Functional and antioxidant responses to natural heat stress in Fagus sylvatica and Quercus robur: effects of ecological adaptation and ontogenetic stage

Heat waves and prolonged droughts affect the ecological and physiological processes of forest trees, yet their responses vary by species and ontogenetic stage. In this study, we analyzed the functional and antioxidant responses of seedlings and mature trees of Fagus sylvatica (shade-tolerant species) and Quercus robur (light-demanding species) after a natural heat stress event and during a 10-day recovery period. Leaves collected immediately after the heat wave were maintained under controlled laboratory conditions. Photosynthetic parameters (chlorophyll a and b, chlorophyll a/b ratio, carotenoids, and the effective quantum yield of photosystem II) and antioxidant parameters (catalase activity and total phenolic content) were measured. Significant differences were observed between species and ontogenetic stages. The mature F. sylvatica exhibited limited recovery of photosynthetic pigments and a marked reduction in catalase activity under heat stress. In contrast, the mature Q. robur maintained high ΦPSII values and initially exhibited elevated total phenolic content, suggesting effective protection of the photosynthetic apparatus and a more robust antioxidant capacity under the conditions analyzed. The F. sylvatica seedling showed rapid recovery of chlorophyll a and ΦPSII, with higher amplitudes of photosynthetic and antioxidant parameters compared with the mature tree. The recovery index highlighted ontogenetic differences: the F. sylvatica seedling displayed rapid recovery of chlorophyll a (+15.2%) and ΦPSII (+19.5%), whereas the mature tree showed minor increases in chlorophyll b and ΦPSII, along with decreases in chlorophyll a (-3.3%) and carotenoids (-10.3%). In Q. robur, the mature tree maintained pigment stability and high ΦPSII values, while the seedling initially had higher chlorophyll and carotenoid amounts but a reduced functional recovery of ΦPSII and antioxidant response. Overall, responses to heat stress and recovery dynamics depend strongly on ontogenetic stage and on the specific functional and antioxidant traits measured, indicating that general ecological strategies of species do not fully reflect actual physiological performance under natural stress. This approach provides relevant insights for assessing forest resilience and developing adaptive management strategies under variable climatic conditions.