Transpiration Dominates Ecosystem Water-Use Efficiency in Response to Warming in an Alpine Meadow

As a key linkage of C and water cycles, water-use efficiency (WUE) quantifies how much water an ecosystem uses for carbon gain. Although ecosystem C and water fluxes have been intensively studied, yet it remains unclear how ecosystem WUE responds to climate warming and which processes dominate the response of WUE. To answer these questions, we examined canopy WUE (WUEc), ecosystem WUE (WUEe) and their components including gross ecosystem productivity, ecosystem evapotranspiration (ET), soil evaporation (E), and plant canopy transpiration (T), in response to warming in an alpine meadow by using a manipulative warming experiment in 2015 and 2016. As expected, low- and high-level warming treatments increased soil temperature (T-soil) at 10cm on average by 1.65 and 2.77 degrees C, but decreased soil moisture (M-soil) by 2.52 and 7.6vol %, respectively, across the two years. Low- and high-level warming increased WUEe by 7.7 and 9.3% over the two years, but rarely changed WUEc in either year. T/ET ratio determined the differential responses of WUEc and WUEe. Larger T/ET led to less difference between WUEc and WUEe. By partitioning WUEc and WUEe into different carbon and water fluxes, we found that T rather than gross ecosystem productivity or E dominated the responses of WUEc and WUEe to warming. This study provides empirical insights into how ecosystem WUE responds to warming and illustrates the importance of plant transpiration in regulating ecosystem WUE under future climate change.