Disturbance Regimes

The main factors disturbing natural boreal forest landscapes are fire, wind throws and disturbance associated with gaps, insect outbreaks, fungal diseases, periodic drought or excess water (Gromtsev, 2002; Selikhovkin, 2005). Fire, which is considered to be the most important disturbance factor in natural boreal forests, determines the structure and dynamics of boreal forests especially in drier pine-dominated forests and in more continental areas (e.g. Sannikov and Goldammer, 1996; Gromtsev, 2002). On the other hand, gap-phase dynamics and wind throws are of utmost importance in moister (e.g. spruce-dominated forests) forest types (Hofgaard, 1993; Kuuluvainen et al., 1998) as well as in temperate, broad-leaved forests (Runkle, 1985). In addition, insect outbreaks and pathogens can be important forces creating gaps and modifying forest structure especially in natural forests dominated by gap-phase dynamics (Castello et al., 1995; Perry and Amaranthus, 1997).

Natural boreal forests typically show a wide variation in the quality, size, severity and repeatability of disturbances in space and time (Engelmark, 1999; Bergeron et al., 2002; Kuuluvainen, 2002). In contrast, in managed forests the disturbance (harvesting) areas and the harvest rotation are relatively constant (Kuuluvainen, 2002). Natural disturbances typically leave large amounts of biological legacies (both living organisms and organically derived structures like dead trees) from the pre-disturbance period which facilitate the recovery of the post-disturbance communities while biological legacies left by silvicultural disturbances (e.g. clear-cutting) are typically less diverse, less abundant and exhibit lower levels of spatial heterogeneity than those left by natural disturbances (Franklin et al., 2000; Bengtsson et al., 2003). Consequently, managed forests with low levels of biological legacies (i.e. dead wood) show much lower species diversity of wood-decaying fungi than natural forests and very seldom host threatened and demanding fungal species (e.g. Lindblad, 1998; Kruys et al., 1999; Sippola et al., 2001; Penttila et al., 2004; Junninen et al., 2006). The continuity of dead wood is important for maintaining the species diversity of wood-decaying fungi, both in natural and managed forests (Stokland, 2001). For example, polypores from the old growth can remain for a long time on logs after final cutting of a stand and also on logging waste (Sippola and Renvall, 1999). A high number of species (and findings of threatened species) is often found in young, logged successional stages in pine-dominated forests which can be partly explained by the residual remains from the old, uncut forest (Junninen et al., 2006).

Besides decreasing diversity and heterogeneity of wood-decaying fungi, silvicultural practises may also enhance the impact and spread of fungal pathogens like Armillaria spp., Heterobasidion annosum and P. weirii (Castello et al., 1995). In natural forests both fire disturbance and gap-phase dynamics, creating spatially heterogenous and diverse stand structures, often control the spread and impact of these pathogenic basidiomycetes (e.g. Froelich et al., 1978; Worrall and Harrington, 1988; Dickman and Cook, 1989). In managed forests, on the contrary, increased cutting and fire suppression in addition to homogenization of tree species composition have increased the spread and thus also the problems caused by these necrotrophic pathogens in many areas (e.g. Sherman and Warren, 1988; Byler et al, 1990; Korhonen and Stenlid, 1998).

In natural boreal forests the prevailing disturbance regimes, and especially the frequency and severity of these disturbances, set the stage for the occurrence of fungal species and communities after disturbance. For example, wood-decaying fungi growing on pine and in pine-dominated forests, which have burned more often than spruce-dominated forests (Zackrisson, 1977; Gromtsev, 2002), are likely to be more adapted to fire disturbance than species preferring spruce as a host. Listing species with preferences for burned areas and charred wood (Table 6), gives evidence for this hypothesis, since most of the coniferous-inhabiting species prefer pine as their host. In addition, many of the fire-favoured species are very rare or red-listed in the Nordic countries (e.g. A. primaeva, Dichomitus squalens, G. carbonarium and protractum, Physisporinus rivulosus, Crust-oderma dryinum), which most probably is a consequence of effective fire control and rarity of burned areas with large dead trees.

