Balancing heat and drought tolerance: evidence for physiological trade-offs in Quercus ilex

The combined occurrence of drought and heatwaves, known as hot droughts, poses a major threat to forest ecosystems by disrupting plant physiological processes and increasing tree mortality. Yet, the key physiological mechanisms underlying tree acclimation to hot droughts remain poorly understood, particularly where potential trade-offs between drought tolerance and thermotolerance may constrain acclimation. In addition, canopy microclimate, especially differences between sun-exposed and shaded leaves, may strongly modulate these responses but is rarely explicitly considered.

We investigated how acclimation to long-term precipitation exclusion (>20 years) in mature Quercus ilex trees affects their drought and heat tolerance. During the summer of 2025 (June, July, and August), we assessed physiological responses under control and drought treatments in sun-exposed and shaded leaves. Key measurements included thermal tolerance of photosynthesis and cell integrity (T_EL), gas exchange, and plant water status.

We observed clear site- and treatment-dependent differences in thermal tolerance. Overall, control trees exhibited higher T_EL than droughted trees, although the magnitude and direction of this effect varied between sites. In contrast, plant water potential showed limited treatment effects, potentially indicating hydraulic acclimation to long-term drought. Across both sites, sun-exposed and shaded leaves differed markedly in thermal tolerance, underscoring the role of microclimate. In droughted trees, sun-exposed leaves had higher T_EL than shaded leaves in Spain, but in France, shaded leaves had higher T_EL than sun-exposed leaves. In the control treatments, shaded leaves consistently had higher T_EL at both sites.

Our results suggest that long-term drought acclimation alters physiological responses to heat stress in a canopy-position-dependent manner. While canopy microclimate strongly shapes thermotolerance, the extent to which drought and heat tolerance are linked by physiological trade-offs remains unclear. Understanding these interactions is critical for predicting forest resilience under future climate change.