Carbon and water fluxes are more sensitive to drought than heat in terrestrial ecosystems in China

Abstract

Climate variables such as heat and drought are generally regarded as the main factors regulating carbon, water, and energy fluxes (i.e., net radiation (Rn), latent flux (LE), and sensible heat fluxes (H)) in terrestrial ecosystems. Heat and drought stress occur more frequently in conditions of higher temperatures and more extreme hydrological cycles. Our research focuses on the joint analysis of both energy partitioning and temperature and evaporative fraction (EF = LE/(LE + H)) anomalies to reveal the responses of carbon and water fluxes to climate anomalies in terrestrial ecosystems with different plant functional types (PFTs) and in different climate zones in China. In this study, eddy covariance (EC) data were used to evaluate differences in Bowen ratio (β = H/LE) values and energy partitioning (H/Rn and LE/Rn) among different sites, PFTs, and climate zones. Energy exchange in the studied ecosystems was characterized by high values of β (mean values of 0.98, 0.88, and 0.91 among the different sites, PFTs, and climate zones, respectively), high LE/Rn ratios (mean values of 0.42, 0.45, and 0.43, respectively) and low H/Rn ratios (mean values of 0.32, 0.29, and 0.31, respectively). Furthermore, to evaluate the sensitivity of carbon and water fluxes in ecosystems in China to anomalies in temperature and water availability, a series of heat and drought sensitivity indices based on the daily temperature and evaporation fraction were developed. The responses of gross primary productivity (GPP) and evapotranspiration (ET) in different PFTs to temperature and EF anomalies were quantified. The results suggest that in terrestrial ecosystems in China, carbon and water fluxes are more sensitive to drought than to heat. Our study recommends that validated sensitivity indicators should play a vital role in quantifying the response of ecosystem carbon and water fluxes to climate anomalies.