## 1 Introduction  
Drought acts as an independent stressor, depleting ecosystem water resources with devastating effects on the environment [1]. Drought reduces tree vigor, making forestsmore vulnerable to insects and pathogens [2]. Warmer temperatures during droughs can exacerbate water depletion through higher evaporative demands or through increased respiration of carbon starvation [3]. The natural mountain forests in semi-arid areas are widely affected by drought [4]. Due to large climatic gradients and interannual fluctuations in semi-arid regions and due to the uneven distribution of precipitation, large areas of white birch forests have experienced climate-driven decline and death [5; 6]. Tree transpiration plays a determining role in the water balance of forest stands and in water yields from forested catchments [7]. It is also central to the construction of an ecosystem-level water balance [8]. A better understanding of the effects of environmental factors on stand transpiration (T) will aid in the assessment of T changes under the influence of climate change. Therefore, the accurate calculation and prediction of T and its changing pattern, as well as revealing the relationship between T and environmental factors are helpful to deeply understand the relationship between forest and water and to provide guidance for local vegetation restoration and forest water management. Monitoring the response of tree species to growing season drought over several years allows a better understanding of not only how plants cope with extreme events but also how these events affect species [9]. The measurement of sap flow is widely used to evaluate water consumption over different time intervals [10; 11; 12]. In addition, this methodology has been used at different biological scales ranging from individuals [13] to the entire forest canopy [10; 14]. Sap flow is controlled by both environmental and physiological factors. Physiological factors mainly include tree size [15] and leaf area index (LAI) [16], while environmental factors mainly include meteorological factors such as air temperature, rainfall, vapor pressure deficit (VPD), and solar radiation (Rn), as well as the soil moisture content, edaphic features, groundwater table, and others [12]. Evaporative demand, available energy, and water availability can be reflected by environmental factors [12]. Meteorological factors are the main power source of tree transpiration, among which Rn and VPD have direct effects on T [17; 18; 19]. VPD reflects the level of atmospheric evaporative demand for water vapor, and Rn provides the available energy for sap flow [12]. Soil moisture is the main source of water for T, and the effect on T depends on the water available in the soil [20]. Soil water was reported as being important during periods of low rainfall [21; 22; 23], such that transpiration was reduced when the plant's available water in the soil was depleted [24]. REW (relative extractable water) reflects the water that can be absorbed and utilized by plant roots [25], and as REW increases, the water available to trees and the leaf water potential increase, as does transpiration [26]. However, at different scales, the environmental factors affecting tree sap flow will change significantly, which in turn affects tree transpiration (Tt) and T. At present, there are many studies on the responses of Tt and T to environmental factors at different time scales. For example, [PERSON] et al. [27] studied the transpiration of Qinghai spruce (_Picea crassifolia_ Kom.) among canopy layers and found that T was strongly influenced by potential evapotranspiration (PET) and soil moisture. A study reported by [PERSON] et al. [28] revealed that with increasing REW, T initially increased quickly and almost linearly. [PERSON] et al. [12] found that the key environmental factor controlling Aspen (_Populus aavidiana_ Dode.) sap flow in the subhumid mountainous region of North China was photosynthetically active radiation. [PERSON] et al.