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Article

Tree- and Stand-Scale Roost Selection and Partitioning by Bats Barbastella barbastellus Schreber, 1774 and Pipistrellus pygmaeus Leach, 1825 in a European Lowland Forest

by
Alek Rachwald
1,
Grzegorz Apoznański
2,
Tomasz Oszako
3,*,
Sandra Krzemińska
2,
Ireneusz Ruczyński
4,
Ewa Komar
4,
Marcin Zegarek
4 and
Andrew Carr
1,*
1
Forest Ecology Department, Forest Research Institute, Sękocin Stary, 05-090 Raszyn, Poland
2
Department of Vertebrate Ecology and Paleontology, Institute of Environmental Biology, Wrocław University of Environmental and Life Sciences, 50-375 Wrocław, Poland
3
Department of Forest Protection, Forest Research Institute, Sękocin Stary, 05-090 Raszyn, Poland
4
Mammal Research Institute, 17-230 Białowieża, Poland
*
Authors to whom correspondence should be addressed.
Forests 2025, 16(7), 1189; https://doi.org/10.3390/f16071189 (registering DOI)
Submission received: 4 June 2025 / Revised: 14 July 2025 / Accepted: 17 July 2025 / Published: 19 July 2025
(This article belongs to the Special Issue Conservation and Restoration of Forest Biodiversity)
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Abstract

Our research focused on the roost preferences of two bat species in a forest environment. Throughout the Anthropocene, people have heavily altered the landscape. Forested habitation has declined, with remaining forests becoming fragmented and often deprived of old trees that provide shelter opportunities for bats. TReMs (tree-related microhabitats) are essential for forest-dwelling bat species as they provide an opportunity to roost and shelter. Following an infestation of Ips typographus L., the Białowieża Forest is saturated with dead spruce trees. We investigated roost selection in two forest-dwelling species, Barbastella barbastellus and Pipistrellus pygmaeus. To examine similarities or differences in roost selection between species, we radio-tracked 24 barbastelles and 13 soprano pipistrelles over three breeding seasons. We located a total of 48 barbastelle roosts and 15 pipistrelle roosts, together with the characteristics of the surroundings. We found that barbastelles select roosts almost exclusively in dead spruce trees (43/48), while pipistrelles selected roosts predominantly in live (n = 8) and dead (n = 5) broadleaved trees. Our results show that both bat species have clear differences in roost tree preference. In our study area, with an abundance of exfoliating bark, barbastelles showed a preference for roosting under flaking bark despite the availability of crevices within broadleaved trees. Our findings provide useful insight into forestry practices, highlighting the importance of standing dead trees.

1. Introduction

Two limiting resources for temperate bat populations during the summer are prey and roosts [1,2,3]. As people continue to impact the landscape, many temperate bats adapt to use man-made structures for roosting. However, some species remain largely dependent on natural roosts such as tree crevices. Providing foraging and shelter opportunities, forested areas remain habitats of critical importance for many bat species [4,5]. Tree crevices, often referred to as tree-related microhabitats (TreMs) [6], are important for the forest ecosystem, as they provide roosting opportunities for bats.
The presence and abundance of TreMs define local bat populations as they are crucial for reproductive success and survival [1,3,7,8,9,10,11]. Forest-dwelling bats use several types of TreMs, including woodpecker cavities, bark shelters, cracks, breaks, double leaders, and hollow heartwood [6,12], which are often species-specific.
The number of old trees and those with cavities suitable for roosting bats has declined in European forests due to the traditional and now partially abandoned silvicultural practices that remove dead or damaged trees by categorising them as weakened or ‘overgrown’, or disease vectors [11,13,14,15]. As a result, the number of tree crevices and standing deadwood is declining when compared to natural forests, as they are routinely removed during thinning cycles to maximise economic yield—posing a threat to bats and other forest-dwelling species [14,16]. Studying the roosting preferences of tree-dwelling bats is particularly valuable in relatively undisturbed forests (i.e., natural or semi-natural), where their behaviour is least influenced by anthropogenic alterations to the habitat [17]. This knowledge will most effectively inform future efforts to protect bats and restore natural processes within human-altered forest ecosystems.
Among forest-dwelling bats in Europe, two species warrant particular attention due to their association with forests or due to gaps in knowledge regarding their ecology and habitat preferences: the western barbastelle (Barbastella barbastellus) and the soprano pipistrelle (Pipistrellus pygmaeus). Although barbastelles exhibit ecological plasticity across their range [18,19,20], the species has maintained a preference for roosting within forest habitats [21,22,23,24]. The population is fragmented throughout its geographic range (which covers Western, Central, and Southern Europe), and is currently classified as Near Threatened (IUCN Red List: https://www.iucnredlist.org/fr/species/2553/22029285, accessed on 16 July 2025). Soprano pipistrelles are a widespread European bat species occurring in high densities in both forested and non-forested habitats (e.g., agriculture, urban areas). However, recent studies indicate that the species prefers the forest environment relative to other habitats, and its distribution is currently increasing [25,26].
In the Białowieża Primaeval Forest (hereafter referred to as BPF), these two species are considered relatively common. They co-occur and occupy a similar ecological niche as members of the guild “mid-range echolocators”, “MRE” [27], or “edge-space aerial foragers” [28]. Although the foraging niches of these species overlap only marginally [29,30,31,32,33,34], it remains unclear whether they compete for roosting sites. Research in heavily managed (and often fragmented) forests in Europe suggests that roosting barbastelles and soprano pipistrelles use similar tree species, preferring deciduous trees, although they often differ in roost type [17,22,24,33,35]. However, research in contiguous and well-preserved forests suggests barbastelles may prefer coniferous trees under suitable conditions [17,25] and may avoid the soprano pipistrelle roosting niche in this respect [36]. Dietz et al. [17] found soprano pipistrelles roosted almost exclusively behind flaking bark of decaying broadleaved trees. In order to fill a current knowledge gap, we decided to investigate roost selection by barbastelles and soprano pipistrelles. We investigated the tree roosting preferences of these two co-occurring bat species in a forest habitat that closely resembles natural forests, such as those present in the BPF.

