Introduction

Awareness of the key importance of dead wood and wood-inhabiting fungi for biodiversity in boreal forests has increased substantially during the last two decades, as a result of extensive research in northern Europe and North America

British Mycological Society Symposia Series © 2008 The British Mycological Society

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(Harmon et al., 1990; Dahlberg and Stokland, 2004; Jonsson et al., 2005; Harper et al., 2006). This improved understanding has also been communicated to decision makers in politics and forestry and resulted in information, guidelines and rules for practical management to enhance biodiversity associated with dead wood.

Of the estimated 10,000 plus non-lichenised fungal species in Fennoscandi-navia, more than 2,500 are wood-inhabiting (Siitonen, 2001; Gardenfors et al., 2003; Dahlberg and Stokland, 2004). Roughly 1,500 of these are basidiomycetes and 1,000 are ascomycetes. These estimates are predominantly based on observations of sporocarps, and the contribution of ascomycetes and other species with inconspicuous sporocarps or rarely fruiting species are probably both qualitatively and quantitatively underestimated. Among the 1,500 wood-inhabiting basidiomycetes, at least 550 are corticoid, 200 polypores, 150 agarics and 100 heterobasidiomycetes. One challenge is to understand the presence of this high diversity given the relatively few tree species present in boreal forests. Niche separation can be the result of spatial, temporal and qualitative differences in the environment (Table 1).

Dead organic matter is of paramount importance as the energy source for the major part of the biological diversity in forest ecosystems. Less than 10% of the

Table 1 The number of potential combinations of factors that affect the quality of dead wood and thus the conditions for the occurrence of wood-inhabiting fungi is almost unlimited

Factors affecting the

Examples

Number

quality of the wood

of types

Tree species

Picea abies, Pinus sylvestris, Betula spp, Populus tremula

>10

Dimensions of the

Fine woody debris and coarse woody

>2

wood

debris

Decay class

Recently dead, initial decay, intermediate decay, advanced decay, very decayed

5

Part of tree

Branches, trunk, root

>3

Part of the wood

Bark, sapwood, heartwood

>3

The wood's cause of

Weakened and slowly dying due to age,

>3

death

insects or fungal pathogens or directly killed after storm, fire or cutting

The increment speed

Slow to rapid

>2

of the wood

Microenvironment

Dry, damp, wet, sunk in water,

>5

around the wood

sunexposed, shaded

Other factors

Associated species

>5

Number of potential

combination

possibilities with

this example of

factors >100,000

approximately 20,000 multicellular species in Fennoscandian boreal forests have photosynthetic ability while the majority are either part of the decomposer or herbivore food web. Fungi are the key drivers in the decomposer food web as they are mainly decomposers of cellulose and lignin (Rayner and Boddy, 1988). Most species associated with dead wood do not live on the wood tissue directly but on species that decompose it. For example, a large number of insect species and other invertebrates feed on fungal mycelium.

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