It was a beautiful autumn day in Algonquin; rain showers made the forest glow, and mushrooms were everywhere. I was out with other forest ecologists, and it reminded me that mushrooms are often valued as a gourmet treat, and sometimes for their beauty, but the enormous ecological importance of fungi to the forest is often overlooked.
Fungi perform a lot of work in forests. Their most obvious role is as decomposers of dead wood. Trees are made up of about 15-30% lignin – a tough, chemically complex molecule that gives much of the rigid structure to wood – and 40 to 60% cellulose, which is formed in long chains and gives wood a lot of its strength and flexibility (it is also what we make paper from). Lignin and cellulose are very hard for most micro-organisms to break down, but fungi are able to decompose both materials and so they are critical in nutrient-cycling in forests. Fungi are found as fine threads (hyphae) throughout the soil and in dead logs, twigs, leaves and needles. When conditions are right, especially during the fall rainy season, they produce mushrooms to spread their spores. This is often the only sign we see of the prolific fungal life hidden just below the surface.
Fungi that are good at decomposing lignin are more specialized than those that break down cellulose. Some species of fungi consume only the cellulose and leave dark brown, blocky wood containing modified lignin as a residue. These ‘brown rot’ fungi are most common in northern temperate forests, especially conifer forests. Because the brown blocky residue has compounds that are resistant to decay, brown rot fungi contribute more to the creation of soil humus than white rot fungi – humus is the dark coloured organic matter that accumulates near the surface of forest soils, which helps retain moisture and nutrients. Humus is so resistant to decay that it has average turn-over times of 400 to 1000 years in many forests.
Fungi that are good at consuming both lignin and cellulose leave behind light-coloured, stringy wood. There are more species of these ‘white rot’ fungi, and they are much more common globally.
Another group of forest fungi are parasites on trees, growing in the roots, trunk, branches or leaves. Parasites range in their impact from simply weakening the tree structurally by decomposing the heartwood, to killing otherwise healthy trees relatively quickly.
| Honey mushrooms (Armillaria spp)
Armillaria species cause root rot of trees, which typically kills the tree and the fungus then spreads up the trunk via thick black ‘bootlace rhizomorphs’ and colonizes the wood. At this point the fungus becomes a saprotroph (wood decomposer), causing a white rot of the trunk. The rhizomorphs are a sign of Armillaria infection, as are the numerous honey coloured mushrooms that may appear in late summer or autumn. The fungus mostly attacks trees that are already stressed or weakened, but may also sometimes kill otherwise healthy trees. Armillaria colonies are among some of the largest organisms on earth, with genetically identical colonies covering up to 960 ha, with estimated ages between 2000-8000 years.
|The northern tooth fungus (Climacodon septentrionale)
This fungus causes a white heart-rot in hardwood trees, especially sugar maple, ash and beech. Because it rots the heart wood of living trees it is considered parasitic, though the heart wood of trees is dead, inactive tissue. The primary damage to the tree is through weakening of the trunk which may cause it to snap in windstorms.
|Tinder conk or clinker fungus (Inonotus obliquus)
Tinder conks are a sterile conk (it does not produce spores) of a fungus that causes white heart rot in living hardwood trees, especially birches. Because the fungus grows in living trees it is classed as a parasite, and does to some extent attack living tissue of the tree, though it mostly decays the dead heartwood of the tree. Infected Trees may live for 80 years or more, but 20 years is more typical. The primary damage to the tree is weakening of the stem, in part by the formation of the conk itself. The conks, known as chaga, are considered to be medicinal. Global use of chaga has been growing dramatically since at least the early 2000’s – it’s not clear what this means in terms of sustainability of harvesting.
The mycorrhizal fungi
Mycorrhizae (which translates as fungus-roots) are fungi which are in a symbiotic relationship with plants. The fungus attaches to the plant roots and takes sugars from the plant, and in return it provides nutrients that it is difficult for the plant to obtain using its own root system. The most important of these nutrients is phosporous.
Mycorrhizae occur on an estimated 95% of all plant species, and many plant species cannot survive without them. A surprising number of the forest mushrooms found in wet autumn weather may be mycorrhizal – but perhaps it is not so surprising considering how important this relationship is to nutrient cycling, and the growth of both forest trees and understory plants.