Decaying wood – an elixir of life for forest species

News |
The leaving of retention trees and the active production of dead wood through the creation of man-made snags have an impact on the total amount of dead wood.

Species, which are dependent on dead wood, include a large number of fungi, in particular polypores and polyporales as well as lichens, mosses and insects. Some rare and endangered 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 Tengman’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.

Redrafted forestry guidelines already have a visible impact on the ground

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, but stacks of softwood logs can cause a much larger risk of mass incidence of bark beetles if they are left in the forest for extended periods.

Dead trees retained

Trees, which have been knocked down by the wind or that have died standing, are by no means useless and the recommendation is to leave them in the forest. Decaying wood is in itself a fresh habitat and decomposing trunks create new forest soil and form an important platform for sapling growth.

Decomposition takes decades. 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.

Species that depend on decaying wood need different kinds of deadwood

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 species of beetle and polypore fungus will only accept rotting wood that has earlier been decomposed by a specific species of fungus. The polypore Pycnoporellus fulgens, for example, only lives on wood that has already been rotted by the polypore Fomitopsis pinicola.

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 ten years or so. In Southern Finland, an average of 3.2 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 double at 7.6 m3/ha on average. The amount of dead wood in natural-state forests varies according to growth stand and location and is about 20-130 m3/ha.

Further information:
Suvi Raivio, Ph.D., Senior Advisor (Biodiversity),
tel. +358 9 132 6671

The UN has declared 2010 the International Year of Biodiversity. This series of articles examines concepts and phenomena that are related to biodiversity.