2. Materials and Methods

2.1. Study Area

The BPF (52°45′29″ N, 23°46′8″ E) is a forest area with a significant degree of naturalness, covering an area of approximately 1500 km2 (divided by a state border between the Republic of Poland and the Republic of Belarus). BPF includes areas of the primaeval forest preserved in the National Park (area approximately 100 km2) and large areas of both natural and managed forests. The managed forest is similar in origin and composition to natural forest characteristics [36,37,38]. Historically, BPF is a remnant of a larger forest complex protected by royal orders as early as the 15th century [39,40], which, despite tree losses incurred in some periods (predatory economy during World War I, the interwar period and forest management after World War II) is still distinguished by a good degree of preservation and the occurrence of natural ecological processes. The significance of the BPF lies not only in its historical continuity but also in its spatial integrity, which has been preserved due to 600 years of protection with only minor interruptions, preventing fragmentation typically associated with human-altered landscapes. Forest fragmentation affects the forest configuration across various spatial scales, disturbs natural processes in the forests, and also threatens the forest-dwelling bat population [16,33,41,42,43].
Some parts of the BPF were significantly transformed during the last outbreak of bark beetle Ips typographus, which occurred between 2012 and 2019 [44]. The beetle outbreak changed the spatial characteristics and age structure of stands with a participation of Norway spruce, reduced the total share of coniferous trees in the BPF stands, and increased the amount of standing and lying deadwood in the forest [44,45,46,47], all factors that have a potentially significant impact on the functioning of the entire ecosystem and resources available to bats.

2.2. Target Species

The Western barbastelle bat (Barbastella barbastellus) is a medium-sized species distributed across the temperate regions of the Northern Hemisphere. Its geographical range covers all Western and Central Europe except for northern Scandinavia, reaching the southern coast of the Mediterranean Sea, in the East to Belarus, Ukraine, and the Eastern coast of the Black Sea as well as the western coast of the Caspian Sea (foot of the Talysh Mountains) (IUCN Red List https://www.iucnredlist.org/fr/species/2553/22029285, accessed on 16 July 2025). In Europe, barbastelles inhabit both forests and forest-field landscapes. In forests, they occupy roosts in trees, but in the agriculturally transformed landscape, colonies in buildings are documented, usually in the wooden walls of farm buildings. (e.g., Ref. [20]). In the forest, they occupy roosting places in cracks of tree trunks, tree hollows and under protruding bark, most often deciduous [22,24,35,48] but also coniferous [17,25,48]. It is a food specialist feeding mainly on nocturnal Lepidoptera moths [29,30,31,34,49,50,51]. The species is included in Annex II of the European Union Habitat Directive and has the IUCN “NT” (“near threatened”) status.
Soprano pipistrelles (Pipistrellus pygmaeus) are a small species widely spread throughout most of Europe, including southern Scandinavia, with enclaves in the Caucasus (https://www.iucnredlist.org/ja/species/136649/21990234, accessed on 16 July 2025). It is a species that commonly inhabits built-up urban areas where it forms nursery colonies in buildings [32,52]. In agricultural and forest landscapes, soprano pipistrelles feed at ecotones and riparian habitats [50]. Recent studies indicate a high abundance of the species in forest habitats [26,53,54]. Therefore, it can be considered an eurytopic species. Soprano pipistrelle diet includes insects from the Chironomidae and Ceratopogonidae families, mostly aquatic flying insects, with a mixture of many other groups [30,51].

2.3. Bat Capture and Telemetry

Bats were captured with monofilament mist nets (ECOTONE, Gdynia, Poland) and fitted with telemetry transmitters (LB2-X, Holohil Systems, Carp, ON, Canada). Two or three nets were placed at single locations along expected bat flight commuting routes within BPF (open patches, small dust roads and forest tracks). To increase the capture of bats flying higher and closer to the tree canopy, a single net set up consisted of two nets, each approximately 2.5 m high, placed one on top of the other [25]. The length of the mist net depended on the width of the patch or dust road; most often, it was 9 m. Biometric data was collected, including forearm length and body weight.
Due to differences in phenology, we diversified our fieldwork schedule. Soprano pipistrelle females give birth earlier in the summer season than female barbastelles. We targeted soprano pipistrelles in the second half of June and barbastelles in the second half of July to target lactating females and avoid catching heavily pregnant individuals and volant pups [55,56]. We considered that attaching telemetry tags on females during the lactation period was ethical and provided data on nursery colony roost preference in addition to roosts selected by males.
Transmitters were attached to the bat’s dorsal side under the hair coat with the use of non-allergenic Torbot® glue (Torbot Group, Inc., Toledo, OH, USA). The animal was held in a clean cloth bag for up to 15 min (to allow the glue to set) and then released. Tracking of tagged bats was conducted by three teams, each equipped with a Lotek Biotracker VHF receiver (Lotek Wireless Inc., Newmarket, ON, Canada) and a directional antenna. The LB2-X transmitters weighed 0.31 g and were selected according to the 5% body weight rule. All animals were tracked every day until transmitter failure or loss. An appropriate permit was obtained from the Regional Directorate for Environmental Protection in Białystok (number WPN.6401.110.2022.KP).

