Controversies about successional dynamics of woodland communities have a long history, dating back to the classical debates between Clements and Gleason and continuing into the present. These debates have largely concerned the predictability or convergence of forest developmental trends as well as the relative importance of different mechanisms and forces driving forest succession. However, opportunities for rigorous testing of competing hypotheses are limited, mainly because plot‐based studies of forest vegetation spanning more than a decade are scarce and even fewer concern late‐successional stands. We exploit a unique long‐term dataset from mesic temperate forests of eastern Poland, spanning c. 80 years (1936–2012) in strictly protected, late‐successional woodlands assigned to seven different ‘structural types’. We use non‐metric multidimensional scaling to assess the stability of species composition over the study period. We examine the predictability of composition and change trajectories over time using Mantel statistics, and we examine the changes in distributions of dissimilarity indices to assess convergence or divergence at the examined time and spatial scales. Tree communities in Białowieża Forest have changed substantially over the last eight decades. Several species (aspen, birch, pine, oak, ash, maple and spruce) exhibited large decreases in density, while a few other species (especially hornbeam and lime) have increased in importance across a wide range of initial compositional types. Forest types recognized in earlier periods have become much less distinguishable. Metrics do not yet show clear successional convergence, mainly due to (a) decline in the previously broadly distributed spruce and (b) persistence of large individuals of intermediate, long‐lived species (even though these species lack significant regeneration). Synthesis. Late‐successional woodland communities of Białowieża Forest are clearly dynamic and do not show quasi‐equilibrial properties often assumed of old‐growth forests. Forest types previously recognized as distinct have become progressively less differentiated. Plausible explanations invoke alteration in competitive relationships due to complex changes in environment, including warming climate, N deposition, natural and human disturbance, and ungulate herbivory over 80 years. Hornbeam and lime have been favoured over spruce, pine, oak and birch, although potential successional convergence is far from complete at this time. In the absence of major disturbance, we expect future decades to show continued successional homogenization.

Forests, both natural and managed, provide a critical habitat for a significant part of global biodiversity. Among many different groups of forest biota, tree species occupy a special position as they create conditions upon which the existence of virtually all other forest organisms depends, either directly or indirectly. To permanently play this role, particular tree species must be demographically stable; i.e., their populations should be distinguished by the balanced, size-dependent proportions of individuals representing different developmental stages (from seedlings and saplings to mature and old trees). In this study, we examined the extent to which this condition is met in the managed part of Białowieża Forest in northeastern Poland, an important biodiversity hotspot in Central Europe. Comparison of species-specific equilibrium vs. actual size distributions revealed that almost half of all trees growing in Białowieża Forest represented “inappropriate” (i.e., occurring in excess compared to the balanced models) species and/or diameter ranges. The amount of deficits was also large (around 30% of the current tree number), concerning primarily the smallest trees. Considering this, we recommend targeted, active management strategies to restore the demographic balance of key tree species and, thus, to enhance the conservation of local biodiversity. We also indicate that the key elements of such strategies should be the gradual removal of trees from surplus diameter ranges and assisted regeneration of species with the greatest deficiencies in small diameter classes.

Forests, both natural and managed, provide a critical habitat to a significant part of a global diversity. Among many different groups of forest biota, trees occupy a special position. They act as foundation species that create conditions upon which the existence of virtually all other forest organisms depends, either directly or indirectly. To permanently play this role, trees must be demographically stable, i.e., their populations should be distinguished by the balanced, size-dependent proportions of individuals representing different developmental stages (from seedlings and saplings to mature and old trees). In this study, we examine the extent to which this condition is met in Białowieża Forest in northeastern Poland, an important biodiversity hotspot in Central Europe. Comparison of species-specific, equilibrium vs. actual size distributions revealed that almost half of all trees growing in Białowieża Forest represented "inappropriate", i.e. occurring in excess compared to the balanced model, species and/or diameter ranges. The amount of deficits was also large (around 30% of the current tree number), concerning primarily the smallest trees. To restore demographic balance of key tree species and thus, to enhance conservation of local biodiversity, we recommend targeted, active management strategies, including gradual removal of trees from surplus diameter ranges and assisted regeneration of species with the greatest deficiencies in small diameter classes.