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> Forest Report 2015 Condition and Use of Swiss Forests FOEN / WSL 2015
4.5 Deadwood
Thibault Lachat, Urs-Beat Brändli, Markus Bolliger
> Deadwood and habitat trees are irreplaceable habitats and sources of food for more than 20 per cent of the species living in the forest. Many of these 6,000 species are threatened.
> The increase in habitat trees and the volume of deadwood is the result of storms, greater understanding of ecological interactions and lower prices for timber.
> According to the National Forest Inventory NFI, the volume of deadwood doubled in the period from 1995 to 2013 and has reached a level in the Swiss forest today of 24 cubic metres per hectare. This quantity is not, however, sufficient everywhere to maintain endangered species – considerable deficiencies are particu- larly apparent on the Swiss Plateau and in the Jura. Deadwood with a large diameter and in later stages of decomposition is still rare throughout Switzerland.
> With the intensified harvesting of energy wood, habitat trees and fresh deadwood could become economically more valuable again. Special measures are therefore needed to ensure they are maintained in the forest.
Deadwood and habitat trees
Deadwood is defined as dead trees or the dead parts of trees.
These may be standing or lying, and in the form of branches or large stems. Deadwood is either produced through the nat- ural death of a tree or a part of it – for example, due to ageing, windthrow, diseases or pests (section 2.4) –, or as residuals from harvesting during forest management. Habitat trees are defined as living trees with habitats for specialised species, for example, hollow trees with mould for certain beetles or trees with cracks for bats (Fig. 4.5.1).
Deadwood and habitat trees are important for the forest as an ecosystem because around 6,000 species depend on them as habitats or food sources. These include over 1,700 beetle species and 2,700 higher fungi, as well as many birds, amphib- ians, mosses and lichens. More habitat trees and deadwood are therefore good for biodiversity. Deadwood has additional functions. It can, when properly arranged, provide protection against rockfall or serve as a seedbed for tree seeds and thus promote natural regeneration (section 4.2) in mountain forests.
Deadwood is increasing
The development of deadwood in Swiss forests is gratifying as its volume and the number of snags have increased since the 1980s. According to the NFI, the volume of deadwood more than doubled from 11 to 24 cubic metres per hectare (m3/ha) between 1995 and 2013. Moreover, the number of giant trees with a diameter of more than 80 centimetres has risen (sec- tion 1.3). This increase in deadwood is, among other things, a consequence of the windstorm ‘Lothar’. Another contribut-
ing factor is that timber harvesting in inaccessible areas is no longer profitable and numerous stands have not been actively managed for decades (section 4.3). In addition, acceptance among forest owners and managers of deadwood and habi- tat trees has grown in recent decades. This has led to more deadwood being tolerated in forests and habitat trees being deliberately left standing.
Many animal and plant species can benefit from the larger quantities of deadwood, snags and large old trees in the Swiss forest, for example, most woodpecker species (Mollet et al. 2009) and some wood-dwelling species. Thus populations of the Alpine longhorn beetle (Rosalia alpina) have grown since the 2nd World War (Lachat et al. 2013), partly due to the good supply of dead beeches exposed to the sun at lower and medium altitudes.
Threshold values
How much deadwood is needed to conserve endangered spe- cies? This question can be answered with the help of so-called threshold values. These describe the minimum amounts of deadwood needed to conserve specialised species. Most saproxylic species require between 20 and 50 m3/ha, but this depends on the forest type. In montane-subalpine conifer for- ests, 20 to 30 m3/ha are needed, whereas in oak-beech for- ests, 30 to 50 m3/ha are necessary (Müller and Bütler 2010).
Particularly demanding species like the rare fungus Antro- diella citrinella need over 100 m3/ha deadwood. Such large quantities are found only in forests that have not been actively managed for a long time. In certain regions, the threshold
> 4 Biodiversity 4.5 Deadwood 81
values of the deadwood volumes for conserving most saprox- ylic species are reached. Nevertheless, there are many forests in which this is not the case. These are mostly located in easily accessible areas at lower altitudes in the Jura and on the Swiss Plateau, where the deadwood volumes are smallest and, corre- spondingly, the ecological deficiencies highest (Fig. 4.5.2). On the Swiss Plateau, only those forests that have been affected by a storm have large quantities of deadwood.
With deadwood, it is not only the quantity but also the quality that counts. The diversity of size classes and stages of decomposition determines the composition of the species communities (Lachat et al. 2014). Experts estimate that at least 5 to 10 snags or habitat trees per hectare forest must be conserved for the species that depend on them to survive (Bütler et al. 2013). In addition, the deadwood and habitat trees should be well distributed and interconnected in the forest, as well as permanently available. From the point of view of biodiversity, a network of large and small forest stands with a large quantity of deadwood and many habitat trees is ideal.
They should be embedded in a near-natural forest landscape, where all forest areas contain some deadwood. Establishing and protecting natural forest reserves and old growth patches (section 4.9) is thus an important measure to ensure that demanding species survive in the long term.
It is difficult to predict how the quantity of deadwood and the number of habitat trees in the Swiss forest will develop in future. The growing demand for energy wood could stop the trend towards more deadwood. The great challenge will be to meet the needs of the species that require habitat trees and deadwood despite the intensified use of wood. Here, com-
promises between economic and ecological goals are needed, which is what forest managers have been trying to achieve for decades with the help of comprehensive forest planning (For- est Development Plan – Waldentwicklungsplan WEP).
Fig. 4.5.2 Average volume of deadwood in the Swiss forest per economic region. Source: NFI 2009/13
Switzerland: 24 m3/ha 21–30 m3/ha
11–20 m3/ha 31–40 m3/ha 41–50 m3/ha >50 m3/ha 19
23
29
30 28
42
25
34
52
13
16
24
18 16
Fig. 4.5.1 Habitat tree with small-scale habitats such as wood- pecker holes, console fungi and bark pockets, which are parti- cularly valuable for specialised species. Photo: Andreas Rigling
