Fungal Firefighters: They're on the Job 24/7

Written by Jerry Hassinger, Chair, Mycological Technical Committee

The most commonly observed and photographed fungal firefighter in Pennsylvania is the turkey tail shelf fungus (seen above). There are over 5,000 photographs of this species on the Fungi of Pennsylvania iNaturalist Project website. Its fruiting structure is occasionally visible and can be photographed, but the body of this fungus is hidden in decaying wood.

In the Appalachians, wood decay fungi help reduce the severity of wildfires. It's that simple and that complex. There's more, a lot more, than meets the eye.

While a teacher at Messiah University in Pennsylvania, Gary Emberger, retired, created a website named "Fungi Growing on Wood: Keys, photographs, and descriptions of macroscopic fungi utilizing wood as a substrate in northeastern North America," currently archived at MushroomExpert.com as a public service. In listing some of the reasons wood decay fungi are important, Dr. Emberger noted, "Wood decay fungi are the preeminent recyclers of wood in ecosystems. Without these fungi, wood would never decay. We would be 'up to our eyeballs' in twigs, limbs, and tree trunks. [In other words, we would be up to our eyeballs in fuel for severe, hot-burn wildfires.] Further, without decomposition, valuable nutrients in wood are locked up and unavailable for new growth."

Bristol Tree Services, a United Kingdom-based tree surgeon company, says on their website, "Wood decay fungi are a particular group of fungi that specialize in decomposing wood. They are able to break down the complex organic materials found in wood, including cellulose and lignin, which are difficult for most other organisms to decompose. This ability makes wood decay fungi a crucial part of forest ecosystems, where they help to recycle nutrients and maintain the health of the forest."

Wood decomposition is a dynamic process that involves a complex of organisms referred to as saprophytes (wood eaters). Fungi initiate the process, followed by microbes (notably bacteria) and eventually larger decomposers like slime molds, insects (e.g., wood boring beetles, maggots, and termites), slugs, snails, and worms. Wood decomposition rates vary widely, dependent on temperature, moisture, oxygen, tree species (hardwood or softwood), the volume of wood (small limbs vs. large tree trunks), and the dynamic nature of the decomposer community.

Leaves, too, are fuel for wildfires; dried leaves are easily ignited. Varying with forest age, tree species, and weather, leaf fall in Eastern deciduous forests can approach 2 to 3 tons per acre in some years. The decomposition community hidden in the litter layer can recycle the nutrients trapped in these leaves within a year or two. Leaves and dead wood are nature's mulch and fertilizer.

A biodiverse and healthy forest includes a component of dead wood. Numerous articles on the web, such as this one, support this statement.

Fires can both consume dead wood and create dead wood; this happens relatively fast, it is an event, and no one can fail to notice a fire. Fungi, too, can create and consume dead wood. This happens constantly, is not an event, and other than the sight of rotting wood or transient fungal fruiting bodies, there’s little visible evidence that fungi are major contributors to the decomposition of dead wood. This is one reason why fungi have been referred to as the "hidden kingdom," unfortunately, out of sight and out of mind. In my book, they're hidden, unsung heroes. 

The body of a fungus species consists of a network of fine filaments (hyphae) that are collectively referred to as a mycelium. Fungal bodies are hidden in soil or in organic matter, like a stump or log. The mushrooms, brackets (or shelves), cups, and other occasionally visible fungal structures are the fruiting bodies of a hidden mycelial network.

During the rainy season, I turned over a small limb that was in contact with the litter layer. The bottom of this limb was covered with a segment of mycelium. The decay process was well underway. Paul Stamets, in his book titled Mycelium Running, remarks, "I calculate that every footstep I take [in a healthy forest] impacts more than 300 miles of mycelium."

It is easy to find rhizomorphs, which are relatively large and very visible, shoestring-like aggregations of many mycelial filaments. I turned over a lot of fallen limbs before I found the rhizomorphs illustrated below.

Part of a mycelial network, photo by Jerry Hassinger.

I found the rhizomorphs in the image below by peeling off the loose bark of a dead oak tree.

