RE-FORMATION OF MYCORRHIZAL SYMBIOSES ON MOUNT ST HELENS, 1980-1990 - INTERACTIONS OF RODENTS AND MYCORRHIZAL FUNGI

The three major types of disturbed habitats resulting from the eruption of Mount St Helens were a sterile pyroclastic flow site, a blast zone with all of the vegetation destroyed and tephra deposited on the surface, and a high ashfall region with deposited tephra over surviving plants. We present data assessing the fate of the invading mycorrhizal fungi and their associated plants by characterizing vesicular-arbuscular and ectomycorrhizal fungi from soil and root samples from sites within the three disturbance zones over the decade following the 1980 eruption. We also assess the importance of gopher-dispersed old soil containing VA mycorrhizal inoculum. Both ecto- and VA mycorrhizas re-established in disturbed areas primarily through inoculation from the buried old soils, a process probably facilitated by gophers. Mycorrhizas always formed at a quicker rate in the less disturbed areas of the eruption site. Detailed observations on the sterilized pumice plain indicated that, through 1990, the ectomycorrhizas were poorly developed and no fruiting structures were seen. Three possible hypotheses are proposed for the slow development of ectomycorrhizas despite their being wind-dispersed on to the site, and may be related to either slow soil development and/or compatibility restrictions. These results indicate that the magnitude of the disturbance regulates the rate of recovery not only of the vegetation, but also of both the VA and ectomycorrhizal fungi.