to the observed gradient in W(P). Phosphorus emerges as more important than nitrogen in accounting for the observed variations in productivity. Implications of these findings are discussed in the context of future tropical forests under a changing climate.
Philosophical Transactions of The Royal Society B Biological Sciences 11/2011; 366(1582):3316-29. · 6.40 Impact Factor
ABSTRACT: The terrestrial carbon sink has been large in recent decades, but its size and location remain uncertain. Using forest inventory data and long-term ecosystem carbon studies, we estimate a total forest sink of 2.4 ± 0.4 petagrams of carbon per year (Pg C year(-1)) globally for 1990 to 2007. We also estimate a source of 1.3 ± 0.7 Pg C year(-1) from tropical land-use change, consisting of a gross tropical deforestation emission of 2.9 ± 0.5 Pg C year(-1) partially compensated by a carbon sink in tropical forest regrowth of 1.6 ± 0.5 Pg C year(-1). Together, the fluxes comprise a net global forest sink of 1.1 ± 0.8 Pg C year(-1), with tropical estimates having the largest uncertainties. Our total forest sink estimate is equivalent in magnitude to the terrestrial sink deduced from fossil fuel emissions and land-use change sources minus ocean and atmospheric sinks.
Science 08/2011; 333(6045):988-93. · 31.20 Impact Factor
ABSTRACT: The future of tropical forests has become one of the iconic issues in climate-change science. A number of studies that have explored this subject have tended to focus on the output from one or a few climate models, which work at low spatial resolution, whereas society and conservation-relevant assessment of potential impacts requires a finer scale. This study focuses on the role of climate on the current and future distribution of humid tropical forests (HTFs). We first characterize their contemporary climatological niche using annual rainfall and maximum climatological water stress, which also adequately describe the current distribution of other biomes within the tropics. As a first-order approximation of the potential extent of HTFs in future climate regimes defined by global warming of 2°C and 4°C, we investigate changes in the niche through a combination of climate-change anomaly patterns and higher resolution (5 km) maps of current climatology. The climate anomalies are derived using data from 17 coupled Atmosphere-Ocean General Circulation Models (AOGCMs) used in the Fourth Assessment of the Intergovernmental Panel for Climate Change. Our results confirm some risk of forest retreat, especially in eastern Amazonia, Central America and parts of Africa, but also indicate a potential for expansion in other regions, for example around the Congo Basin. The finer spatial scale enabled the depiction of potential resilient and vulnerable zones with practically useful detail. We further refine these estimates by considering the impact of new environmental regimes on plant water demand using the UK Met Office land-surface scheme (of the HadCM3 AOGCM). The CO(2)-related reduction in plant water demand lowers the risk of die-back and can lead to possible niche expansion in many regions. The analysis presented here focuses primarily on hydrological determinants of HTF extent. We conclude by discussing the role of other factors, notably the physiological effects of higher temperature.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 01/2011; 369(1934):137-60. · 2.77 Impact Factor
ABSTRACT: *Second-generation Dynamic Global Vegetation Models (DGVMs) have recently been developed that explicitly represent the ecological dynamics of disturbance, vertical competition for light, and succession. Here, we introduce a modified second-generation DGVM and examine how the representation of demographic processes operating at two-dimensional spatial scales not represented by these models can influence predicted community structure, and responses of ecosystems to climate change. *The key demographic processes we investigated were seed advection, seed mixing, sapling survival, competitive exclusion and plant mortality. We varied these parameters in the context of a simulated Amazon rainforest ecosystem containing seven plant functional types (PFTs) that varied along a trade-off surface between growth and the risk of starvation induced mortality. *Varying the five unconstrained parameters generated community structures ranging from monocultures to equal co-dominance of the seven PFTs. When exposed to a climate change scenario, the competing impacts of CO(2) fertilization and increasing plant mortality caused ecosystem biomass to diverge substantially between simulations, with mid-21st century biomass predictions ranging from 1.5 to 27.0 kg C m(-2). *Filtering the results using contemporary observation ranges of biomass, leaf area index (LAI), gross primary productivity (GPP) and net primary productivity (NPP) did not substantially constrain the potential outcomes. We conclude that demographic processes represent a large source of uncertainty in DGVM predictions.
