Transpiration and water balance in Quercus ilex forests under different management

Climate change is intensifying drought stress in Mediterranean forests, threatening ecosystem functioning and long-term resilience. In holm oak (Quercus ilex) forests, thinning is thought to mitigate drought effects, but its impact on water balance is still uncertain, highlighting the need to understand management effects for climate adaptation.

This study was conducted during 2024–2025 in paired control and thinned holm oak stands in the Gúdar–Javalambre region in eastern Spain. Tree transpiration was monitored with sap flow sensors in five control and five thinned trees, while ecosystem evapotranspiration and gross primary productivity were measured using eddy covariance techniques. Individual sap flow measurements were upscaled to ecosystem level based on stand basal area.

Thinning strongly modified forest water partitioning and drought recovery dynamics. From the transpiration response, we identified three main phases: drought stress (02/2024–09/2024), early recovery (10/2024–05/2025) and late recovery (06/2025–12/2025). Stand transpiration differed strongly between sites, with the thinned stand showing lower drought sensitivity and higher transpiration during the recovery phases compared to the control stand. Additionally, transpiration contribution to evapotranspiration increased in the control site (58–81%) from drought to recovery, while remaining low and stable in the thinned site (16–21%). The significant site × period interaction showed that thinning effects depended on recovery phase. Although evapotranspiration-based water use efficiency was generally lower in the thinned site, transpiration-based water use efficiency was consistently higher, reaching 11.0–12.8 g C kg H₂O⁻¹ compared with 3.3–4.0 g C kg H₂O⁻¹ in the control.

Our results show that thinning reduces transpiration pressure while increasing carbon gain per unit of transpired water. These results suggest that thinning promotes a more conservative and efficient tree water use strategy in holm oak forests, enhancing drought resistance and post-drought recovery under increasing drought risk.