Tree size and site environment affects sessile oak growth and intrinsic water-use efficiency response to wet-dry years

Drought is a significant global environmental stressor that impacts tree growth and survival, often causing substantial die-offs in temperate forests. Hence, contemporary forest management strategies increasingly aim to transition from spruce-dominated to mixed forest with more climate-resilient species such as oaks. Sessile oak (Quercus petraea (Matt.) Liebl.), a key forest tree species, is widespread across central Europe, but its response to climate extremes, especially individual intraspecific variability, remains poorly understood. In this study, we analysed tree-ring cores from 404 sessile oak trees in Thayatal National park, Austria. We assessed radial growth through tree-ring width and evaluated intrinsic water-use efficiency (iWUE) by analyzing δ¹³C in latewood from wet (1987) and dry (1994) years. Further, we investigated the effects of site conditions, specifically light and water status inferred from potential daylight duration and the topographic wetness index (TWI), on key tree characteristics such as diameter at breast height (DBH), height, and the height-to-diameter ratio (HDR). We then examined how these factors affected iWUE and radial growth under wet and dry years, assessing radial growth's resistance, resilience, and recovery. As expected, iWUE was increased and radial growth was decreased during the dry year compared to the wet year, but there was high intraspecific variability. Tree age and size influenced these responses; age positively affected growth during the dry year, followed by decreased drought recovery. Increased daylight duration improved iWUE and reduced growth during the dry year but supported drought recovery. During the dry year, iWUE positively affected growth and resistance to drought. The TWI correlated with increased tree height and DBH but did not affect iWUE and growth responses under extreme years. Our findings suggest promoting mixed-age stands could enhance forest resilience against drought. Moreover, forest management strategies should integrate specific environmental conditions, including light availability and water status, to effectively mitigate impact of climate extremes.