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Article

A Demographic Imbalance of Tree Populations in the Managed Part of Białowieża Forest (NE Poland): Implications for Nature-Oriented Forestry

by
Bogdan Brzeziecki
1,*,
Jacek Zajączkowski
1 and
Marek Ksepko
2
1
Department of Silviculture, Institute of Forest Sciences, Warsaw University of Life Sciences, Nowoursynowska 159/34, 02-776 Warsaw, Poland
2
Bureau for Forest Management and Geodesy, State Enterprise, Białystok Division, Lipowa 51, 15-424 Białystok, Poland
*
Author to whom correspondence should be addressed.
Forests 2025, 16(9), 1382; https://doi.org/10.3390/f16091382
Submission received: 30 July 2025 / Revised: 24 August 2025 / Accepted: 26 August 2025 / Published: 28 August 2025
(This article belongs to the Section Forest Biodiversity)
Browse Figures
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Abstract

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.

1. Introduction

The high threat and ongoing loss of many components and facets of biodiversity are currently matters of widespread concern. The main causes of this phenomenon and possible countermeasures are widely discussed and analyzed on global [1,2,3,4,5], regional [6,7,8,9], and local scales [10].
In Central Europe, as in other intensively developed regions, a significant part of the native biodiversity is associated with managed forests, which, despite centuries of exploitation and many transformations, constitute the most “natural” form of land use [8,9,11,12,13]. Among the numerous taxa associated with forest ecosystems, trees occupy a particularly important position [12,14,15,16]. Although trees may be significantly inferior to other forest organisms in terms of the number of species and individuals, due to their specific features and parameters—such as age and size at maturity—they form the key components of communities and ecosystems, significantly influencing their functioning [17,18,19,20]. They play a predominant role in basic ecosystem processes, such as biomass production, water balance, the rate of organic matter decomposition, nutrient cycling, carbon sequestration, and energy flow [17,19,21]. For this reason, the existence and survival of all other forest organisms depend, either directly or indirectly, on trees. Consequently, trees very often act as foundation species, i.e., species that play a crucial role in structuring a forest ecosystem by creating or enhancing habitats and influencing the diversity and abundance of other forest organisms [17,18,19]. The importance of trees for other forest species can be measured by the number of taxa representing numerous groups of forest biota (fungi, insects, lichens, etc.) associated with various tree species [22,23,24,25,26,27]. Although the significance of particular tree species as hosts for individual groupings of forest-dwelling organisms may differ, it can generally be assumed that safeguarding the natural values of forest ecosystems requires, first of all, maintaining a high species diversity of tree stands [12,15,28,29,30,31].
An important issue that has not received enough attention so far is the fact that trees, in order to permanently act as key driving forces of overall forest biodiversity [12], must be demographically stable, at a given/assumed/desired spatial and temporal scale [32, see also 15]. The traditional criterion used to judge the demographic sustainability of tree populations are their respective size distributions [32,33,34,35,36]. Recently, Halpin and Lorimer [32] adopted a model-based approach to demonstrate that, from a long-term perspective, among different types of tree size distributions—flat, shallow descending monotonic, unimodal, and steeply descending—only the latter (resembling a reversed J shape) type is sustainable and stable.
In this study, we check to which extent the requirement of demographic sustainability is fulfilled in the case of tree species occurring in the managed part of Białowieża Forest (BF). BF, located on the border between Poland and Belarus, is widely recognized as an important biodiversity hotspot in Central Europe. High diversity, richness, uniqueness, and rarity are features characteristic of virtually all groups of plant, animal, and fungal organisms inhabiting the forest [10,37,38,39]. In contrast to the great diversity and richness of many forest-associated organisms, the group of tree species found in BF appears relatively modest. According to Faliński [40,41], the dendroflora of BF includes 28 tree species, of which only c. 10 species play a significant role in local woodland communities. Until very recently, the majority (approximately 80%) of the Polish part of BF was actively managed. This means that for a long time, the species composition of individual tree stands and the demography of local tree populations were deliberately shaped. Nevertheless, considering that the managed part of BF occupies a quite large area (520 km2), which is distinguished by a high spatial diversity of soil conditions and water regimes and subjected to varied disturbance regimes, we hypothesize that the actual size distributions of most local tree species, as determined for the whole study object, are at least approximately balanced and sustainable. In other words, we assume that they can permanently play the role of key driving forces of local biodiversity. To check (to confirm or to reject) this hypothesis, we look for the answers to the following two main research questions:
  • To what extent has forest management carried out until recently in BF influenced the current demographic status (reflected by the respective actual size distribution) of individual tree species?
  • Compared with an ideal equilibrium structure, ensuring the best conservation effects, how many surplus or deficit trees exist for each species and DBH class, and what conservation measures are needed to restore the optimal structure?
To determine actual, species-specific size distributions, we use data from a large-scale forest inventory conducted in BF in 2018 [42]. To investigate potential discrepancies between the desired and actual states, we first develop theoretical equilibrium size distributions for Białowieża tree species and then compare them with actual distributions. Based on the identified differences (deficits and surpluses in particular size classes), we suggest the actions needed to improve the demographic status of specific tree species and to secure their key role in preserving local biodiversity. Finally, we discuss the possible consequences of currently adopted solutions aimed at preserving the natural values of Białowieża woodland communities by referring to the situation in the “Strict Reserve” of Białowieża National Park (BNP), the part of BF that has been under strict protection for over 100 years.

