Environmental Factors Driving the Transpiration of a Betula platyphylla Sukaczev Forest in a Semi-arid Region in North China during Different Hydrological Years

More and more droughts happened during the last decades, threatening natural forests in the semi-arid regions of North China. The increase in drought pressure may have an impact on stand transpiration (T) in semi-arid regions due to rising temperature and changes in precipitation. It is unclear how the transpiration of natural forest in semi-arid regions respond to drought, which is regulated by environmental factors. In this study, a relatively simple but mechanism-based forest stand T model that couples the effects of the reference T, solar radiation (Rn), vapor pressure deficit (VPD), and relative extractable water (REW) in the 0–80 cm soil layer was developed to quantify the independent impacts of Rn, VPD, and REW on T. The model was established based on the observed sap flow of four sample trees, and environmental factors were observed from May to September in different hydrological years (2015, 2017, 2018, and 2021) in a pure white birch (Betula platyphylla Sukaczev) forest stand in the southern section of the Greater Khingan Mountains, northeastern China. The sap flow data were used to calculate tree transpiration (Tt) and T to calibrate the T model. The results indicated that (1) The Tt sharply declined in the ‘dry’ year compared with that in the ‘wetter’ year. The daily Tt for small trees in the ‘dry’ year was only one-fifth of that in the ‘wetter’ year, and the daily Tt of large trees was 48% lower than that in the ‘normal’ year; (2) Large trees transpired more water than small trees, e.g., the daily Tt of small trees was 89% lower than that of the large trees in the ‘normal’ year; (3) Daily T increased with the increase in Rn, and the response conformed to a binomial function. Daily T responded to the rise of VPD and REW in an exponential function, first increasing rapidly, gradually reaching the threshold or peak value, and then stabilizing; (4) The driving factors for the T shift in different hydrological years were the REW in the ‘dry’ year, but the Rn and REW in the ‘wet’, ‘normal’, and ‘wetter’ years. The REW in the ‘wet’ and ‘wetter’ years exerted positive effects on T, but in the ‘normal’ and ‘dry’ year, exerted negative effects on T. Thus, the environmental factors affecting T were not the same in different hydrological years.