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

Mediterranean mountain regions are facing significant challenges due to climate change, including declining annual rainfall, prolonged dry spells, and increasingly frequent summer storms. These challenges pose serious threats to ecosystem resilience and the sustainable management of water resources and tackling them requires effective ecohydrological strategies. However, understanding water flow through the critical zone remains challenging due to the intricate water partitioning processes shaped by soil and vegetation heterogeneities. In an attempt to somewhat diminish this complexity, this study aims to investigate the water use dynamics of montane Scots pine (Pinus sylvestris L.) under varying wetness conditions by integrating ecohydrological data, stable water isotope (²H and ¹⁸O), and numerical modeling with Hydrus 1D.

We conducted a comprehensive plot-scale field investigation in the Vallcebre research catchments (NE Spain), monitoring two sets of three Scots pine trees since May 2022. Data collection included throughfall, sap flow, stem diameter variations, and soil water potential and content down to 70 cm depth, all at 5-minute intervals. Weekly sampling of different water pools (throughfall, bulk and mobile soil water down to 100 cm, groundwater, and xylem water) provided isotope data across the growing season of 2022. The analysis of these datasets revealed dynamic tree water uptake behavior, with shifts in source water contributions across variable wetness conditions. We observed that tree water uptake predominantly contained winter precipitation, even after a large summer storm delivering more than 60 mm of rainfall in a single day after a 20-day dry spell. However, later in the growing season, the isotopic composition shifted to reflect a roughly equal contribution from both summer and winter precipitation.

We used the Hydrus 1D model to test three distinct root distribution estimation methods and utilizing our field ecohydrological, and isotopic data as inputs. The simulations revealed that the choice of root distribution significantly influenced model performance. The model captured the patterns of soil moisture and atmospheric demand, particularly emphasizing how shifts in these factors influence tree water use efficiency and water stress responses. These findings demonstrate the importance of accurately representing root distribution in ecohydrological models to improve our understanding of tree water uptake processes. Our integrated approach provides a reliable framework for exploring the complex water dynamics in montane Scots pine ecosystems, offering insights into tree resilience under future climate scenarios.

Keywords: Ecohydrology; Soil-plant-water interactions; Stable isotopes; Modelling; Root distribution, Scots pine