Detritus, or the Living Dead

Tim Iverson

(10/2015) If you’ve been fortunate to see some of the changing fall colors on a drive, or better yet a walk in the woods, you may have wondered exactly what happens to all those leaves. If they all just piled up shouldn’t we be up to our eyes in fallen leaves? An entire forest’s worth might even go over our heads. Imagine how large the pile would be if we raked it all up! Nature has much bigger plans for all that leaf litter though. It may not seem all that important, but it plays an integral role in the broader ecosystem and the creation of new soils. This decomposition cycle fuels the life cycle, and what is the end stage for some are the birth stages of other cycles.

Deciduous trees in the temperate forests of North America drop an estimated 3 to 5 tons of leaves per hectare (roughly 2.5 acres) per year. In broad general terms trees are broken down into two main categories: coniferous and deciduous. Coniferous trees have needles and produce cones. Deciduous trees have broad leaves which die off in cooler temperatures in a process called abscission. Deciduous means "to die off", which is where the name comes from, and this usually coincides with winter and cooler temperatures. Abscission happens when leaves stop producing chlorophyll and green pigmentation. During the summer the broad leaves are essential for survival, but in the winter those leaves become a lethal liability. The leaves are a costly source of water loss and consumption, and during the winter months water is hard to come by. To prepare for the change in seasons deciduous trees will shed their leaves. Conifer needles have less surface area and retain water better than leaves because of a waxy coating and smaller pores, which can allow for year round photosynthesis.

All the twigs, acorns, leaves, and other organic matter that accumulates on the ground is collectively called leaf litter or detritus. This is valuable for two primary reasons in the forest ecosystem - food and shelter. All this organic matter provides shelter from the elements and ample habitat to hunt or hide in for small organisms like insects and worms, but also reptiles like snakes and toads, and amphibians like frogs and salamanders. For decomposer organisms this is an impressive bounty to feast upon. Their job, while not glamorous, is vital. Common decomposers include fungi, slime molds, earthworms, springtails, flies, maggots, beetles, beetle larva, slugs, millipedes, snails, and bacteria. The list of decomposing critters could go on ad infinitum, but their job is simple. It’s to breakdown any and all dead organic matter.

The decomposers consume all the accumulated leafy and woody material and excrete the nutrients out thus continuing the cycle. It starts with the hyphae of fungus spreading out across the forest floor. Hyphae is a white filament that looks similar to thread or string, and is collectively called mycelium. The hyphae spurs vegetative growth for fungi, and is the first principal agent in breaking down all those leaves and other things in the detritus litter. Enzymes are secreted from these which breaks down the dead plant material, then nutrients are absorbed and carried throughout the body of the fungus. It’s a lot like chewing and swallowing.

For tougher and more fibrous plants and logs the process is more involved and can take a lot longer. A basic deciduous leaf will usually decompose within a year. Conifer needles, woody material, and logs can take years to fully break down. The process is greatly affected by overall climatic conditions. Wetter warmer climates tend to have higher decomposition rates due to increased fungal, insect, and invertebrate activity. Woody material is usually beset by rot over extended periods of time, but the first decomposer on the scene is still fungi. As the fungi spreads throughout a fallen tree access is made easier for others and the next phase of decomposition begins.

It’s at this point that termites, springtails, millipedes, and more begin to move in. They eat away at the fallen and decaying wood. Not to miss out on an opportunity for an easy meal, predators like wasps and parasites like robber flies, move in to feed on the insects and invertebrates feeding on the bounty provided by the detritus. As tunnels and pockets are created within the downed trees moisture continues to creep in accelerating the rot process. At the final stage of decomposition earthworms begin to appear on the scene. Once this happens the wood is beginning to be assimilated into the soil. As it breaks down into the soil other plants and trees will draw from these nutrients in order to flourish themselves before they too reach this stage of their life cycle.

This entire process of decomposition takes place in the open air, and is referred to as aerobic decomposition. Aerobic decomposition is generally odorless and similar to composting. If you have a compost pile at home for gardening you utilize the same processes that occur naturally in the forest, but with some minor modifications. By concentrating a compost pile heat is generated and accelerates the decomposition process making for nutrient rich fertilizer.

Animal decomposition differs primarily in that doesn’t rely heavily on fungus. Decomposition is facilitated primarily by scavengers and bacteria. These carrion eaters consume the dead tissue and use it as sustenance and energy. For carcasses that aren’t immediately consumed by scavengers bacteria will move in to break down and putrefy the remains. This breakdown process occurs within the body of the deceased and causes the form to swell from the pressure of gas build up. This type of decomposition is called anaerobic because it occurs without the presence of oxygen, and is the type of decomposition associated with a particularly rotten smell. Eventually the body will rupture and be consumed by flies, maggots, beetles, and more. Still this is essential for the life cycle and the return of the nutrients to the environment and clearing of waste. After time only bones will remain, but even these serve a purpose. Mice and other rodents will chew on these and consume them for the calcium they provide.

While all of this may seem unpleasant it is important to consider that everything serves a purpose and continues to serve a purpose even after it ceases to exist. There’s some comfort in knowing that the continuation of the cycle is served even after death. The living may utilize the energy from those that came before. Decomposition provides essential nutrients and is a key aspect in the cycle of life and ecological processes. When you see all those ephemeral fall colors fade and float toward the ground remember all the work tiny unseen organisms do.