- Coarse woody debris
Coarse woody debris (CWD) is a term used in English-speaking countries for fallen dead trees and the remains of large branches on the ground in forests. Some prefer the term coarse woody habitat (CWH). A dead standing tree is known as a snag and provides many of the same functions as coarse woody debris. The minimum size required for woody debris to be defined as "coarse" varies by author, ranging from 2.5–20 cm (1–8 in) in diameter.
Since the 1970s, forest managers worldwide have been encouraged to allow dead trees and woody debris to remain in woodlands, recycling nutrients trapped in the wood and providing food and habitat for a wide range of organisms, thereby improving biodiversity. This natural process of woody growth, decomposition, and recycling may aid in the renewal of ancient forests.
Natural tree mortality, disease, and insects, as well as catastrophic events such as fires and storms, are all sources of CWD. The ancient, or old growth, forest, 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 healthy temperate forests, dead wood comprises up to thirty per cent of all woody biomass.
In recent British studies, woods managed for timber had between a third and a seventh less fallen debris than unmanaged woods that had been left undisturbed for many years, while in recently coppiced woods the amount of CWD was almost zero.
In old growth Douglas-fir forests of the Pacific Northwest of North America, CWD concentrations were found to be from 72 metric tons/hectare (64,000 pounds/acre) in drier sites to 174 t/ha (155,000 lb/acre) in moister sites. Australian native forests have mean CWD concentrations ranging from 19 t/ha (17,000 lb/acre) to 134 t/ha (120,000 lb/acre), depending on forest type.
Coarse woody debris and its subsequent decomposition recycles 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.
Scientific studies show that coarse woody debris can be a significant contributor to biological carbon sequestration. Trees store atmospheric carbon in their wood using photosynthesis. Once the trees die, fungi and other saprotrophs transfer some of that carbon from CWD into the soil. This sequestration can continue in old-growth forests for hundreds of years.
By providing both food and microhabitats for many species, coarse woody debris helps to maintain the biodiversity of forest ecosystems. Up to forty percent of all forest fauna is dependent on CWD. Studies in western North America showed that only five per cent of living trees consisted of living cells by volume, whereas in dead wood it was as high as forty percent by volume, mainly fungi and bacteria. Colonising 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 metabolising the biomass.
The list of organisms dependent on CWD for habitat or as a food source includes bacteria, fungi, lichens, mosses and other plants, and in the animal kingdom, invertebrates such as termites, ants, beetles, and snails, amphibians such as salamanders, reptiles such as the slow-worm, as well as birds and small mammals. One third of all woodland birds live in the cavities of dead tree trunks. Woodpeckers, tits, chickadees, and owls all live in dead trees, and grouse shelter behind woody debris.
Plants use coarse woody debris as habitat, often covering them with mosses and lichens. In some forests with dense ground cover or waterlogged soils, germination on top of CWD is a competitive advantage. Large fragments of CWD that provide such habitat for herbs, shrubs, and trees are called nurse logs. CWD can also protect young plants from herbivory damage by acting as barriers to browsing animals.
Fallen debris and trees in streams provide shelter for fish and mammals by modifying the flow of water and sediment through the channel. This stream debris may be referred to as large woody debris.
Coarse woody debris, particularly on slopes, stabilizes soils by slowing downslope movement of organic matter and mineral soil. Leaves and other debris collect behind CWD, allowing for decomposition to occur. Infiltration of precipitation is improved as well. During dry weather, CWD slows evaporation of soil moisture and provides damp microhabitats for moisture-sensitive organisms.
In Glen Affric, Scotland, the Trees for Life group found the black tinder fungus beetle (Bolitothorus reticulatus) is dependent on a particular fungus (Fomes fomentarius), which itself grows only on dead birch. Another insect, the pine hoverfly (Blera fallax), requires rotting Scots Pine in order to reproduce.
In certain subtropical areas such as Australia where bushfire constitutes a major hazard, the amount of CWD left standing or lying is determined by what may be considered safe in the course of reasonable fire prevention. However, when fires do occur, invertebrates find shelter either within or beneath dead tree logs.
In Canada, bears seek out dead tree logs to tear open and look for and feed on ants and beetles, a fact that has encouraged the authorities to reserve a sufficient amount of coarse woody debris for these purposes. In North America, too, CWD is often used as barriers to prevent browsing deer and elk from damaging young trees.
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