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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.

Background Forest management affects the habitat conditions for many forest-dwelling species. Among them, the capercaillie ( Tetrao urogallus ) is a rare forest grouse inhabiting old, mature forests. We compared the structure of forest habitat among 9 active and 9 abandoned leks in the Augustów Forest (North-Eastern Poland), within a radius of 1 km of the leks, defined as the Key Areas for the capercaillie in lowland temperate forest. Habitat measurements were conducted on 1779 circular plots. Assessments made on all plots related to 13 habitat variables measured or noted in the field, including stand structure, canopy closure, stand developmental stage, percentage of Scots pine ( Pinus sylvestris ), soil fertility and soil moisture, the share of undergrowth, the cover of shrubs, the cover of bilberry ( Vaccinium sp.), and the presence of certain habitat elements important to the capercaillie. Results To compare the still-occupied and the abandoned KAs for the capercaillie, a logistic regression model was developed. The variables best explaining differences between these two categories were: the occurrence of undergrowth layers, canopy closure in the second canopy layer, and stand age. According to the model, with the increase of the shrub-layer cover as well as the density of trees, the probability of the presence of the capercaillie decreased. The capercaillie in the area of the Augustów Forest occupy mainly dry and poor, middle-aged, pine-dominated forests, with a moderate extent of stand canopy closure and only weakly-developed layers of undergrowth. Conclusions The filling-in of mature stands with sub-canopy trees and shrubs (the process which is stimulated by climate change and site eutrophication) causes structural changes, which are unfavourable to the capercaillie. This might explain why in the course of the recent decades the capercaillie has abandoned the oldest stands, distinguished by the presence of bigger shares of undergrowth. The capercaillie has shifted to younger stands, which reveal a lesser extent of canopy closure and a more limited development of understorey vegetation.

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.

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. Streszczenie Kontrowersje dotyczące charakteru zmian sukcesyjnych zachodzących w zbiorowiskach leśnych mają długą historię, sięgającą czasów klasycznych debat między Clementsem i Gleasonem. Kontrowersje te do dzisiaj nie zostały jednoznacznie rozstrzygnięte. Głównym przedmiotem sporów i dyskusji jest problem przewidywalności oraz konwergencji (zbieżności) trendów rozwojowych mających miejsce w zbiorowiskach leśnych oraz kwestia względnego znaczenia różnych mechanizmów i czynników wpływających na sukcesję lasu. Okazji do empirycznej weryfikacji konkurujących ze sobą teorii i modeli jest jednak niewiele, ponieważ badania dynamiki lasu, oparte na stałych powierzchniach próbnych, obejmujące odpowiednio długie przedziały czasowe, są bardzo nieliczne. Jeszcze mniej jest badań dotyczących wieloletnich zmian zachodzących w zbiorowiskach reprezentujących późne stadia sukcesji ekologicznej. W pracy poddano analizie zbiór danych przedstawiających rozwój, w okresie blisko 80 lat (1936–2012), późnosukcesyjnych zbiorowisk leśnych (zaliczonych do siedmiu różnych ‘typów strukturalnych’) objętych ochroną ścisłą, reprezentujących lasy strefy umiarkowanej występujące we wschodniej Polsce. W celu oszacowania stabilności składu gatunkowego poszczególnych typów lasu w okresie objętym badaniami wykorzystano metodę niemetrycznego skalowania wielowymiarowego. Stopień przewidywalności trajektorii opisujących zakres i kierunek zmian składu gatunkowego zbadano przy pomocy statystyki Mantela. Określono także zmiany rozkładów współczynników niepodobieństwa analizowanych płatów roślinnych i na tej podstawie oceniono stopień konwergencji/dywergencji badanych zbiorowisk w skali czasowo‐przestrzennej objętej badaniami. W zbiorowiskach leśnych występujących w Puszczy Białowieskiej (poddanych ochronie ścisłej) w okresie ostatnich blisko osiemdziesięciu lat nastąpiły bardzo duże zmiany. Liczebność wielu gatunków (osiki, brzozy, sosny, dębu, jesionu, klonu i świerka) znacząco zmalała, przy jednoczesnym wzroście znaczenia niewielkiej liczby innych gatunków (lipy i grabu), obejmującym praktycznie wszystkie zbiorowiska leśne. Początkowo wyraźne różnice pomiędzy poszczególnymi typami zbiorowisk, z czasem stają się coraz mniejsze. Mimo to, obliczone parametry i wskaźniki nie wykazały wyraźnej konwergencji sukcesyjnej, co, jak można sądzić, wynikało z generalnego ustępowania świerka, szeroko rozpowszechnionego na początku badań, oraz względnej trwałości występowania dojrzałych osobników długowiecznych gatunków drzew o pośrednich wymaganiach świetlnych (przy braku istotnego odnowienia tych gatunków). Synteza. Późnosukcesyjne zbiorowiska leśne występujące w Puszczy Białowieskiej charakteryzują się dużą dynamiką i nie posiadają właściwości homeostatycznych często zakładanych w przypadku lasów o charakterze pierwotnym. Typy lasu, początkowo znacznie różniące się pod względem składu gatunkowego, z czasem stały się dużo bardziej podobne pod tym względem do siebie. Lista potencjalnych czynników środowiskowych odpowiedzialnych za obserwowaną dynamikę zbiorowisk leśnych jest długa i obejmuje zmiany klimatyczne, rosnącą depozycję związków azotowych, zmianę reżimu zaburzeń naturalnych i antropogenicznych oraz zmienną presję dużych roślinożerców na naturalne odnowienie lasu w okresie minionych blisko 80 lat. W wyniku tych wszystkich zmian, lipa, a zwłaszcza grab uzyskały przewagę konkurencyjną nad świerkiem, sosną, dębem, brzozą i innymi gatunkami drzew. Rosnąca dominacja grabu prowadzi do konwergencji sukcesyjnej, tym niemniej proces ten nie został jeszcze zakończony. Można się jednak spodziewać, że jeżeli nie dojdzie do większych zaburzeń czy istotnych zmian w układzie obecnie panujących warunków środowiskowych, to proces homogenizacji sukcesyjnej badanych zbiorowisk będzie postępował nadal. Our long‐term study in Białowieża Forest (NE Poland) shows that late‐successional woodland communities 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 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 the resulting successional convergence is far from complete at this time. In the absence of major disturbance, we expect future decades to show continued and successional homogenization.

