Comprehensive Response of Daily Transpiration from Armeniaca sibirica Plantations to Meteorological and Soil Moisture/Temperature Conditions on the Semi-Arid Loess Plateau, China

Forest transpiration plays a vital role in the regional water budget and water supply security of the semi-arid Loess Plateau of northwest China. A thorough understanding and accurate predictions of the variation in the transpiration of forests with important tree species, e.g., Armeniaca sibirica (L.) Lam., are critical for land and water management. Owing to the extreme climatic seasonality and interannual variability, detailed information on the seasonal variation in the transpiration of Armeniaca sibirica plantations and its response to climatic and soil moisture/temperature fluctuations is limited. Therefore, in this study, the sap flux density and meteorological and soil moisture/temperature conditions were continuously monitored during the growing season (May to October) in 2019–2020. The results show the four following features: (1) The mean daily transpiration of the Armeniaca sibirica plantation was 0.31 mm·day−1; (2) the daily transpiration varied nonlinearly with increasing potential evapotranspiration (PET). Transpiration first increased rapidly until reaching the PET threshold of 4 mm·day−1 and then slowly increased within the PET range of 4–8.5 mm·day−1, but thereafter, it decreased slightly when PET exceeded 8.5 mm·day−1; (3) the daily forest transpiration varied with increasing relative extractable soil water content (REW) and soil temperature (ST) following a saturated exponential function; i.e., it first increased until reaching a threshold of 0.5 of REW or 14 °C of ST, but thereafter tended to stabilize; (4) models for estimating the daily forest transpiration were established. According to these models, PET had the greatest limiting effect (32.17%) on forest transpiration during the observation period, while REW and ST showed lower limits at 7.03% and 3.87%, respectively. The findings of this study are useful for understanding and managing the hydrological effects of forests in the semi-arid Loess Plateau as a typical dryland with seasonal droughts.