Atmosphere, Vegetation, and Soil Water Coupling Determined by Stomatal Regulation of Transpiration

Abstract Stomatal regulation plays a critical role in controlling tree water loss and mediating atmosphere–vegetation–soil water coupling, yet the implications of species-specific differences in stomatal regulation on this coupling remain poorly understood. Drimys species possess primitive leaf anatomy with limited stomatal closure capacity, while Nothofagus exhibits more effective stomatal control. We compared multi-year sap flux data from these two co-occurring Southern Chilean species to evaluate how stomatal traits influence water-coupling dynamics across timescales. Using boosted regression tree modeling and wavelet coherence analysis, we found that while both species showed similar functional responses to environmental drivers, the relative importance of these drivers differed between them. Both Drimys and Nothofagus responded to VPD, but Drimys sap flux was more strongly influenced by soil moisture, particularly during early season wet periods and late-season drought. In contrast, Nothofagus showed greater dependence on light and vapor pressure deficit (VPD), reflecting tighter stomatal regulation. Wavelet coherence analyses further confirmed stronger soil moisture control of sap flux in Drimys , especially at weekly to sub-seasonal timescales, and provided evidence that stomatal regulation can either buffer or amplify late-season soil moisture deficits. These findings suggest Drimys follows a high water use, low-conservation strategy closely tied to soil moisture, whereas Nothofagus demonstrates more conservative water use governed by atmospheric conditions. The strong soil moisture dependence of Drimys may increase its vulnerability to future warming and drying trends, with implications for forest composition and hydrological modeling in a changing climate.