2.4. Forest Characteristics

Netting points were in tree stands with a mix of coniferous and deciduous trees. Forest plots were identified using publicly available maps (Forest Data Bank, https://www.bdl.lasy.gov.pl/portal/mapy, accessed on 6 July 2022) that showed the boundaries of forest sub-compartments with similar species composition and age. The maps provided detailed data on species structure and age of each stand. We recorded the following roost characteristics: roost tree species, dominant tree species, and stand age. For each roost tree, the four nearest trees (one per cardinal direction) with a DBH exceeding 25 cm were selected, and the tree species and condition (living/dead) were recorded according to a point-centred quarter method [56]. The selection of the research site was preceded by previous studies, during which the two dominant species of bats (soprano pipistrelle and barbastelle) were observed in BPF stands [54], and barbastelles were also found to occupy roosts in dead spruce trees [25].

2.5. Data Analysis

To evaluate whether barbastelles and pipistrelles selected roosts non-randomly with respect to available habitat features, we conducted a series of chi-square goodness-of-fit tests comparing observed roost characteristics with expected distributions based on collected or availability data. For each analysis, we constructed a one-way contingency table, where each row represented a category (e.g., tree species or stand type), and the columns represented observed and expected frequencies. Expected frequencies were based on independently collected availability data (detailed below), and degrees of freedom (df) were calculated as the number of categories—1. For each test, expected frequencies were calculated by multiplying the total number of observed roosts by the proportional availability of each category (e.g., tree species or forest stand type). These proportions were from neighbouring tree data or derived from independent datasets categorising the proportion of forest stand types in the study area, or the dominant tree species across stands.
To identify which categories differed significantly from expected frequencies, we applied Bonferroni-adjusted confidence intervals to the observed versus expected counts for each chi-square test, allowing for the identification of specific categories with statistically significant over- or under-representation.

2.5.1. Roost Tree Selection

To examine whether barbastelles and pipistrelles selected roosts in a particular tree species, we used chi-square analyses (goodness-of-fit) to determine whether the use of a tree species (number of roosts of each tree type divided by the total number of roosts in the study) departed from that expected (number of neighbouring tree type of each tree type divided by the total number of neighbour trees in the study). Tree species were divided into three categories for barbastelles as (1) Picea abies, (2) Pinus sylvestris, and (3) ‘other’ tree species, and four categories for pipistrelles as (1) P. abies, (2) P. sylvestris, (3) Quercus robur and (4) ‘other’ tree species.

2.5.2. Forest Stand Type Selection

To examine whether barbastelles and pipistrelles selected roost trees within a particular forest stand type, we used a chi-square analyses (goodness-of-fit) to determine whether the use of forest stand type (number of roosts within a stand type divided by the total number of roosts in the study) departed from that expected (the proportion of different habitat types from prior habitat surveys), and a Fisher’s exact test. Stand type was determined from the dominant tree species and categorised as either coniferous or broadleaved. The current distribution and composition of habitats within BPF were determined from the literature as 55% coniferous and 45% broadleaved [45,57]. We assumed that the sampled bats had equal access to all areas within the contiguous forest complex, based on the absence of significant movement barriers and the known ranging abilities of both species, which allow them to traverse the entire study area [23,24,35,50].

2.5.3. Dominant Tree Species Stand Selection

To examine whether barbastelles and pipistrelles selected roosts within forest stands with a particular dominant tree species, we used chi-square analyses (goodness-of-fit) to determine whether the use of a forest stand type with a particular dominant tree species (number of roosts within a stand with a particular dominant tree species [barbastelles: P. abies or ‘other species’, pipistrelles: Q. robur or ‘other species’] divided by the total number roosts in the study) departed from that expected (the proportion of different dominant trees in all stands with roosts), and a Fisher’s exact test. We selected the dominant roosting tree for each bat species as one category and all other dominant tree species as the other. We consider this a suitable approach to explore whether bats sought out tree stands dominated by their most used roost trees (i.e., barbastelle = P. abies, pipistrelle = Q. robur) and whether this reflected the availability of that tree species at the forest stand level.
All statistical analyses were conducted in R version 4.4.2; R Core Team, 2024 [58]). Chi-square and Fisher’s exact tests were performed using the chisq.test() and fisher.test() functions from base R. Post hoc comparisons with Bonferroni correction were conducted using the pairwise.prop.test() function, and multinomial confidence intervals were calculated using the MultinomCI() function from the DescTools v. 0.99.60 package.

