Hydro-meteorological forcing modulates sap flux dynamics in beech trees along a steep Mediterranean hillslope

Understanding how forest ecosystems respond to environmental stressors is crucial for forest management strategies in the climate-sensitive Mediterranean region. This study examines the spatio-temporal variability of sap flux in response to hydro-meteorological drivers along a steep hillslope in a mountain Mediterranean forest. Sap flux rates and soil moisture were measured in a pure European beech stand ( Fagus sylvatica L.) during the 2021 growing season (May-September) at three different locations along a steep hillslope in the Re della Pietra catchment, central Italy. Results indicated a distinct topographic gradient in physiological control. Transpiration in riparian trees was mainly limited by solar radiation and vapor pressure deficit, independent of soil moisture seasonality. Moving upslope, the trees shift from energy-limited to water-limited conditions as the growing season advances. By using an independent, replicated low-cost sensor network combined with moving-window correlation and regression-tree analysis, we determined soil moisture thresholds (0.15 m³m⁻³ and 0.17 m³m⁻³ at 15 and 35 cm depth, respectively) at which the relationship between sap flux and solar radiation decreased, making soil water availability the main factor influencing transpiration. Therefore, while soil moisture, mediated by topography, affected tree transpiration, emerging as the key predictor for hillslope positions, radiation was identified as the main factor influencing trees in the riparian position. Results show that forest stands act as a "dynamic ecohydrological mosaic" in which soil water redistribution dictates variability in ecological thresholds for the same species. This research emphasizes the importance of accounting for fine-scale topographic and soil moisture variations in hydrological models and precision forestry strategies to effectively forecast forest resilience under future climate change scenarios.