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 Biaowiea 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 Biaowiea 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 Biaowiea 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. We developed theoretical, equilibrium tree-size distributions to assess a degree of demographic stability of tree species, occurring in strictly protected forest stands in Biaowiea National Park (NE Poland). We examine the possible reasons for the large and steadily growing departures between the steady-state and observed distributions. We also discuss the most important community-level implications of the long-lasting tree demographic patterns.

The competition, especially for light, is a fundamental determinant of the structure and composition of a forest. Large-scale forest models must balance real-world complexity with computational demand and poorly constrained parameters. The LPJ-GUESS dynamic global vegetation model has a strong track record of simulating forest composition and tree demography with a simple representation of forest canopies. However, its current approach is limited in its ability to explore the functional coexistence of trees within forest patches or to represent the full implications of forest management actions that create heterogeneous light conditions on the forest floor. This is because LPJ-GUESS currently represents forest canopy light transmission with vertically overlapping crowns, neglecting any horizontal structural heterogeneity. Whilst computationally efficient, this scheme does not allow for a realistic representation of light distribution on forest floor following tree death or harvest. Here we describe the implementation of a new scheme with spatially explicit canopies, where tree cohorts have a fixed position within a patch, enabling simulation of forest floor light conditions that better captures spatial variation, especially following disturbances such as tree death or harvest. Additionally, we test a lower-complexity canopy scheme based on the perfect plasticity approximation. To evaluate these developments, we conducted four assessments. First, we evaluated the model's performance against field observations of aboveground woody biomass, mortality, and productivity across diameter size classes. Second, we examined their ability to represent tree functional coexistence. Third, we explored how forest harvest influenced the re-establishment of a woody understory. Lastly, we conducted two sensitivity tests. Results show that the spatially explicit canopy scheme improves the representation of forest tree size structure and dynamics across boreal, temperate, and tropical regions. It enables functional coexistence without the influence of large-scale disturbances, captures the interplay of forest gap dynamics with the establishment of a recruitment layer, and produces more realistic understory light environments and competitive interactions, capabilities not achievable with the standard canopy scheme. By capturing these dynamics without requiring explicit individuals, the scheme expands methodological options for bridging individual-based and cohort-based models, while avoiding abrupt canopy-layer transitions and enabling a more gradual and ecologically consistent representation of canopy reorganization. This improves the representation of stand structure and key demographic processes, enhancing the model's capacity to simulate forest dynamics, resource fluxes, and responses to environmental change, while improving alignment with observational data.