3. Results

3.1. Roost Tree Parameters

During three seasons of research, a total of 14 barbastelle females and 10 males, as well as 13 soprano pipistrelles females, were captured, fitted with telemetry tags and tracked. All females were adult and lactating. From the tracked bats, we obtained a total of 48 barbastelle roosts and 15 soprano pipistrelle roosts (Figure 1). We recorded roost tree species from all roosts for barbastelles and pipistrelles, and further nearest neighbour trees and stand type from all soprano pipistrelle and 29 barbastelle roosts (Figure 2, Table 1 and Table 2).
Tree selection by both bat species, compared to the composition of nearby trees (Table 1), revealed a marked preference for dead spruce for barbastelles and a less pronounced but still evident preference for oak by soprano pipistrelles. The mean diameter at breast height (DBH) of the roost trees was 43.57 cm for barbastelles and 82.07 cm for soprano pipistrelles.
Thermal imaging observations of barbastelle nursery roosts (n = 6) during the evening emerging period found that the size of groups of females roosting under the protruding bark of Norway spruce trees ranged from 8 to 17 individuals (Figure 3).

3.2. Selection of Roost Trees

Observed barbastelle roost trees consisted of standing dead P. abies (n = 43) and P. sylvestris (n = 4) trees, and one C. betulus. Expected roost trees consisted of P. abies (n = 23), P. sylvestris (n = 14) and ‘other’ tree species (n = 11). Tree species were not selected at random (χ233, d.f.= 2, p < 0.001) by roosting bats. Bonferroni-corrected p-values show that the sampled barbastelles selected P. abies significantly more often than expected based on availability (χ2 = 16.6, p < 0.001), while P. sylvestris and other tree species were selected significantly less often than expected (χ2 = 7.3, p = 0.02 and χ2 = 8.6, p < 0.01, respectively) (Table 3).
Observed soprano pipistrelle roost trees consisted of P. abies (n = 2), Q. robur (n = 11), B. verrucosa (n = 1) and A. pseudoplatanus (n = 1). Expected roost trees consisted of P. abies (n = 5), P. sylvestris (n = 1), Q. robur (n = 3) and ‘other’ tree species (n = 7). Tree species were not selected at random (χ224, d.f.= 3, p < 0.001) by roosting bats. Bonferroni-corrected p-values show that the sampled pipistrelles selected Q. robur significantly more often than expected based on availability (χ2 = 21.3, p < 0.001), while P. abies, P. sylvestris and ‘other’ tree species were used in line with availability (Table 3).

3.3. Selection of Forest Stand Type

Barbastelle roost trees were located in coniferous (n = 14) and broadleaved (n = 15) forest stands. Using the documented proportions of coniferous and broadleaved tree stands throughout the forest complex, the expected number of barbastelle roost trees in coniferous stands was 16, and in broadleaved tree stands, it was 13. Chi-square goodness-of-fit test showed no significant difference between observed and expected values (χ20.6, d.f = 1, p = n.s). A Fisher’s exact test was additionally performed, which confirmed no significance (p = 0.79). Despite a preference for roosting within conifer trees, the sampled barbastelles do not appear to select coniferous dominant stands and use coniferous and broadleaved forest stands in line with their availability.
Soprano pipistrelle roost trees were located in coniferous (n = 2) and broadleaved forest stands (n = 13). Using the documented proportions of coniferous and broadleaved tree stands throughout the forest complex, the expected number of pipistrelle roost trees in coniferous stands was eight, and in broadleaved tree stands, it was seven. Chi-square goodness-of-fit test showed a significant deviation from expectations (χ210, d.f = 1, p < 0.01). Bonferroni-corrected p-values show the sampled soprano pipistrelles selected roost trees in broadleaved forest stands significantly more often than expected based on availability (χ2 = 5.2, p < 0.05), whilst selecting roost trees in coniferous stands in line with availability.

3.4. Selection of Dominant Tree Species Stand

Barbastelle roost trees were not located in P. abies-dominated forest stands, leaving all roosts within ‘other’ (n = 29) dominated forest stands (the proportion of the dominant tree species within each forest stand was variable). Using the documented proportions of dominant tree species composition throughout the forest complex, the expected number of barbastelle roost trees in P. abies-dominated stands was seven, and roost trees in stands dominated by ‘other’ tree species were 22. Chi-square goodness-of-fit test showed a significant deviation from expectations (χ29.7, d.f = 1, p < 0.01). Given the low observed count in one category (0 P. abies), a Fisher’s exact test was also conducted, which confirmed the significance of the deviation (p = 0.01). Despite a preference for roosting within P. abies trees, the sampled barbastelles select roost trees in stands dominated by P. abies significantly less often than expected based on availability (χ2 = 7.25, p < 0.05).
Soprano pipistrelle roost trees were located in Q. robur (n = 2) and ‘other’ (n = 13) dominated forest stands (the proportion of the dominant tree species within each forest stand was variable). Using the documented proportions of dominant tree species composition throughout the forest complex, the expected number of pipistrelle roost trees in Q.robur-dominated stands was two, and roost trees in stands dominated by ‘other’ tree species were 13. Chi-square goodness-of-fit test showed no significant difference between observed and expected values (χ20.08, d.f = 1, p = ns). A Fisher’s exact test confirmed the lack of significant difference (p = 1.00). Despite a preference for roosting within Q. robur trees, the sampled pipistrelles do not appear to select stands dominated by Q. robur and use roost trees within Q. robur-dominated stands in line with their availability.

