Henryk Żybura

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.

While some tree species can regenerate naturally without mechanical site preparation (MSP), Scots pine has been shown to benefit from this process. We compared three methods: using a double-mouldboard forest plough (FP), an active single-disc plough (AP), and a forest mill (FM), as well as a no-MSP control, in terms of growth, survival and density of occurrence of pines during the first 4 years of natural regeneration. Moisture conditions were expressed via calculated de Martonne aridity indices, while the microhabitats generated via different MSP methods were further characterised by the total contents of N and C, and the C/N ratio, P₂O₅, and base cations, as well as bulk density and actual moisture. The trials showed inferior regeneration without MSP in terms of the density and cover of young pines. Any of the studied treatments influenced survival, though the best growth was achieved by seedlings using the FP and AP methods, while the best density and evenness results were obtained using AP. The factors most influencing regeneration features were high precipitation during the first growing season after sowing and reduced competition with other vegetation in the cleared area. This impact seems far more important than the capacity of different MSPs to produce differentiation in soil microhabitats in terms of nutrient status or bulk density.

• Key message: Long-term strict protection of woodland communities may lead to their compositional simplification and homogenisation. • Context: In the past, it has often been postulated that structures and processes typical for natural forests should be mimicked by silvicultural activities in the case of managed tree stands. • Aims: To determine which features and traits of natural woodland communities (alongside typical old-growth attributes) should be imitated in managed forests, as well as which should not (and for what reasons). • Methods: Tree data from five permanent study plots (of a total area of 15.44 ha) established in 1936 in the core area of the Białowieża National Park (NE Poland) are used to calculate several quantitative indices describing the temporal dynamics (in terms of stand structure and composition) of eight major woodland community types. • Results: Most structural attributes revealed rather high stability over time. In contrast to these, during the observation period, noticeable changes in the composition of particular Białowieża woodland communities have been taking place, related to declining occurrence and reduced roles characteristic for a large number of tree species. • Conclusion: In many ways, natural forests can serve as an important model for managed forest stands. However, in certain circumstances, silvicultural treatments counteracting natural developmental trends may appear to be indispensable, especially when more diverse and stable tree species composition (at a given spatial and temporal scale) is indicated or desirable.

Successful tree regeneration is a key process in ensuring forest sustainability and one of the most crucial investments made in silviculture. This study compared the effects of three reforestation methods (planting, direct seeding, and natural regeneration) and three mechanical site preparation methods (double mould-board forest plough (FP); active plough (AP); and forest mill (FM)) on biometric parameters, survival, and density of Scots pine (Pinus sylvestris L.) seedlings in the first 4 years of growth in a clear-cut area in south-eastern Poland. Planted seedlings were higher, thicker in root collar, and had higher survival rates after the fourth growing season than trees from natural regeneration and direct seeding. Site preparation methods did not affect the density of planted seedlings. After natural regeneration and direct seeding, seedling density was lower and less homogeneous (plots with no seedlings) in FM soil preparation in comparison to other methods. The survival of pines in all reforestation methods was not affected significantly by site preparation methods. Our results indicate that the best mechanical site preparation method for planting is FM, as this is the one that least disturbs the soil environment. For direct seeding the best results were achieved after AP preparation. Natural regeneration of Scots pine was most effective after FP use, and in relatively wet years also after AP use.

Currently, Brzeziecki et al. 2016 (Journal of Vegetation Science 27: 460–467.) are using data from permanent study plots established in 1936 in Białowieża National Park (NE Poland) to develop theoretical equilibrium tree size distributions and to then compare modelled and actual distributions with a view to assessing the population dynamics of the species involved. As part of their discussion, the authors address the question of possible consequences for the overall diversity of forest ecosystems under strict protection if long‐term trends relating to tree population densities and size structures are maintained. In the overall context of the above, the goal of the present paper is to respond to Jaroszewicz et al. (Journal of Vegetation Science 28: 218–222.) who suggest that the paper of Brzeziecki et al. (2016) is not representative for the whole Białowieża National Park, and that – in this connection – strict protection should not be seen as a cause for concern. In this paper, we show that the data analysed by Brzeziecki et al. (2016) adequately characterize conditions in the wider Park. We also point out that the thorough scientific understanding of the long‐term dynamics of woodland communities under strict protection should indeed be taken into account as efforts are made to arrive at an effective conservation strategy capable of ensuring that the uniquely valuable features of the Białowieża Forest are retained.

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 Białowieża 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 shade‐intolerant and intermediate species.Conclusions: The populations of several tree species occurring in the Białowieża 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 Białowieża 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.

Forest succession depends strongly on the life history strategies of individual trees. An important strategic element is the ability to survive unfavourable environmental conditions that result in strongly reduced tree growth. In this study, we investigated whether the relationship between growth and mortality differs among tree species and site conditions. We analysed 10 329 trees of nine tree species (Picea abies , Taxus baccata , Fagus sylvatica , Tilia cordata , Carpinus betulus , Fraxinus excelsior , Quercus robur , Betula spp. and Alnus glutinosa ) from unmanaged forests of Europe: the continental Białowieża forest (Poland) and several oceanically influenced Swiss forest reserves. For each species, we calculated a set of flexible logistic regression models with the explanatory variables growth (as measured by relative basal area increment), tree size and site. We selected the species‐specific model with the highest goodness‐of‐fit and calculated its discriminatory power (area under the receiver operating characteristic curve, AUC) and calibration measures. Most models achieved at least a good discriminatory power (AUC>0.7) and the AUC ranged from 0.62 to 0.87; calibration curves did not indicate any overfitting. Almost all growth–mortality relationships differed among species and sites, i.e. there is no universal growth–mortality relationship. Some species such as F. excelsior showed reduced survival probabilities for both unfavourable and very good growth conditions. We conclude that the growth–mortality relationships presented here can contribute to the life‐history classification of trees and that they should also help to improve projections of forest succession models.

This paper presents results of a long‐term study on natural forest dynamics in the Białowieza National Park (BNP), northeastern Poland. Five permanent sample areas were used, each consisting of a transect of varying width (40 ‐ 60 m) and length (200 ‐ 1380 m). The total sample area is 14.9 ha. The study covers the period 1936–1992. During this period measurements were made on five occasions at approximately 10‐yr intervals. On each measurement date all trees with DBH > 5 cm were identified and their spatial location, diameter, crown condition and position in the canopy determined.During the study period the stands underwent noticeable changes, mainly in terms of tree species composition. The major change was a quantitative increase of the late‐successional species: Tilia cordata and Carpinus betulus, also to a lesser degree Fraxinus excelsior and, in the last period, of the early successional Alnus glutinosa. Declining species included both early‐ and late‐succession species. Among the latter group, Picea abies ranked first. This species lost much of its importance during the last few decades. P. abies was followed by Pinus sylvestris which is an important component of the climax vegetation under the conditions prevailing in Bialowieza, at least on more oligotrophic sites. Still, this species has not been able to regenerate during the whole study period. Some other late‐succession species, Acer platanoides and Quercus robur, were also amongst the declining species. Although the basal area of Q. robur increased, its population was getting older and the process of natural regeneration was markedly impeded.All typical pioneer, short‐lived species: Betula pendula and B. pubescens, Salix caprea and Populus tremula also decreased, which was probably caused by a lack of major disturbances during the study period.In general, the results obtained for the semi‐natural conditions of Bialowieza during the 56‐yr observation period suggest a rather high compositional instability of the forest stands there. A more precise identification of the role of particular factors in the observed stand dynamics is difficult because of the paucity of appropriate historical and environmental data which refer directly to the study plots; moreover, the data are generally incompatible and of different resolution.