New Phytologist 08/2010; 187(3):666-81. · 6.64 Impact Factor
ABSTRACT: *The large-scale loss of Amazonian rainforest under some future climate scenarios has generally been considered to be driven by increased drying over Amazonia predicted by some general circulation models (GCMs). However, the importance of rainfall relative to other drivers has never been formally examined. *Here, we conducted factorial simulations to ascertain the contributions of four environmental drivers (precipitation, temperature, humidity and CO(2)) to simulated changes in Amazonian vegetation carbon (C(veg)), in three dynamic global vegetation models (DGVMs) forced with climate data based on HadCM3 for four SRES scenarios. *Increased temperature was found to be more important than precipitation reduction in causing losses of Amazonian C(veg) in two DGVMs (Hyland and TRIFFID), and as important as precipitation reduction in a third DGVM (LPJ). Increases in plant respiration, direct declines in photosynthesis and increases in vapour pressure deficit (VPD) all contributed to reduce C(veg) under high temperature, but the contribution of each mechanism varied greatly across models. Rising CO(2) mitigated much of the climate-driven biomass losses in the models. *Additional work is required to constrain model behaviour with experimental data under conditions of high temperature and drought. Current models may be overly sensitive to long-term elevated temperatures as they do not account for physiological acclimation.
New Phytologist 08/2010; 187(3):647-65. · 6.64 Impact Factor
ABSTRACT: Given the importance of Amazon rainforest in the global carbon and
hydrological cycles, there is a need to use parameterized and validated
ecosystem gas exchange and vegetation models for this region in order to
adequately simulate present and future carbon and water balances. Recent
research has found major differences in above-ground net primary
productivity (ANPP), above ground biomass and tree dynamics across
Amazonia. West Amazonia is more dynamic, with younger trees, higher stem
growth rates and lower biomass than central and eastern Amazon (Baker et
al. 2004; Malhi et al. 2004; Phillips et al. 2004). A factor of three
variation in above-ground net primary productivity has been estimated
across Amazonia by Malhi et al. (2004). Different hypotheses have been
proposed to explain the observed spatial variability in ANPP (Malhi et
al. 2004). First, due to the proximity to the Andes, sites from western
Amazonia tend to have richer soils than central and eastern Amazon and
therefore soil fertility could possibly be highly related to the high
wood productivity found in western sites. Second, if GPP does not vary
across the Amazon basin then different patterns of carbon allocation to
respiration could also explain the observed ANPP gradient. However since
plant growth depends on the interaction between photosynthesis,
transport of assimilates, plant respiration, water relations and mineral
nutrition, variations in plant gross photosynthesis (GPP) could also
explain the observed variations in ANPP. In this study we investigate
whether Amazon GPP can explain variations of observed ANPP. We use a sun
and shade canopy gas exchange model that has been calibrated and
evaluated at five rainforest sites (Mercado et al. 2009) to simulate
gross primary productivity of 50 sites across the Amazon basin during
the period 1980-2001. Such simulation differs from the ones performed
with global vegetation models (Cox et al. 1998; Sitch et al. 2003)
where i) single plant functional type parameter values are assigned and
assumed invariant with environmental condition but also ii) these models
use leaf N as a factor that limit photosynthesis. Instead, since leaf P
may also limit photosynthesis of the tropical forest (Reich et al.
2009), we use a more specific description of photosynthetic capacity
across the basin based on the model evaluation done in Mercado et al.
(2009) in which canopy photosynthetic capacity is related to foliar P
but also using the relationships derived between canopy photosynthesis
and leaf nutrients (N and P) from measurements in tropical trees
(Domingues et al.In review). A study of this kind can inform the
global vegetation/climate community as to the need for variability in
key model parameters in order to accurately simulate carbon fluxes
across the Amazon basin. Baker, T. R., et al. 2004. Increasing
biomass in Amazonian forest plots. Philosophical Transactions of the
Royal Society of London Series B-Biological Sciences 359 (1443):353-365.
Phillips, O. L. et al. 2004. Pattern and process in Amazon tree
turnover, 1976-2001. Philosophical Transactions of the Royal Society of
London Series B-Biological Sciences 359 (1443):381-407. Malhi, Y. et
al. 2004. The above-ground coarse wood productivity of 104 Neotropical
forest plots. Global Change Biology 10 (5):563-591. Mercado, L.M. et
al. 2009. Impact of changes in diffuse radiation on the global land
carbon sink. Nature 458 (7241), 1014. Cox, P. M. et al. 1998. A
canopy conductance and photosynthesis model for use in a GCM land
surface scheme. Journal of Hydrology 213 (1-4):79-9 Sitch, S. et al.
