INFLUENCE OF SEASONAL DROUGHT ON ECOSYSTEM WATER USE EFFICIENCY IN A SUBTROPICAL EVERGREEN CONIFEROUS PLANTATION

Abstract

Seasonal drought (low precipitation combined with high atmospheric temperature) typically occurs during the active vegetative growth season in subtropical regions of southeast China. We examined the responses of water use efficiency (WUE) and its components, gross primary productivity (GPP) and evapotranspiration (ET), to water limitations and high atmospheric temperature (T-a) in a subtropical evergreen coniferous plantation. The effect of high temperature and low water availability on the water use efficiency (WUE) at the site was analyzed using six years (2003-2008) of eddy covariance flux observations. During the active growing season (June-August), GPP and ET did not decrease remarkably and successively unless extreme drought occurred, i.e. mean soil relative extractable water decreased below 0.4. Low precipitation and high T-a (26-32 degrees C) at the peak of the active growing season decreased GPP. Furthermore, the response of water consumption (i.e. ET) to high Ta and drought lagged behind that of carbon fixation (i.e. GPP). Whether daily WUE increases or decreases in response to drought depends on temperature. For the present study site, at daily time scale, variations of WUE during the active growing season were determined mostly by air temperature while at longer (annual) time scales the influence of solar radiation may exert greater control over interannual variation in WUE than air temperature.