Long-Term Changes in Species Composition, Stand Structure, and Natural Regeneration in Selected Oak-Rich Nature Reserves of Northeastern Poland

Research Highlights: Long-term observations reveal a shift in stand species composition and regeneration, including an increasing importance of shade-tolerant species and weak oak recruitment in protected oak–hornbeam forests of northeastern Poland, highlighting the declining role of pedunculate oak. Background and Objectives: Natural forest dynamics and regeneration are important for biodiversity and the conservation objectives of protected areas. We assessed long-term changes in species composition, stand structure, and regeneration in selected nature reserves containing mature oaks (Quercus robur L. and, in Stara Dębina, potentially Q. petraea (Matt.) Liebl.). Materials and Methods: Archival records and repeated field measurements from 1970–2019 were analysed for 14 permanent plots in five nature reserves: Stara Dębina, Grąd Radziwiłłowski, Koryciny, Kalinowo, and Rycerski Kierz. Tree density, basal area, species composition, dead wood, and regeneration of seedlings and saplings were assessed. Changes were evaluated using non-parametric statistical tests. Results: Stand structure generally changed only slightly, while hornbeam (Carpinus betulus L.) increased in importance in several reserves. Oak regeneration was scarce, and the density of upper saplings declined in most reserves, indicating weak recruitment into the advanced regeneration layer. Maple, lime, elm, and hazel frequently became more prominent in younger stand layers. Conclusions: The observed scarcity of advanced oak regeneration suggests limited recent recruitment on many monitored plots but does not establish the long-term fate of oak populations or the causes of regeneration failure. Continued monitoring and assessment of potential limiting factors are needed before site-specific conservation interventions are considered.