2003. Evaluation of ecosystem dynamics, plant geography and terrestrial
carbon cycling in the LPJ dynamic global vegetation model. Global Change
Biology 9 (2):161-185. Reich B. R. et al. 2009. Leaf phosphorus
influences the photosynhtesis-nitrogen relation: a cross-biome analysis
of 314 species. Oecologia, doi 10.1007/s00442-009-1291-3. Domingues,
T. et al. In review. Co-limitation of photosynthetic capacity by
nitrogen and phosphorus along a precipitation gradient in West Africa.
Plant Cell and Environment.
ABSTRACT: We model future changes in land biogeochemistry and biogeography across East Africa. East Africa is one of few tropical regions where general circulation model (GCM) future climate projections exhibit a robust response of strong future warming and general annual-mean rainfall increases. Eighteen future climate projections from nine GCMs participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment were used as input to the LPJ dynamic global vegetation model (DGVM), which predicted vegetation patterns and carbon storage in agreement with satellite observations and forest inventory data under the present-day climate. All simulations showed future increases in tropical woody vegetation over the region at the expense of grasslands. Regional increases in net primary productivity (NPP) (18–36%) and total carbon storage (3–13%) by 2080–2099 compared with the present-day were common to all simulations. Despite decreases in soil carbon after 2050, seven out of nine simulations continued to show an annual net land carbon sink in the final decades of the 21st century because vegetation biomass continued to increase. The seasonal cycles of rainfall and soil moisture show future increases in wet season rainfall across the GCMs with generally little change in dry season rainfall. Based on the simulated present-day climate and its future trends, the GCMs can be grouped into four broad categories. Overall, our model results suggest that East Africa, a populous and economically poor region, is likely to experience some ecosystem service benefits through increased precipitation, river runoff and fresh water availability. Resulting enhancements in NPP may lead to improved crop yields in some areas. Our results stand in partial contradiction to other studies that suggest possible negative consequences for agriculture, biodiversity and other ecosystem services caused by temperature increases.
Global Change Biology 01/2010; 16(2):617 - 640. · 6.86 Impact Factor
ABSTRACT: Global terrestrial ecosystems absorbed carbon at a rate of 1-4 Pg yr(-1) during the 1980s and 1990s, offsetting 10-60 per cent of the fossil-fuel emissions. The regional patterns and causes of terrestrial carbon sources and sinks, however, remain uncertain. With increasing scientific and political interest in regional aspects of the global carbon cycle, there is a strong impetus to better understand the carbon balance of China. This is not only because China is the world's most populous country and the largest emitter of fossil-fuel CO(2) into the atmosphere, but also because it has experienced regionally distinct land-use histories and climate trends, which together control the carbon budget of its ecosystems. Here we analyse the current terrestrial carbon balance of China and its driving mechanisms during the 1980s and 1990s using three different methods: biomass and soil carbon inventories extrapolated by satellite greenness measurements, ecosystem models and atmospheric inversions. The three methods produce similar estimates of a net carbon sink in the range of 0.19-0.26 Pg carbon (PgC) per year, which is smaller than that in the conterminous United States but comparable to that in geographic Europe. We find that northeast China is a net source of CO(2) to the atmosphere owing to overharvesting and degradation of forests. By contrast, southern China accounts for more than 65 per cent of the carbon sink, which can be attributed to regional climate change, large-scale plantation programmes active since the 1980s and shrub recovery. Shrub recovery is identified as the most uncertain factor contributing to the carbon sink. Our data and model results together indicate that China's terrestrial ecosystems absorbed 28-37 per cent of its cumulated fossil carbon emissions during the 1980s and 1990s.
Nature 05/2009; 458(7241):1009-13. · 36.28 Impact Factor
ABSTRACT: Plant photosynthesis tends to increase with irradiance. However, recent theoretical and observational studies have demonstrated that photosynthesis is also more efficient under diffuse light conditions. Changes in cloud cover or atmospheric aerosol loadings, arising from either volcanic or anthropogenic emissions, alter both the total photosynthetically active radiation reaching the surface and the fraction of this radiation that is diffuse, with uncertain overall effects on global plant productivity and the land carbon sink. Here we estimate the impact of variations in diffuse fraction on the land carbon sink using a global model modified to account for the effects of variations in both direct and diffuse radiation on canopy photosynthesis. We estimate that variations in diffuse fraction, associated largely with the 'global dimming' period, enhanced the land carbon sink by approximately one-quarter between 1960 and 1999. However, under a climate mitigation scenario for the twenty-first century in which sulphate aerosols decline before atmospheric CO(2) is stabilized, this 'diffuse-radiation' fertilization effect declines rapidly to near zero by the end of the twenty-first century.
