Characteristics of Changes in Sap Flow-Based Transpiration of Poplars, Locust Trees, and Willows and Their Response to Environmental Impact Factors

The sap flow and transpiration of three typical tree species (poplar, locust tree, and willow) in Ningxia are crucial for sustaining the ecosystem in the Ningxia Yellow River Irrigation area. However, there is a lack of clarity regarding the variations in sap flow and transpiration of these trees and their corresponding responses to environmental factors. From February to December 2021, this study selected 30 samples representing the three typical trees in the irrigation area and monitored their tree sapwood sap flow continuously and dynamically using the Thermal Diffusion Probe method. This study yielded several key findings: (1) Variations exist in sap flow density and transpiration among the three typical trees, with willows exhibiting higher sap flow density and transpiration than poplars and locust trees. (2) Tree transpiration showed a highly significant positive correlation with net radiation, temperature, and vapor pressure deficit, along with a highly significant negative correlation with relative humidity. (3) Soil moisture content undergoes changes under precipitation and artificial drip irrigation, but its correlation with tree transpiration is limited. (4) The primary environmental factors influencing poplars, locust trees, and willows are temperature, soil moisture content at a depth of 30 cm, and soil moisture content at a depth of 60 cm.