Patterns of sessile oak regeneration and its main drivers in an old-growth sessile oak–European beech forest

Abstract Understanding the regeneration dynamics of sessile oak ( Quercus petraea ) under unmanaged forest conditions is crucial for sustaining oak-dominated ecosystems, particularly in the context of close-to-nature silviculture and climate change. In this study, we assessed the natural regeneration of sessile oak in a well–preserved old-growth forest reserve in Western Romania. Detailed inventories of seedlings and saplings across 27 canopy gaps of varying sizes revealed that while oak regeneration is currently abundant—accounting for 48% of total regeneration density–seedlings are predominantly confined to first regeneration development stage, with fewer individuals reaching sapling size. Principal component analysis and multiple regression modelling based on Boruta feature selection, identified light availability, browsing pressure, competition from European beech ( Fagus sylvatica ), and ground vegetation cover as critical factors influencing regeneration success, with their importance varying by development stage. Light was the principal driver for advanced regeneration stages (R 2 = 0.14), whereas seedling density was primarily shaped by browsing intensity and overstory oak composition (R 2 = 0.28). Despite the successful establishment of seedlings, the transition to larger size classes remains constrained by suboptimal light conditions and biotic competition, suggesting a bottleneck in sessile oak recruitment under the current disturbance regime. These findings highlight the need for targeted management interventions—such as promoting canopy openness, managing competing vegetation, and mitigating browsing pressure to support oak regeneration and ensure long-term stand sustainability.