Oaks and extremes: Contrasting responses of Quercus robur to drought in Southeastern Europe, shaped by the Carpathians

Recent extreme climate events have severely impacted forests worldwide. Deciduous forests, in general, and oak-dominated forests, in particular, are more frequently and severely affected by repeated and intensified drought events than other biomes. In this context, updated insights into oak responses to drought events are needed to understand their resilience and adaptability capacity in order to promote forest management practices that mitigate the adverse effects of climate change. To analyze the response of pedunculate oak (Quercus robur L.) to droughts, we used an extensive tree-ring network comprising more than 2100 trees from 90 sites across Romania and the Republic of Moldova. Given evidence that the Carpathian Mts. significantly influence regional climate patterns, we split our network into three clusters based on sites’ positions relative to the Carpathians: western, eastern, and southern sites. We used resilience indices, following Lloret et al. (2011), to quantify oak responses to droughts, while multinomial logistic models were used to assess the occurrence probability of positive or negative pointer years in growth rates in relation to the Standardised Precipitation-Evapotranspiration Index. Regarding the resilience index, we found no significant differences between the clusters; however, eastern sites exhibited lower resistance and higher recovery rates. By contrast, the western sites exhibited the highest resistance and the lowest recovery rate. Multinomial logistic models indicated that, at southern sites, there is a higher probability of negative pointer years during winter droughts, whereas spring droughts are associated with a higher probability at eastern sites. Overall, our findings highlight spatial differences in growth plasticity and drought adaptability of pedunculate oak in Southeastern Europe in relation to the Carpathian Mts.