Environmental and stomatal control on evapotranspiration in a natural oak forest

Abstract

Evapotranspiration (ET) is an important component in water budgets in forest ecosystems, and the associated latent heat flux has important implications for regional climate. However, biophysical control mechanisms on variations in ecosystem ET on intra‐annual scales may have year‐to‐year variations in different years with contrasting precipitation and soil water contents. Related research is lacking in natural oak forests in Central China. Based on eddy‐covariance technique, we investigated variations of ET and controlling factors in a natural oak forest in central China over three years (2017–2019) with contrasting soil water contents (SWCs) and precipitation. The ET of the oak forest was mainly affected by surface conductance (gs) in growing seasons, and the restriction from stoma on ET was aggravated in dry year. High vapour pressure deficit (VPD) promoted gs and ET in wet year but decreased gs and ET in dry year. There was a positive relationship between shallow soil water contents and gs in dry year but no relationship between the two variables in wet year. The ET were 738.1, 750.6 and 513.1 mm, respectively, for 2017–2019, with a coefficient of variation (CV) of 20%, while the corresponding precipitation (P) were 1239.4, 855.8 and 645.6 mm, with a CV of 33%. Annual ET was the lowest in the year with the lowest P, SWC and gs. Short‐term droughts (periods with relative extractable water content less than 0.1 for less than 15 days) only constrained gs and ET in short timescales (e.g., hours). Long‐term droughts (periods with relative extractable water content less than 0.1 for more than 15 days) evidently decreased the gs, thus reducing the ET and affecting energy partitioning in the corresponding period. ET/P were 0.6, 0.88 and 0.79 in 2017–2019, respectively, indicating that ET consumed most of the rain. The ratios of latent heat flux (LE) to net radiation (Rn) were 0.86, 0.88 and 0.6 during growing seasons in 2017–2019, respectively. Our study suggested that the natural oak forest has high ET rate and soil moisture affects the variations of ET.