Mycorrhizae and Mount St. Helens:Story of a Symbiosis

Mycorrhizae are symbioses between plants and fungi localized in the roots. These mutualisms represent important components in the recovery of vegetation because most plants depend on their fungal symbiont for a large portion of their soil resources, such as water, nutrients, and sometimes carbon. Although these organisms are generally rather cryptic and often unnoticed, they regulate many processes in ecosystems. The fungal hyphae, consisting of microscopic threads 2 to 10 μm in diameter, form the body of the fungus and ramify through the roots, forming a large surface area for exchanging nutrients and carbon. The hyphae then extend outward into the soil to provide nutrients and water to the host, become a sink for carbon, bind soil particles into soil aggregates, and produce sporocarps that are food for animals (Allen 1991; Smith and Read 1997; van der Heijden and Sanders 2002). External hyphae can be several meters to more than a kilometer per gram of soil. Thus, hyphae magnify the surface area of soil available for nutrient uptake and for soil-particle binding manifold compared with the roots alone.