Ecohydrological and isotopic insight into Mediterranean montane Scots pines water use dynamics under different wetness conditions

Study region: The study was conducted in the Vallcebre research catchment, Pyrenees, NE Spain. Study focus: Mediterranean mountain forests are increasingly facing altered precipitation regimes and extended droughts, yet the seasonal dynamics of tree water uptake under varying wetness conditions remain insufficiently understood. This study investigates Scots pine (Pinus sylvestris L.) water use dynamics in a Mediterranean montane environment by integrating high-resolution hydrological monitoring with stable water isotope data. Throughout one growing season, we monitored sap flow, tree water deficit, soil water content and potential and meteorological variables. We sampled key water pools - (throughfall, soil water, groundwater, and xylem water) weekly – and analyzed their δ¹ ⁸O and δ²H isotopic signatures. The Seasonal Origin Index (SOI) was calculated to trace the seasonal origin of tree water sources. New hydrological insights for the region: During drought, tree water uptake was primarily constrained by low soil water potential. Stable isotope and SOI analyses indicated that Scots pines predominantly sourced water from winter season precipitation tightly bound in small soil pores, even after large convective summer precipitation events. Only after sustained rewetting xylem water reflected a partial shift towards more recent (summer season) precipitation. Our findings highlight that trees preferably root in more stable water pools rather than sourcing transient precipitation inputs, highlighting the importance of winter precipitation for Scots pines sustaining hydraulic functioning in drought-prone Mediterranean mountain ecosystems.