Janean Creighton

Large organic debris, also known as coarse woody debris, is the dead woody material that accumulates on the forest floor after windstorms, snow slides, timber harvests and flooding. The role of coarse woody debris in the forest environment is one of economy: if allowed to remain on the ground, very little will go to waste. In fact, woody debris illustrates quite well the concept of "reduce, reuse and recycle!" Dead wood provides habitat for small microorganisms that break down and ingest the woody material. These critters feed invertebrates, which are small organisms including insects, spiders, slugs and mollusks that in turn feed small mammals, birds, amphibians and reptiles. These animals provide food for large carnivores and birds of prey.

Woody debris is a tree that was once alive and standing. When a tree dies and remains standing, it becomes a snag. During the decay process, the snag provides important habitat for over one-third of all the native wildlife species in the Pacific Northwest, both terrestrial and aquatic. Eventually, these snags fall down and become coarse woody debris.

As dead and downed material, woody debris continues to supply nesting, denning and foraging habitat for many species of wildlife. It also increases the structural diversity of the forest floor; houses over 400 species of known insects that provide food for many animals; and in wetland and riparian zones, provides an essential component of aquatic habitats. When dead wood remains rotting on the ground, it also provides forest soils with nutrients that might be limited, such as nitrogen.

As wood decays, increasing amounts of nitrogen become available to the soil. The nitrogen is "released" from the wood back into the soil by the action of microbes involved in the decay process. In addition, the microbes and bacteria living within the rotting wood are able to capture or "fix" atmospheric nitrogen. These bacteria change the nitrogen from the air into a form that plants can use. Because of this recycling process, the highest concentrations of nutrients tend to reside in the upper soil layer. Plants are most likely to take root in this layer where high concentrations of nutrients are found. If dead woody debris is removed, those nutrients are gone from the system forever.

Woody debris is undeniably an important part of forest functioning. However, what function does woody debris have in wetlands and riparian areas? Debris that falls naturally into streams and ponds is essential for good fish and amphibian habitat, but if we travel down the food chain toward its base, we find aquatic invertebrates. These smaller organisms that lack a backbone also require coarse woody debris for their survival.

Stream environments are home to many aquatic invertebrates such as daphnia, snails and water gliders. They can be found in all parts of the stream system from the smallest headwaters to large meandering rivers emptying into the sea. These organisms provide food for fish, amphibians, birds and mammals that occupy riparian and wetland areas. In turn, many aquatic insects feed on algae and microscopic organisms that live on aquatic vegetation and on the surface of submerged rocks and logs.

The requirements of good water quality and suitable streambed substrate can be quite specific for aquatic invertebrates. Just like the fish they feed, invertebrates are intolerant of changes in their habitat such as fluctuations in water temperature, too much or too little oxygen, increased siltation and sediment deposition. Woody debris in the form of logs and root wads creates microhabitats within the stream system that both fish and invertebrates need. Large structures in streams create pools of still water where insect larvae develop, frogs lay their eggs, and fish rest and hide from predators.

Riparian invertebrates that live on land need fallen trees and decaying logs in proximity to the water's edge to create the microhabitats that they need. Cool temperatures, consistent relative humidities and moist soils are crucial components for many riparian zone inhabitants, including invertebrates. Woody debris ties aquatic and terrestrial habitats together and provides a bridge for animal movements from one side of the stream to the other.

What can you do on your land?

How can landowners provide the coarse woody debris that is so important in riparian and wetland systems? It's important to understand that the woody debris found naturally in riparian zones comes from the upland zones. Snags and logs upslope from the stream contribute directly to in-stream debris, thus the upland and streamside regions of riparian zones are inherently connected. If dead wood is absent from the stand, it may be necessary to add structures directly into the stream to improve its function. The addition of coarse woody debris and snags in the uplands will provide a source of material for the future.

Current forest practices regulations in Washington require a minimum of 2 logs per acre, both at least 12" diameter at the small end, with a minimum length of 20 feet. In Oregon, 2 logs per acre are required on harvested units greater than 25 acres, and each downed log must be at least 6 feet long and contain a gross volume of at least 10 cubic feet. However, determining the most effective number of logs is really dependent on the specific wildlife present, and the type of forest and its condition. Recent studies indicate that more logs of larger sizes are necessary to maintain healthy populations of woody debris-dependent wildlife in managed forests than what is currently required.

Beyond the requirements, the volume of debris that is appropriate to leave depends on your own sense of aesthetics and tolerance for a messy forest, as well as desired fuel load. While you want to provide enough debris to be useful for wildlife, too much makes it impossible to work and move around in, and could provide an additional source of fuel in areas with high fire potential.

Avoid damaging the logs by placing them away from areas where future harvesting may occur. Larger structures are preferred over small ones because big logs decay slower. The longer the log is present in the environment the more useful it is, both in terms of the wildlife habitat it supplies over time and the continued infusion of nutrients it recycles back into the system.

If debris is needed in a riparian zone, contact your local Natural Resources Conservation Service or state forestry office for the technical help you will need to appropriately place the debris in the stream channel.

Managing for coarse woody debris in riparian habitats is an effective way to provide for a large number of wildlife species with little effort. The best way to provide these structures is to retain any that currently exist on the site. This goes for existing snags, too. After all, snags will provide you with a steady supply of dead and downed debris for future generations of aquatic organisms, fish and wildlife that live in your forest.

Janean H. Creighton is Wildlife Extension coordinator for the Department of Natural Resource Sciences, Washington State University, in Pullman.

This article appeared in the Northwest Woodlands Magazine, Summer 2000- Published quarterly by the World Forestry Center as a benefit of membership in the Oregon Small Woodlands Association, Washington Farm Forestry Association, Idaho Forest Owners Association and Montana Forest Owners Association.