Oak growth is increasingly constrained by drought across Romania

Oak species are sensitive to local site conditions, soil moisture levels, and increasing drought. The presence of a geophysical barrier, the location within the southeastern European distribution range, aridification, episodes of transcontinental oak decline, and inconsistent stand management create a unique setting for studying complex, time-dependent relationships between drought and growth. We conducted a detailed evaluation of growth trends and responses to climate using a full Bayesian framework. We built a robust database with 132 tree-ring width chronologies from 2998 Quercus robur and 2817 Quercus petraea trees across Romania. Here, the Carpathians separate continental, Mediterranean, and temperate air masses, creating a unique setting to assess the impacts of drought on tree growth, which has risen since 1985. The Bayesian framework unveiled spatio-temporal patterns and estimated space-varying growth-climate responses before and after 1985. A time-dependent and non-monotonic growth-climate response was found for both oak species, mainly associated with drought stress. Under warmer and drier conditions, West and South Q. petraea trees became more sensitive, while East and South Q. robur trees were decoupled and less sensitive, although the relationship became unstable. Intensified spring and summer droughts constrained growth across regions, thereby reducing resilience and causing growth declines in the most responsive sites. This study highlights the effectiveness and reliability of Bayesian statistics in managing large dendroecological datasets. Our findings demonstrate the susceptibility of oak stands to drought primarily in the South, an ecosystem characterized by limitations in both energy and water, and in the West, which is more complex and necessitates increased attention, as it indicates a transition from being energy-limited to water-limited. The results are used to inform future management strategies tailored to regional requirements, based on regional responses to aridification, thereby promoting the selection of mixed stands and monitoring of sensitive sentinel stands.