The asynchronous response of carbon gain and water loss generate spatio-temporal pattern of WUE along elevation gradient in southwest China

Abstract. The ratio of carbon gain to water loss, or water use efficiency (WUE), is a key characteristic of ecosystem function, and reflects the trade-off relationship between carbon gain and evapotranspiration (ET). The carbon and water cycles are very complex at the ecosystem scale, especially along altitudinal gradients. A stable carbon isotope method and the AVIM2 model were used to study the temporal and altitudinal patterns of carbon gain, water loss and ecosystem WUE in dark coniferous forests (Abies fabri) in subalpine, Southwest China. WUE was defined using different components of carbon gain and water loss. The temporal and altitudinal variations of WUE were found. WUE in the vegetative season was larger than in the dormant season. Climatic conditions such as monthly precipitation, air temperature, vapor pressure deficit (VPD) and net radiation contributed to the monthly variation in ecosystem WUE. Both the carbon gain and water loss decreased as the altitude increased, and the carbon gain exhibited a moderately decreasing rate compared with that of the water loss along the altitude gradient. WUE exhibited an increasing trend along the elevation gradient, but discrepancies appeared when different definitions of WUE were used. Variations in WUE indicated different coupling relationships between the carbon gain and water loss on different temporal and altitudinal scales. The asynchronous response of the carbon gain and water loss to climatic and physiological variables determined the temporal and altitudinal patterns of ecosystem WUE.