4. Discussion

The literature on roosting in buildings opposed to trees is richer for soprano pipistrelles and barbastelles [20,27,59]. Both species have a degree of environmental flexibility, often sheltering in buildings (e.g., on farms) in human-modified landscapes. Even the barbastelle, a species that is considered a forest-dwelling species [21,23], roosts in man-made structures or other natural roosts such as rock crevices [60], and commutes between forests, travelling considerable distances to forage [20,24]. As such, any direct comparison of roosting preferences between soprano pipistrelles and barbastelles in the forest environment is not possible without conducting studies in an appropriately selected forest complex where both species occur together. The natural tree stands of BFP (and the bordering Belovezhskaya Pushcha National Park in Belarus) [17] are suitable areas for studying the selection of tree roosts by bats due to the large area of uninterrupted stands and due to their diversified species, age and spatial structure, which corresponds to the natural conditions of the European lowland forest.

4.1. Roost Trees

In the forest habitat, the barbastelle bat is a species that occurs most frequently in old stands, where it has a wide range of natural roosting opportunities. Barbastelles prefer roosting in crevices categorised as cracks in tree trunks [24] and the space under protruding bark (boot crevices and bark shelters) [6]. In studies conducted in relatively undisturbed forests in northern Italy, this species favoured old forests and roosted mainly under protruding bark in beech trees [22]. However, in forests rich in coniferous trees, especially diseased, old and dead trees, barbastelles roost under the bark of dead spruce trees [25,48]. We located barbastelles roosting almost exclusively under the flaking bark of Norway spruce trees despite the opportunity to roost in the crevices of broadleaved trees. This is evidence that barbastelles prefer roosting in relatively uninsulated roosts over more insulated trunk crevices. Tolerance for uninsulated roosts could be explained by their behavioural trait of social thermoregulation [61] and fission-fusion roosting strategy. However, in a forest with an abundance of persistent roosts, such as trunk cracks, barbastelles show fidelity for ephemeral roosts, which has been shown across multiple studies [17,48]. We can conclude that shallow roosts provide optimal conditions for barbastelles.
Soprano pipistrelles are considered eurytopic species. Recent research indicates considerable ecological flexibility and highlights potential displacement of closely related, competing species [26]. In contrast to their “sibling” species Pipistrellus pipistrellus (Schreber, 1774), soprano pipistrelles occur in high densities in European forests and compete with P. nathusii (Keyserling et Blasius, 1839), which is considered a forest and wetland species [62]. In Scotland, soprano pipistrelles appeared to have an advantage over common pipistrelles in conifer-dominated forests, while the latter was more strongly associated with agricultural landscapes [50,53].
In studies conducted in BPF, soprano pipistrelles were found to be by far the most abundant among bats of the genus Pipistrellus [54]. Although colonies of soprano pipistrelles have been found in forest villages [63], they likely roost within trees in BPF. Competition between bats can be based on food, shelter and foraging sites. We investigated whether soprano pipistrelles compete with barbastelles for roosting resources (shelter), as both species occur in the BPF and are among the most common bats in the area.
A previous study in BPF on roost selectivity of male and female barbastelles [25] showed that barbastelles roosted almost exclusively in dead spruce trees. Although five years have passed since this previous study in BPF (the fieldwork was carried out in 2019), we show that the resource of standing dead spruce trees remaining after the 2012–2018 bark beetle outbreak continues to be the main roost type for barbastelles. This indicates a certain persistence of this seemingly ephemeral resource, consisting of protruding bark on dead spruce trees. Since the number of standing dead spruce trees has certainly decreased during this time (bark beetle outbreaks in the BPF occur cyclically every 10 or more years, and there is no indication of a new outbreak), we consider that the availability of this type of roost feature may be less ephemeral if the original resource was sufficiently large.
We found that the two bat species seem to be avoiding each other in terms of roost tree preference, most obviously that pipistrelles select roosts in broadleaved trees and barbastelles in coniferous trees. Given the lack of overlap between their foraging and prey niches [29,30,31,34,49,50], there is a lack of competition for resources between these two species. The already known selection of roosting shelter between these two species (barbastelle: crevices, flanking bark, gaps, soprano pipistrelle: tree cavities, but also flanking bark, e.g., Ref. [17], was complemented by a difference in the choice of inhabited tree species. While the soprano pipistrelles favoured deciduous trees, almost exclusively choosing old oaks, barbastelles chose dead spruce trees and roosted under protruding bark. It is noteworthy that, based on previous studies [22,24,35], barbastelles are expected to roost in broadleaved trees similarly to what we found for soprano pipistrelles. Unlike the Italian high forests, where barbastelles selected beech (Fagus sylvatica L.) trees for roosting in the BPF, where beech is absent, deciduous tree species comprise primarily mature oaks and hornbeams [36]. Selection of mature oak trees by barbastelles in southern England [24] indicates barbastelles in BPF would select these species given the choice, as would soprano pipistrelles [17]. In contrast, we found that the roost preferences of the two species for roost trees are different. On this basis, we hypothesise that the recent conditions in BPF (high availability of dead conifers) provided optimal roosting conditions for barbastelles in a mixed lowland forest of diversified structure. The BPF offers a particularly rich selection of roosts, which we consider to be natural habitats for forest bats. Our findings do not rule out the possibility that, under conditions of less diverse roosts, these two species may coexist in closer proximity and in more similar shelters.

