Multi-year trends and interannual variation in ecosystem resource use efficiencies in a young mixedwood plantation in northern China

Understanding the long-term trends and interannual variability in ecosystem resource use efficiencies (RUEs) and their controlling factors is central to the prediction of ecosystem response to climate change. However, knowl-edge in this area, particularly as it pertains to young urban plantations, remains largely absent. In this study, we used continuous eddy covariance and site-specific climatic data collected from a 12-year-old mixedwood plan-tation in Beijing, China, to assess (i) long-term trends, (ii) interannual variability, and (iii) key controlling factors of RUEs with respect to plantation use of carbon (i.e., CUE), water (WUE) and light (LUE). On average, annual CUE and WUE over the 2012-2020 growing seasons increased by about 5.2 and 7.6%. Trends in RUEs corre-sponded with increases in normalized difference vegetation index (NDVI) when the plantation transitioned from a carbon source to a sink. Analysis of controlling factors indicated that RUEs were regulated by NDVI and other environmental factors at varying degrees dependent on season. Interannual variability in (i) CUE was largely controlled by spring soil temperature, (ii) WUE by summer water vapor pressure deficit and spring soil water content (SWC), and (iii) LUE by photosynthetically active radiation, summer SWC and spring air temperature. Relative seasonal changes in paired RUEs were positively correlated, with soil water availability being the most limiting. Long-term summer drought was shown to evoke severe and persistent decline in same-year RUEs. Ample mid-summer rainfall lessened the impact of early spring drought on plantation performance. Our results emphasized the need to account for the combined effects of climatic controls and canopy-physiological char-acteristics in the management of young plantations.