Influence of immediate rainfall on sap flux in cypress trees in the Southwest hilly area of China

Rainfall intensity and duration pose significant challenges to the stability of plantations in mountainous areas prone to soil erosion. Accurately understanding how tree transpiration responds to atmospheric evaporation demands under real-time rainfall conditions is essential for enhancing our understanding of tree water use. Since 2015, the alder-cypress forest has been completely replaced by a pure cypress forest, which led us to employ the heat dissipation probe method to measure sap flux of cypress wood. To analyze the responses of cypress trees to real-time and delay-time rainfall, we utilized no- and seasonal autoregressive integrated moving average with exogenous variables models to analyze the responses of sap flux to external environmental factors. Our analysis revealed that past values of sap flux and vapor pressure deficit jointly accounted for 79.6% (wet season) and 34.6% (dry season) of current sap flux values in the delay-time condition. However, under a real-time condition, past sap flux values and rainfall did not explain the variance in current sap flux values (R² = 7.01 × 10^{- 4}). This indicates a decoupling between present and past sap flux and the other three environmental factors in real-time conditions. The findings demonstrate significantly different hydraulic drive patterns of trees under real-time rainfall conditions, providing a new basis for forest managers to optimize irrigation plans and allocate water resources effectively.