Mycorrhizal mat communities in forest soils

The role of ectomycorrhizal (EM) fungi in forest soil function and tree productivityhas become an increasingly important topic for research in light of possible directlarge-scale degradation of EM populations in the forests of Europe (Arnolds, 1991).Although there is a rapidly growing body of information being assembled fromlaboratory studies using pure culture syntheses, there is relatively little knownabout how these fungi function in the field. EM species of Gautieria and Hyster-angium that form distinctive hyphal or rhizomorph mats have been observed inforests ranging from the subtropical (Eucalyptus in Australia) to boreal forests inAlaska (Castellano, 1988; Griffiths et al., 1991b; Griffiths, unpublished observa-tions). The actual quantitative impact of these mat communities on the forest floorremains largely unknown, although Cromack et al. (1979) reported up to 27 % oftemperate coniferous forest mineral soils could be colonized by a single species,H. setchellii.
These mats present a novel solution to the problem of how to measure theimpact of an EM fungus on its immediate surroundings by greatly magnifying theinfluence of a single fungal species. The mat communities studied are generallydominated by a single EM species which can have a biomass equivalent to up tohalf the mass of the soils with which they are associated (Ingham et al 1991). Thislevel of impact permits one to study the influence of these fungi on associatedsoil by comparing the biology and chemistry of soils colonized by these fungi withsoils that are not so colonized. This approach has now been used to document largedifferences between mat and non-mat soils over seasonal cycles and in differentfungi located in different areas of the Pacific Northwest (Cromack et al., 1988;Griffiths et al., 1990, 1991a).