Long−term dynamics of changes in birch stand density on a drained fen peatland – case study of Bagno Wizna I reserve (NE Poland)

Peatlands are among the rarest and most valuable ecosystems due to their high biodiversity and crucial ecosystem service functions, particularly carbon sequestration and water regulation. However, during the twentieth century, many peatlands were extensively drained for agricultural and forestry purposes. Therefore, their hydrological regimes were profoundly and permanently altered leading to degradation of the peat deposit and substantial transformations of the vegetation cover including forest ecosystems established on organic soils. This study examines the long−term dynamics of structural changes in a sixty−year−old forest stand composed of Betula pubescens and B. pendula located on a degraded fen peatland in the Bagno Wizna I nature reserve (Narew River valley, north−eastern Poland). In addition to assessing stand disintegration, the study evaluates the practical applicability of various remote sensing techniques for chronosequence analysis and the investigation of environmental dynamics. The research was conducted using multi−temporal remote sensing data acquired from airborne laser scanning (ALS) and low−altitude unmanned aerial vehicle (UAV) surveys combined with relevés (phytosociological sample plots). The Individual Tree Detection (ITD) method was applied to quantify changes in stand structure. Between 2011 and 2023, the number of detected trees decreased by more than 22%, while total canopy area declined by approximately 30% indicating substantial stand degradation. In contrast, mean and maximum tree height slightly increased reflecting selective mortality and structural reorganisation. Such patterns represent a natural and expected process of stand development, but in this case, they are likely amplified by long−term disturbance conditions under which the stand developed. The spatial pattern of decline was heterogeneous with more pronounced canopy loss observed in the forest interior than near reserve edges. Furthermore, phytocoenotic analyses revealed a decline of more than 50% in floristic species diversity over the past two decades accompanied by the disappearance of several characteristic fen species. No natural tree regeneration was recorded. The ground layer dominated by nitrophilous herbaceous species, particularly Urtica dioica, likely suppresses seedling establishment and contributes to the interruption of forest continuity. A chronosequence of disturbance processes was reconstructed suggesting that hydrological regime alteration, groundwater instability, and species composition not fully adapted to site conditions are the primary drivers of stand decline. 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