Ectomycorrhizal fungi are, economically, one of the most important groups of fungi. These are the fungi that form a symbiotic relationship with a plant forming a sheath around the root tip of the plant. The fungus then forms a Hartig Net which means that there is an inward growth of hyphae (fungal cell growth form) which penetrates the plant root structure. There are actually seven types of mycorrhiza and 90% of plants form mycorrhiza with fungi, but ectomycorrhizal refers to this sheath forming type.
The fungus then gains carbon and other essential organic substances from the tree and in return helps the trees take up water, mineral salts and metabolites. It can also fight off parasites, predators such as nematodes and soil pathogens. Indeed, most forest trees are highly dependant on their fungal partners and in areas of poor soil, could possibly not even exist without them. Thus in forest management, if we do not manage for the mycorrhizal fungi, we could be damaging the trees. A good example of forest mismanagement is given by excessive burning of forests which kill off the mycorrhizal fungi and leave it open to attack from pathogens such as Phytophthora cinnamomi.