4.2. Forest Stand Type

We found that barbastelles avoided spruce-dominated stands despite a significant preference for roosting within spruce trees. We consider two possible reasons for this. The first is that the spruce-dominated stands had been infested by bark beetles, and many Norway spruce trees had already fallen. This resulted in forest plots that had very little canopy but remaining single dead spruce trees. These remaining trees provide an abundance of roosting opportunities under flaking bark. However, most tracked bats roosted within spruce trees that were surrounded by other trees. Our observation was that single dead spruce trees in spruce-dominated stands (potential roost trees) had full exposure to the sun in all aspects, while roost trees mostly had some shade from the canopy. These trees were also less isolated, with a greater ability for the bats to emerge amongst the cover of surrounding tree canopies (potential predator avoidance). The second explanation is that the collected data was current and so reflected the current species composition of the stands. The forest databank (where we collected stand data for analyses) provides information on dominant living trees, excluding dead ones. After the beetle outbreak, there were numerous dead spruces in the studied stands, while the dominant species, according to the databank, remained alive. Damaged spruce stands in the BPF were mainly pines or some deciduous species, and according to current data, they are now dominant. The result is an apparent inconsistency of stand preferences with the preference for roost trees.
Norway spruce is an economic tree species throughout northern Europe and, as such, is almost everywhere subject to economic activities aimed at (among other things) the removal of old and dead trees, with significant consequences for the ecosystem [11,13,14,15]. Therefore, there are no realistic conditions to experience the role of these trees as elements of a mature ecosystem, including as a source of TReMs. Only the work in protected areas allows us to verify some previous statements about the preferences of barbastelles and their niche in the forest ecosystem. Despite the large supply of dead spruce as a result of the beetle outbreak (a new resource that emerged within a few years), soprano pipistrelles chose more “conservative” sites for nursery colonies. We also found that both bat species chose mature trees with a relatively high DBH. For the barbastelle, this value was on average lower than for the soprano pipistrelle, which can be attributed to the typically smaller trunk diameter of the favoured tree species (spruce). In the case of barbastelle, the factor that determined the age of roost trees (and thus the size of their DBH) was the fact that Ips typographus generally does not inhabit young trees [63,64]. It should be noted that at the time of the study, there was an exceptional situation in the forest, namely the mass death of old spruce trees, including “natural monuments” [65] that are more than a hundred years old. Exfoliating bark continues to slowly fall off infested and dead spruce trees, and, in the future, the wind will topple these standing dead trees. Even if they remain standing, it is unlikely that they will provide suitable roosting sites for bats (although research has not yet confirmed this).

5. Conclusions

The results of our study agree with earlier findings regarding the preference of roosting barbastelles in a lowland mixed forest with natural dynamics and show how two forest bat species utilise available roost resources in different ways. Our research could be supported by analysing habitat preferences in relation to foraging areas and investigating the diet composition of both species. Additional preliminary results by the authors suggest the possibility of niche divergence also in terms of foraging area selectivity. In the future, it is important to explore behavioural changes (including roosting) occurring in both species when the spruce trees naturally disappear from the species composition of the stands.

Author Contributions

Conceptualisation, A.R., A.C., G.A. and I.R.; methodology, A.R., A.C., G.A. and I.R.; formal analysis, A.C.; investigation, A.R., A.C., G.A., S.K., I.R., E.K., M.Z. and T.O.; resources, A.R. and I.R.; data curation, A.C.; writing—original draft preparation, A.R., A.C. and G.A.; writing—review and editing, A.R., A.C., G.A., S.K., I.R., E.K., M.Z. and T.O.; visualisation, A.R. and A.C.; supervision, A.R.; project administration, A.R.; funding acquisition, A.R., I.R. and T.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by grant 2021/41/B/NZ9/01397 financed by National Science Centre (NCN, Poland). The APC is financed by Forest Research Institute.

Data Availability Statement

All the data needed for our analysis is presented in the manuscript in the tables, with the exception of the geographical coordinates of bat roosts, due to the precautionary principle in nature conservation.

