Water stress controls on carbon flux and water use efficiency in a warm-temperate mixed plantation

Water stress can cause the structural and physiological changes of plants to prevent excessive water loss according to species-specific water use strategies. CO2 and water vapor fluxes were measured by the eddy covariance in a mixed plantation during a 4-year period. The potential evapotranspiration (PET) was obtained by the Shuttleworth-Wallace model. Evapotranspiration (ET) versus PET was used to determine the plant water stress index (PWSI). The objective of this study is to quantify the effect of PWSI on net ecosystem productivity (NEP), gross primary productivity (GPP) and water use efficiency (WUE). The relationship between PWSI and soil water content in sunny and cloudy skies was examined. Mean seasonal PWSI values varied substantially among years. The average annual value of PWSI was 0.49 +/- 0.07. PWSI was strongly correlated with vapor pressure deficit, illustrating that the moisture limitation through high evaporative demand was more remarkable than water supply limitation. Water stress can substantially reduce canopy conductance and leaf area index, and hence decreasing photosynthesis. Water stress suppressed GPP while having little influence on ecosystem respiration, resulting in the increase of net carbon uptake. NEP decreased as a reaction to water stress when PWSI was more than 0.4, whereas WUE was enhanced under moderate and severe water stress (PWSI > 0.6). Compared to NEP, the difference of ET between cloudy and sunny sky conditions was more evident at the same PWSI value. Therefore, cloudy sky conditions were beneficial to enhancing WUE. Water stress restricted the mixed plantation in the period of spring-early summer. At the annual scale, the mixed plantation was exposed to moderate water stress. The PWSI approach is a promising tool quantifying water stress in forest ecosystems.