1. Fires are nowadays small, yet frequent, in temperate Central European conifer forests, but little is known about the fire history in this region. This is likely due to the lack of intact forests that contain old trees and dead wood from which fire history may be reconstructed. An exception is the Białowieża Primeval Forest (BPF) in Poland for which we were able to reconstruct the fire history in detail. 2. From 886 tree ring samples collected in a 13-ha conifer-dominated area, we reconstructed fire events and tree establishment back to the mid 1600s. From 1653 to the late 1700s fires were very frequent, with mean point (single tree) fire intervals of 18 years and mean stand scale fire intervals of 6 years. After 1781, the intervals between the fires increased dramatically, and since 1874 no major fire was recorded. 3. Tree establishment underwent substantial changes, closely tracking shifts in fire frequency. When fires were frequent, Pinus sylvestris establishment occurred only sporadically. Later, less frequent fires promoted massive establishment of both P. sylvestris and Picea abies. At present, only P. abies and a few deciduous trees regenerate. 4. Synthesis. We present the first high-resolution fire history in the Central European temperate lowland forest area. The discovery of old P. sylvestris trees and stumps with fire scars in many conifer-dominated parts of BPF show that fire was a major component in the past dynamics of this forest. We also show that historically, fires were recurring at very close intervals, supporting an open, Pinus-dominated forest. These result contrasts with the written history of BPF, which focus on a few, large fires from the past. Human influence on the fire regime was probably substantial, although the disentangling of climatic and human impacts needs further studies. We propose that fire should be increasingly taken into consideration in models of disturbance, vegetation development and forest openness in the whole Central European lowland forest region.

Currently, Brzeziecki et al. 2016 (Journal of Vegetation Science 27: 460-467.) are using data from permanent study plots established in 1936 in Biatowieza 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 Biatowieza 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 Biatowieza Forest are retained.

Question: What are the main driving factors in 70 years of natural dynamics in tree recruitment in the Białowieża National Park? Location: Białowieża National Park, Poland, is one of the least disturbed temperate, lowland forest systems in Europe. Methods: We tested whether fluctuations in large herbivore populations, changes in climate and openness of the forest explained compositional dynamics. Tree recruitment (to size class DBH≥5 cm) was measured on permanent transects (in total, 14.9 ha) six times between 1936-2002. These data were related to existing data on ungulate density, climatic parameters and estimates of forest openness collected during the same period. Results: Total recruitment of all tree species combined was negatively correlated with total ungulate density and red deer density. The variation in response between species was related to the preferences of herbivores; the more preferred forage species (especially Carpinus betulus) were positively and the less preferred species negatively related to herbivore density. Total tree recruitment rates were not related to climatic parameters and openness of the forest. Only Alnus glutinosa recruitment was significantly related to climatic parameters, and Ulmus glabra related to forest openness, but there were no predictable patterns in recruitment among species in relation to these factors. Conclusion: The present study indicated that changes in large herbivore density have played an important role in driving patterns in tree recruitment and species composition during the last 70 years in Białowieża National Park. In contrast to other studies, increasing herbivore numbers were associated with higher recruitment of preferred and browsing-tolerant species. Periodical crashes in ungulate numbers, whether human-induced or caused by natural factors, may offer windows of opportunity for regeneration of a range of tree species and facilitate more diverse and dynamic forest development.

Currently, Brzeziecki et al. 2016 (Journal of Vegetation Science 27: 460-467.) are using data from permanent study plots established in 1936 in Biaowiea 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 Biaowiea 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 Biaowiea Forest are retained.