2. Material and Methods

2.1. Study Area

BF is situated on both sides of the national border between Poland and Belarus. It encompasses a total of 1475 km2 (625 km2 in Poland and 850 km2 in adjacent Belarus). The Polish sector of BF is divided into the protected area (BNP, approximately 105 km2) and the managed area (around 520 km2), which is subdivided into three forest districts: Białowieża, Hajnówka, and Browsk.
The local climate exhibits characteristics of both continental and Atlantic influences [43]. The average air temperature for the period 1951–2019 was 6.7 °C, with a clear temperature increasing trend of 0.34 °C/10 years. During this period, the coldest month was January with an average air temperature of –4.4 °C, while the warmest month was July (17.7 °C). The total annual precipitation averaged 637 mm.
BF lies on a flat, undulating terrain ranging from 135 to 190 m a.s.l., composed of glaciofluvial sands, gravels, and clays [44]. BF contains a variety of woodland community types, including coniferous forests, mixed coniferous/broadleaved forests, mixed broadleaved/coniferous forests, and broadleaved forests. Additionally, there are streamside alder-ash forests and black alder bog forests. A detailed phytosociological description of these basic vegetation units has been provided by Paczoski [45], Matuszkiewicz [46], Faliński [37], and Sokołowski [47,48], among others. According to current phytosociological investigations, a total of 36 different woodland community types are present in BF (Supplementary Table S1).

2.2. Determination of the Actual DBH Distributions of Tree Species

In order to determine the actual DBH distributions of the main tree species occurring in the managed part of BF, data from measurements carried out in 2018 on permanent circular sample plots were used. The plots were established as part of a project aimed at the comprehensive inventory of the natural and cultural values of BF [42]. Within this project, a total of 1373 permanent test plots were set up, including 855 plots in managed stands (the remaining plots were located in BNP (240 plots, including 116 plots in the “Strict Reserve”) and in forest reserves excluded from forest management (278 plots)). The sample plots had a radius of r = 11.28 m (size 0.04 ha) and were arranged in a regular grid of 650 m × 650 m (Supplementary Figure S1). During measurements performed on these plots, among others, the species affiliation of all trees with a DBH of ≥7 cm was determined, and their diameters were measured. In this paper, we used these data to calculate the number of trees in successive, 4 cm wide diameter classes with midpoints of 9 cm, 13 cm, 17 cm, etc. The list of species that occurred most frequently and were included in the analyses presented below comprised Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karsten), pedunculate oak (Quercus robur L.), common hornbeam (Carpinus betulus L.), small-leaved lime (Tilia cordata Miller), Norway maple (Acer platanoides L.), common ash (Fraxinus excelsior L.), black alder (Alnus glutinosa (L.) Gaertner), aspen (Populus tremula L.), silver and downy birch (Betula pendula Roth and Betula pubescens Ehrh.), and wych elm (Ulmus glabra Hudson).

