Abstract
Projected changes in the frequency and severity of droughts as a result of increase in greenhouse gases have a significant impact on the role of vegetation in regulating the global carbon cycle. Drought effect on vegetation Gross Primary Production (GPP) is usually modeled as a function of Vapor Pressure Deficit (VPD) and/or soil moisture. Climate projections suggest a strong likelihood of increasing trend in VPD, while regional changes in precipitation are less certain. This difference in projections between VPD and precipitation can cause considerable discrepancies in the predictions of vegetation behavior depending on how ecosystem models represent the drought effect. In this study, we scrutinized the model responses to drought using the 30-year record of Global Inventory Modeling and Mapping Studies (GIMMS) 3g Normalized Difference Vegetation Index (NDVI) dataset. A diagnostic ecosystem model, Terrestrial Observation and Prediction System (TOPS), was used to estimate global GPP from 1982 to 2009 under nine different experimental simulations. The control run of global GPP increased until 2000, but stayed constant after 2000. Among the simulations with single climate constraint (temperature, VPD, rainfall and solar radiation), only the VPD-driven simulation showed a decrease in 2000s, while the other scenarios simulated an increase in GPP. The diverging responses in 2000s can be attributed to the difference in the representation of the impact of water stress on vegetation in models, i.e., using VPD and/or precipitation. Spatial map of trend in simulated GPP using GIMMS 3g data is consistent with the GPP driven by soil moisture than the GPP driven by VPD, confirming the need for a soil moisture constraint in modeling global GPP.
Highlights
Estimation of global vegetation Gross Primary Production (GPP) and Net Primary Production (NPP) and their interannual variations are critical for understanding the feedbacks between the biosphere and the atmosphere
For each climate variable analysis, only S_vpd showed a consistent decreasing trend, while the other simulations all produced increasing trends in global GPP (Figure 1). These results suggest that land models solely relying on Vapor Pressure Deficit (VPD) may overestimate the reduction in GPP caused by water stress in 2000s
Modeling and Mapping Studies (GIMMS) 3g data to evaluate the impacts of drought on the interannual variation of Gross Primary Production (GPP) simulated either in terms of VPD or soil moisture effects
Summary
Estimation of global vegetation Gross Primary Production (GPP) and Net Primary Production (NPP) and their interannual variations are critical for understanding the feedbacks between the biosphere and the atmosphere. Inversion models, and inventories have been used for assessing global land primary production, generating total annual global estimates of GPP and NPP converging around 120 [1] and 60 [2] Pg∙C∙yr−1, respectively. Productivity (NBP), the net carbon accumulation by ecosystems [3], was estimated just 2% of GPP for the 1990s [4]. Estimation of interannual variations of GPP and NPP are important as well as their total magnitudes for understanding NBP response to CO2 emissions and changes in climate. In contrast to total magnitude of GPP, there is no consensus on interannual variation in global GPP or NPP even for the last few decades with satellite observations (for example, [6,7])
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