Combined effects of drought intensity and heat stress impair physiological performance and recovery capacity in Populus nigra L. seedlings.

Climate change is increasing the frequency and intensity of droughts and heatwaves, posing a major threat to forest regeneration and tree survival. Young trees are particularly vulnerable to these stressors due to their limited root systems and reduced physiological buffering capacity. In this study, we investigated the individual and combined effects of progressive drought intensity and elevated temperature on the physiological, morphological, and oxidative stress responses of Populus nigra seedlings, as well as their capacity to recover following rewatering. Two-year-old seedlings were grown under controlled optimal (20-25 °C) or high (30-35 °C) temperature regimes and subjected to moderate, severe, and extreme drought, followed by recovery phases. Plant performance was assessed through measurements of water relations, gas exchange, chlorophyll fluorescence, chlorophyll content, oxidative stress (O₂- accumulation), and aboveground growth traits (i.e., leaf area and biomass, and stem height and biomass). Both drought and heat stress individually impaired plant water status, photosynthetic efficiency, and growth, whereas their combination produced synergistic effects, leading to more severe physiological dysfunction, greater oxidative stress, and greater growth suppression. Recovery capacity depended strongly on drought severity and temperature conditions. Moderate drought allowed near-complete recovery of physiological function, whereas severe drought resulted in partial recovery, and extreme drought often caused irreversible damage and mortality, particularly under high temperatures. Physiological traits generally recovered faster than morphological traits, indicating that structural growth requires sustained carbon availability beyond initial functional restoration. Multivariate analyses revealed a shift from acquisitive to conservative resource-use strategies under increasing stress, with incomplete reorganization of trait relationships during recovery under high temperature. Our results highlight the vulnerability of young P. nigra seedlings to compound drought-heat stress and suggest that future climate extremes may severely constrain seedling establishment, forest regeneration, and ecosystem resilience in riparian and temperate forest systems.