VESICULAR-ARBUSCULAR MYCORRHIZAE INFLUENCE MOUNT ST. HELENS PIONEER SPECIES IN GREENHOUSE EXPERIMENTS

Vesicular-arbuscular mycorrhizae (VAM) are present in most terrestrial ecosystems and play a major role in community structure and function. However, their role in primary succession remains poorly understood. Two greenhouse studies examined the role of VAM in Mount St. Helens pi oneer species under three nutrient regimes and four competitive scenar ios. Nutrient levels were complete, complete without phosphorus(-P), a nd tap water (very low nutrient levels). In tap water a negative effec t from VAM colonization was observed perhaps due to parasitic action o f the VAM fungi. A weak but apparent benefit from VAM occurred in the -P treatment since plants in the -P treatment were usually not less in biomass than those in the complete nutrient treatment and VAM coloniz ation levels were greater in the -P treatment. VAM colonization was mo re beneficial to plants under the complete nutrient treatment than und er the tap water treatment. VAM assisted the facultatively mycotrophic Hypochaeris radicata in competition with the non-mycotrophic Carex me rtensii. Lack of VAM improved the competitive ability of Carex mertens ii when in competition with facultatively mycotrophic species. However , VAM did not significantly influence competitive outcomes between fac ultatively mycotrophic species.