2.3. Construction of Equilibrium/Sustainable Tree Size Distributions

To calculate the number of trees in successive diameter classes of equilibrium, sustainable species-specific DBH distributions, the so-called demographic approach was used [35,36,49,50,51,52,53]. Under the demographic approach, the long-term sustainability of tree populations is determined by a simple rule: over time, the number of trees moving into a given size/diameter class through growth (in-growth) must equal the number of trees leaving that size class due to growth (out-growth) or mortality due to natural causes or management [35]. In accordance with this rule, to determine species-specific equilibrium DBH distributions, the following formula was used [35,52,53]:
n i + 1 = ( 1 − l i ) ⋅ n i ⋅ p i / ( ( 1 − l i + 1 ) ⋅ p i + 1 + l i + 1 )
where
ni, ni + 1 → The numbers of trees in diameter classes i and i + 1, respectively;
pi, pi + 1 → The outgrowth rates, i.e., proportions of trees moving from diameter class i to class i + 1 and from diameter class i + 1 to class i + 2, respectively, during a given period;
li, li + 1 → The mortality rates, i.e., the proportions of trees in diameter classes i and i + 1, respectively, which died or were harvested during the same period.
The outgrowth rate (pi) was determined by multiplying the annual absolute diameter growth rate (gi) by the length of the corresponding census period Δt (we used a 10 yr period) and dividing by the diameter class width b (4 cm diameter classes were used):
p i   =   ( g i ⋅   Δ t ) / b
The absolute diameter growth rate (g) itself was estimated by means of the first derivative of the Chapman–Richards growth function, using DBH instead of time [52]:
g   =   A   · k   · q   · e   −   k · D B H   ·   ( 1   −   e   −   k · D B H ) q   −   1
where A, k, q are model parameters.
The species-specific mortality rate (l) was estimated using a binary logistic function:
l   =     e a   +   b · D B H   +   c · D B H 2 1   +     e a   +   b · D B H   +   c · D B H 2
where l is the probability of tree mortality and a, b, c are model parameters.
The reduction terms (1–li and 1–li + 1) in Formula (1) were introduced by Brzeziecki et al. [52] to account for the mortality of trees that moved out of classes i and i + 1 during the observation period but died before the end of that period. Both natural mortality and growth functions were parameterized using empirical data from permanent study plots established in BNP and surveyed over the period 1936–2012 (cf. Appendix S9 in Brzeziecki et al. [52]).
The calculation of tree numbers in consecutive diameter classes was performed, always starting from n1 (the number of trees in the first, smallest diameter class). The value of n1 was fixed using a trial-and-error method, such that the resulting basal area of the model DBH distribution amounted to the target basal area of a given species. To determine the latter parameter, first, the overall value of the basal area (i.e., the value remaining after performing regulatory cuts) was established. In the next step, this value was distributed among individual tree species proportionally to their assumed roles in the managed part of BF, calculated on the basis of their model shares in particular woodland communities (Supplementary Figure S2), weighted by their area (Supplementary Table S1).
All calculations were performed by means of our own procedures written in Microsoft Visual Studio’s NET programming environment.

2.4. Calculation of Surplus (Total and Reduced) and Deficit Trees of Individual Species by Diameter Class and Determination of the Area of Regeneration Units by Species and in Total

The total amount of surplus and deficit trees for a given species was calculated in a simple way as the arithmetic difference between the number of trees in individual diameter classes of the corresponding real and model size distributions. In the next step, for each species, the amount of reduced surpluses was calculated, under the assumption that the basal area of the residual DBH distribution (i.e., the distribution that would remain after the completion of regulatory cuts) should correspond to the model value of basal area determined for this species.
The information on the amount of species-specific deficits in the first diameter class (DBH = 9 cm) was used to calculate the area of the regeneration units (ARU, in ha) that would be needed in order to establish (either naturally or artificially) groups and patches of young individuals of tree species that are currently in short supply. The value of the ARU parameter was calculated per 100 ha (which is the approximate standard size of large forest compartments into which the area of BF is divided) using information about the growing space (in m2) occupied by a single tree with DBH = 9 cm (midpoint of the first diameter class). It was assumed that the latter corresponds to the vertical crown projection area, calculated using allometric functions developed for Białowieża tree species [54]:
A R U   =   D E F × G S 10,000 × F C S
where
ARU → The total area of regeneration units for a given tree species (in ha);
DEF → The number of deficit trees of a given species in the smallest diameter class (in trees·ha−1);
GS → The growing space (in m2) of a single tree with DBH = 9 cm of a given species (corresponding to its crown projection area);
FCS → The approximate size of a standard forest compartment (amounting to 100 ha in this case).

