Coarse woody debris (CWD) is a term used to describe fallen dead trees and the remains of large branches on the forest floor and in rivers or wetlands. A dead standing tree is known as a snag and provides many of the same functions as CWD.
Since the 1970’s forest managers worldwide have been encouraged to allow dead trees and woody debris to remain in the forest, recycling nutrients trapped in the wood and providing food and habitat for a wide range of organisms, thereby improving biodiversity. The amount of course woody debris is considered to be an important criterion for the evaluation and restoration of temperate deciduous forest.
Course woody debris comes from natural tree mortality, disease, and insects, as well as catastrophic events such as fires, storms and floods.
Ancient, or old growth, forests with its dead trees and woody remains lying where they fell to feed new vegetation, constitutes the ideal woodland in terms of recycling and regeneration. In temperate forests dead wood comprises up to thirty per cent of all woody biomass.
Coarse woody debris and its subsequent decomposition recycle nutrients that are essential for living organisms, such as carbon, nitrogen, potassium, and phosphorus. Saprotrophic fungi and detritivores such as bacteria and insects directly consume dead wood, releasing nutrients by converting them into other forms of organic matter which may then be consumed by other organisms. CWD, while itself not particularly rich in nitrogen, contributes nitrogen to the ecosystem by acting as a host for nonsymbiotic free-living nitrogen-fixing bacteria.
Up to 40 percent of all forest fauna is dependant on CWD. Colonizing organisms that live on the remains of cambium and sapwood of dead trees, aid decomposition and attract predators that prey on them and so continue the chain of metabolizing the biomass.
One third of all woodland birds live in the cavities of dead tree trunks. Woodpeckers, chickadees, and owls all live in dead trees, and grouse shelter behind woody debris.
Some plants use coarse woody debris as habitat. Mosses and lichens may cover logs, while ferns and trees may regenerate on top of logs.
CWD, particularly on slopes, stabilizes soils by slowing down movement of organic matter and mineral soil. Leaves and other debris collect behind CWD, allowing for decomposition to occur. During dry weather CWD slows evaporation of soil moisture and provides damp microhabitats for moisture-sensitive organisms.