Community Development In Wood Resource Units

The development of the fungal community can be described at different scales. At the resource unit level, initially, dispersal and ability to utilize fresh wood are important characters shaping the decay community. This has been called primary resource capture (Rayner and Boddy, 1988; Boddy, 2001). As the wood-inhabiting community is building up, the mycelia will start to make contact and microbial interactions (Chapter 7) become increasingly important.

When the initial colonization of the resource is completed and the wood is occupied by primary colonizers, new entries into the unit will be in competition with already established mycelia (Boddy, 2001). This is called secondary resource capture. Mycelia already established in wood have a competitive advantage, not least by having access to a large resource pool (Holmer and Stenlid, 1993; Lindahl et al., 2001). However, new species do enter the closed community, and these are often better at decomposing recalcitrant material, for example lignin, or are good at parasitizing mycelia. Renvall (1995) reported that white-rot fungi that can decompose lignin are more common at later stages of wood decomposition as compared to brown rotters with only marginal ligninolytic abilities.

Species specialized in combative mycelial interactions, leading to takeover of resources, have either a broad spectrum or a narrow range of species that are out-competed. Species with a general high ability to replace others also tend to produce hyphal aggregates, for example cords and rhizomorphs, that can grow through soil and interconnect between resource units (see Chapter 1). This increases the ability to find resources and also allows for import of energy and nutrients into the new resource, thereby increasing the inoculum potential and the chances of out-competing other mycelia. From an evolutionary strategy point of view, the costs of producing extensive mycelial aggregates for foraging can only pay off if the potential gains are high, for example in terms of being able to out-compete other organisms and thereby capture valuable resources. In other words, species with a strong ability to forage might also be expected to be relatively strong in intermycelial encounters with a broad range of target species. Competitive hierarchies can be found among species indicating this (Holmer and Stenlid, 1997). In the everlasting struggle for resources, fungi have various means of defending the resources already captured, including forming outer mycelial boundaries such as thick melanized mycelial rinds called pseudo-sclerotial plates.

Secondary resource capture sometimes also involves species-specific interactions. This typically happens when a species is specifically able to target a common species that frequently appears in the early stages of resource capture, and to invade that species' mycelial domain. This strategy is prompted by targeting species that are successful in primary resource capture and relatively common in the ecosystem. Based on spatiotemporal positioning of fruiting bodies, Niemela et al. (1995) reported on a large number of species pairs where one species was presumably specifically parasitizing the other, and suggested specific interactions at late stages of decay. Holmer et al. (1997) confirmed this view and showed that secondary resource capture by mycelia in wood under laboratory conditions was more likely to take place by species close to fruit bodies of primary decay fungi than vice versa. Moreover, invasion was more likely to be via the specific primary decayer than by primary decayers from combinations not observed in the field.

With time, the wood resource inevitably disappears. During the latest stages of decomposition, energy will be in short supply and nutrients will be relatively tightly bonded to macromolecules.

The number of basidiomycetes fruiting on wood has been reported to be relatively low at initial decay stages, to be highest at intermediate stages, then decrease again at later stages of decomposition, probably due to a lack of energy resources when the cellulose in the wood is depleted (Table 5; Figure 1). Apparently, the living tree is inimical to colonization by the majority of decay

Table 5 Number of species found in logs of different stages of decomposition in boreal forests

Stage of decay

Early

Intermediate

Late

References

Norway spruce

67

77

60

Lindblad (l998)

Norway spruce

27

68

30

Renvall (l995)

Norway spruce

l5

25

l0

Bader et al. (l995)a

Scots pine

ll

68

20

Renvall (l995)

aRecalculated data.

aRecalculated data.

Living Newly dead Light decay Intermediate Strong decay decay

Decay class

Figure 1 The number of species of wood-decaying basidiomycetes on Picea abies occurring at different states of decay. Data are from the Swedish Species Information Centre and are based on 264 species that have Picea abies as their principal resource in Sweden.

Living Newly dead Light decay Intermediate Strong decay decay

Decay class

Figure 1 The number of species of wood-decaying basidiomycetes on Picea abies occurring at different states of decay. Data are from the Swedish Species Information Centre and are based on 264 species that have Picea abies as their principal resource in Sweden.

fungi. The pathogenic decay fungi that are present may contribute to gap formation in the forest (Johannesson and Stenlid, 1999). Once the tree has died it represents an enrichment disturbance in the forest ecosystem and colonization can increase (Figure 1).

A full cycle of decomposition has been reported to take 60-70 years for spruce logs in the southern boreal region (Liu and Hytteborn, 1991) and approximately 150 years in northern areas (Hofgaard, 1993). Makinen et al. (2006) reported that for spruce and pine stems, it takes 60-80 years and for birch stems 25-40 years for total decomposition in southern Finland. The retention time increases depending on the tree remains standing after death (remaining as snag); snags (white and red pine) remaining for 90 years, fallen trees for 55-60 years (Vanderwel et al., 2006). Storaunet and Rolstad (2002) agreed that decay-rate estimates need to be computed from two regressions, one for the time standing and another for the time fallen. On average decomposition takes 100 years in old growth and 64 years in managed forests in Norway, and an additional 22 years (range 0-91 years) on average to fall. Our own observations indicate that a snag that has been colonized by brown rotters will break faster than those colonized by white-rot fungi, indicating an important role for brown-rotting species, for example Fomitopsis pinicola, in shortening the retention time of coarse woody debris.

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