Dead wood is a very important arena for growth and sporulation of saprotrophic basidiomycetes, supporting thousands of different species with variable adaptations to the wood environment. Fallen trunks and branches often constitute the main bulk of dead wood in long unmanaged deciduous forests (Christensen et al., 2005). Heartwood of ancient trees is another major source of dead wood, though less common in plantations managed for wood production, as trees are often harvested long before substantial heartwood develops. Attached dead branches, stumps and buried roots are other important dead wood habitats, which may constitute an important fraction of the total dead wood volume, not least in managed or formerly managed stands (Norden et al., 2004a). The balance between different dead wood types varies depending on forest types and dominating tree species. In a survey of unmanaged forest stands in Lithuania dead standing wood predominated in alder stands (Alnus spp.), but not in birch (Betula spp.), aspen (Populus tremuloides) and oak (Quercus spp.) (Vasiliauskas et al., 2004). Similarly, the relative proportion of standing dead wood was considerably higher in montane European beech forest reserves (41-47% of total dead wood volume) compared with lowland reserves (23-29% of total dead wood volume) (Christensen et al., 2005), probably reflecting a combination of higher windstorm damage in lowland areas in NW Europe and the presence of silver fir (Abies alba) in most evaluated montane stands. Even the structure of fungal communities may have a marked impact on the frequency of snags (natural snags). In the beech forests of Halland, Sweden, Fomes fomentarius is dominant in the primary decay community and due to its decay, usually concentrated in the mid-stem section, almost all trees experience tops breaking and falling to the floor, while uprooted trees, common in most other beech forest zones, are scarce (HeilmannClausen, 2005).

The wide variety of dead wood habitats is known to support a vast variety of more or less specialized fungi, but the global number of fungal species involved in wood decay is unknown. In Sweden alone, more than 2,500 species have been recorded as primarily associated with dead wood (Dahlberg and Stokland, 2004), corresponding to ^20% of an estimated total of 12,000 species. With an estimated 1.5 million fungal species worldwide (Hawksworth, 2001) the total number of wood-inhabiting fungi on the global scale is likely to exceed 100,000 species, even if the wood-inhabiting fraction is considerably lower than in the Swedish case.

Fungal community development in dead wood has been investigated intensively, especially since the 1980s, and a general understanding of principles and factors affecting the process is beginning to emerge. Here we first review ecological strategies in wood decay fungi, in terms of dispersal, life-history and colonization strategies. Second, important habitat factors influencing community development are considered. Third, community development pathways and the ways in which environmental factors determine community composition are discussed.

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