Deciduous forests play a crucial role in preserving forest biodiversity in Central Europe. In Poland, native oak forests – dominated by pedunculate oak Quercus robur and sessile oak Q. petraea – are of significant ecological and economic importance. Since natural forests are rare, managed stands may contribute to biodiversity conservation, provided their ecological characteristics are properly understood. Natural and old−growth forests are characterised by complex vertical stratification, frequent canopy gaps and large amounts of deadwood, which sustain a variety of ecological processes. However, even the oldest managed forests rarely achieve these characteristics. This study investigates how the forest vertical structure, deadwood stock and vascular plant species diversity change along an age gradient in managed oak stands aged between 40 and 180 years. The research was conducted in the Bory Stobrawskie mesoregion in southwestern Poland. Data were collected from 100 plots with a minimum of 50% oak cover. The following structural features were recorded: tree height and diameter (DBH), number of trees, proportion of oak trees, cover of forest layers. Moreover, deadwood objects, i.e. logs, stumps, and fine woody debris were recorded, and all vascular plant species and their cover were inventoried. Statistical analyses, including Spearman’s rank correlation and LOESS smoothing, were performed to determine the relationships between all features, species diversity and stand age. We identified several evident trends in forest structure, i.e. positive correlations between stand height and age, and between average DBH and age, and negative correlation between the number of trees in the high tree layer and stand age. Further results showed that the number of stumps per plot revealed a slight negative correlation with age. Plant species diversity decreased as the forest stands aged, with reductions observed across most forest layers. Most species exhibited declining abundance with age, particularly disturbance−tolerant trees and shrubs, whereas only a few shade−tolerant or competitive species increased in abundance. Minor changes in vertical structure and species diversity suggest that managed old forests may not offer the same level of conservation as old−growth forests. While extending the lifespan of forests and allowing natural tree mortality would probably enhance biodiversity, this is not economically viable when forest management objectives are met efficiently. Nevertheless, given that our present study focuses on only a limited aspect of biodiversity, the importance of old managed forest stands in sustaining biodiversity should not be overlooked.

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