A Rotten Story: Forest Decomposition

The woods can be pretty messy, with all of those leaves, branches, twigs and logs falling over and cluttering up the forest floor, making it hard to walk around.

What is Mother Nature thinking? Oh wait, now where does soil come from? Do these organic inputs actually help fuel the amazing and rich forest ecosystems we nurture, exploit, and enjoy?

Douglas-fir log subject to fungal work in Eastern Washington (Photo: Ken Bevis)

According to U.S. Forest Service research, one acre of managed conifer forest in Washington averages around 107 tons of above-ground organic material at any given time (Campbell et al. 2010).  In the lush, west-side timberlands Douglas-fir and hemlock/spruce forests often contain 300-450 tons per acre, and this doesn’t even count what is below ground!

Photosynthesis and growth are continuously adding material to this system by pulling carbon dioxide out of the atmosphere and creating tissue through this amazing, miraculous process. This material continuously builds up, and we might wonder, why aren’t we simply overwhelmed with old tree and plant parts? A complex ecosystem under our feet breaks all of this material down and turns it back into basic components that recycle and feed the system.

Most of the essential break-down work is done by fungus. Yep, good old fungus.

In fact, the moist forests of the Pacific Northwest are a great kingdom of fungus, with enormous biomass of fungal life all around us. Kneel down and pull apart some of the forest duff and rotting wood. See the white threads of fungal mycelium down there? These tender “roots” are the main body of fungus, and move through the downed material, breaking it down to basic components using enzymes and other chemicals. The fungus uses some as food and nutrients, processes it again, and releases further broken-down material that is picked up by the next organism in the soil ecosystem.

There is a zone called the “rhizosphere,” which refers to the immediate area around roots and root tips, where a high degree of the biological activity in the soil occurs. Here, fungus, bacteria, nematodes, springtails, worms and many other organisms interact with each other and the plants to support the forest (Molina & Amaranthus, 1990). Sometimes, nitrogen (a limiting nutrient in soil) is pulled from the atmosphere and “fixed” by nodules of bacteria attached to the roots of plants, including alder. The alder doesn’t do it, the bacteria do. An amazing example of symbiosis.

It gets even more complicated …

There are also mycorrhizal fungi. They form an essential symbiotic relationship with plants by attaching to root systems with fungal hyphae (threadlike fungal “roots”). These help the plant by absorbing moisture and nutrients. In return, the fungus gets sugar from the plant’s photosynthesis. This is an amazing relationship that is being rigorously studied, and is now known to be critical element of a healthy forest.

Mushrooms on the forest floor near Sequim, Washington. (Photo: Ken Bevis)

And don’t forget the mushrooms. Most of a fungus is in the form of hyphae, invisible above the ground, performing the quiet, unglamorous work of digesting organic material. But when conditions are right, the fungus needs to reproduce, mix genetics and spread their growth around, the mushrooms appear.

Mushrooms are the fruiting bodies of fungi. These can provide food for a variety of forest-dwelling animals, including the northern flying squirrel, which then spreads the spores through their droppings and help the fungus move about the forest.

Some of these above-ground fruiting bodies are tasty to us homo sapiens too, including chanterelles, matsutake, and morel mushrooms. Some fungi have below-ground fruiting bodies, such as truffles, but the effect is the same. These “fruits” are often meant to be eaten by something to help spread the fungal wealth in the ecosystem.

Forest floor duff near Adna, Washington (Photo: Ken Bevis)

Fungi are the “engines of decay” in the forest ecosystem, beautifully described in David George Haskell’s excellent 2012 book, The Forest Unseen. Haskell looks closely at one square meter of a Tennessee forest over the course of a year, and shares observations of life, both large and small, that apply to forests everywhere.

Meanwhile, glamorous megafauna such as our banana slug, will eat fungal elements, including mushrooms, and can play their own role in breaking down organic material by shredding, eating, and defecating as they slide around the forest floor (see recent article in DNR’s Small Forest Landowner News).

There are a lot of other animal characters acting in the forest duff as well, including mites, millipedes, centipedes, springtails, protozoans, worms, spiders, snails and gophers. The food chain down there is stunningly elaborate, with fungus (and bacteria) in foundational roles, breaking leaves and wood down initially in a myriad of ways. Then small animals such as springtails, (some smaller than the period at the end of this sentence!) feed on the fungus and the released compounds. These critters are in turn fed upon by larger creatures such as millipedes, who are in turn fed upon by larger critters, such as shrews, who could be eaten by a small owl, who could be eaten by a larger owl or coyote, etc. It is boggling when the various energy pathways are identified.

Forest floor megafauna – Red-backed salamander. (Photo: Ken Bevis)

Salamanders live in the decaying duff and wood of the forest floor. These ancient life forms live slow and deliberate lives, mostly in the dark tunnels underground, making dramatic appearances on wet days in our forests. They eat small insects and worms, and are the apex of the rotting wood food pyramid.

How our forests are managed can greatly influence what happens in the soil. The soils we began managing in the late 19th and 20th centuries were the products of many thousands of years of development by way of fire, ice, geology, organisms and time. Human impacts in forestlands today are generally in the form of changing vegetative structure, removing organic material and soil compaction.

What can small woodland owners do to help promote and protect the precious habitats and soil ecosystem below our feet? A few suggestions:

  1. Retain organic material across the land, especially decaying logs
  2. Keep portions of the land shaded to lessen soil drying
  3. Minimize compaction from machinery
  4. Scatter large organic pieces after harvest
  5. Retain patches of natural forest at regular intervals on managed landscapes to enable soil flora and fauna to persist and return to managed areas
  6. Get down on your hands and knees with a hand lens and marvel at the life in a patch of duff, rotting wood, and forest soil


This is only the proverbial tip of the soil ecosystem. I hope you got down low on the forest floor, looking under some duff with dirty hands after reading this article. Send me your photos, ideas and experiences!

Ken Bevis, Wildlife Biologist, Washington State Department of Natural Resources, ken.bevis@dnr.wa.gov


Campbell, S.; Addell, K.W.; Gray, A.; (2010). Washington’s forest resources, 2002-2006: Forest inventory and analysis report.  Gen. Tech. Rep. PNW-GTR-800.  Portland, OR: U.S. Department of Agricultur, Forest Service, Pacific Northwest Research Station.

Molina, Randy & Amaranthus, Michael. (1990). Rhizosphere biology: ecological linkages between soil processes, plant growth, and community dynamics.