Preserving natural biodiversity of forest ecosystems is one of the greatest challenges of modern, multifunctional forest management. Among the many different groups of forest organisms trees are of major importance as they often act as the so-called foundation species, i.e. species that create conditions on which the existence of virtually all other organisms depends, either directly or indirectly. The essential condition for trees to permanently play this role is their demographic sustainability, which is reflected in the balanced proportions of individuals representing different developmental stages (from seedlings and saplings to mature trees). The main goal of this study was to assess the possibility of tree species occurring in the Białowieża Forest (important biodiversity hotspot) to maintain the role of foundation species in a sustainable manner. Comparison of species-specific, equilibrium vs. actual size distributions revealed that almost half of all trees growing in the Białowieża Forest represent "inappropriate", i.e. occurring in excess compared to the balanced model, species and/or diameter ranges. Similarly high was also the amount of deficits, concerning primarily the smallest trees. It is suggested that, in order to permanently preserve a high biological diversity of the Białowieża Forest, active actions should be taken to bring the current tree diameter distributions as close as possible to the desired sustainable model.