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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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