3. Results

3.1. Actual vs. Theoretical DBH Distributions

The actual DBH distributions (trees × ha−1) of tree species found in the managed part of BF are shown in Supplementary Table S2, along with their overall density and basal area, basal area of very large trees (DBH ≥ 67 cm; trees excluded from potential regulatory activities), reduced basal area (obtained after subtracting very large trees), and the percentage share of a given species in the reduced basal area.
Model (target) values of the basal area, overall and for individual tree species (reflecting their supposed roles in the stands of BF, determined on the basis of model tree species composition of the local woodland community types and their spatial extent), are presented in Supplementary Table S3.
The species-specific equilibrium DBH distributions, calculated taking into account the model values of basal area assigned to each species, are given in Supplementary Table S4.
The current DBH structure in the stands occurring in the managed part of Białowieża Forest is presented, in the form of the share of particular species in subsequent DBH classes, in Figure 1 (top). As can be seen, at the moment, four tree species play the most significant role in the composition of the Białowieża stands: alder, hornbeam, spruce, and pine. While pine and alder occur most frequently in intermediate DBH classes, hornbeam trees dominate in the small diameter range (from 9 to 29 cm) and spruce is represented rather evenly over the whole diameter range. Birch and oak are also relatively common, and are represented, similarly to spruce, fairly evenly throughout the entire tree’s diameter range. Other species, perhaps with the exception of aspen, which appears quite abundantly in the five largest classes (midpoint 49 cm and higher), are practically absent. In contrast, the results obtained for model equilibrium distributions (Figure 1, bottom) show that all tree species should be more or less evenly represented over the prevailing range of diameter classes, proportionally to their assumed total shares.
Comparing the real DBH distributions obtained for particular tree species with their model (equilibrium) distributions reveals that, at the level of individual tree species, some characteristic patterns could be recognized (Figure 2). Aspen, maple, elm, ash, and oak are all distinguished by the mere presence of deficits, occurring, to a greater or lesser extent, over the entire range of tree diameters. Large amounts of deficits, involving mainly initial DBH classes (9–17 cm), also occur in the case of birch and, to a lesser extent, in the case of pine (in the first DBH class with a midpoint amounting to 9 cm). In turn, the largest total surpluses occur in the case of pine, spruce, hornbeam, and alder. For pine and alder, the surpluses occur mainly in intermediate DBH classes, for hornbeam they appear mostly in the small DBH classes, and for spruce they occur rather evenly over the whole range of tree sizes.
In addition, in Figure 2 so-called hypothetical (or help) distributions are shown. They were constructed to calculate the amount of reduced surpluses, i.e., trees that should be removed during the next 10-year-long planning period. The overall basal areas for hypothetical DBH distributions were selected in such a way that the basal areas of the residual DBH distributions (obtained with the smaller value between the real and hypothetical number of trees) corresponded to the target basal areas assumed for each species (cf. Supplementary Table S3).

3.2. Overall Amount and Distribution of Total and Reduced Surplus Trees

The total number of surpluses, by species and diameter class, is shown in Figure 3A. The overall density of surplus trees was 311.7 ha−1, which constituted approximately 49% of the current overall density of all trees, while their basal area was 12.8 m2 × ha−1, which corresponded to 47% of the total actual value of basal area after subtracting very large trees.
The DBH distribution of reduced surpluses was similar to that represented by total surpluses and involved mainly small and intermediate diameter classes (with midpoints from 9 to 41 cm) (Figure 3B). In the smallest diameter classes (9–17 cm), hornbeam trees clearly prevailed. Most of the spruce trees to be removed belonged to the 17–33 cm classes, while alder and pine trees primarily came from the 21–41 cm classes. The total density of trees planned for removal over the next 10-year planning period was 108.9 trees × ha−1, while their basal area amounted to 4.1 m2 × ha−1 (i.e., 15% of the actual total basal area after subtracting very large trees).

3.3. Overall Amount and Distribution of Deficit Trees

The greatest deficits (calculated by subtracting real distributions from their model counterparts) occurred in the three initial diameter classes (with midpoints between 9 and 17 cm) (Figure 4). The first diameter class was particularly prominent in this regard, with the greatest shortages concerning birch, oak, and pine. A group of species characterized by deficiencies in the first diameter class also included aspen, maple, ash, elm, and alder. The total deficit was 185.4 trees × ha−1, which constituted around 29% of the current density of all trees, while the total basal area of the deficits was 8.7 m2 × ha−1, which was around 32% of the current total basal area after excluding very large trees.

3.4. Estimated Area of Regeneration Units (Regeneration Patches) Required to Reduce Species-Specific Deficits in the Smallest Diameter Class

The total size of necessary regeneration units, as calculated per 100 ha of forest area, was approximately 9 ha (Table 1). The greatest needs in this regard concerned birch, oak, and pine, while the lowest concerned lime and alder. For hornbeam and spruce the estimated area of regeneration spots amounted to zero.

