Retention trees add to the structural diversity of forest stands and conserve original species. The purpose of retention trees is to ensure the continued presence of decaying wood over the turnover time of a forest stand and in this way uphold biodiversity.
Forestry guidelines recommend the leaving of live retention trees at suitable places on all regeneration felling sites, such as in damp depressions or adjacent to ecologically valuable habitats. Retention trees also help in the conservation of species that were found on them originally. For example tree lungwort, a species of lichen that grows in old-growth forests, is capable of surviving on retention trees for quite some time. Retention trees cushion changes to the landscape and provide shade, which supports the conservation of valuable habitat characteristics.
Retention trees are never harvested or thinned, even though all of them die and fall down in due course. Retention trees add to biodiversity as snags that have died standing or as decaying wood that has fallen to the ground. They are left undisturbed in conjunction with soil preparation measures.
Retention trees left in groups
The recommendation is to leave an average of 5-10 retention trees per hectare of regeneration area. The diameter of these trees must exceed 10 cm. Retention trees are usually left standing in groups, but it is also possible to leave individual, sturdy retention trees. The number of retention trees varies according to the objectives of the land owner, the size of the logging area, the amount of known species occurrences in the area as well as depending on the landscape and recreation use values of the location. If the intention is to burn over the site at a later date, the recommendation is to leave a larger number of retention trees than normally.
It is usually recommended that aspen, sallow, rowan, alder and other valuable hardwood species are used as retention trees, but old spruce, pine and birch trees are also important to biodiversity. Sturdy hard- and softwood trees with a lot of branches or crooked trunks or that have begun to rot or are otherwise of little economic value make especially good retention trees.
Retention trees can also be man-made snags, which are created by cutting them at a height of about 3 - 5 metres. Researchers have identified up to 300 and more different beetle species in such man-made snags, some of which were threatened species.