Water-filled tree holes (WHs), commonly referred to as dendrotelmata, develop when water accumulates in cavities related to tree architecture or in rotten parts of the tree. These structures can occur in forest ecosystems and may represent essential microhabitats in the life cycle of various animal species. WHs form temporary microhabitats during wet periods and sometimes sustain water throughout dry seasons. Research on the use of WHs by organisms mainly focuses on invertebrates developing in these while vertebrates are rarely mentioned. A literature search on the use of WHs by vertebrates revealed that attention has been given only to vertebrates in tropical and subtropical systems, mostly to groups with aquatic stages, such as amphibians. To fill the knowledge gap on the use of WHs by vertebrates in temperate forests, we conducted a camera study in three sites across Germany. We identified a total of 28 vertebrate species including 11 mammal, 17 bird and one amphibian species using the WHs. The recorded videos showed that vertebrates use the WHs mainly for nutrition and hydration. With an expected future increase in frequency and intensity of dry spells in central Europe, these microhabitats may sustain healthy forest ecosystems by providing resources for wildlife. Reliable, updated data about the importance of WHs for vertebrates is required to urge forest managers and stakeholders to enforce the preservation of these microhabitats.
Water-filled tree holes (hereafter WHs) may constitute an integral part of forest ecosystems by providing space for development, prey and a source of freshwater to forest organisms (Kitching 2000). Due to water accumulation in open crevices, tree stem cavities and root systems, these WHs, also referred to as dendrotelmata, may carry water throughout the year or only during distinct time periods. Leaves and detritus accumulate in WHs, enriching them with organic material and nutrients (Fish and Carpenter 1982). So far, research on the use of WHs by organisms primarily focuses on invertebrates developing in these aquatic microhabitats (Fashing 1975, Schmidl et al. 2008, Gossner et al. 2016). Invertebrates without aquatic life stages may also use WHs for drinking (e.g. wasps, bees and beetles, Gossner 2018, Petermann unpubl.). Vertebrates are sometimes targeted in WH studies from tropical and subtropical regions (Yanoviak 2001, Walters and Kneitel 2004, Vickers et al. 2014, Sharma et al. 2016), but to our knowledge, there is no study carried out in temperate regions on the use of WHs by vertebrates.
In the tropics and sub-tropics, researchers reported mainly amphibians, reptiles, marsupial gliders and primates using the WHs. Possible uses include life cycle development, typical for amphibians (Yanoviak 2001); drinking, which is documented for primates (Sharma et al. 2016) and suggested for bats (Boyles et al. 2006); and bathing, as it has been observed for birds (Baker 1983). A field study by Gossner et al. (2020) in temperate beech forests revealed that fake larvae displayed in WHs are frequently attacked by small mammals and birds, indicating WHs are used and harbor potential food resources.
To document the use of WHs by vertebrates in temperate forests, we performed a field study, with the aim of providing a first inventory of vertebrate species using WHs, including notes on their behavior.
The field study included two different data collections located in three study areas in Germany: the UNESCO Biosphere Reserves Schwäbische Alb (ALB; south-west) and Schorfheide-Chorin (SCH; north-east), two sites that are part of the Biodiversity Exploratories project (Fischer et al. 2010); and the southern Black Forest (BF), a site of the ConFoBi project (Storch et al. 2020). The forests of ALB were either dominated by European beech Fagus sylvatica or Norway spruce Picea abies, those in SCH by either F. sylvatica, Scots pine Pinus sylvestris or sessile oak Quercus petraea, and those of BF by P. abies, silver fir Abies alba and F. sylvatica.
WHs were observed at ALB and SCH (10 forests stands each) using camera-traps (Berger + Schröter 31277, Digital Trail Camera 12 MP) with a motion sensor, night vision and a 32 GB SD-card. Recording began in April 2015 and ended in August 2015, totaling 143 and 116 days at ALB and SCH, respectively. WHs included a total of 48 pan (ALB 25, SCH 23) and 8 rot holes (ALB 7, SCH 1) of different origin from ground level to 23 m height in the canopy. All cameras were moved to a new tree hole in each forest stand after three to four weeks. A