4. Discussion

4.1. Using Equilibrium Distributions as an Indicator of the Demographic Sustainability of Tree Populations

To permanently play the role of key drivers of overall forest biodiversity, tree populations must be demographically sustainable; i.e., their size/diameter distributions should have a steeply descending monotonic form [32]. The number of trees that should be present in subsequent diameter classes to ensure the demographic sustainability of a given population depends, first of all, on two (species-specific) demographic processes: growth and mortality. In this work, to characterize the growth and mortality rates of trees, we used functions parameterized by Brzeziecki et al. [52], based on data collected during research conducted on permanent plots in the strictly protected part of BF, covering a period of almost 80 years. Such a long study period provides an opportunity to eliminate the influence of random factors on the examined processes and increases the chance of correctly determining the main trends and patterns [34].
In addition to the tree growth and mortality functions, an important element of the demographic approach used in this work was the model value of the basal area, overall and assigned to the individual tree species. We assumed that the basis for determining the overall value of this parameter (characterizing the condition of stands after cuttings aimed at approximating the real distributions to the modeled ones) should be the actual value of basal area in BF stands (after excluding very large trees), reduced by a predetermined figure (15% or 4.1 m2 × ha−1 in our case). The above number was chosen somewhat arbitrarily but was adopted at such a level as to ensure that the basal area of trees harvested during a standard 10-year planning period would be markedly below the estimated value of the periodic 10-year increment of basal area in Białowieża stands amounting to around 7.3 m2 × ha−1 [55,56]. The total model value of the basal area was then divided among individual tree species, in proportion to their assumed percentage shares in Białowieża communities. For this purpose, data on the model tree species composition of the main types of Białowieża woodland communities were used (adopted after BULiGL [55]; cf. Supplementary Figure S2). These data constitute a synthesis of many previous phytosociological studies conducted at different times in the communities of BF by numerous researchers [37,45,46,47,48]. Although these data are, at least to some extent, arbitrary, they seem to constitute a reasonable approximation of what can be assumed in relation to individual tree species, based on the knowledge of their life strategies, ecological requirements, and biological properties. Nevertheless, in the future, the assumptions made in this respect (which are necessarily based on past experience) may be changed. In the face of climate change and modifications to other important environmental parameters (e.g., caused by the deposition of nitrogen compounds), this may turn out to be necessary, indeed [57,58,59,60]. Nevertheless, in any situation, striving to ensure a balanced demographic status of a given species, regardless of its assumed participation in community structure, remains an issue of fundamental importance [15,32,61].

4.2. Common Deviations Between Actual and Theoretical Tree Size Distributions: Major Causes and Implications

For many, or even the majority, of tree species in BF, their actual diameter distributions deviated, to a greater or lesser extent, from the desired equilibrium state, as shown by large amounts of total surpluses and deficits. In certain, usually middle, ranges of tree diameters of some species, there were surpluses, and in others (mainly for the smallest trees, and to some extent also for the largest trees) there were deficits in the actual number of trees in relation to the model values. Together, almost half of all trees in BF represented “inappropriate” species and/or diameter ranges; i.e., they were found in excess in relation to the balanced model. This problem mainly concerned hornbeam, spruce, alder, and pine. The scale of the deficits was also high, estimated at over 30% of the current total number of trees.
The question arises as to why such a large area covered by this study (over 520 km2) was not sufficient to allow for a demographic balance at the level of individual tree species. There is certainly no single, simple answer to this question. What is of fundamental importance here is the fact that, in the area in question, exploitation of wood resources has been carried out for over 100 years—since World War I. Initially, this was in the form of plundering cuts (German cuttings during World War I; the activity of the “Century” timber company during the interwar period), and then, after World War II, in the form of regulated forest management, favoring certain tree species at the expense of others [37,62].
The legacy of events that took place at the beginning of the 20th century, and probably also in the post-war years, which has been preserved to this day, is, among other things, the excessive occurrence of alder-dominated stands in the eutrophic and wet habitats of BF. Such habitats could potentially be occupied by much more diverse tree stands, consisting of ash, maple, elm, oak, spruce, hornbeam, and lime. The heritage of forest management is also the overrepresentation of pine, and to some extent spruce, in certain stands, which is the result of excessive favoring of these (important from a commercial point of view) species in the past.
Hornbeam is also found in great abundance. In this case, it is primarily the result of the strong competitive ability and shade tolerance of this species. It is worth emphasizing that, from an ecological and economical point of view, the appearance and growth of hornbeam under the canopy of other species (such as pine or oak) is generally a desirable phenomenon. The problem arises when a given forest area is completely taken over by hornbeam, because, over a long period of time, practically no other tree species are able to effectively compete with it.
The influence of forest management can also explain the currently low abundance of birch and especially aspen in the managed part of BF. The populations of these species, being less valuable from an economic point of view, have been systematically reduced over time during thinning operations carried out in favor of species that are more valuable economically (oak, pine, spruce) [63].
In turn, the currently low density of species such as elm and ash is the result of mass die-off caused by factors on which forest management had practically no influence (Dutch elm disease in the case of elm and the invasive fungus Chalara fraxinea in the case of ash [64,65]).