The immediate, short-term effect of fire on fungal communities is typically destructive (Pugh and Boddy, 1988; Watling, 1988). Intense fire destroys fungal mycelia and decreases the inoculum potential of many fungi by reducing the amount and quality of dead woody material and by creating extreme environmental conditions (Pugh and Boddy, 1988). The few existing studies on the effects of fire on communities of wood-decaying fungi (Penttila and Kotiranta, 1996; Penttila, 2004; Junninen et al., 2007) show, that immediately (1 year) after the fire the number of fruiting species is much lower than in the pre-fire communities. However, although fire destroys or inhibits fruiting of resident fungal

Table 6 Boreal wood-decaying fungi that seem to favour burned areas and charred wood (Anthracophilous Species sensu Moser, 1949)

Species group

Species name

Main host

Polypores

Antrodia primaeva

Coniferous

A. sinuosa

Coniferous

A. xantha

Coniferous

Ceriporiopsis subvermispora

Coniferous (deciduous)

Dichomitus squalens

Coniferous

Gloeophyllum carbonarium

Coniferous

G. protractum

Coniferous

G. sepiarium

Coniferous

Physisporinus rivulosus

Coniferous (deciduous)

Postia placenta

Coniferous

Pycnoporus cinnabarinus

Deciduous

Trametes hirsuta

Deciduous

Corticoid fungi

Crustoderma dryinum

Coniferous

Phanerochaete raduloides

Deciduous

Hyphoderma sp.

Coniferous

Ascomycota

Daldinia loculata

Deciduous

Source: The main sources of information were Eriksson (1958), Penttila and Kotiranta (1996), Johannesson et al. (2001), Penttila (2004) and Junninen et al. (unpublished), but also other published and unpublished information dealing with forest fires and habitat preferences of wood-decaying fungi. Nomenclature of polypores follows Ryvarden and Gilbertson (1993, 1994), of corticoid fungi Hansen and Knudsen (1997).

Source: The main sources of information were Eriksson (1958), Penttila and Kotiranta (1996), Johannesson et al. (2001), Penttila (2004) and Junninen et al. (unpublished), but also other published and unpublished information dealing with forest fires and habitat preferences of wood-decaying fungi. Nomenclature of polypores follows Ryvarden and Gilbertson (1993, 1994), of corticoid fungi Hansen and Knudsen (1997).

communities, it also provides a large input of new resources for decomposers in the ecosystem, and in doing so, acts as enrichment disturbance (Pugh and Boddy, 1988). Indeed, the amount of dead wood in natural boreal forests is usually at its highest in early successional stages immediately after large-scale disturbances, such as forest fires and storms (Spies et al., 1988; Siitonen, 2001). A long-term study on the effects of fire on wood-decaying fungi in eastern Finland (Penttila, 2004) clearly shows the enriching effect of fire disturbance on fungal communities in the long run. In this study, a pine-dominated old-growth forest stand with considerable amounts of dead wood was burned in 1989, and for 2 years after the fire the number of polypore species was lower than before the fire. However, 6 years after the fire the burned forest hosted an equal number of species as before the fire, and finally 13 years after the fire the number of species had increased strongly, including a very large number of threatened and near-threatened species in Finland. This increase in species number results partly from the enrichment given by the fire-killed trees but, especially in the case of red-listed species, is also due to the rich legacy of pre-fire dead trees both in the burned stand and in the surrounding old-growth forest.

Examination of fungal life strategies may also be helpful in understanding the species composition and structure of fungal communities after disturbance (Pugh and Boddy, 1988). In general, disturbances are expected to favour ruderal species at the cost of competitive species (Rayner and Boddy, 1988). Also stress-tolerant species (Cooke and Rayner, 1984) may flourish following some types of disturbance (e.g. burned). Evidence from the existing studies (Penttila and Kotiranta, 1996; Penttila, 2004; Junninen et al., 2007) mainly support these general patterns, for example many non-competitive pioneer colonizers of fresh wood flourish in burned and in clear-cut areas, while competitive species usually utilizing logs in more advanced stages of decay decrease (fruiting is prohibited) after fire disturbance, at least in the short term. In addition, good examples of stress-tolerant species thriving in disturbed areas are the heat tolerant Gloeophyllum species (Loman, 1965) occurring in burned and clear-cut areas (Table 6). However, species life strategies are not always straightforward and there is evidence that the same species can exhibit different life strategies at different times during its life-cycle (Pugh and Boddy, 1988). For example, the post-fire ascomycete Daldinia loculata (see Table 6) seems to show such a behaviour. According to the tentative life-cycle presented in Johannesson et al. (2001), D. loculata lives outside fire areas as a latent mycelia established by sexual ascospores, which disperse from fruit bodies (stromata) emerging in scattered burned forest sites. Besides D. loculata, a corticoid species Phanerochaete raduloides (see Table 6) may show a similar pattern, since it often produces extensive fruit bodies on dead birch after fire, but is very seldom found outside fire areas (Penttila and Kotiranta, 1996; Johannesson et al., 2001).

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