[Characteristics of soil moisture limitation and non-limitation in the response of sap flow to transpiration driving factors].

Transpiration is a significant part of water cycle in forest ecosystems, influenced by meteorological factors and potentially constrained by soil moisture. We used Granier-type thermal dissipation probes to monitor xylem sap flow dynamics of three tree species (, , and ) in a semi-arid loess hilly region, and to continuously monitor the key meteorological factors and soil water content (SWC). We established the SWC thresholds delineating soil moisture-limited and -unlimited sap flow responses to transpiration drivers. The results showed that mean sap flux density () of and was significantly higher during period with higher soil moisture compared to lower soil moisture, while the difference in for between the two periods was not significant. We used an exponential saturation function to fit the relationship between the of each tree species and the integrated transpiration variable (VT) which reflected solar radiation and vapor pressure deficit. The difference in the fitting curve parameters indicated that there were distinct response patterns between and VT under different soil moisture conditions. There was a threshold in soil moisture limitation on sap flow for each species, which was identified as 0.129 m·m for , 0.116 m·m for , and 0.108 m·m for . Below the thresholds, was limited by soil moisture. Above these points, the normalized sensitivity index (NSI) for and reached saturation, while that of did not reach saturation but exhibited a significant reduction in moisture limitation. Among the three species, was the most capable of overcoming soil moisture constraints.