The greatest asset in tree care is a healthy tree that fulfills its functions as an air filter, a source of shade, and a photosynthesizing plant. If, during a tree inspection, it is determined that a tree is at risk of breaking, that the obligation to ensure public safety cannot be met, and that animal habitats are visible, an expert opinion is prepared in consultation with the local nature conservation authority in the area of species protection. One reason for the risk of breakage may be, for example, a severe fungal infestation.

Following metrological investigations, the tree’s stability and risk of failure are assessed. If the tree is at risk of failure, a decision must be made as to whether it can remain standing as a habitat tree after safety pruning, or whether the risk is so high that it must be removed.

Special pruning techniques can be used to create additional artificial habitats on a tree. These include so-called “tears,” which mimic natural branch breaks. The resulting crevices, cracks, and rough surfaces provide ideal conditions for fungal spores, insects, and numerous microorganisms. By increasing the surface area, valuable microhabitats are created, which in turn provide food and habitat for many species.

The pruning technique is also specifically tailored to species conservation: Some cuts are made with a chainsaw at an angle of about 35 degrees to the crown. This creates additional potential niches and refuges, including for bats.

Of course, such a measure also comes with limitations. Due to the severe pruning, the tree’s original functions—such as its filtering capacity or photosynthetic output—are significantly reduced. In some cases, such heavily pruned trees may sprout new growth. However, they generally do not regain the size of their original crown.

Whether created naturally or artificially through pruning techniques, these refuges for animals and plants are quickly inhabited and embraced. In addition, various nesting boxes for open-nesting birds, cavity-nesting birds, and bat boxes can be installed.

Habitat trees fulfill an important ecological function. They serve as daytime and winter quarters for many animal species, as well as nesting sites—especially for bats. In addition, they provide habitat for insects, beetles, arachnids, birds, and dormice. One of the most important functions of a tree is not only its ecological role as a producer of oxygen or an air filter, but also its role in creating habitat for plants and animals.

Within the city limits of Mainz, the Drususwall in the Oberstadt is a hotspot for habitat trees. Overall, the Drususwall is characterized by an old and ecologically valuable tree population. As a result, there are already several habitat trees there that make an important contribution to biodiversity in the urban area.