Mycorrhiza

July 9, 2006

When we walk in the forest we see the trees, the plants and perhaps mushrooms, mammals, reptiles, amphibians and birds, but beneath our feet is an ecosystem just as active and vibrant as the one we see above ground. Within this ecosystem, is a vast network of mycorrhiza, which literally means "fungus root". Mycorrhiza is the naturally occurring, non-pathogenic prolonged symbiotic association between fungi and the roots of vascular plants, in particular trees.

Mycorrhizae are separated into two groups, those that penetrate the cells of the root cortex, called endomycorrhizae and those that don't, called ectomycorrhizae. Douglas-fir can be a host to 2,000 different species of ectomycorrhizal fungi over the course of its life. Individual species of mycorrhizal fungi exist and behave as a function of a number of variables including soil fertility, aeration, soil temperature, pH, the presence of certain micro organisms, the presence of a specific host and the presence of specific mycorrhizal fungi mycelia or spores.

Mycorrhizae obtain simple carbohydrates from the host tree. In return, mycorrhizae improve nutrient availability and uptake by, in part, increasing the surface area placing more of the root in contact with soil nutrients. Carbonic acid, the result of fungal respiration and the solvent properties of fungal metabolic byproducts assist in the biochemical degradation of primary minerals in infertile soils. Pine seedlings, inoculated with mycorrhizal fungi, grown in prairie soil, aside from showing an increase in nitrogen concentration, demonstrated a two-fold increase in potassium and a three-fold increase in phosphorus. In addition, the exudates of the mycorrhizal fungus can stimulant growth. Moreover, mycorrhizae can serve as a biologic deterrent to pathogenic root infection by, in part, utilizing carbohydrates and other chemicals attractive to pathogens, by the fungal sheath acting as a physical barrier and by secreting antibodies which can inhibit or destroy pathogens.

Most healthy forest trees, conifers and broad-leaved trees alike, appear to have mycorrhizae. Moreover, their importance is greater felt in certain ecosystems. For example, the poor soils in the Amazon rain forest characterized by thin layers of litter and humus, below which are poor water logged clays, are able to support such a lush environment because the mycorrhizae efficiently recycle nutrients in the fallen leaf litter which would otherwise be leached away in heavy rains.

I welcome responses as well as ideas for future Tree Tips. You can also get an email version by contacting me at thembi@mcn.org or go to www.nwwg.org, click on Resources and click on Tree Tips.

Thembi Borras is a Registered Professional Forester living and working in Mendocino County, CA USA.