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Questions: Does the size structure characterizing particular tree species in natural forest stands converge towards a steady state? What is the main reason for departures of observed tree size distributions from equilibrium models? What is the consequence of long-term strict protection for the demography of tree species and the conservation of associated biota?Location: Stands of natural forest in Białowie_ za National Park (NE Poland) that have been under strict protection for about 90 yr.Methods: Data on recruitment, growth and mortality of approximately 10 000 trees representing 11 species (aspen, birch, alder, pine, oak, maple, ash, elm, spruce, lime and hornbeam), from seven replicated surveys of five permanent monitoring plots (total size = 15.44 ha), established in 1936, were used to derive theoretical, species-specific equilibrium distributions as a basis for evaluating the demographic changes of the species involved.Results: Over a monitoring period of 76 yr, there were strong downward trends in the population densities of several tree species of different successional status and light requirements. A vast majority of the conspecific equilibrium curves, modelled on the basis of growth and mortality data, revealed a 'rotated sigmoid' shape when plotted in semi-log plots. In contrast to these equilibrium curves, the observed tree size distributions of most species were unimodal, at both the beginning and the end of the study period. Departures between theoretical and observed distributions increased over time, particularly for shadeintolerant and intermediate species.Conclusions: The populations of several tree species occurring in the Białowie_ za National Park are demographically unstable -a fact reflected in large and steadily increasing differences between theoretical and actual tree size distributions. The main reason for this are the insufficient recruitment rates typical of most species. Some tree species may very soon disappear almost entirely, and this will certainly lead to far-reaching changes in the functioning of local forest communities. Among other consequences, the current demographic trends, as observed for Białowie_ za tree species, are very likely to have a strong effect on the long-term survival of numerous forest organisms (such as insects, fungi, lichens, etc.), which depend, directly or indirectly, on the presence and balanced demography of particular tree species.

Indices quantifying spatial forest structure are frequently used to monitor spatial aspects of tree attributes including biodiversity in research plots of limited size. The treatment of edge trees, which are close to the plot boundaries, can affect the estimation of such indices that include neighbour effects, since some of their neighbours are likely to fall outside the plot. This paper investigates whether and under what circumstances edge-correction methods are necessary and evaluates the performance of six different approaches: no edge correction, translation, reflection, buffer zone, and two new nearest-neighbour methods. The performance of edge-correction methods depends strongly on the algorithmic structure of the indices and the spatial pattern of tree positions involved. Some edge-correction methods introduce more error than ignoring edge bias altogether. For indices accounting for the diversity of tree positions and especially for those computing angles, translation or buffer zone methods reduce the estimation error regardless of the sample size. The use of the reflection method is associated with large bias values. One of the new nearest-neighbour edge-correction methods proves to be capable of reducing the bias considerably. The results confirm the need for sufficiently large monitoring plots to avoid bias from edge effects. Where this is impossible, neighbours beyond the plot boundary need to be included in the survey, thus providing unbiased estimates but at the cost of extra measurements. Sensitivity analysis is required for newly introduced indices prior to their first application.