Title: Integrating ecohydrological, isotopic, and numerical approaches to assess water use by montane Scots pine under varying wetness conditions

The predoctoral researcher Loujain Alharfouch, from the Surface Hydrology, Ecology and Erosion group, will defend her thesis on 19th June at 10:30h in Escola Tècnica Superior d’Enginyeria de Camins, Canals i Ports de Barcelona (UPC, Campus Nord Building C1. Classroom: 002)

Directors: Jérôme Latron, Juan J. Hidalgo and Maarten Willem

Thesis Committee: Natalie Orlowski, Adrià Barbeta and Jean Vaunat

Abstract:
Ecohydrology provides a key framework for understanding interactions between vegetation and the hydrological cycle, particularly in forest ecosystems that regulate water fluxes and ecosystem resilience. In Mediterranean mountain regions, increasing climate variability and recurrent droughts are pushing forests close to their hydraulic limits, while uncertainties remain regarding how trees access and use water under contrasting environmental conditions.

This thesis investigates tree water uptake dynamics in Scots pine (Pinus sylvestris L.) in the Vallcebre experimental catchments (NE Spain) through the integration of high-resolution field observations, stable water isotopes, and process-based modeling. Results show that during droughts, tree water use is strongly constrained by soil water availability, with trees relying primarily on winter-recharged soil water and internal stem water storage. Summer rainfall contributed to tree water uptake only after sustained soil rewetting, highlighting the critical importance of winter precipitation for maintaining tree hydraulic functioning in Mediterranean montane forests.

Building on these findings, a physics-based, calibration-free root water uptake parameterization was developed and successfully reproduced soil water dynamics, outperforming commonly used empirical approaches. Independent isotope-based validation confirmed the robustness of the proposed framework. Overall, the integration of ecohydrological observations, stable isotopes, and process-based modeling provides a consistent and reliable approach for improving our understanding and representation of tree water uptake processes.