Importance for vertebrates in temperate forests

A total of 184 interaction events between vertebrates and WHs were observed at BF, 246 (ground ≤ 2 m: 160; canopy > 2 m: 86) at ALB and 176 (111; 65) at SCH.

In total, we found 28 vertebrate species visiting WHs (Table 1), including one amphibian, 17 bird and 11 mammal species. We provide information on the frequencies of observation per day and tree. Among mammals, at BF and ALB the Eurasian squirrel was among the most frequently observed species (observed on average 0.3 times per WH and day). Yellow-necked mouse was the most frequently observed mammal at BF (0.4), and edible dormouse at SCH (0.6). Among birds the treecreeper (ALB, 0.3), bullfinch (ALB, 0.2), European robin (SCH 0.3), great spotted woodpecker (SCH, 0.2) and Eurasian blackcap (BF, 0.1) were observed most frequently. Interestingly, a bat was found in our study, representing probably the first evidence of use of temperate WHs by this taxon (Supplementary material Appendix 1). We identified two main activities at the WHs (Table 1): 1) direct use in form of drinking (Supplementary material Appendix 2) or feeding (either larvae or plant seeds, Supplementary material Appendix 3); 2) bathing (Supplementary material Appendix 4). In addition, we observed species sitting (European robin, Eurasian blackcap, great tit, tawny owl, treecreeper, edible dormouse, unidentified mouse) or sleeping in a hole (great tit), and intra-specific interactions for pine martens (Supplementary material Appendix 5). Previous studies have suggested martens defending their territories quite vividly (Genovesi et al. 1997). The study was conducted during hot and dry summer months. The frequent consultation with numerous visits per day suggests the WHs' importance as a hydration source in an otherwise dry environment, which may lead territorial animals to actively defend them as a potentially limited resource. Differently from the tropics (Yanoviak 2001), we did not record reptiles visiting the WHs. The activities carried out at WHs by other vertebrate species, however, resembled those recorded in the tropical and subtropical environment, including drinking, bathing and feeding (Yanoviak 2001, Walters and Kneitel 2004, Vickers et al. 2014, Sharma et al. 2016). The importance of vertebrate predation on the aquatic insect larvae in WHs have recently been shown by experiments with fake larvae in European beech forests (Gossner et al. 2020). In contrast to the tropics and subtropics (Scheffers et al. 2013), we did not find evidence of life cycle development of amphibians in these microhabitat. However, the observation of frog development in a big dendrotelm of a lying deadwood log in the National Park Hainich suggest that large dentrotelmata on trees might serve as suitable habitats for amphibian development (Nöllert 2012).

Coping with drought and management

Various studies modeling climatic scenarios predict more intense and longer-lasting droughts in the future which will put stress on ecosystems worldwide (Ciais et al. 2005, Bolte et al. 2009, Forzieri et al. 2014). Accordingly, Germany is experiencing increasingly dry summers with prolonged periods without or only with minimal rainfall and above-average temperatures (Mühr et al. 2018). These climatic extremes will affect vertebrates as well as invertebrates and will have far reaching impacts on ecosystems including changes in the provisioning of microhabitats (Augustynczik et al. 2019). Small water bodies such as WHs might provide one of the few available water sources during dry periods (Gossner 2018).

WHs thus not only promote biodiversity by providing specific microhabitats for a specialized invertebrate fauna with aquatic life stages (Gossner 2018) but might additionally increase the survival of terrestrial invertebrates and vertebrate species in both temperate and tropical forests. We found on average 1.3 ± 0.22 SE (BF, recorded on pre-selected trees, Storch et al. 2020), 15.7 ± 2.5 SE (ALB, with capacity > 200 ml), 11.0 ± 2.2 SE (SCH, with capacity > 200 ml) WHs per ha that can potentially be used by vertebrates. This, in combination with the frequent visits observed, impressively shows how important these microhabitats for vertebrates likely are.

Water-filled tree holes and forest management

A promising management tool, the tree-related microhabitats typology (Larrieu et al. 2018), can be helpful to inventory WHs, mostly on the ground level as WHs in the canopy are difficult to assess. This typology is an established tool (Asbeck et al. 2019) for the selection of ‘habitat trees’, which have a long-lasting tradition in temperate Europe (Mölder et al. 2020). Applications of the typology can be used to assess many aspects of forest biodiversity and tradeoffs between timber production and biodiversity conservation (Paillet et al. 2019, Santopuoli et al. 2019). Habitat trees and their conservation recently gained increasing attention in the context of retention forestry in temperate forests (Basile et al. 2020, Gustafsson et al. 2020). WHs are an integral selection criterion for habitat trees and their importance for insects and nematodes have repeatedly been shown (Gossner et al. 2016, Petermann et al. 2020). Our results point out the importance of WHs beyond this by providing sources of food and water for vertebrates. Increased knowledge about the role of these microhabitats for vertebrate species (e.g. the use of WHs by bats) in temperate forests could allow forest managers to explicitly consider these in their management plans to protect species relying on WHs.