Divergent forcing of water use efficiency from aridity in two meadows of the Mongolian Plateau

Abstract

The future climate change predicts higher drought risks in deteriorating the functions and services of dryland ecosystems. Interpreting the sensitivity of ecosystem water use efficiency (WUE) to aridity could provide important environmental implications to water conservation and carbon budgets of drylands. The effects of temporal aridity changes on WUE in semi-arid ecosystems, particularly in meadow steppes, have been poorly investigated. In this study, we used eddy-covariance data from two representative temperate meadows on Mongolian Plateau, Changling (2007-2015, China) and Bayan (2014-2018, Mongolia), to understand the inter- and intra-annual changes of WUE by evaporative fraction (EF), as well as the climatic and biotic controls on gross ecosystem production (GEP), evapotranspiration (ET) and partitioned evaporation (T) and transpiration (E). Results showed that the response of annual WUE to EF was positive in Changling meadow with dry atmosphere but insignificant in Bayan with cold climate; the contrasting WUEs were driven by divergent GEP and ET responses to EF in two meadows. Monthly T/ET increased linearly with EF in both meadows, but logarithmically with leaf area index (LAI). Bayan and Changling meadows had a similar instantaneous WUE (WUEp, GEP/T) that were insensitive to EF change. We also found that the frequency of growing season precipitation affected WUE at both sites. We conclude that contrasting GEP responses to EF determined the meadow WUE variations. The magnitude and changes in transpiration and evaporation provide further insights on the biophysical regulations (i.e., LAI) of ecosystem water use and WUEp. This study indicated that differences in ecosystem water yield, soil water status and sensitivities of GEP and ET partitioning jointly contributed to the divergent WUE at the two meadow steppes in this semi-arid region.