Alharfouch et al. (2025) Ecohydrological and isotopic insight into Mediterranean montane Scots pines water use dynamics under different wetness conditions

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2025
• Date: 2025-10-11
• Authors: Loujain Alharfouch, Pilar Llorens, Marius G. Floriancic, Francesc Gallart, Jesús A. Castro-López, Juan J. Hidalgo, Jérôme Latron
• DOI: 10.1016/j.ejrh.2025.102791

Research Groups

• Institute of Environmental Assessment and Water Research (IDAEA-CSIC), Barcelona, Spain
• Department of Civil, Environmental and Geomatic Engineering, ETH Zürich, Zürich, Switzerland
• Department of Environmental Systems Science, ETH Zürich, Zürich, Switzerland

Short Summary

This study investigated Scots pine water use dynamics in a Mediterranean montane environment by integrating high-resolution hydrological monitoring with stable water isotope data. It found that Scots pines predominantly sourced water from winter precipitation tightly bound in small soil pores, even after large convective summer precipitation events, highlighting the critical importance of winter precipitation for sustaining hydraulic functioning in drought-prone ecosystems.

Objective

• To understand how variations in meteorological and soil hydrological conditions affect Scots pine tree water use in a Mediterranean montane environment.
• To provide insights from stable water isotopes regarding Scots pine water uptake sources and the seasonal origin of water available for transpiration.

Study Configuration

• Spatial Scale: Vallcebre research catchment, Pyrenees, NE Spain. Study plot of 174 square meters at approximately 1300 meters above sea level, with a Scots pine density of ~1839 trees per hectare. Six individual trees were monitored.
• Temporal Scale: One growing season (May to October 2022) for ecohydrological monitoring. Weekly water sampling for stable isotopes was conducted from July 4 to October 10, 2022 (14 sampling dates). High-resolution (5-minute to 10-minute intervals) measurements were collected for meteorological, soil, and tree water use variables.

Methodology and Data

• Models used:
  • Seasonal Origin Index (SOI) for assessing the seasonal dynamics of water sources.
  • Penman-Monteith equation for calculating reference evapotranspiration (ET₀).
  • TREX R package for processing sap flow data.
  • Double regression method (Lu et al., 2004) for heat index (K) in sap flow calculations.
  • Zero-growth concept (Zweifel et al., 2016) for partitioning stem diameter changes into irreversible growth and reversible water-related changes (Tree Water Deficit, TWD).
• Data sources:
  • Field Monitoring: Sap flow (Granier thermal dissipation sensors), tree water deficit (metal band dendrometers), soil water content (TDR probes at 10, 20, 30, 40, 40-70 cm depths), soil water potential (probes at 10, 20, 40 cm depths), throughfall (tipping-bucket rain gauges), groundwater level (pressure sensor), and meteorological variables (air temperature, relative humidity, net radiation, wind speed and direction, bulk precipitation).
  • Isotopic Sampling: Weekly samples of throughfall, bulk soil water (at 10, 20, 30, 40, 60, 80, 100 cm depths), mobile soil water (suction cup lysimeters at 10, 20, 30, 40, 60 cm depths), groundwater, and xylem water (from twigs).
  • Isotopic Analysis: Cryogenic vacuum distillation for water extraction, followed by analysis using a Picarro L2120-i analyzer for δ¹⁸O and δ²H isotopic signatures.
  • Statistical Analysis: Shapiro-Wilk, D’Agostino-Pearson, Student’s t-test, Mann–Whitney U test, Kruskal–Wallis test, Dunn test with Bonferroni correction, Wilcoxon signed-rank test, and Spearman’s rank correlation coefficients.

Main Results

• Scots pine sap flow (SF) was significantly limited by low soil water potential (SWP) during drought, showing strong negative correlations with SWP at all measured depths.
• Tree water deficit (TWD) was strongly and positively correlated with SWP during dry periods, indicating that soil water availability was the primary control on tree hydration status.
• After rewetting, SF became less limited by SWP and more coupled with vapor pressure deficit (VPD), but plateaued at VPD values above approximately 1.5 kilopascals, suggesting an internal hydraulic limitation.
• TWD decreased rapidly after minor precipitation events and reductions in atmospheric demand, even before significant soil rehydration, indicating reliance on internal stem water storage. Stem growth, however, only resumed after several weeks of sustained wet conditions and major stem rehydration.
• Stable isotope analysis revealed significant differences between bulk soil water and mobile soil water/groundwater/throughfall. Mobile soil water, groundwater, and throughfall isotopic compositions were similar, reflecting rapid movement of recent precipitation.
• Bulk soil water was isotopically distinct and consistently dominated by winter precipitation across all depths (10-100 cm).
• Xylem water isotopic signatures were similar to shallow bulk soil water (10-20 cm) during the dry period.
• The Seasonal Origin Index (SOI) indicated that Scots pines predominantly sourced water from winter precipitation (negative SOI values) during the dry period. After intense summer precipitation events, xylem water SOI values gradually shifted towards a mix of winter and summer precipitation (SOI values around zero), but still showed a sustained reliance on winter-dominated sources.
• This suggests Scots pines preferentially access stable water pools (tightly bound water in small pores) primarily recharged during winter, rather than transient summer precipitation inputs.
• Ecohydrological separation was observed, with mobile soil water and groundwater linked to recent summer precipitation, while bulk soil water and xylem water were linked to older, winter-dominated sources.

Contributions

• Provided an integrated ecohydrological and isotopic understanding of Scots pine water use dynamics and drought recovery strategies in a Mediterranean montane environment.
• Demonstrated the time-scale dependence of Scots pine water use responses, highlighting the importance of short-term analyses for capturing physiological adjustments to stress and recovery.
• Revealed that Scots pines rely on internal stem water storage during early drought recovery and prioritize transpiration and stem rehydration over growth.
• Applied the Seasonal Origin Index (SOI) to show that Scots pines predominantly source water from winter precipitation stored in small soil pores, even when recent summer precipitation is available.
• Reinforced the concept of ecohydrological separation, illustrating distinct water transport pathways for groundwater recharge versus tree water uptake in Mediterranean mountain ecosystems.

Funding

• MICIU/AEI/10.13039/501100011033 through grants PID2019–106583RB-I00 (RHYSOTTO) and PID2019–106887GB-C31 (HydroPore).
• MICIU/AEI/10.13039/501100011033 and FEDER/UE through grants PID2022–141868NB-I00 (WARMed) and PID2022–137652NB-C41 (HydroPore II).
• Project 202330E155 (Severo Ochoa–IDAEA), funded by the Spanish Research Council (CSIC).
• COST Action Water isotopes in the critical zone: from groundwater recharge to plant transpiration (WATSON), CA19120.

Citation

@article{Alharfouch2025Ecohydrological,
  author = {Alharfouch, Loujain and Llorens, Pilar and Floriancic, Marius G. and Gallart, Francesc and Castro-López, Jesús A. and Hidalgo, Juan J. and Latron, Jérôme},
  title = {Ecohydrological and isotopic insight into Mediterranean montane Scots pines water use dynamics under different wetness conditions},
  journal = {Journal of Hydrology Regional Studies},
  year = {2025},
  doi = {10.1016/j.ejrh.2025.102791},
  url = {https://doi.org/10.1016/j.ejrh.2025.102791}
}