Mycorrhiza

There is a great deal of evidence that clearly demonstrates the great importance of mycorrhizae to plant nutrition, especially in nutrient-poor soils. For example, many species of trees and shrubs can be grown in the greenhouse, but if these are then transplanted into an abandoned pasture, prairie, or some other non-forested habitat, they commonly fail to survive or grow well, and will exhibit signs of nutritional distress. This happens because the soils of those habitats do not have populations of appropriate species of mycorrhizal fungi to colonize the roots of the tree seedlings. If the seedlings are to do well in such habitats, they must be deliberately inoculated with an appropriate mycorrhizal fungus.

On the other hand, if the seedlings are transplanted into a recent clear-cut that was previously a forest dominated by the same or closely related species of trees (for example during post-harvest regeneration of the stand), they will generally do well. This happens because the clear-cut still has a population of mycorrhizal fungi that are suitable to inoculate the tree seedlings.

In some cases, plants will not do well in the absence of a mycorrhiza, even when they are growing in apparently fertile soil. In such a case, the mycorrhizal mutualism is said to be obligate. In most cases, however, plant species can survive in fertile soil in the absence of a mycorrhizal relationship with a fungus. In less fertile soil, however, these species may do very poorly without a mycorrhiza. In this case, the mutualism is said to be facultative.

When natural ecosystems are being converted into some sort of system that is managed for the benefits of people, it may be critically important to consider the mycorrhizal fungi on the site. In tropical forests, for example, almost all of the tree species depend on mycorrhizae to supply them with nutrients from the typically infertile soils on which these ecosystems develop. Moreover, the mycorrhizal fungi are critical to retaining nutrients within the forest biomass, and in preventing these chemicals from being washed away by the abundant tropical rains. If the forest is cleared and burned to develop new agricultural lands, much of the nutrient capital of the land is lost due to the reduction of growing biomass, including the destruction of much of the mycorrhizal fungi by the disturbance. Moreover, the fungi that do manage to survive are not necessarily appropriate symbionts for the species of grasses and other crops that farmers attempt to grow on the cleared land. Therefore, the conversion of the tropical forest, with its efficient recycling and use of the scarce nutrient resources, into a more open agricultural system with large losses of nutrient capital, often leads to a rapid degradation of the fertility of the site. This is a major reason why so many conversions of tropical forests into agriculture prove to be unsustainable.

Many mycorrhizal fungi in the Basiodiomycete group develop edible mushrooms, and these are gathered by many people for use in gourmet cooking. Perhaps the most famous of these mushrooms are the truffles, such as Tuber melanosporum, which is commonly mycorrhizal on species of oaks (Quercus spp.). The spore-bearing bodies of the truffle fungi develop underground, and must be found using specially trained, truffle-sniffing pigs or dogs. Mushroom collectors must be careful, however, because some mycorrhizal fungi develop fruiting bodies that are deadly poisonous, as is the case of the death or destroying angel ( Amanita virosa).

Orchids (family Orchidaceae) are examples of plants that can only grow if they have an endomycorrhizal mutualism. Orchid seeds are tiny and dust-like, and they have virtually no stored energy to support the seedling when it germinates. The tiny orchid seedlings will only grow if they manage to become inoculated by an appropriate endomycorrhizal fungus. Until this fact was discovered by horticulturalists, orchids were extremely difficult to propagate and grow in greenhouses. Now, orchids are relatively easy to breed and cultivate.

Some species of vascular plants do not contain chlorophyll, and are incapable of photosynthesis. The whitish shoots of species such as the Indian pipe (Monotropa uniflora) depend entirely on their mycorrhizal fungus to supply the plant with organic nutrients needed for growth and reproduction. This is an unusual case in which the balanced, reciprocal dependence of the plant and fungal symbionts of a mycorrhiza have become unbalanced, to the degree that the plant is now parasitic on the fungus.