Soil water availability determining the responding mechanism of water use efficiency of an plantation ecosystem to extreme drought

Background The regional shortage of available water for a plantation has been becoming increasingly severe, due to global warming. The objective of this study is to monitor seasonal and interannual responding mechanism of water use efficiency (W_ue) of a plantation ecosystem to the periodic or extreme drought events of soil conditions.

Methods The eddy covariance technique and the system of micrometeorological sensors were used to 1) statistically analyze carbon, water flux, and micro meteorological data, and 2) clarify the responding mechanism of W_ue of a plantation ecosystem to its major environmental factors, at different time scales, including daily, seasonal, annual, and interannual scales.

Results 1) The soil condition of this plantation ecosystem was very dry due to the low annual precipitation during the 3-year period (i.e., 2012–2014), and the evaporation efficiency of the ecosystem was increasing year by year, especially in 2014, in this year the evaporation efficiency was 1.6. 2) The carbon (gross productivity, G_ep) and water (evapotranspiration, E_t) variables in the ecosystem was closely coupled and linearly correlated. Daily G_ep and E_t values reached the maximum in summer, with daily maximum values of 14.4 g/(m²·d) and 10.2 mm/d; both occurring in year 2012, respectively. 3) Annual variation of the carbon/water coupling index, i.e., water use efficiency (W_ue), was 2.2–2.9 g/kg among these years, with (a) the maximum occurring in autumn, (b) the values of W_ue in rainy days being higher than those in non-rainy days, and (c) the highest value of a day being 8.2 g/kg during the morning period of 07:00–08:00. 4) Soil water availability in spring was more significant than those in summer or autumn for adjusting and improving W_ue values. 5) High air temperature environment, combined with extreme drought in soil and low precipitation conditions during this 3-year period, causing the closure of vegetation stomata to reach the minimum level, of which the effect finally reducing G_ep more than E_t, and finally W_ue decreasing.

Conclusions The available water from the soil determines the seasonal and interannual variability of W_ue of an artificial forest ecosystem in response to climate change, under different air temperature conditions.