Differential Responses of Sap Flow to Environmental Factors Under Contrasting Rainfall Amounts During the Rainy Season in a Boreal Birch Forest

Abstract

Sap flow dynamics are critical for understanding how vegetation consumes water and adapts to environmental stress. The response of sap flow in boreal birch secondary forests to rainfall variations during the rainy season, however, has been inadequately explored. Our study indicated that photosynthetically active radiation (PAR) and vapour pressure deficit (VPD) are the primary drivers of sap flow density in birch trees across different diameter classes (F_ds: small trees, F_dm: medium-sized trees, F_dl: large trees). Soil water content (SWC) significantly reduces sap flow when it falls below the 0.18 cm³/cm³. Sap flow density increased with PAR and initially with VPD but plateaued at higher VPD levels due to saturation. A hierarchy of sap flow density was observed, with F_dl > F_dm > F_ds, each responding differently to PAR, VPD and SWC. With decreasing rainfall across rainy seasons, the influence of PAR on F_ds and F_dm weakened, while the influence of VPD strengthened. For F_dl, the impact of VPD peaked and then declined, while the influence of PAR showed an inverse pattern. In the dry season, F_dl was primarily driven by PAR and influenced by VPD and SWC, whereas F_ds was mainly controlled by VPD, with minimal effects from PAR and SWC. The response of F_dm to SWC was similar to that of F_dl, but it mirrored the response of F_ds to PAR and VPD. These findings suggest that sap flow in boreal birch forests may become increasingly susceptible to SWC stress as global climate change intensifies.