How Mycorrhizae Fungi Affect Trees

An unlikely relationship exists between some fungal species -- called mycorrhizae -- and the trees in your backyard. Although the fungi attach themselves to the tree roots, they are not parasitic. They work together so that each benefits from the relationship.

The two primary types of mycorrhizae colonize tree roots, but they target different parts of the roots. Endomycorrhizae, also called arbuscular mycorrhizae -- abbreviated AM -- grow inside the tree roots. Ectomycorrhizae – abbreviated EM -- grow around the outside of tree roots. As the fungi grow, they send out thin strands called hyphae into the soil. Because the mycorrhizae are attached to tree roots, their hyphae act like extensions of the roots. This increases the absorptive capacity of a tree’s root system to take up water and minerals. The payoff for the mycorrhizal fungi is an exchange of sugars -- the products of photosynthesis that a tree manufactures -- which mycorrhizae must have to live.

Trees that typically host AM fungi include sweet gum (Liquidambar styraciflua) and catalpa (Catalpa bignonioides), both of which are perennials in U.S. Department of Agriculture plant hardiness zones 5 through 9. Redbud (Cercis canadensis), hardy in USDA zones 4 through 8, and persimmon (Diospyros virginiana), hardy in USDA zones 4 through 9, are other AM hosts. New seedling sweet gum trees typically experience low growth rates in the first three to five years after planting. The seedlings are significantly healthier, however, when grown in AM-rich soil during this same establishment period.

A tipoff that EM fungi are hard at work is the presence of mushrooms, which are fungal fruiting bodies, around trees. Host trees for EM fungi include the common beech (Fagus sylvatica) and Douglas fir (Pseudotsuga menziesii), both hardy in USDA zones 4 through 7. These host trees may have more than one type of EM fungi attached to their roots that produce plant growth regulatory hormones, called auxins, to enhance root growth. Pinus species plants, including whitebark pine (Pinus albacaulis) in USDA zones 6 through 8, depend on mycorrhizal fungi for their survival.

Most plants that share beneficial relationships with mycorrhizal fungi experience increased growth, higher survival rates, increased disease resistance and enhanced nutrition compared to non-mycorrhizal plants, according to The Ohio State University Extension. Purdue University Cooperative Extension Service notes that mycorrhizal fungi can boost the growth of some tree seedlings by 400 percent. Mycorrhizae also enable seedlings to better withstand drought and poor soil and to grow as large as non-mycorrhizal plants with half the nutrients.