While plants vary in their dependence on mycorrhizal fungi, the fungi are generally obligate mutualists (Lewis 1973). Interestingly, patterns of mycorrhizal diversity do not always follow those of plant diversity (Allen et al. 1995). There are approximately 150 species of AM fungi (Morton et al. 1995; Schenck and Perez 1990) forming associations with about 70% of plants worldwide. Virtually any AM fungus can associate with any vascular plant capable of forming an AM (Allen et al. 1995). Despite low fungal species richness, many AM communities can exhibit high plant species diversity. In Wyoming sagebrush-steppe, 11 species of AM fungi were found where plant species richness exceeded 150 species (Allen et al. 1993). In a survey of 80 sites across the Great Basin in the western United States, Allen et al. (1993) found only 48 species of AM fungi, with no one site containing more than 12 species. In a seasonal tropical forest, Allen et al. (1998) found approximately 25-30 species of AM fungi in a forest with plant species richness exceeding 1,000.
Unlike AM fungi, the formation of EM involves a greater diversity of fungal species (> 5400 species), exhibiting varying degrees of host specificity (Molina et al. 1992). For example, the EM fungal genera, Hydnangium, is found only on Eucalyptus and Suillis and Rhizopogon are restricted to Pinaceae, while Amanita and Laccaria associate with most EM hosts (Molina et al. 1992). Further, EM fungal diversity can be high in areas where plant community diversity is low. Early studies by Trappe (1977) estimated 2000 species of EM associated with Douglas Fir alone. In the Jarrah forest of southwestern Australia dominated by Eucalyptus marginate and E. calophylla, 90 species of EM fungi were found (Hilton et al. 1989). Over 50 species of EM fungi were identified in a Quercus agrifolia stand near Temecula in southern California. These included truffle fungi in the genera Hydnotryposis, Hydnotrya, and Tuber, as well as epigeous mushrooms such as Amanita, Boletus, Cortinarius, Laccaria, and Russula. Still, most studies underestimate EM diversity because many of these fungi fruit irregularly, and fungi with abundant sporocarps may not form functional EM (Gardes and Bruns 1996; Gehring et al. 1998).
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