Ten-year variability in ecosystem water use efficiency in an oak-dominated temperate forest under a warming climate
The impacts of extreme weather events on water–carbon (C) coupling and ecosystem-scale water use effi-ciency (WUE) over a long term are poorly understood. We analyzed the changes in ecosystem water useefficiency (WUE) from 10 years of eddy-covariance measurements (2004–2013) over an oak-dominatedtemperate forest in Ohio, USA. The aim was to investigate the long-term response of ecosystem WUEto measured changes in site-biophysical conditions and ecosystem attributes. The oak forest producednew plant biomass of 2.5 ± 0.2 g C kg−1of water loss annually. Monthly evapotranspiration (ET) and grossecosystem production (GEP) were tightly coupled over the 10-year study period (R2= 0.94). Daily WUEhad a linear relationship with air temperature (Ta) in low-temperature months and a unimodal relation-ship with Tain high-temperature months during the growing season. On average, daily WUE ceased toincrease when Taexceeded 22◦C in warm months for both wet and dry years. Monthly WUE had a strongpositive linear relationship with leaf area index (LAI), net radiation (Rn), and Taand weak logarithmicrelationship with water vapor pressure deficit (VPD) and precipitation (P) on a growing-season basis.When exploring the regulatory mechanisms on WUE within each season, spring LAI and P, summer Rnand Ta, and autumnal VPD and Rnwere found to be the main explanatory variables for seasonal varia-tion in WUE. The model developed in this study was able to capture 78% of growing-season variationin WUE on a monthly basis. The negative correlation between WUE and P in spring was mainly due tothe high precipitation amounts in spring, decreasing GEP and WUE when LAI was still small, adding ETbeing observed to increase with high levels of evaporation as a result of high SWC in spring. SummerWUE had a significant decreasing trend across the 10 years mainly due to the combined effect of seasonaldrought and increasing potential and available energy increasing ET, but decreasing GEP in summer. Weconcluded that seasonal dynamics of the interchange between precipitation and drought status of thesystem was an important variable in controlling seasonal WUE in wet years. In contrast, despite the neg-ative impacts of unfavorable warming, available groundwater and an early start of the growing seasonwere important contributing variables in high seasonal GEP, and thus, high seasonal WUE in dry years.