“Oh, man,” I muttered to myself a couple weeks ago, “there’s another one!”

My thought was triggered by seeing another log pulled apart by a black bear, the third one in just a couple days. The bruins have been increasing in number in the area around our New York cabin for a decade, and I have seen scat or tracks multiple times. This year the bears were more active, ripping up downed logs and shredding bark on standing dead snags in search of insects or grubs. While I have yet to see a bear in the area, I dread the day when one of them decides to raid the cabin or barn in search of food because a hungry bruin can do a lot of damage.

What really caught my eye with this sugar maple log was the teal green color of the rotting wood. I have encountered the phenomenon many times but never followed up with research to determine what caused the unusual coloration. Rob Bryan is an expert forester/naturalist and when we saw a similar log in our explorations, he speculated that the color was caused by a fungus of some sort. As usual, it turns out that Rob was correct.

While there are untold numbers of fungal species out there, the culprit in this case is quite likely Chlorociboria aeruginascens or the closely related species C. aeruginosa. It would take a microscopic examination of samples to distinguish between them, and an eye far more experienced with fungi than mine. The Latin name is quite logical in this case, meaning “becoming blue-green.” Wikipedia offers three vernacular names for C. aeruginascens—green elfcup, green wood cup, or blue stain. I will go with the imaginative name elfcup for this article to keep things simple.

Most of the time the cup-shaped fungus is not visible, at least not to the naked eye. The erect fungus shown in the second photo probably stands less than one half of an inch high and it generally appears in the winter months for a relatively short period of time. Once it has dropped its microscopic spores, measured in a couple microns, the cup and stem wither away. But those spores trigger more growth of the fungus and soon the log looks like someone brushed on leftover decorative paint, not just on the surface but throughout the piece of wood.

So, what is the point? The point is that trees have a limited life span and end up toppling to the ground. Trees do not always grow strong and healthy, so defective limbs can break off. Many species of trees like the sugar maple in this story are called self-pruners, meaning that they shed their lower limbs as the tree grows under thick forest canopy. Storms regularly damage trees and insects follow along to make a home in the resulting cracks and crevices, further degrading the trees. Something happens to all that forest debris or we would find our wooded areas nearly impassable.

There are multiple routes for the gradual breakdown of trees in a natural process of recycling. Excess moisture and ultraviolet light contribute to this in modest ways. Insects like termites and beetles also play a role. However, the primary responsibility for reducing a dead tree into reusable nutrients falls to the world of fungi. The decay of wood is dependent upon many factors like temperature and humidity, but this rotting process is at best a gradual one. There are in fact different forms of wood rot, primarily what are known as brown rot and white rot, determined by the fungi and their enzymes driving the process. The key is how the components of wood are being broken down. The largest component of most trees is cellulose, a form of the sugar glucose. Hemicellulose is composed of a mix of sugars. The most difficult part of a tree to deal with is lignin, as any pulp processing company will attest.

Those fungi involved in brown rot, like the colorful orange chicken-of-the-woods, readily break down cellulose and hemicellulose, absorbing the leftover sugars as energy to drive their metabolic functions. Brown lignin is left behind as the remains of the tree gradually crumble into fine particles within the soil, playing an important role in water retention by the soil. Fungi involved in white rot, such as the delicious oyster mushroom, can break down all three tree components with some lingering white cellulose responsible for the lighter colors of the decaying wood.

According to an interesting blog called Mushroom Appreciation, the green elfcup can be found widely in North America and Europe. It grows only on wood, never from the ground up. The fungus prefers bark-free logs and sticks, mostly hardwoods, with oak a particular target. Often the fungus is white and fluffy but in the presence of some sort of stressor, the fungus turns bluish-green inside and out, perhaps to defend a food source from other fungi. Mycologists report the fungus has little odor and is not particularly edible. Yet it plays a small part in decomposition in a third form of rot known as soft rot (no, not soft rock).

The lovely elfcup turns wood that lovely teal color via a pigment called xylindein. As far back as the 14th century, creative woodworkers in Italy learned to benefit from the natural pigment, both by using stained or “spalted” wood in their projects or by collecting the pigment for select pieces of wood. The stained wood was known broadly as “green oak” and was often used to create beautiful hardwood inlays. Craftsmen today are going back to traditional ways and are using the pigment to stain not just wood, but also fabrics such as silk, while scientists explore ways to stimulate commercial scale growth of the fungi for higher production of natural xylindein.

A fallen tree through all the years of its gradual decay provides important habitat for many of the denizens of the forest. This includes small mammals, many species of insects, untold numbers of microbes, and birds. The products of decay become new components of the forest soil, supporting the vast network of tree roots and other plant growth below the surface. If the decay process also happens to paint the rotting wood in a palette of lovely colors, all the better for our viewing pleasure!

If you like Ed Robinson’s writing, check out his two Nature Notes books! Click here for more information.