Clarifying the effects of potential evapotranspiration and soil moisture on transpiration in secondary forests of birch in semi-arid regions of China

Stand transpiration (T) is an important process in the forest hydrological cycle, influenced by environmental factors such as potential evapotranspiration (PET) and relative extractable water (REW). Clarifying the independent effects and interactions of environmental factors on T can help to better understand the mechanisms of ecohydrological control of forest water use. In this study, the independent contributions of PET and REW to T and their interactions were quantified through field observations, modeling, and factor analysis in the secondary birch forest of the southern Greater Khingan Mountains. First, observations were made of sap density, meteorological variables, and REW over three growing seasons. Second, a T-coupled model (R² = 0.68, NSE = 0.7) was developed based on long-term observations of T, PET, and REW. Finally, the factorial analysis showed that PET was the main driver affecting T changes compared to REW and the interaction between PET and REW. The relative contribution of PET to T was 7.8%, 11.1%, and 10.7% in 2017, 2018, and 2019, respectively. REW limited 1.8%, 12.3%, and 9.7% of T change in 2017, 2018, and 2019, respectively, compared to the mean of these three years. Our study provides a simple and feasible way to separate the effects of PET and REW on T change and may deepen the understanding of the effects of different environmental factors on T.