Species, which are dependent on dead wood, include a large number of fungi, in particular polypores (bracket fungi) as well as lichens, mosses and insects. Some rare and threatened beetle species use decaying wood for nourishment or as a habitat. Woodpeckers and tits hollow a nest cavity into trees, usually into snags that have died standing. Species such as Tengmalm’s owl, the common goldeneye and the flying squirrel as well as the redstart and the pied flycatcher also nest in hollows that have been excavated by woodpeckers.
Revised forestry guidelines are already visible in forests
The biodiversity of the forest environment began to be taken into better consideration in forest management during the 1990s. Before this, dead trees would be carefully removed in conjunction with forestry measures because they were thought to spread disease and pests. This is not so, however, especially in the case of broadleaved trees. Decaying sallows and aspens are valuable maintainers of biodiversity. On the other hand, recently logged coniferous trees attract bark beetles, which can cause damage in the surrounding woodlands. An individual wind-felled tree or even several such trees are not of great significance in this respect.
Dead trees retained
Retention trees that are left in the logging sites of today will eventually die and fall, creating decaying wood long into the future. The decomposition process can be speeded up by cutting the tree at a height of a few metres. Research indicates that such man-made snags form a suitable habitat for several threatened beetle species.
Fallen dead trees should be bypassed and left intact in conjunction with soil preparation measures so that various species can take advantage of them. Up to hundreds of different species of organisms can live in a trunk during decomposition.
About a quarter, or some 4,000 – 5,000, of the forest species of Finland are dependent on decaying wood. Some species can survive on standing dead trees, but the majority require sturdy, downfallen dead trees. Many species that depend on dead wood are extremely specialised and can only survive in particular tree species, which are of a specific size and whose decomposition has progressed to a degree that is specifically suitable to them. Once decomposition progresses further, these species need to find a new nearby dead tree that suits their particular requirements with regard to species, size and degree of decomposition. Some beetle and polypore species will only accept rotting wood that has earlier been decomposed by a specific species of fungus.
The microclimate of the location also affects the decomposition process substantially. If dead wood dries too much, it can become useless to several species of polypore fungi, for example. On the other hand, certain species of beetle require trees that have died standing at dry and sun-baked locations.
Amount of dead wood has increased in recent years
According to the Finnish Forest Research Institute, the amount of dead wood in Finnish forests has increased over the last twenty years or so. In Southern Finland, an average of 4.3 m3/ha of deadwood can now be found, while the corresponding amount was 2.8 m3/ha at the end of the 1990s. In Northern Finland, the amount of deadwood is around at 7,3 m3/ha on average. The amount of dead wood in natural-state forests varies according to productivity and location and is about 20-130 m3/ha.