Responses of tree water uptake to soil water content variability under combined threshold effects

In the context of global climate change, seasonal droughts significantly impact tree water use and soil water source contribution (WSC) in forest ecosystems. However, studies on the threshold effects of the soil water content (SWC) and previous precipitation on tree water uptake are scarce. In the study, the focus was placed on the two tree species Platycladus orientalis and Quercus variabilis , which grow in the mountainous areas of northern China. Using eco-hydrological monitoring data from 3 years, we investigated the nonlinear threshold relationship between the SWC and the WSC in different precipitation treatments (zero, half, natural, and double precipitation). A combination of structural equation modeling, random forest modeling, and S-shaped curve threshold analysis was applied to evaluate the indirect effects of previous precipitation on the SWC and WSC. S-shaped threshold analysis identified SWC thresholds spanning approximately 3–26 %; Quercus variabilis showed a consistent deep-layer transition at low SWC (φ1 = 6.3–8.4 % across zero, half and natural treatments), whereas Platycladus orientalis exhibited clear cross-layer thresholds mainly under half precipitation (φ1 = 7.0–9.8 %). Thresholds generally shifted upward under wetter conditions. Based on the results, SWC, as the dominant factor influencing the WSC, exhibited complex and significant threshold effects at different soil depths. The two species displayed contrasting water use strategies at similar threshold levels, effectively reducing competition for water. The indirect influence of previous precipitation on the SWC and WSC also varied significantly with soil depth and precipitation amount. The results of this study highlight the complex threshold effects of the SWC on WSC in different precipitation scenarios, providing a scientific basis for understanding forest water dynamics under climate change and developing adaptive management strategies. • Across treatments, SWC is the primary control on water source contribution (WSC). • Uptake thresholds shift upward from low (∼3–10 %) to high (∼12–26 %) SWC with increasing wetness. • P. orientalis and Q. variabilis show specific threshold patterns, suggesting niche differentiation . • Precipitation pulses trigger abrupt strategy shifts by moving SWC across critical thresholds.