Acknowledgments

The authors would like to thank (in alphabetic order) Natalia Białecka, Gabriela Bielecka, Kane Colston, Wojciech Godlewski, Kinga Gołost, Monika Górska, Regina Grugel, Ruby Hill, Marc Khoueiry, Olga Łuczak, Natalia Małecka, Ewa Marszałek, Tomasz Marszałek, Emilia Rakowska, Jakub Ryczek, Bartosz Sarnowski, Henry Schofield, Mikołaj Świątek and Felix Tuff for valuable help with the fieldwork.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 16 01189 g001
Figure 1. Map of Białowieża Forest (BPF) showing the location of 48 Barbastella barbastellus, and 15 Pipistrellus pygmaeus roosts. Points have been jittered.
Figure 1. Map of Białowieża Forest (BPF) showing the location of 48 Barbastella barbastellus, and 15 Pipistrellus pygmaeus roosts. Points have been jittered.
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Forests 16 01189 g002
Figure 2. A dead Norway spruce tree inhabited by Barbastella barbastellus nursery colony (left) and a dead oak tree inhabited by Pipistrellus pygmaeus nursery colony (right) (photo: A. Rachwald).
Figure 2. A dead Norway spruce tree inhabited by Barbastella barbastellus nursery colony (left) and a dead oak tree inhabited by Pipistrellus pygmaeus nursery colony (right) (photo: A. Rachwald).
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Figure 3. Thermal imaging photo showing the emerging of a Barbastella barbastellus (Schreber, 1774) summer colony. Vertical streaks are tree trunks, the red dot represents exit from the roost as the place with the highest temperature. Blue spots represent places with the lowest temperature (photo I. Ruczyński).
Figure 3. Thermal imaging photo showing the emerging of a Barbastella barbastellus (Schreber, 1774) summer colony. Vertical streaks are tree trunks, the red dot represents exit from the roost as the place with the highest temperature. Blue spots represent places with the lowest temperature (photo I. Ruczyński).
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Table 1. The main parameters of the trees in which bat roosts were found.
Table 1. The main parameters of the trees in which bat roosts were found.
Bat SpeciesRoost Tree SpeciesTree StatusDBHNearest Four TreesStatus (D/L)
B. barbastellusP. abiesDead42Pabi/Bver/Pabi/PabiD/D/D/D
B. barbastellusP. abiesDead60Qrob/Aglu/Pabi/BverL/L/D/L
B. barbastellusP. abiesDead-Aglu/Aglu/Aglu/QrobL/L/L/L
B. barbastellusP. abiesDead50Psyl/Psyl/Ptre/PabiL/L/L/D
B. barbastellusP. abiesDead50Qrob/Qrob/Pabi/BverD/L/D/L
B. barbastellusP. abiesDead45Pabi/Psyl/Qrob/BverD/L/L/D
B. barbastellusC. betulusLive20Psyl/Psyl/Pabi/PabiD/L/L/D
B. barbastellusP. abiesDead28Qrob/Pabi/Qrob/PabiL/D/L/D
B. barbastellusP. abiesDead51Pabi/Cbet/Qrob/CbetD/L/L/L
B. barbastellusP. abiesDead47n.d.n.d.
B. barbastellusP. abiesDead31Pabi/Psyl/Pabi/PabiD/L/D/D
B. barbastellusP. abiesDead47Pabi/Psyl/Pabi/BverD/L/D/D
B. barbastellusP. abiesDead33Pabi/Psyl/Pabi/QrobD/L/D/L
B. barbastellusP. abiesDead45Psyl/Psyl/Psyl/PabiL/L/D/L
B. barbastellusP. abiesDead30Pabi/Pabi/Pabi/PabiD/D/D/D
B. barbastellusP. abiesDead51Psyl/Psyl/Psyl/PsylL/L/L/L
B. barbastellusP. abiesDead33Psyl/Pabi/Pabi/PabiL/D/D/D
B. barbastellusP. abiesDead26Pabi/Psyl/Pabi/PabiD/L/D/D
B. barbastellusP. sylvestrisDead50Psyl/Psyl/Pabi/PsylL/L/D/L
B. barbastellusP. abiesDead26Qrob/Qrob/Psyl/PsylL/L/L/L
B. barbastellusP. sylvestrisDead55Psyl/Pabi/Psyl/PsylD/D/D/D
B. barbastellusP. sylvestrisDead51Psyl/Pabi/Psyl/QrobD/D/D/L
B. barbastellusP. sylvestrisDead41Pabi/Psyl/Pabi/PsylD/D/L/L
B. barbastellusP. abiesDead55Pabi/Pabi/Pabi/PabiD/D/D/D
B. barbastellusP. abiesDead46Pabi/Pabi/Pabi/UglaD/D/D/L
B. barbastellusP. abiesDead42Pabi/Qrob/Ptre/PabiD/L/L/D
B. barbastellusP. abiesDead52Cbet/Pabi/Pabi/AgluL/D/D/L
B. barbastellusP. abiesDead53Pabi/Pabi/Cbet/PabiD/L/L/D
B. barbastellusP. abiesDead60Qrob/Aglu/Bver/PabiL/L/L/D
P. pygmaeusQ. roburDead110Ptre/Ptre/Ptre/BverL/L/L/L
P. pygmaeusQ. roburLive65Qrob/Qrob/Qrob/PabiL/D/L/L
P. pygmaeusQ. roburDead86Apse/Cbet/Pabi/TcorL/L/L/L
P. pygmaeusQ. roburDead75Qrob/Bver/Bver/PabiD/L/L/D
P. pygmaeusA. pseudoplatanusLive70Pabi/Pabi/Ptre/PabiD/D/L/D
P. pygmaeusP. abiesDead80Ptre/Ptre/Ptre/PtreL/L/L/L
P. pygmaeusQ. roburDead57Pabi/Apla/Pabi/QrobL/L/L/L
P. pygmaeusQ. roburLive120Bver/Cbet/Qrob/PabiL/L/L/L
P. pygmaeusB. verrucosaDead33Bver/Cbet/Pabi/PabiL/L/D/D
P. pygmaeusQ. roburDead130Qrob/Aglu/Pabi/CbetL/L/L/L
P. pygmaeusQ. roburLive72Qrob/Psyl/Pabi/QrobL/L/D/L
P. pygmaeusQ. roburLive100Pabi/Pabi/Cbet/BverD/L/L/L
P. pygmaeusP. abiesDead79Qrob/Pabi/Psyl/AgluL/D/L/L
P. pygmaeusQ. roburDead84Pabi/Pabi/Qrob/QrobL/D/L/L
P. pygmaeusQ. roburDead70Bver/Pabi/Psyl/QrobL/L/L/L
DBH—roost tree diameter at breast height (cm); nearest four trees (North, South, East, and West of the roost tree); status—tree live (L) or dead (D); Pabi—Picea abies (L.) H. Karst. (1881); Psyl—Pinus sylvestris L. (1753); Bver—Betula verrucosa Roth (1788); Cbet—Carpinus betulus L. (1753); Qrob—Quercus robur L. (1753); Aglu—Alnus glutinosa (L.) Gaertn. (1791); Ptre—Populus tremula L. (1753); Apla—Acer platanoides Linne 1753; Apse—Acer pseudoplatanus Linne 1753; Ugla—Ulmus glabra Huds. 1762.
Table 2. Total number of bat roosts discovered during whole research period with relevant tree species.
Table 2. Total number of bat roosts discovered during whole research period with relevant tree species.
Sp.PabiPsylBverCbetQrobApse
LDLDLDLDLDLD
Bbar0430400100000
Ppyg020001007410
L—living tree; D—dead tree; Bbar—Barbastella barbastellus; Ppyg—Pipistrellus pygmaeus; Pabi—Picea abies; Psyl—Pinus sylvestris; Bver—Betula verrucosa; Cbet—Carpinus betulus; Qrob—Quercus robur; Apse—Acer pseudoplatanus.
Table 3. Summary of chi-square and Fisher’s exact tests examining non-random roost selection by B. barbastellus and P. pygmaeus across tree species, forest stand types, and dominant stand-level tree species. Degrees of freedom (df), test statistics, and Bonferroni-corrected post hoc comparisons are provided to identify specific categories with significant over- or under-selection.
Table 3. Summary of chi-square and Fisher’s exact tests examining non-random roost selection by B. barbastellus and P. pygmaeus across tree species, forest stand types, and dominant stand-level tree species. Degrees of freedom (df), test statistics, and Bonferroni-corrected post hoc comparisons are provided to identify specific categories with significant over- or under-selection.
Selection TypeBat SpeciesCategories (k)dfTest UsedTest Results (χ2, p)Bonferroni Post Hoc
Tree species used as roostsB. barbastellusP. abies, P. sylvestris, Other2Chi-squareχ2 = 33.0, p < 0.001P. abies: χ2 = 16.6, p < 0.001; P. sylvestris: χ2 = 7.3, p = 0.02; Other χ2 = 8.6, p < 0.01
P. pygmaeusP. abies, P. sylvestris, Q. robur, Other3Chi-squareχ2= 24.0, p < 0.001Q. robur: χ2 = 21.3, p < 0.001; P. abies: χ2 = 1.6, p = ns; P. syvestris: χ2 = 0.8, p = ns; Other χ2 = 3.1, p = ns
Forest stand type (conifer vs. broadleaf)B. barbastellusConiferous, Broadleaved1Chi-square + Fisher’s exactχ2 = 0.6, p = ns; Fisher’s p = 0.79Coniferous: χ2 = 0.25, p = ns; Broadleaved: χ2 = 0.3, p = ns
P. pygmaeusConiferous, Broadleaved1Chi-squareχ2 = 10.0, p < 0.01Coniferous: χ2 = 4.5, p = ns; Broadleaved: χ2 = 5.2, p < 0.0
Dominant tree species in forest standsB. barbastellusP. abies, Other1Chi-square + Fisher’s exactχ2 = 9.7, p < 0.01; Fisher’s p = 0.01P. abies: χ2 = 7.25, p < 0.05; Other: χ2 = 2.4, p = ns
P. pygmaeusQ. robur, Other1Chi-square + Fisher’s exactχ2 = 0.08, p = ns; Fisher’s p = 1.0Q. robur: χ2 = 0.07, p = ns; Other: χ2 = 0.01, p = ns
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Rachwald, A.; Apoznański, G.; Oszako, T.; Krzemińska, S.; Ruczyński, I.; Komar, E.; Zegarek, M.; Carr, A. Tree- and Stand-Scale Roost Selection and Partitioning by Bats Barbastella barbastellus Schreber, 1774 and Pipistrellus pygmaeus Leach, 1825 in a European Lowland Forest. Forests 2025, 16, 1189. https://doi.org/10.3390/f16071189

