Investigating Interactions

Interactions between wood decay fungi have most commonly been investigated in pairs under controlled in vitro conditions using agar-based media (Cooke and Rayner, 1984; Rayner and Webber, 1984; Dowson et al., 1988; Pearce, 1990; Murphy and Mitchell, 2001; see references in Boddy, 2000) where they can be clearly visualised (Figure 1e and f). Although it has been argued that valuable information can be gained from this approach (Henningson, 1967; Rayner, 1978; Boddy and Rayner, 1983; Rayner and Webber, 1984), interpretation of the results from such artificial environments may be misleading as responses are not always the same as in more natural systems (Boddy, 2000; Woods et al., 2005, 2006). Contrasting results have been obtained between tests carried out on high sugar media and woody resources (Lundborg and Unestam, 1980; Nicolotti and Varese, 1996; Highley, 1997; Woods et al., 2005). Moreover, differences in outcome often occur between different natural substrata; thus, the outcome of interaction in wood may be different from when mycelia encounter one another in soil (Dowson et al., 1988). In research utilizing fungi isolated from stumps of Sitka spruce, interaction outcomes between fungi differed greatly on the defined, high sugar Norkrans agar compared to spruce-based wood media (Woods et al., 2005). R. bicolor, a cord-forming wood colonizer, proved particularly striking in this respect, showing poor competitive ability on Norkrans agar, whereas on substrates based on Sitka spruce wood, including a sawdust-agar medium and auto-claved root blocks, it was the most aggressively competitive of all the species tested. This species rapidly colonizes spruce stumps in the field (Kirby et al., 1990; Holmer and Stenlid, 1997b), out-competing other species of decay-causing hymenomycetes, even in co-inoculations (Woods et al., 2006). In contrast, Phaeolus schweinitzii, cause of brown cubical rot in standing conifers, was the most aggressively competitive hymenomycete when tested on Norkrans agar, whereas on the wood-based substrates it was considerably less aggressive (Woods et al., 2005).

Higher levels of complexity, and consequent difficulties in interpreting results, arise when attempting to include more than two species of fungi within an interaction test (e.g. White et al., 1998; Boddy, 2000; Sturrock et al., 2002; Sudin, 2005). Increasing size and complexity in in vitro multi-species interaction tests lead to less consistency in the interaction outcomes, and outcomes in three species interactions cannot be predicted based on the relevant two species interactions (White et al, 1998; Boddy, 2000; Sudin, 2005).

In vitro systems based on natural resources may enable better assessment of the in vivo nature of interactions between saprotrophic fungi, despite the inherent difficulties in emulating microenvironmental conditions occurring in the field (Rayner and Webber, 1984). Use of trays of compressed, non-sterilized soil maintained under laboratory conditions has been particularly successful for emulating conditions of saprotrophic cord-forming Basidiomycota (Chapter 1) that extend between organic resources in woodlands at the soil litter interface (Donnelly and Boddy, 2001; Figure 1a and b).

When field inoculations into natural resources are utilized to test interspecific interactions, results can be rather difficult to interpret. Uncontrollable variations in environmental conditions, along with differences relating to timing of inoculation, may lead to highly variable results. Despite these possible pitfalls, it is important to consider interactions with respect to natural resources and environmental conditions, to develop a more complete understanding of the processes underlying community development.

Appropriate resources can be directly inoculated with pairs of interacting fungi in the field. Complications can arise, however, for several reasons, but particularly as species which were not deliberately inoculated may colonize the resource after the inoculations, confounding the results. Sitka spruce stumps in Scotland were inoculated with pairs of decay-causing hymenomycetes, as either pre-colonized sawdust inoculum or more discrete pre-colonized woody dowels (Woods et al., 2006). In both inoculation types, many stumps also became colonized by wild strains of the decay-causing agaric M. proteus. However, this apparent contamination had little noticeable impact on the types of interactions observed between inoculated species.

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