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

Abstract

Water use efficiency (WUE) is a key characteristic of ecosystem function, and reflects the trade-off relationship between carbon gain and water loss. The coupled relationship between carbon and water is complex for ecosystems, especially along elevation gradient. A foliage stable carbon isotope method and the AVIM2 model were used to study the temporal and elevational pattern of carbon gain, water loss and WUE in subalpine dark coniferous forests (Abies fabri). The results showed that monthly WUE in wet season was larger than in dry season. The diverse response curves between carbon gain, water loss and meteorological factors, such as monthly precipitation, air temperature (Ta), vapor pressure deficit (VPD) and net radiation (Rn), determined the monthly variation of WUE. Both the carbon gain (gross primary production, GPP; net primary production, NPP) and water loss (evapotranspiration, ET; transpiration, Et) decreased as elevation increased. The carbon gain exhibited moderately decreasing rate compared with that of water loss along the elevation gradient. Plant WUE (GPP/Et or NPP/Et) exhibited an increasing trend along the elevation gradient, comparing with ecosystem WUE (GPP/ET or NPP/ET) only increased from 2800 to 3500 m a.s.l. and slightly decreased as elevation continue increasing to 3700 m a.s.l. The ratio of canopy interception to ET determined the varied elevational pattern of WUE between plant and ecosystem scale. Air temperature contributed 79.27% and 68.66% for elevational pattern of GPP and transpiration (Et), respectively. While Ta and ci/ca (ratio of intercellular and air CO2 concentration) contributed 50.67% and 47.67 to the variation of GPP/Et along elevation gradient, respectively. The canopy height and canopy coverage were significantly negative with WUE along the elevation gradient. The asynchronous response of the carbon gain and water loss to climatic and physiological variables determined the temporal and elevational patterns of ecosystem WUE.