Species diversity in mulm caves: Influence of forest structure and cave characteristics

Animals that are directly or indirectly dependent on deadwood during at least one phase of their life cycle are known as saproxylic. It is estimated that 34 % of all species occurring in central European forests are saproxylic, and in the case of forest-dwelling beetles, more than 50 % are regarded as saproxylic. Saproxylic species thus make a major contribution towards species diversity in the forest. They are also important for the nutrient cycle in the forest, as they play a key role in the decomposition of deadwood.

Mulm caves provide habitats for numerous groups of animals. The largest group consists of arthropods, especially flies and mosquitoes (Diptera), beetles (Coleoptera), but also mites (Acari) and springtails (Collembola). Mulm caves are also a habitat for cavity-nesting birds and mammals such as bats. Consequently, mulm caves are key structures for a high level of biodiversity in the forest. As old hollow trees were often removed in the past due to their low economic value, mulm caves are now among the rarest structures in central European forests. This habitat shortage means that cave-dependent tree hollow specialists among the beetles, i.e. those species whose larvae can only develop in mulm caves, are among the most endangered groups of animals. Around 75 % of them are on the Red List.

In terms of their characteristics, mulm caves are as diverse as their inhabitants. There are large and small mulm caves, caves with large or small entrances, moist and less moist caves, caves with coarse, less decomposed mulm or those with dark, heavily decomposed mulm. The habitat requirements of the mulm cave dwellers are also very different. Some species only colonise very damp hollows, others only hollows low down on the tree, in contact with the forest floor, or only large hollows. These special requirements of many species further reduce the supply of suitable mulm caves.

A total of 283 saproxylic beetle species from 48 families (5,880 individuals) were collected in the 134 mulm caves sampled in the three study areas. Of these, 62 species are on the Red List, corresponding to a 22 % share of threatened species. 16 species were identified as mulm cave specialists, all of which are on the Red List. The number of species detected as well as the composition of the species communities differed significantly in some cases between the three forest areas studied (Figure 3 and 4).

Mulm caves in the Ebrach state forestry area showed the greatest species diversity per hollow (an average of 13.2 beetle species per hollow, Fichtelberg: 4.6, Kelheim: 7.9), as well as the highest number of endangered species and tree cavity specialists. It was found that the species diversity was particularly influenced by the local characteristics of the mulm caves themselves as well as the hollow trees (Figure 2). For example, more beetle species were thus found in mulm caves with large entrances and in hollows close to the ground. Furthermore, there were more beetle species in caves with still slightly decomposed and correspondingly lighter and coarser-grained mulm than in caves with already more decomposed mulm. The still fresh mulm can probably be used by a larger number of generalist saproxylic species, too, while the strongly decomposed, dark mulm is mainly accepted by just a few species that specialise in these stages of decomposition.

The species diversity in the mulm caves was mainly investigated with regard to the influence of the forest structure within a radius of up to 500 m around the tree hollows. For this purpose, we were able to use the regularly collected forest inventory data of the BaySF state forestry company, which use a grid of inventory points (200 m x 200 m) to record numerous parameters of forest structure (e.g. amount of deadwood, breast height diameter of the surrounding trees, tree species composition). After interpolation of the point data and evaluation of the maps created especially for this purpose, it became apparent that the above described influence of local mulm cave characteristics on the species diversity in the hollows always also depends on the forest structure of the area in question. In the forest areas with a higher proportion of deciduous trees (Ebrach and Kelheim), the influence of local mulm cave characteristics was more pronounced than in the Fichtelberg forestry area, which is dominated by conifers.

In the Fichtelberg area, there are only a few isolated mulm caves in deciduous trees. If mulm caves are rare and the only ones in an area, the quality of the individual hollows hardly seems to matter - mulm cave dwellers take what they can get. In the coniferous forest, only the proportion of beech trees in the immediate vicinity of the investigated tree caves thus had a positive influence on the species diversity in the hollows (Figure 2). Since beech trees in the Fichtelberg forest area mostly stand on their own in an environment of conifers, the presence of other beech trees in the closer vicinity of the cave trees (within a radius of up to 100 m) was the most important influencing factor for mulm cave dwelling beetles in this forest area.

A consequence of the study for forestry practitioners is the protection and preservation of each developing and existing tree mulm caves, since each of these cavities offers a unique range of valuable microhabitats in its specific structure and characteristics. Given the rarity of mulm caves in commercially used forests and the very different habitat requirements of their inhabitants, all existing tree caves should be protected. Furthermore, in view of the decades it takes for mulm caves to develop, all trees that contain structures from which mulm caves could develop (e.g. bark injuries over large areas, broken-off branches), should be preserved.

The study shows that, in addition to the quality of the individual tree caves, the forest structure is also important for the species diversity of mulm cave dwellers. Consequently, an increase in the amount and quality of deadwood in commercially managed forests is also conducive to the protection of ecological communities in mulm caves.

Especially the amount of deadwood of deciduous tree species should be increased, as saproxylic beetles are sensitive not only to the quantity but also to the tree species, diameter, degree of decomposition and exposure to sunlight of the deadwood. The tree species composition of the respective forest area has also proven to have great influence on saproxylic species diversity in mulm caves. An increase in the proportion of deciduous trees in conifer-dominated commercially managed forests would thus also help to increase the species diversity of saproxylic beetles in tree cavities.