Appendix 1 — Soil biodiversity
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British Columbia
Ministry of Forests
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Appendix 1
   

Fungi

 

Approximately 69,000 species of fungi have been identified worldwide so far and it is estimated that there are about 1.6 million species in total.

A typical ratio of fungi to vascular plants for a given ecosystem is about six to one. Many of the fungi live in the soil where they function as pathogens, parasites, saprophytes, and mycorrhizal fungi.

In temperate and boreal forests, fungi often constitute the largest biomass component of the forest floor. They play very important roles in decomposing organic matter that contributes to nutrient cycling.

As disease organisms, they have important functions, such as thinning of weak and suppressed trees in overstocked forests.

The health of the forest ecosystem is even more directly linked to ectomycorrhizal fungi, which form a symbiosis with most of the trees and shrubs that grow in temperate and boreal forests.

The few species without ectomycorrhizal fungi usually have other types of mycorrhizae. All trees in the ectomycorrhizal forest are dependent on mycorrhizae for survival and growth, and will either die or will not grow without their ectomycorrhizal symbiont.

The feeder roots of ectomycorrhizal trees are the organs of the tree that is responsible for nutrient and moisture uptake. Essentially every feeder root of an ectomycorrhizal tree is completely covered by a dense covering of fungus called the mantle or sheath, and every cortical cell of the tree's feeder roots are also individually surrounded by a layer of fungus called the Hartig net.

This combination of fungus and tree root is what constitutes a mycorrhiza (literally fungus root). It is estimated that up to about 50% of the energy that trees store goes directly to its mycorrhizal fungi, which indicates the significance of the mycorrhizal fungus in this symbiotic relationship.

There are currently estimated to be 5000-6000 species of ectomycorrhizal fungi. The fungi and mycorrhizae formed by them are highly variable in appearance and other characteristics.

They have different preferences for pH, temperature, moisture levels, season, and substrate type. The different types exude different kinds of antibiotics and take up nutrients in different forms, such as nitrate, ammonium, or organic nitrogen.

Some kinds exude siderophores, which are organic molecules can aid in the uptake of iron; others exude enzymes to mobilize phosphorous or micronutrients.

Some kinds show specific antagonism towards various disease-causing organisms, while others are capable of directly breaking down organic matter without an intervening saprophyte.

Some kinds of mycorrhizal fungi form fungal mats, which are dense layers of fungus that physically, if not biologically or chemically, dominate some types of forest floors.

Some kinds of mycorrhizal fungi connect overstorey plants to understorey plants and can transfer carbohydrates between trees of the same species or different species of plants.

These associations of interconnected plants are called plant guilds.

Some types of mycorrhizae prefer trees of different ages while others persist over the life of a forest. It is clear that the ectomycorrhizal fungi do many different things and are not equal.

Even within a given species of fungus, there is considerable adaptation, just as with provenances of trees. A typical tree has many different types of ectomycorrhizae to allow it to exploit the various conditions that may be found even within the rooting zone of a single tree.

There have been reports of up to 150 types of mycorrhizae on a single Douglas-fir tree.

It is common to find 30 to 40 species of ectomycorrhizal fungi in a stand, but that may be an underestimate because of the difficulties in sampling for them.

Mycorrhizal fungi are dispersed by aerial spores, by spores transported in the guts of animals and by root contact.

Some species have no known spore dispersal mechanism and are dependent on mycelia contact for dispersal.

Many small plants, such as Arctostaphylos, are ectomycorrhizal and can act as refugia for mycorrhizae after logging or other disturbances, though not all ectomycorrhizal fungi can colonize these species.

Approximately 1000 species of ectomycorrhizal fungi form subterranean fruiting bodies (truffles or truffle-like mushrooms) are meant to be eaten so that spores can be dispersed in dung.

Squirrels, voles, deer, bears, and other animals eat these types of mushrooms. The flying squirrel, which lives throughout the forested areas of Canada, is very dependent on the underground fungi.

Mushrooms, above and below ground, are good sources of nutrients, often containing 25 to 30% protein. They vary in vitamin and nutrient content and a diverse population of fungi helps create a balanced diet for the animals that utilize them.

Many fungi are very important food sources for humans as well. Several species of mushrooms are harvested in BC, including chanterelles, pine mushrooms, morels, brain mushrooms, king boletus and giant puffballs.

Chaterelles, pine mushrooms and king boletus are ectomycorrhizal and require specific types of forests to produce mushrooms. Some types of mushrooms, such as morels, fruit abundantly under specific conditions, such as after a wildfire or broadcast burning.

In Europe, dramatic declines (in the order of 50%) in the number of species of ectomycorrhizal fungi in forests have been reported. The reasons for this are unclear but factors that have been suggested include pollution, over harvesting of mushrooms, and fertilization of forests.

Many of the forests of Europe are currently in a state of decline. It is not known if the decline in ectomycorrhizal species is contributing to the decline in the forest or if stress to the forest is causing decline in types of mycorrhizae.

However, there is ample evidence now to suggest that complex populations of ectomycorrhizae are synonymous with a healthy and stable forest.

     
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Appendix 1

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