AMA Style

Rachwald A, Apoznański G, Oszako T, Krzemińska S, Ruczyński I, Komar E, Zegarek M, Carr A. Tree- and Stand-Scale Roost Selection and Partitioning by Bats Barbastella barbastellus Schreber, 1774 and Pipistrellus pygmaeus Leach, 1825 in a European Lowland Forest. Forests. 2025; 16(7):1189. https://doi.org/10.3390/f16071189

Chicago/Turabian Style

Rachwald, Alek, Grzegorz Apoznański, Tomasz Oszako, Sandra Krzemińska, Ireneusz Ruczyński, Ewa Komar, Marcin Zegarek, and Andrew Carr. 2025. "Tree- and Stand-Scale Roost Selection and Partitioning by Bats Barbastella barbastellus Schreber, 1774 and Pipistrellus pygmaeus Leach, 1825 in a European Lowland Forest" Forests 16, no. 7: 1189. https://doi.org/10.3390/f16071189

APA Style

Rachwald, A., Apoznański, G., Oszako, T., Krzemińska, S., Ruczyński, I., Komar, E., Zegarek, M., & Carr, A. (2025). Tree- and Stand-Scale Roost Selection and Partitioning by Bats Barbastella barbastellus Schreber, 1774 and Pipistrellus pygmaeus Leach, 1825 in a European Lowland Forest. Forests, 16(7), 1189. https://doi.org/10.3390/f16071189

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