A pool, a drink, and a hunting ground – What else water-filled tree holes can be for forest animals?

Submitted by editor on 25 March 2021. Get the paper!
Water-filled tree holes may represent an important source of water and food for vertebrates of temperate forests, such as this Eurasian squirrel (Sciurus vulgaris). Credit: J.Kirsch/M. Basile

By Marco Basile

What makes a tree a resource for wildlife? In many cases, the possible answers include food and shelter. However, as we recently learned during this pandemic, hygiene is very important. In the animal kingdom, bathing can also represent an important activity for wildlife. Animals inhabiting temperate forests may not always have access to water because availability is subject to seasonal variation and drought events. To deal with seasonal changes, small vertebrates such as songbirds rely on water-filled tree holes for maintaining their hygiene. These tree trunk structures facilitate water and detritus accumulation and create a microhabitat rich in organic matter.

Sequence of a Eurasian nuthatch (Sitta europaea) bathing in a water-filled tree hole located in a tree bifurcation. Credit: J. Petermann.

However, the most important aspect of water-filled tree holes is their provision of water and food, considering the rich invertebrate communities that they can host. Indeed, they can represent a hunting ground and a source for hydration, as we showed in our study. Using camera-traps, we surveyed three forests in Germany to compile a list of vertebrate species that use water-filled tree holes. We also assessed the type of use and, in many cases, distinguished between feeding, drinking, and bathing activities.

The importance of these tree microhabitats is well known for tropical and sub-tropical forests. However, in temperate regions, most research has focused on invertebrate communities inhabiting water-filled tree holes. During our research, we found some unexpected species on our camera-trap recordings. First, we documented a bat that seemed to be feeding or drinking at the water hole, a type of use never documented before. Second, we found a European tree frog Hyla arborea. Amphibians are known for using water-filled tree holes for their life cycle development in the tropics. Sometimes tree frogs are observed around holes, especially in warmer Mediterranean regions, within the temperate zone. We wonder whether more research could reveal further, unknown potential of water-filled tree holes.

Temperate forests face an increasing risk of drought events, largely due to climate change. Also, in areas that previously did not experience frequent and intense droughts, water may soon become a scarce resource. From this perspective, it is even more important to study the role of water-filled tree holes for wildlife, considering their potential importance as a water source in an ever-drier forest. Forest management schemes are starting to consider these important structures in their inventory activities and we hope that further research will help understand all the linkages between water-filled tree holes and forest organisms.