Topography controls the response of beech trees to atmospheric demand during soil moisture droughts

Droughts can have significant impacts on forest ecosystems. To better understand how droughts affect tree transpiration across catchments, we studied the combined effects of landscape position (topography and related variability in soil moisture) and atmospheric demand (vapour pressure deficit, VPD) on sap flow velocities across a forested hillslope in central Italy. Our results show that sap flow velocities for trees on the upper part of the hillslope decreased during the hottest period in summer but remained relatively stable for the trees in the riparian zone. The variation in the mean daily sap flow velocity for trees on the hillslope was best explained by the variation in soil moisture, while for trees in the riparian zone, it was best explained by variations in VPD and temperature. The analysis of the time lag between daily peak VPD and peak sap flow velocity confirmed that the hillslope trees experienced moderate stress during the period with low water availability. On the contrary, the wet riparian zone limited tree water stress and early stomatal closure in summer. These results highlight the need to account for topography and related hillslope scale differences in soil moisture when analysing the response of forests to droughts and when simulating the effects of soil moisture on transpiration in catchment-or landscape-scale ecohydrological models. However, these results need to be validated for different hillslopes and different tree species.