4.3. Measures Which Need Be Taken to Bring the Local Tree Populations Closer to This Desired State

Considering the large discrepancies between actual and theoretical sustainable distributions, two main types of actions would be required to bring the current situation closer to the desirable sustainable state: (1) a gradual elimination of existing surpluses, taking into account those species and diameter ranges where they are currently the largest; (2) a systematic reduction in existing deficits, primarily by actively supporting the process of regeneration and recruitment to the stand stage for those tree species that show the greatest deficiencies in this regard.
From the point of view of maintaining the demographic stability of individual tree species, the biggest problem today is the significant deficit of trees in the initial diameter classes for a large group of species. As pointed out by Ohse et al. [15], the early life stages are crucial for maintaining population sustainability. This suggests that, first and foremost, efforts should focus on creating conditions that enable the regeneration and subsequent recruitment to the stand phase of species that are currently in short supply. This could partly be achieved through (assisted) natural regeneration; however, in many cases, artificial regeneration would also be necessary. In fact, the need for human intervention to support tree recruitment processes has often been recognized by researchers who have studied BF at different times [45,46,48,66,67,68]. For example, almost 100 years ago, Paczoski [45] already suggested that oak should be promoted at the expense of hornbeam through active silvicultural measures (planting, release cuttings). Similar suggestions, regarding several different tree species (Scots pine, Norway maple, ash, elm, and even alder), were also made by other authors later.
Actions aimed at reducing existing deficits in some species can, or even should, be combined with actions aimed at eliminating current surpluses. Places where there is a particularly large concentration of species and diameter classes occurring in excess in relation to the equilibrium curves should be selected for the establishment of regeneration units (in form of smaller or larger artificial canopy gaps) for species in short supply. The need to eliminate surplus trees is justified by the necessity to create space for missing tree species. Excesses and deficiencies are often interrelated in the sense that the occurrence of excesses in certain species and diameter ranges causes deficiencies in other species and diameter ranges. It is known, for example, that an overrepresentation of trees in higher diameter classes may strongly inhibit regeneration processes, thus leading to shortages in the initial diameter classes [32,69].
Undoubtedly, one of the priority tasks in the active protection of the natural resources of BF should be the restoration of populations of those tree species that are currently very sparsely represented (elm, maple, ash, and aspen; see Figure 1 and Supplementary Table S2). They all are important host species for many valuable elements of forest biodiversity [64,70,71]. To be effective, potential actions aimed at improving the demographic status of the aforementioned species should cover extensive areas of BF.

