THE ROLE OF MYCORRHIZAL FUNGI AND MICROSITES IN PRIMARY SUCCESSION ONMOUNT ST. HELENS

This study was designed to examine the role of vesicular-arbuscular my corrhizae (VAM) and microsites on the growth of pioneer species. Flat, rill, near-rock, and dead lupine microsites were created in plots in barren areas of the Pumice Plain of Mount St. Helens. VAM propagules w ere added to the soil in half of the plots. Six pioneer species were p lanted into both VAM and non-VAM inoculated microsites. Plants in dead lupine microsites were greater in biomass than those in flat, rill, a nd near-rock microsites. Significant effects of VAM on plant biomass d id not occur. Microsites continue to be important to plant colonizatio n on the Pumice Plain, but VAM do not yet appear to play an important role. This may be due to limited nutrient availability and the faculta tively mycotrophic nature of the colonizing plant species. It is unlik ely that VAM play an important role in successional processes in newly emplaced nutrient-poor surfaces.