I , Mycorrhizal Mat Communities in 3 Forest Soils

The role of ectomycorrhizal (EM) fungi in forest soil function and tree productivity has become an increasingly important topic for research in light of possible direct large-scale degradation of EM populations in the forests of Europe (Arnolds, 1991). Although there is a rapidly growing body of information being assembled from laboratory studies using pure culture syntheses, there is relatively little known about how these fungi function in the field. EM species of Gautieria and Hysterangium that form distinctive hyphal or rhizomorph mats have been observed in forests ranging from the subtropical (Eucalyptus in Australia) to boreal forests in Alaska (Castellano, 1988; Griffiths et al., 1991b; Griffiths, unpublished observations). The actual quantitative impact of these mat communities on the forest floor remains largely unknown, although Cromack et al. (1979) reported up to 27 % of temperate 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 the impact of an EM fungus on its immediate surroundings by greatly magnifying the influence of a single fungal species. The mat communities studied are generally dominated by a single EM species which can have a biomass equivalent to up to half the mass of the soils with which they are associated (Ingham et al 1991). This level of impact permits one to study the influence of these fungi on associated soil by comparing the biology and chemistry of soils colonized by these fungi with soils that are not so colonized. This approach has now been used to document large differences between mat and non-mat soils over seasonal cycles and in different fungi located in different areas of the Pacific Northwest (Cromack et al., 1988; Griffiths et al., 1990, 1991a).