Two types of fungi visible on a tree trunk. Photo by Jerry Hassinger.

These black rhizomorphs are visible evidence of Armillaria-caused root rot, also known as shoestring root rot. This is most commonly caused by the honey mushroom (Armillaria mellea). The honey mushroom is both a parasite and saprobe. Pathologists try to prevent or manage this tree disease. Ecologists, on the other hand, credit parasitic fungi for natural forest thinning by removing weak and diseased trees, thereby initiating and sustaining the nutrient recycling process.

Meet Less than 1% of the Fungal Fire Fighters

You'll never meet the vast majority of wood decay fungi. They are hidden in organic matter and do not produce the visible fruiting structures of the "extroverted minority." There is a short but very revealing popular article titled "What Lurks in Logs," by Carl Zimmer. I borrowed the phrase "extroverted minority" from this article. In the article, he wrote, "These days, scientists do not have to rely on their eyes alone to observe the fungus on a log. They can drill into the wood, put the sawdust in a plastic bag, go to a lab, and fish the DNA out of the wood. A group of scientists did just this in Sweden recently, sequencing DNA from 38 logs in total. They published their results...in the journal, Molecular Ecology. In a single log, they found up to 398 species of fungi. Only a few species of fungi were living in all 38 logs; many species were limited to just one. Consider that on your next walk in the woods. The one or two types of mushrooms you see on a log are an extroverted minority."

Two different kinds of fruiting bodies on pitch pine logs. Photos by Jerry Hassinger.

I've kept my eye on certain logs on Ned Smith Center lands for at least 20 years. Over these years, three pitch pine logs, averaging about 12 inches in diameter, produced nine different fungi fruiting bodies that I can remember (The image above illustrates two of these species). Very likely there were an equal number that I never saw. Nevertheless, given the aforenoted research, we can presume I'll never see the vast amount of fungal firefighting species hidden in these logs over the years. At the rate these logs have been decaying, I guesstimate they'll no longer provide any fuel for a wildfire within the next 10 years.

The cost of this free service to the Ned Smith Center? Patience.

The Intricacies of Decay

To obtain nutrients and sugars, wood-inhabiting fungi secrete enzymes that break down cellulose, hemicellulose, and lignin (the components of wood), which are not possible—or at least are extremely difficult—for other organisms to decompose. 

Essentially, you and I ingest food and then digest it. But a fungus digests food and then ingests it.

The intricacies of decay are beyond the scope of this article. It's enough to know that, when it comes to wood decay, fungi are important contributors, and the net result over time is less fuel for wildfires. Still, I would be remiss if I did not touch on the two most frequently mentioned categories of rot fungi: namely, brown rot fungi (BRF) and white rot fungi (WRF).

BRF consume cellulose, but the lignin is only modified; the remaining wood is brown from the lignin, cube-like, and very fragile. BRF live primarily—but not exclusively—within coniferous species and likely comprise less than 7% of all wood decay species. BRF have an outsize importance in that:

1) softwoods are more flammable than hardwoods;

2) softwoods are the backbone of the construction industry;

3) this type of decay creates a water- and nutrient-retaining mulch that persists on the forest floor for a long time; and

4) BRF facilitate the creation of cavities for wildlife.

The image shows two fungi: chicken-of-the-woods, Laetiporus sulphureus and red-belted polypore, Fomitopsis mounceae

In Pennsylvania, the red-belted polypore, Fomitopsis mounceae, common on conifers, is a good example of a brown rot fungus. Another example is the chicken-of-the-woods, Laetiporus sulphureus, guilty of brown heart rot in hardwoods, often oak. 

Note that, in general, live trees will rot from the inside out, and dead trees rot from the outside in.

The vast majority of fungal firefighters are white rot fungi, which break down lignin, leaving behind the whitish cellulose. Some WRF produce enzymes that can break down both lignin and cellulose.  Some WRF species prefer conifers, some prefer hardwoods, and still others can occur on both conifers and hardwoods.

Check out this website for a more detailed description of fungal-induced rot and decay.