Interannual variability and trends of gross primary production and transpiration in savannas and grasslands from 2000 to 2021

Abstract

Carbon and water fluxes of savannas and grasslands have large seasonal dynamics and inter-annual variation. In this study, we selected five savanna and grassland sites, each of them having 10+ years (11−21 years) of eddy covariance (EC) data, and a total of 85 site-years at these five sites which offers a unique opportunity for data analyses and model evaluation. We ran a long-term simulation (2000−2021) of the vegetation photosynthesis model (VPM, v3.0) and vegetation transpiration model (VTM, v2.0) to investigate the seasonal dynamics, interannual variation, and decadal trends of modeled gross primary production (GPP_VPM) and transpiration (T_VTM) at these sites. The seasonal dynamics of daily GPP_VPM and T_VTM track well with the seasonal dynamics of EC-based GPP (GPP_EC, R²: 0.76−0.93) and evapotranspiration (ET_EC, R²: 0.69−0.92). The inter-annual variation of annual GPP_VPM tracked well that of annual GPP_EC, with the linear regression slopes for GPP_EC versus GPP_VPM-EC ranging from 0.89 to 1.11. The simulation results of GPP_VPM and T_VTM using two different climate data sets (in situ climate data and European Center for Medium-Range Weather Forecasts Reanalysis v5 data set (ERA5)) were similar, suggesting that ERA5 data can be used for VPM/VTM simulations at large spatial scales. From 2000 to 2021, annual GPP_VPM and T_VTM had no significant inter-annual trends at one savanna and three grassland sites but increased significantly at one savanna site. The results demonstrate the potential of using VPM (v3.0) and VTM (v2.0) to predict the seasonal dynamics and inter-annual variation of GPP and T in savannas and grasslands.

Graphical abstract

Keywords

vegetation photosynthesis model / vegetation transpiration model / ERA5 / MODIS / carbon fluxes

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