Turning points in the impact of earlier green-up on evapotranspiration and gross primary productivity in a semi-arid grassland watershed

Abstract

Warming-induced shifts in vegetation phenology affect ecosystem functions and the hydrological cycle. Research has so far focused on phenological trends, drivers and feedback across temperate and tropical forests, while their impacts on carbon assimilation and hydrological processes in temperate semi-arid grasslands remain poorly understood. Based on remote-sensing datasets from the Moderate Resolution Image Spectroradiometer (MODIS), we examined the trends in vegetation phenology, gross primary productivity (GPP) and evapotranspiration (ET) in the Chabagou watershed of the Loess Plateau, China, for 2001–2020. The results revealed that the start of the growing season (SOS) had advanced significantly between 2001 and 2020, at an average rate of –1.47 days yr−1 (p < 0.05), while the end of the growing season (EOS) had delayed non-significantly by about 0.04 days yr−1. SOS robustly regulated growing season length (GSL), which was extended by an average rate of 1.60 days yr−1. Regression analysis between phenology and two main climatic factors (temperature and precipitation) reveals that precipitation is the main climatic factor affecting SOS, while temperature plays a relatively important role in EOS delay in the study area. Consistent with the phenological trend, annual ET and GPP increased steadily from 2001 to 2020. Based on the Regional Hydro-Ecological Simulation System (RHESSys), we further investigated the net effects of phenological change on carbon and water fluxes. The simulation results indicated that, for our study site, the correlation between SOS and summer ET would transform from positive to negative once SOS had advanced to around 129 days of the year (DOY) (and 125 DOY for summer GPP). Summer ET and GPP increased as SOS advanced (and GSL extended) until the SOS reached the turning points, and decreased if the SOS (and GSL) advanced beyond the turning points. The mechanism of variations in water and carbon fluxes responding to phenology dynamics could be more complex than we acknowledged to date because the climate is now approaching turning points. We, therefore, suggest improving the dynamic phenologic models for better predicting trends of ecosystem carbon gain and water loss with ongoing climate change.