Mycorrhizae are symbiotic fungus-root associations. The colonization of roots by mycorrhizal fungi can benefit the host by improving nutrient and water uptake. In exchange, the host plant provides the mycorrhizal fungi carbohydrates (carbon) from photosynthesis. A substantial portion of this carbon is ultimately transferred to the rhizosphere and is estimated to account for up to 15 percent of the organic matter in forest soils. Under most environmental conditions, trees and other plants are naturally colonized by mycorrhizal fungi. These mycorrhizal associations are highly complex and dynamic, a result of the great diversity of mycorrhizal fungi, hosts, and terrestrial systems that interact and evolve with changes in hosts and environmental conditions.
Cram, Michelle M.; Dumroese, R. Kasten. 2012. Mycorrhizae in forest tree nurseries. In: Cram, Michelle M.; Frank, Michelle S.; Mallams, Katy M., tech. coords. Forest nursery pests. Agriculture Handbook 680 rev. 2012. Washington, DC: U. S. Department of Agriculture, Forest Service. p. 20-23.