4.4. Conserving Natural Values of BF: What Strategy to Choose?

The debate about the most appropriate management strategy aimed at preserving the high natural values of BF on a sustainable basis lasted for a very long time. Essentially, this debate took place between proponents of an active policy [72] and supporters of a passive approach, associated with the concept of strict protection [73]. When BF was granted the status of a World Heritage Site in 2014, the balance clearly shifted in favor of the concept of strict protection, as most of its area was, by political decision, excluded from any activities. The concept of strict protection implies that allowing natural processes is the best way to maintain the high biological diversity of BF. In relation to trees, this concept assumes, at least implicitly, that their balanced demographic structure will spontaneously recover, enabling them to lastingly play the desired role of key supporters and drivers of the whole forest biodiversity. The question, however, is how realistic this assumption is.
At least a partial answer to this question is provided by the research conducted in the “Strict Reserve” of BNP (occupying an area of nearly 50 km2), i.e., that part of BF which has been “left to nature” for more than 100 years. The long-term studies conducted in the “Strict Reserve” clearly show that under conditions of strict protection, for the vast majority of tree species, deviations from the equilibrium state did not decrease; on the contrary, they increased significantly [52]. A fundamental role here was played by a phenomenon often referred to as “tree recruitment failure” [15,50,61,74,75,76]. In BNP, there were many reasons for this phenomenon [52,77,78]. A particularly important role was played by the strong, uncontrolled pressure of large herbivores (bison, elk, deer, and roe deer), resulting in the almost complete elimination of many tree species from regeneration processes [79]. As a result, regeneration and recruitment processes were dominated by a limited number of species, which lead to the increasing simplification and homogenization of woodland communities and the disappearance of many precious elements of biodiversity, such as numerous rare vascular plants [68,80,81] and lichens [82].
At this moment, the demographic status of several tree species in the managed part of BF (where, among other things, the access of ungulates to young stands has been effectively limited by the use of fences) is more favorable than in the “Strict Reserve” of BNP [42]. A remarkable example in this respect is the current diameter distributions of two important species: pine and oak. In the “Strict Reserve”, both pine and oak are currently represented by aging, strongly decreasing populations (Figure 5). At the same time, in the managed part of BF, the present demographic situation of both species is much more favorable, particularly in the range of young and middle-aged trees (with DBH between 7 and 67 cm). This is the result of deliberate support (through various silvicultural measures, including artificial regeneration and fencing) for both species over a long period of time (formerly pine, and in recent years mainly oak). Although these species were mainly favored for economic reasons, both, and especially oak, also play an important ecological role, due to the long list of different forest taxa associated with them, including rare and endangered saproxylic beetles [25,70,71]. The example of pine and oak illustrates the potential of appropriate forest management to create woodland communities distinguished by a diversified structure and a balanced demographic structure of individual tree species. This capacity can and should be used to restore the appropriate demographic status and ecological role of not only the species traditionally favored in forest management but also of all others, while maintaining appropriate quantitative proportions between them.
Thus, a viable alternative to the strategy based on the concept of strict protection is the active approach outlined in this study. This approach is based on the assumption that one should not passively wait and hope that the desired state of BF ecosystems will spontaneously recover (in an unspecific future), but that appropriate actions should be initiated to restore this desirable state here and now. The foundation of the approach proposed here is the concept of stocking control or stocking regulation [83]. The aim of stocking regulation is to allocate the available growing space between individual forest components (tree species and their development stages) in a way that ensures the (relative) permanence of the desired structure of woodland communities at a given spatial scale. The primary goal of regulation can also be defined as the transformation of a given forest structure into another (more desirable one) [83]. The potential scope of regulatory activities may also include other structural elements contributing to the complexity of the forest (tree stand), such as dead standing and lying trees. The structure of the forest is constantly changing due to the appearance of new trees (the recruitment process), dying of and the use of available space by existing trees (the mortality and growth processes). The primary task of regulation in this situation is to ensure that these processes do not lead to excessive changes or deviations from the structure of the forest, considered as optimal.
The approach presented in this work is an example of the application of the concept of regulating forest structure in order to achieve effects primarily desired from the point of view of nature conservation. As part of this approach, the condition that the forest ecosystems in BF should achieve was first defined, primarily in terms of the species diversity of tree stands and the demographic structure of individual tree species (Supplementary Table S4). Next, the desired state was compared with the current situation, and the nature and type of discrepancies between them were determined. These discrepancies constituted the basis for planning actions and treatments aimed at gradually bringing the current state closer to the desired conditions. Although our analysis was carried out in relation to the entire area of BF, it can be repeated in a similar way in relation to smaller spatial units, e.g., forest compartments, which could play the role of basic spatial units of forest silvicultural planning aimed at preserving demographic sustainability of tree species [56].
It is worth emphasizing that the need for an active approach to preserving the high natural values of forest ecosystems has recently been emphasized by an increasing number of authors [12,15,28,32,64,84,85,86,87,88]. As shown, for example, by Cole and Yung [84], today, in the era of global change, it is not possible to effectively protect nature by enclosing it within “protected areas” (reserves, national parks, and wilderness areas), drawing a line around them, and leaving them alone (cf. also [85]). The approach presented in this work meets the demands put forward by all these authors. We suggest that in order for it to bring the desired effects, it should be applied to the majority of the area of BF. Obviously, the application of the approach described here is not limited to the particular case of BF but is also possible in all instances where the preservation of high natural values of forest ecosystems is important.

5. Summary and Conclusions

The analysis of the current demographic status of individual tree species in the managed part of BF revealed that the actual diameter distributions of most of them are characterized by large deviations, both in excess and in deficit, from the desired equilibrium state, characterized by gradually decreasing proportions of individuals representing particular developmental stages (from young/small trees, through middle-aged/intermediate-sized trees, to old/large trees). Thus, our analysis did not confirm the hypothesis that tree species occurring in the managed part of BF are demographically stable.
From the point of view of demographic sustainability, a major problem is the large shortages in the number of trees in the smallest diameter classes, as observed for many Białowieża species. This fact critically undermines the possibility of these species playing a key role in maintaining high levels of overall forest biodiversity in the middle- and long-term perspective.
The concept of protecting natural processes, currently adopted in relation to most of the area of BF, indirectly assumes that the desired demographic balance of individual tree species will be restored spontaneously, although in an unspecified future. An alternative to this concept is an active strategy, which assumes actions aimed at the gradual elimination of identified excesses and deficiencies of trees in relation to the desired equilibrium state. A primary goal of such actions would be to gradually bring the current tree diameter distributions closer to the model distributions, ensuring the demographic sustainability of tree species and, ultimately, more efficient preservation of forest-associated biodiversity.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f16091382/s1 Figure S1: Spatial distribution of 1373 permanent, circular sample plots (size 0.04 ha) in Białowieża Forest; Figure S2: Model tree species composition (in % of basal area) of forest stands in the woodland community types occurring in Białowieża Forest; Table S1: Woodland community types occurring in the managed part of Białowieża Forest and their area, in absolute and relative terms; Table S2: Actual DBH distributions (trees·ha−1) of tree species in the managed part of the BF; Table S3: A model (target) value of the basal area, total and for individual tree species; Table S4: Model (target) number of trees of individual species in subsequent classes of theoretical (equilibrium) DBH distributions.

