## 1 Introduction  
In the context of global climate change, the intensity and frequency of drought have been observed to increase, which is expected to lead to water shortages around the world [1]. As an important limiting factor for plant growth [2], water not only affects plant growth rate and species composition, but also plays an important role in plant photosynthesis and other physiological metabolic processes [3]. Moreover, plants are an important part of ecosystems that play an important role in regulating the climate and maintaining the carbon and water balance [4]. Therefore, it is of great importance to understand the impact of water shortage on plant growth adaptability in the context of global climate change. Water-use efficiency (WUE) is defined as the amount of carbon gained by plants per unit of water lost [5]. WUE not only reflects the intrinsic water consumption mechanisms of plants but also reveals the atmosphere-leaf interactions in the water and carbon cycles and their response mechanisms to environmental factors [6]. Studies on WUE involve different scales from leaf to ecosystem, and there are multiple methods to estimate WUE at different spatiotemporal scales. The eddy covariance method is widely used to analyze total primary productivity (GPP) across the ecosystem [7]. The thermal diffusion method is usually used at the single-plant scale to analyze the transpiration of the stand [8]. On the leaf scale, the traditional research method is the transient water-use efficiency (WUE\({}_{\text{inst}}\)) determined by the gas exchange method. However, this method can only measure the ratio of plant photosynthetic rate to transpiration rate at a certain point in time, which is greatly influenced by environmental changes, and it is difficult to explain the physiological response that occurs when plants are affected by meteorological factors [9]. In recent years, the development of stable isotope has provided a new method to study the WUE of leaves [10]. [PERSON] et al. [11] found a significant positive linear correlation between WUE and its carbon isotope abundance ratio of \({}^{13}\)C/\({}^{12}\)C, which provides the method using the stable carbon isotope ratio (\({}^{13}\)C) to indicate the plant WUE. The physiological information reflected by the stable isotope method is that the plant WUE changes over a certain period. For example, the \({}^{13}\)C of the soluble sugars in leaves that can indicate the response of leaf physiological conditions to environmental changes can be used to derive plant short-term water-use efficiency (WUE\({}_{\text{s}}\)) during their formation time (2-3 days) [12]. Compared with the gas exchange method, the method using WUE\({}_{\text{s}}\) has the advances of needing a smaller sampling amount and not causing harm to plants. Therefore, WUE\({}_{\text{s}}\) can directly reveal the plant response to the current climatic environment. Currently, a number of studies on plant-leaf WUE\({}_{\text{s}}\) have been conducted by researchers. [PERSON] et al. [13] and [PERSON] et al. [14] found that vapor pressure deficit (VPD) had the greatest effect on plant WUE\({}_{\text{s}}\) in the beech, Douglas-fir and Platycladus orientalis forests. [PERSON] et al. [15] found that the increase in the ratio of intercellular CO\({}_{2}\) concentration to ambient CO\({}_{2}\) concentration (C\({}_{\text{i}}\)/C\({}_{\text{a}}\)) can lead to a decrease in plant stomatal conductivity, improve Rubisco enzyme activity and carboxylation efficiency and consequently increase the net photosynthetic rate, which can significantly affect its WUE\({}_{\text{s}}\). However, [PERSON] et al. [16] found that leaf structure has the greatest effect on WUE\({}_{\text{s}}\) in poplar forests. [PERSON] et al. [17] and [PERSON] et al. [18] found that WUE\({}_{\text{s}}\) was significantly influenced by photosynthetic rate (Pn) and the leaf structure of plant leaves. However, there are few studies on the main factors affecting the variability of WUE\({}_{\text{s}}\) in different canopy leaves and their multiple factors.