Shades of Green: Earth's Forests -- Fungi

The mushrooms that you can find on almost any walk through a forest are the "fruits" of an underground network of tiny strands of fungus. Many of these fungi strands, called "micorrhizal fungi" grow around tree roots. The trees and the fungi are both dependent on each other for certain nutrients. If the tree is cut down, the fungi strands around its roots die. If a new tree is then planted in the same spot, it will not grow as well because the soil now lacks this important fungus.

A bracket fungus growing on a tree trunk in mixed temperate forest. Photo by Maya Walters.

Unlike many types of fungi, bracket fungi do damage trees. Photo by Maya Walters.

Fungi depend on obtaining nutrients from organic material such as decaying leaves and animals. Unlike green plants, fungi can not produce their own energy from sunlight and inorganic substances. This is because fungi lack the chemical called chlorophyll, which allows plants to photosynthesize*. The fungi strands, called "hyphae" grow around dead leaves and animals, and decaying logs and branches. The hyphae then secrete enzymes that make it possible for them to digest food molecules.

There is an enormous amount of food available for most fungi, which aren't terribly picky eaters. Huge numbers of leaves fall every year in a deciduous forest. Some of these leaves have been damaged by insects but the vast majority simply fall to the ground at the end of summer. Once on the ground, these leaves enter the food chain of decomposers. The fungi are part of this food chain, along with bacteria and soil-dwelling insects.

In a deciduous forest, over 90% of the organic material, such as leaves, produced by forest plants every year is not eaten by herbivorous animals and insects. Instead, when leaves die and branches fall, they go directly to the fungi under the forest floor.

Tiny mushrooms begin to grow on rotting logs, helping to decompose the wood. Photo by Maya Walters.

It can take as long as three years for fungi and bacteria to decompose fallen leaves. Thick, tough needles of coniferous trees can take even longer. Because of cold temperatures, fungi can't actively decompose material during the winter. They also can't survive in areas with extremely acidic soils, such as bogs. Because of the lack of fungi, organic material in bog forests does not decompose easily.

The main body of a fungus, called the "mycelium" can live for several hundred years and grow new fungus strands as old ones break or die back.

The reproductive structures of the fungi, which include mushrooms and puffballs, are especially easy to find in the fall after a heavy rain. They can be all shapes, sizes, and colors, and each one contains literally billions of tiny spores*. When the mushrooms mature, they release these dust-like spores, which are transported by the wind and animals. One or two spores of the billions released might land in a suitable place and begin to grow.

Most fungi are not parasites, and very few types can harm trees. Most are actually beneficial. There are exceptions, however, and some fungi, such as bracket fungi, can be found growing in living trees. Instead of growing strands through the soil, they grow under the bark of the tree, taking nutrients from the host. When these fungi produce mushrooms, they grow out of the tree trunk. Spores from these mushroom spread to other trees and find their way inside through woodpecker holes and other damaged areas. Another example of harmful fungi is the group of closely related species called "honey mushrooms", which can damage living trees. This fungus causes live bark and tree roots to rot, and the honey mushrooms are therefore also known as "root rot fungus".

Bracket fungus. Photo by Maya Walters.

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