Author Contributions

Conceptualization, B.B.; formal analysis, J.Z.; investigation, B.B., J.Z. and M.K.; methodology, B.B. and J.Z.; software, J.Z.; writing—original draft, B.B.; writing—review and editing, B.B., J.Z. and M.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the General Directorate of State Forests in Poland (GA No. EO.271.3.1.2019).

Data Availability Statement

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Acknowledgments

We are very thankful to two anonymous reviewers who provided very helpful comments on an earlier draft of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Forests 16 01382 g001
Figure 1. Actual (A) and model (B) share of the number of trees of individual species occurring in the managed part of Białowieża Forest by subsequent diameter class.
Figure 1. Actual (A) and model (B) share of the number of trees of individual species occurring in the managed part of Białowieża Forest by subsequent diameter class.
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Forests 16 01382 g002
Figure 2. Surplus (overall and reduced) and deficit trees shown against the background of model and hypothetical (help) distributions developed for individual tree species occurring in Białowieża woodland communities.
Figure 2. Surplus (overall and reduced) and deficit trees shown against the background of model and hypothetical (help) distributions developed for individual tree species occurring in Białowieża woodland communities.
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Forests 16 01382 g003
Figure 3. The amount of the total (A) and reduced (B) surpluses (in relation to the equilibrium distributions) of trees of individual species by diameter class. Reduced surpluses are trees intended for removal during the next 10-year planning period.
Figure 3. The amount of the total (A) and reduced (B) surpluses (in relation to the equilibrium distributions) of trees of individual species by diameter class. Reduced surpluses are trees intended for removal during the next 10-year planning period.
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Forests 16 01382 g004
Figure 4. The amount of tree shortages (deficits in relation to equilibrium models) of individual species by diameter class.
Figure 4. The amount of tree shortages (deficits in relation to equilibrium models) of individual species by diameter class.
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Forests 16 01382 g005
Figure 5. Actual diameter distributions of pine and oak in the managed part of BF (MF) and in the “Strict Reserve” of BNP (SR). Data for MF and SR were determined on the basis of 855 and 116 circular sample plots (size 0.04 ha), respectively.
Figure 5. Actual diameter distributions of pine and oak in the managed part of BF (MF) and in the “Strict Reserve” of BNP (SR). Data for MF and SR were determined on the basis of 855 and 116 circular sample plots (size 0.04 ha), respectively.
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Table 1. The average amount of deficits in the 9 cm class and the approximate average size of regeneration units (total and by species).
Table 1. The average amount of deficits in the 9 cm class and the approximate average size of regeneration units (total and by species).
Tree SpeciesArea of Growing Space for a Tree with DBH = 9 cm (GS, in m2) *Average Number
of Deficit Trees in the
Smallest Diameter Class
(Midpoint = 9 cm)
(DEF, in Trees × ha−1)		Estimated Area
of Regeneration Spots per 100 ha (RS, in ha)
Pine8.1312.501.02
Birch12.5534.464.32
Aspen12.554.610.58
Spruce10.820.000.00
Oak6.1423.431.44
Hornbeam25.240.000.00
Lime15.980.110.02
Maple13.293.850.51
Elm23.102.660.62
Ash10.753.590.39
Alder10.140.800.08
Total 86.008.97
* Accord. to [54].
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Brzeziecki, B.; Zajączkowski, J.; Ksepko, M. A Demographic Imbalance of Tree Populations in the Managed Part of Białowieża Forest (NE Poland): Implications for Nature-Oriented Forestry. Forests 2025, 16, 1382. https://doi.org/10.3390/f16091382

AMA Style

Brzeziecki B, Zajączkowski J, Ksepko M. A Demographic Imbalance of Tree Populations in the Managed Part of Białowieża Forest (NE Poland): Implications for Nature-Oriented Forestry. Forests. 2025; 16(9):1382. https://doi.org/10.3390/f16091382

Chicago/Turabian Style

Brzeziecki, Bogdan, Jacek Zajączkowski, and Marek Ksepko. 2025. "A Demographic Imbalance of Tree Populations in the Managed Part of Białowieża Forest (NE Poland): Implications for Nature-Oriented Forestry" Forests 16, no. 9: 1382. https://doi.org/10.3390/f16091382

APA Style

Brzeziecki, B., Zajączkowski, J., & Ksepko, M. (2025). A Demographic Imbalance of Tree Populations in the Managed Part of Białowieża Forest (NE Poland): Implications for Nature-Oriented Forestry. Forests, 16(9), 1382. https://doi.org/10.3390/f16091382

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