Study on the Water Mechanism of Sparse Grassland Decline of Ulmus pumila L.

Ulmus pumila L. occupies an important niche in arid ecosystems. This study aimed to investigate the sap flow characteristics of declining Ulmus pumila L. in arid regions and its relationship with environmental factors. During the 2023 growing season (June to October), continuous sap flow monitoring was conducted using thermal dissipation probes (TDPs) on Ulmus pumila L., along with measurements of soil moisture, air temperature, relative humidity, solar radiation, wind speed, and vapor pressure deficit (VPD). The results showed that when the sap flow rate of elm individuals reached 0.92 mL/cm2/h, the trees entered an extremely severe decline stage. Sap flow rates were significantly positively correlated with net solar radiation, relative humidity, VPD, and soil moisture, but negatively correlated with wind speed and real-time rainfall. VPD was identified as the key factor influencing sap flow across different decline stages, while solar radiation was critical in assessing the severity of decline. A weakened correlation between sap flow and solar radiation marked the onset of severe decline. Additionally, soil moisture exhibited a significant positive effect on sap flow rates overall. These findings not only advance our theoretical understanding of plant ecology in arid areas but also offer practical insights for managing Ulmus pumila L. decline, thus contributing to more sustainable resource management and environmental protection strategies.