The Influence of Precipitation and Temperature on GPP in a Semi-arid Grassland

Abstract

As a continuous drying trend is predicted in California, changes in precipitation and temperature may significantly impact various ecosystem processes, and subsequently, the gross primary production (GPP), a measure of the amount of carbon assimilated by plants from the atmosphere. This study investigates the sensitivity of GPP to variations in precipitation and temperature in a semi-arid grassland in California by analyzing AmeriFlux tower observation and using the process-based biogeochemical model, Biome-BGCMuSo. Analyses of the observation demonstrate that GPP is affected by both precipitation and temperatures, but precipitation plays a larger role than temperatures in controlling soil moisture, vapor pressure deficit (VPD), and incoming shortwave radiation in spring, and therefore, annual GPP; compared to summer, fall and winter seasons, spring GPP dominates annual GPP. Over the period 2002 – 2019, Biome-BGCMuSo-simulated GPP is in good agreement with eddy covariance tower-based GPP. Model results also reveal that precipitation and temperature primarily affect GPP by regulating leaves’ stomatal conductance through their influences on soil moisture and VPD. Precipitation reduction and temperature rise projected by CMIP5 North America Coordinated Regional Climate Downscaling Experiment (NA-CORDEX) under the high representative concentration pathway (RCP 8.5) scenario decreased annual GPP by 8% and 7%, respectively, when applied separately, and 16% when applied together. These results suggest a reduction of terrestrial primary production in the semi-arid ecosystems when the climate warms.