In temperate regions, winter is characterized by cold temperatures and low food availability. Heterothermic animals can bridge this period by entering a state of torpor characterized by decreased body temperature and reduced metabolic rate. Hibernation site choice is crucial since temperature conditions in the hibernaculum will impact torpor. We analysed temperature-dependent hibernation site use of Barbastella barbastellus. Bats and temperature were monitored in an underground system (1999–2019) and standalone bunkers (2007–2019) in Western Poland. During the winter of 2017–2018 we analysed the thermal variability of the hibernacula. Seasonal variation is higher in bunkers and thus temperatures get colder in winter than in the underground system. On the other hand, short-term variability (thermal variability index) in the bunkers was lower than in the underground system. This makes bunkers a more stable environment to hibernate for cold dwelling bats in warm winters, when temperatures in the bunkers do not get below freezing. Bats use both the warm underground system and the colder bunkers. During the last decade, a continuous series of warm winters occurred and the population of barbastelle bats partly moved from the underground system to the bunkers. These present temperature increases broadened the range of potential hibernation sites for barbastelles. Our study indicates that long-term trends, seasonal variation and short-term variability in temperatures are all important and should be analysed to investigate hibernaculum use by bats. Our study shows that small hibernation sites may become more important in the future.

Białowieża Primeval Forest (BPF, approx. 1700 km²) is an important forest area in Europe from the point of view of the protection of natural diversity. BPF is covered with old mixed tree stands of mostly natural origin. Norway spruce is a tree species in BPF and covers approx. 27% of its area. Between 2012 and 2017 a large outbreak of the bark beetle Ips typographus (Linnaeus, 1758) took place in the forest, which transformed the stands and left many dead standing trees. At that time salvage logging had begun but was stopped due to protests by scientists and activists and for legal reasons. As a result of research conducted using a radiotelemetry method in 2020, we found that the Western barbastelle bat Barbastella barbastellus (Schreber, 1774) chooses nursery roosts in dead Norway spruce trees, showing ecological plasticity by colonizing a newly available resource. Based on this, we found that the Western barbastelle has a preference for a type of roost rather than a tree species. Insect outbreaks in forests of primary, natural, or semi-natural origin are one of the natural factors that shape the habitat. Removal of dead standing trees disrupts these processes, and in this particular case results in the disappearance of a newly appeared ecological niche.

The occurrence of the Common pipistrelle in the bat fauna of the Białowieża Primeval Forest was expected, because the taxon P. pipistrellus was previously comprised of two separate species. Earlier observations of P. pipistrellus in the Białowieża area can mostly be counted for by the new species Soprano pipistrelle, which is considered equally widely distributed in the area. In this study, we were able to record and distinguish several passes of the Common pipistrelle by means of bat detecting techniques. Bats were found in two locations, the first one being the Białowieża village, whereas the second one is the woodland in close proximity of the Czerlonka village. It is very likely, that this species is more widely distributed in the Białowieża area, especially in the vicinity of human settlements.

Key message: An outbreak of the European spruce bark beetle (Ips typographus L.) in spruce (Picea abies L.) dominated forest stands in the Białowieża Forest, Poland, has led to fundamental changes in forest characteristics. The species composition of regenerating stands now differs from earlier records findings, with oak, birch and rowan becoming dominant, while hornbeam numbers have decreased and Norway spruce is now sporadic, suggesting an ongoing period of reorganisation. Among the forest attributes showing the greatest degree of change are canopy architecture and sheltering opportunities—factors known to be most influential for volant-dependent wildlife. Context: The European spruce bark beetle (Ips typographus L.) targets Norway spruce (Picea abies (L.) H.). Once attacked, susceptible trees weaken and die. In planted forests with high proportions of Norway spruce, beetle outbreaks profoundly alter forest structure. The implications of such outbreaks on forest regeneration and their influence on dependent wildlife require ongoing research. Aims: We investigated how bark beetle-induced changes in Norway spruce stands alter forest structure characteristics and regeneration, and explored the potential consequences for forest-dependent volant vertebrates. Methods: We used a static comparative approach to assess differences in forest structure and regeneration between infested and non-infested Norway spruce stands in the Białowieża Forest, Poland. Eighteen forest variables were measured or derived, including indicators of canopy structure, tree composition, bark features, cavity presence, and microclimate. Sapling composition and regeneration frequency were also recorded to evaluate vegetation dynamics. Data were collected during 2022 and 2023. Results: Fifteen forest structural characteristics differed significantly post-infestation, including reduced tree density (trees per hectare), decreased vegetation clutter, and increased light transmittance. Daytime temperatures increased and humidity decreased, while nighttime conditions remained similar. The number of animal shelters in exfoliating bark increased, whereas tree crevices became less frequent. Sapling regeneration was higher in infested stands and dominated by oak (Quercus spp. L.), birch (Betula pendula Roth.), and rowan (Sorbus aucuparia L.). In contrast to earlier research, hornbeam was not the dominant sapling species, and Norway spruce saplings were infrequent. Conclusion: While our static comparative approach did not capture temporal dynamics or fully disentangle complex ecological interactions, it provided clear evidence that bark beetle outbreaks dramatically alter Norway spruce-dominated stands. The forest attributes most altered by infestation are well-established drivers of habitat suitability for forest-dwelling volant wildlife. We propose that cyclical beetle outbreaks in spruce-dominated forests act as drivers of accelerated natural succession, shifting species composition from planted Norway spruce to more naturally occurring species. Our findings highlight that bark beetle outbreaks represent natural change, not forest destruction, within this unique European forest ecosystem.