Hundreds of thousands of snags created for species depending on decayed wood

With growing volumes of logging, the conservation of biodiversity will be an increasingly important part of forestry. Standing dead or dying trees, called snags, are vital to many birds, beetles and polypores. In Finnish commercial forests, snags are now being created by their thousands.

In Finland, the main actors in the forest sector have begun to take modest but extremely effective measures to promote biodiversity by creating snags. Living trees are cut at a height of a few metres, with the result that the tree will decay rapidly but remain standing.

Until now, the presence of deadwood in the forest has been ensured by leaving retention trees in connection with regeneration fellings. Snags, however, produce deadwood even at the thinning stage.

“Thanks to snags, we will have deadwood of different ages and therefore with more diversity. Snags decay in a couple of years, while retention trees need ten to fifteen years,” says Mr Janne Soimasuo, Environment Manager at Metsä Group, a Finnish forest industry group with operations in about 30 countries.

In addition, retention trees left on a felling site will easily fall down in a storm. Of course, they will then continue to decay, but species such as the willow tit (Poecile montanus), which is rare in Finland, prefers to nest in standing dead wood, such as a snag.

Soimasuo stresses that deadwood is needed at all growth stages of the forest. Already at the thinning stage, Metsä Group will ask forest owners about their willingness to create snags.

“Snags can also be used to mark future retention tree groups and thickets for game animals. When snags are placed among such trees, many birds using them as nesting trees, for example, benefit from the shelter of the thickets,” says Soimasuo.

Two snags per hectare

In Finland 85 percent of commercial forests are certified. The certification makes it obligatory to leave retention trees during regeneration fellings, and snags are included in their number. Cutting trees to create deadwood is not a new invention: at the turn of the millennium, this was a much-talked topic in Finland.

Currently, the creation of snags is booming in Finland. As one of Finland’s biggest wood buyers, Metsä Group has announced that it will ask for the willingness of the forest owners to create two snags for every hectare thinned or felled. Metsähallitus, the state-owned forest company, is going to create 100,000 snags this year alone and to include making them in its permanent operating model.

In addition, the Central Union of Agricultural Producers and Forest Owners (MTK) and the Central Union of Swedish-speaking Agricultural Producers in Finland (SLC) have agreed with the Finnish Energy Organisation to increase the amount of deadwood in forests next to power lines.

The creating of snags is currently tested in pilot projects together with power transmission companies. At the same time, the interest of forest owners in the activity will be surveyed, because it is ultimately the forest owner who decides if decaying wood should be created.

First off the mark with creating snags was Metsä Group, which began to implement its operating model last October. According to Soimasuo, the idea came about when the company discussed the development of nature management in forestry and the growth stage at which there is the least decayed wood. The result was that there is a shortage of decaying wood especially in young forests. More particularly, there should be more of standing dead wood.

Need for active nature management

Metsähallitus also aims to create more diverse decayed wood. Until now, snags have been created in state-managed forests mainly to mark cultural heritage sites, for example.

“Snags are a good example of a more active nature management. Retention trees and thickets for game animals come about when something is left undone, but snags must be actively made,” says Ms Maarit Kaukonen, environment expert at Metsähallitus. “In the best case, a tree is cut to make a snag and its crown is left to decay on the ground.”

There is little difference in the various organizations’ instructions on how to create a snag. However, a deciduous tree always decays more easily than a conifer, and the tree crown should be cut at a height of 2–5 metres.

“Creating snags from trees valuable for nature but often with little financial value is a cost-effective way of combining the safeguarding of biodiversity with forestry,” says Ms Lea Jylhä, forestry specialist at MTK.

High-rises for species depending on decayed wood

The thicker and higher a snag, the bigger its surface area and volume, which means more space for species depending on decayed wood.

When the Finnish Forest Research Institute Metla (now merged into the Natural Resources Institute Finland) researched species in sixteen artificial snags at the turn of the millennium, they found approximately 250 beetles dependent on decaying wood. A university study carried out around the same time found that dead trees on felling sites had more rare and endangered species than trees in deep forest.

In addition to beetles, many insects, bracket fungi and lichens prefer snags. Hollow-nesting birds, such as the willow tit, which is vulnerable in Finland, make their nests on the tops of snags.

The species living on decaying wood change as decay progresses, and decaying wood on the ground will harbour other species than standing dead trees do. Of all Finnish forest species, about a quarter – or up to 5,000 species – are dependent on decaying wood.

Environmental organizations criticize the shortage of deadwood in commercial forests. It is, in fact, the most significant cause of endangeredness of forest species.

On average, there are over three cubic metres of decaying wood per hectare in commercial forests. In terms of biodiversity, a sufficient amount is frequently estimated at 20 cubic metres, while natural forests may have even a hundred cubic metres of dead wood per hectare.