Ecological Significance Of Fungalfungal Interactions

Monospecific populations of fungi rarely form within soil and organic resources; thus, interspecific mycelial interactions, with competition for resources, continually occur. These processes affect fungal community dynamics. Since different species effect decomposition, mineralization and nutrient translocation to different extents, outcome of interactions will impact directly on these processes. Moreover, the actual interaction may also affect these processes. For example, decay rate (measured as CO2 evolution) increased when Stereum gausapatum was replaced by Bjerkandera adusta, and during deadlock between L. betulina and Flammulina velutipes in wood (Owens, 1989; Boddy, 2001). Occasionally there was a decrease in CO2 evolution during interactions, e.g. during replacement of Chondrostereum purpureum by S. hirsutum. Interactions also affect movement and partitioning of carbon within mycelial systems. 14C monitoring during interactions between saprotrophic cord-forming Basidiomycota in soil provided evidence of a carbon cost of encounter with a competing mycelium (Wells and Boddy, 2002). P. velutina switched reliance from the carbon available in the wood from which it grew to that available in the captured wood. It opportunistically utilized carbon previously mobilized by the opponent. In interactions that resulted in deadlock in soil, there was evidence of interspecific carbon exchange, presumably following leakage into soil from damaged hyphae in the interaction zone.

Mineral nutrient uptake, partitioning, movement and release are also affected by interspecific mycelial interactions. 32P-uptake kinetics of cord-forming bas-idiomycetes was significantly affected by the presence of a competing mycelium (J.M. Wells and L. Boddy, unpublished). Changes in uptake capacity or rate constants were not related to the outcome of the interaction, but probably reflected the ability of species to divert effort preferentially from phosphorus scavenging to territory defence. In some interactions this ability was over-ridden when there was concurrent supply of uncolonized wood resources. There was reciprocal 32P exchange between R. bicolor, P. velutina and H. fasciculare mycelia in soil which, as with carbon, presumably occurred via leakage in the interaction zone. In contrast, labelled mycelial systems of P. impudicus lost 32P only to R. bicolor, and this was only detectable 39 days after 32P supply. In addition to nutrient exchange between mycelia, there was movement of nutrients within mycelia during interactions. For example, P. velutina and one isolate of H. fasci-culare preferentially translocated mobilized P to the zone of interaction in soil, whereas a less robust isolate of H. fasciculare preferentially translocated mobilized P away from the interaction zone (J.M. Wells and L. Boddy, unpublished).

Interspecific mycelial interactions are, together with grazing by invertebrates (Chapter 9), probably the main factor resulting in release of nutrients to soil. For example, interactions between P. velutina and H. fasciculare, in soil microcosms, resulted in significantly greater losses of 32P to soil than self-pairings (J.M. Wells and L. Boddy, unpublished). Non-self-pairings within a species also resulted in significant losses to soil. In both cases leakage occurred not only in the interaction zone, but also elsewhere.

Many saprotrophic and ectomycorrhizal (EM) Basidiomycota are closely related, and the EM relationship is evolutionarily unstable, having been both gained and lost (Hibbett et al., 2000). It is, therefore, not surprising that many EM hymenomycetes retain key enzyme systems of saprotrophic fungi (Leake and Read, 1997). Since EM hymenomycetes often colonize wood at very late stages of decomposition, and are found in organic soil and litter along with foraging saprotrophic Basidiomycota, there is likely to be intense competition for more labile nitrogen and phosphorus, and for antagonistic mycelial interactions (Lindahl et al., 1999; Leake et al., 2001, 2002, 2004). As with interactions between saprotrophic mycelia in soil, there can be significant transfer of nutrients between the two trophic groups (Lindahl et al., 1999). There were also often marked effects of the saprotrophic P. velutina on carbon allocation to extra-radical EM mycelium of Paxillus involutus and Suillus bovinus (Leake et al., 2001, 2002; Figure 3).

In boreal forest ecosystems, there may be partitioning of niche, reducing interactions between saprotrophic and EM hymenomycetes. Saprotrophs appear to dominate in the upper, organic soil horizons, whereas in lower, mineral layers EM species dominated; these findings suggest that the saprotrophic species lead in the release of carbon from organic materials, whereas the mycorrhizal fungi are the principal agents responsible for mobilization of nitrogen (Lindahl et al., 2007).

In addition to the critical process of nutrient recycling, several hymen-omycetes cause serious economically damaging disease in forest trees; although these fungi are often classified as pathogens, they spend much of their lives acting saprotrophically. Examples include species in the genera Heterobasidion, Armillaria and Phellinus. Increased understanding of the dynamics of interspecific competition may lead to the development of additional management intervention techniques for reducing the impact of these pathogens. The application of spore suspensions of P. gigantea to stumps of felled conifers, as a biological control of H. annosum, is well established (Holdenrieder and Greig, 1998). Other aggressive combatants, e.g. R. bicolor, may also be candidates for use as biological control agents (Holmer and Stenlid, 1997b).

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