Moldy Rotters

by Fred Rhoades

Mushrooms That Specialize in Decomposition 

Part 1
In a previous Whatcom Watch article (Oct./Nov. 2021), I discussed some mushrooms that perform nutrient gathering services for trees. The mycorrhizal connection. About half the mushrooms, and many of the larger species, form these connections to the roots of many of our trees, and, in exchange for some of the carbohydrate made by the tree by photosynthesis, act as a fine root system for the tree.

In this article, I will begin to review the other important role of many mushroom species — the breakdown of plant matter in the environment — decomposition. This time I’ll review some of the mushrooms that work on small plant parts. I did introduce the topic in an earlier article on the “fuzzy foot” (Oct. 2019 — formerly Marasmiellus peronatus now has another new name, Collybiopsis peronata). See that article for a brief description of how that nonnative species has worked to take over some of the decomposition of needles and leaves in our lowland forests.

Next time, I will complete the story by talking about decomposers of woody substrates. One final ecological role of mushrooms that just a few gilled species play (but some other mushroom groups specialize in) is as parasites of plants (and a few animals). I’ll cover that role in another article.

Mushrooms are particularly important in the breakdown of parts of dead plants. The process is a complex one and involves many kinds of organisms. Complete decomposition of a part is a many-year process, and it involves many groups of organisms: bacteria, protozoa, small and large animals, and a variety of fungi including molds and mushrooms. Each organism is responsible for decomposing a small set of chemicals in the part or helping to physically reduce the material.

Specific to Each Plant Type
Because plants and their parts are so different from one another chemically and physically, often the mushrooms that decompose them are specific to each type. Beginning mushroom identifiers learn that habitat is a very important clue to the identity of many species.

Figure 1. Baeomyces myosura mycelium (enlarged in inset to show hyphae) and mushrooms on the end of a spruce cone.
All photographs by Fred Rhoades

Baeomyces myosura (Figure 1) is only found on the cones of spruce and a few other conifers in our area. Mycena pterigena (Figure 2) is only found on the decaying fronds of lady fern.

Figure 2. Mycena pterigena on decomposing lady fern frond.

On the other hand, some mushrooms seem to be quite cosmopolitan in their habitat preferences. Lepista nuda, the wood blewit (Figure 3) can be found in a wide variety of habitats that accumulate disturbed plant litter, including gardens. These are produced by a copious mycelium that was illustrated in the Oct. 2019 article. Wood blewits can be found in these habitats throughout the warm times of the year. They are edible and considered good, but there are poisonous look-alikes to be avoided, and, if you think you have found this species, consult an expert.

Figure 3. Lepista nuda mushrooms.

Mushroom species are particularly good at breaking down the big molecules that build plants. Their hyphae are perfectly adapted to gain entry into these hard substances. Hyphae are the tiny tubular units of construction of their mycelium (Figure 1), which is the spread-out, mesh-like body that produces the mushrooms. By adhering to their substrate, hyphae can gain a foothold and literally drill through the hard parts of plant cells. Partly, this is done by force.

But entry into these hard substances is also eased by releasing specific enzymes that react with and weaken the molecules. Enzymes are biological catalysts that ease the breakdown or formation of other chemicals. Once a branched mycelium is spread out within these hard parts, decomposition of them begins in earnest as enzymes are released from the hyphal tips, digestion of the material takes place outside the mycelium, and the breakdown products (various sugar molecules, minerals, etc.) seep back into the mycelium and help it grow.

Targets: Cellulose and Lignin
Plants are made of two types of chemicals that are the targets of mushroom enzymes: cellulose and lignin. Cellulose is the major structural molecule in the cell walls of all plants. Lignin is an even harder molecule that strengthens certain types of cells, particularly in wood, but also in the cells which pipe water through plant parts. Different fungi specialize in the breakdown of each of these types of chemicals. Certain kinds of mushroom-producing fungi are particularly good at decomposing lignin. Figure 4 shows two stages in the decomposition of a maple leaf, visually showing the difficulty of lignin decomposition.

Figure 4. Beginning and later decomposition of a maple leaf. The veins contain lignin and take longer to decompose.

Let’s look at two more specific examples to get a better feeling about how many mushroom-producing fungi integrate their decomposition activities into their lives. The species in the genus Mycena are common decomposers of many types of plant parts. Different species appear throughout the year on leaves, dead grass, beds of fallen conifer needles, and on various kinds of woody substrates. This group is defined by small, cone-shaped caps, and producing white spores. Although their mycelia are usually not seen, the extent of these bodies can often be decerned from the extent of the mushrooms.

Figure 5. Mycena robusta. Dotted line shows the inferred location of the mycelium.

In Figure 5, the extent of the mycelium of Mycena robusta can be inferred if you outline the set of mushrooms that comes up in the needle beds upon which this species grows. A closer view of the species is seen in Figure 6. This species is relatively large (for a Mycena, hence the species name) gray species and is common in Whatcom County from late December to February when temperatures are above freezing.

Figure 6. Closer view of Mycena robusta.

You can’t see the attachment of the individual mushrooms to their mycelium in the needle bed (Figure 5) because their mycelium (they are all produced by the same, perennial mycelium) is decomposing the lignin in deeper, later-stage decomposing needles. New needles “rain” down on this environment every year, so the set of active organisms, including the M. robusta mycelium moves up into the appropriately aged, building layer of needles or their decomposing products. In this and other Mycena species working on decaying needles and leaves, the areal extent of the mycelium increases very slowly. Compare that behavior to the fuzzy foot invader species described in the Oct. 2019 “Life Before Flowers” article.

Mycologists have studied in detail the players, but, in these environments, Mycenas seem to come in a few years into the reduction of the needle and are involved in breaking down the lignin components. The whole process from newly fallen needle to 95 percent completely decomposed products takes about 11 years.

Fairy Ring Mushroom
Another decomposer that works in a different environment and in a different way is the common fairy ring mushroom (Marasmius oreades, Figures 7 and 8). The mycelium of this species decomposes dead grass-leaf litter and the namesake fairy rings it produces are particularly evident in an even environment such as a lawn with its accumulation of dead leaf thatch that accumulates after repeated mowing.

Figure 7. Marasmius oreades “fairy ring” (arc).

Each year the mycelium of M. oreades works outward into newly available dead grass. The previously decomposed leaves are less rich in the things M. oreades needs, so the mycelium works outward, not inward. Once a year, the mycelium produces mushrooms. Because the mycelium soon becomes donut shaped as it works out away from its center starting point, when mushrooms appear, they appear in a ring.

The size of this ring grows in diameter each year by the amount of distance the mycelium grows. I have seen rings close to 20 feet in diameter. Studies in European locations have found radial growth rates vary from about 4 inches to over a foot per year. I think around here they grow a bit more slowly because of our dry summers.

An interesting feature of this arrangement is that, as the mycelium works on the dead grass mat, and decomposes the parts of the plant cells it needs (not sure if this is cellulose or lignin), the other nutrients in the leaf cells are released, and, in part, fertilize the grass in the region of the mycelium, so it appears greener. This effect can be seen somewhat in Figure 7.

Figure 8. Marasmius oreades.

Marasmius oreades is identified by its light brown, flattish caps often with a central swelling, a tough stem, almost white spore print, distinctive odor and habit of growing in rings in lawns. It is a good edible (you use just the softer caps). But, again, proper identification is important for novices. There are quite poisonous species that grow in rings in lawns.

Litter decomposition is an essential ecological service provided, in part, by many mushrooms. Without them we would be knee-deep in plant parts. 

Part 2: Next month
I will discuss the important role of mushrooms in the decomposition of wood.

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