Response of tree sap flow rate to soil water and atmospheric environment, and adaptability to drought in the Loess Plateau region of China

Due to continuous changes in climate, the impacts of the environment on tree transpiration and productivity are becoming stronger. However, the interaction between soil and atmospheric drought with transpiration in plantation forests is still unclear. Understanding the changes in the sap flow rate (Fs) and environmental control is very important for the sustainable and healthy development of plantation forests. In this study, we selected Mongolian pine and Chinese arborvitae plantation forests to examine the impacts of the environment on Fs during the growing season (May–September) from 2022 to 2023 in order, to determine the soil water content (SWC) threshold for drought stress and to analyze the responses of plantation forests to drought periods in the Loess Plateau region of China. The results showed that in sandy and loess areas, soil water was replenished after abundant rainfall and the depths of soil water replenishment reached 2 m and 1.2 m, respectively. In addition, during the dry season, trees could transfer water from deep moist soil to shallow dry soil through hydraulic redistribution. The seasonal distribution of Fs was controlled by irregular precipitation events and the associated soil water supply. The sap flow rate/vapor pressure deficit ratio (Fs/VPD) was significantly correlated with SWC, and Fs was significantly correlated with solar radiation (Rs) and VPD. The transpiration amounts in four plantation forests during the growing season totaled 313, 214, 198, and 328 mm in 2022, and 325, 268, 237, and 346 mm in 2023 (from north to south respectively). The percentage of transpiration relative to rainfall in plantation forests ranged from 44% to 94%. Precipitation may not be able to meet the water requirements of plantation forests. The SWC threshold trended to increase from north to south, with values of 0.04, 0.04, 0.05, and 0.07 cm3 cm–3, respectively. Both tree species showed a tendency towards isohydric behavior. On sunny days, the Fs for Mongolian pine peak at around 11:00 AM, which was 2 h earlier than the peak for Rs and 3–5 h earlier than the peaks for VPD and air temperature (Air T). The drought sensitivity of Mongolian pine was determined by the soil moisture availability. The increasing water requirements of forests and increasingly arid climate will accelerate soil water depletion, hinder gas exchange in trees, and inhibit tree growth. Therefore, it is necessary to focus on water consumption and sustainability issues in forests in the Loess Plateau region and areas with similar climates around the world.