Winter insect defoliation modifies tree hydraulic responses to environmental stress in Mediterranean forests

Insect-driven defoliation can alter tree regulation of canopy water loss, thus affecting forest water, carbon and nutrient cycles. However, the significance of insect defoliation on tree water relations remains poorly understood in regions characterized by seasonal water deficits. Here, we evaluated whether tree hydraulic responses to seasonal variation in relative soil water content (RSWC) and vapor pressure deficit (VPD) were altered by the pine processionary moth ( Thaumetopoea pityocampa ; PPM), a main defoliator of coniferous forests in the Mediterranean Basin. We measured stem sap flux density (J s ) and calculated relative canopy stomatal conductance (G s ) in mature Pinus nigra trees with contrasting defoliation degrees over two years (2023–2024), alongside continuous measurements of VPD and soil moisture. Four important results emerged. First, medium- and high-defoliated trees exhibited a lower sensitivity to declining RSWC in shallow soil layers relative to low-defoliated trees. Second, high-defoliated trees exhibited a higher stomatal sensitivity to VPD when soil moisture was not limiting. Third, J s was generally higher in medium- and high-defoliated trees, with the highest contrasts achieved in spring when soil moisture was abundant. Finally, defoliation in 2023 resulted in legacy effects on seasonal water-use patterns in 2024, as evidenced by higher J s during peak physiological activity periods in medium- and high-defoliated trees. Overall, our study shows that PPM defoliation induces prolonged changes in water-use regulation that could ultimately amplify tree vulnerability to climate change and may require forest management practices to prevent high levels of defoliation.