Nature 05/2009; 458(7241):1014-7. · 36.28 Impact Factor
ABSTRACT: We examine the evidence for the possibility that 21st-century climate change may cause a large-scale "dieback" or degradation of Amazonian rainforest. We employ a new framework for evaluating the rainfall regime of tropical forests and from this deduce precipitation-based boundaries for current forest viability. We then examine climate simulations by 19 global climate models (GCMs) in this context and find that most tend to underestimate current rainfall. GCMs also vary greatly in their projections of future climate change in Amazonia. We attempt to take into account the differences between GCM-simulated and observed rainfall regimes in the 20th century. Our analysis suggests that dry-season water stress is likely to increase in E. Amazonia over the 21st century, but the region tends toward a climate more appropriate to seasonal forest than to savanna. These seasonal forests may be resilient to seasonal drought but are likely to face intensified water stress caused by higher temperatures and to be vulnerable to fires, which are at present naturally rare in much of Amazonia. The spread of fire ignition associated with advancing deforestation, logging, and fragmentation may act as nucleation points that trigger the transition of these seasonal forests into fire-dominated, low biomass forests. Conversely, deliberate limitation of deforestation and fire may be an effective intervention to maintain Amazonian forest resilience in the face of imposed 21st-century climate change. Such intervention may be enough to navigate E. Amazonia away from a possible "tipping point," beyond which extensive rainforest would become unsustainable.
Proceedings of the National Academy of Sciences 03/2009; 106(49):20610-5. · 9.68 Impact Factor
ABSTRACT: Global environmental changes may be altering the ecology of tropical forests. Long-term monitoring plots have provided much of the evidence for large-scale, directional changes in tropical forests, but the results have been controversial. Here we review evidence from six complementary approaches to understanding possible changes: plant physiology experiments, long-term monitoring plots, ecosystem flux techniques, atmospheric measurements, Earth observations, and global-scale vegetation models. Evidence from four of these approaches suggests that large-scale, directional changes are occurring in the ecology of tropical forests, with the other two approaches providing inconclusive results. Collectively, the evidence indicates that both gross and net primary productivity has likely increased over recent decades, as have tree growth, recruitment, and mortality rates, and forest biomass. These results suggest a profound reorganization of tropical forest ecosystems. We evaluate the most likely drivers of the su...
Global Ecology and Biogeography. 06/2008; 10(6):661 - 677.
ABSTRACT: Simulations with the Hadley Centre general circulation model (HadCM3), including carbon cycle model and forced by a 'business-as-usual' emissions scenario, predict a rapid loss of Amazonian rainforest from the middle of this century onwards. The robustness of this projection to both uncertainty in physical climate drivers and the formulation of the land surface scheme is investigated. We analyse how the modelled vegetation cover in Amazonia responds to (i) uncertainty in the parameters specified in the atmosphere component of HadCM3 and their associated influence on predicted surface climate. We then enhance the land surface description and (ii) implement a multilayer canopy light interception model and compare with the simple 'big-leaf' approach used in the original simulations. Finally, (iii) we investigate the effect of changing the method of simulating vegetation dynamics from an area-based model (TRIFFID) to a more complex size- and age-structured approximation of an individual-based model (ecosystem demography). We find that the loss of Amazonian rainforest is robust across the climate uncertainty explored by perturbed physics simulations covering a wide range of global climate sensitivity. The introduction of the refined light interception model leads to an increase in simulated gross plant carbon uptake for the present day, but, with altered respiration, the net effect is a decrease in net primary productivity. However, this does not significantly affect the carbon loss from vegetation and soil as a consequence of future simulated depletion in soil moisture; the Amazon forest is still lost. The introduction of the more sophisticated dynamic vegetation model reduces but does not halt the rate of forest dieback. The potential for human-induced climate change to trigger the loss of Amazon rainforest appears robust within the context of the uncertainties explored in this paper. Some further uncertainties should be explored, particularly with respect to the representation of rooting depth.
Philosophical Transactions of The Royal Society B Biological Sciences 06/2008; 363(1498):1857-64. · 6.40 Impact Factor