Coarse woody debris
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Coarse woody debris (CWD) is a term used in all English-speaking countries for the dead trees left standing or fallen, as well as the remains of branches on the ground in forests. Note that many scientists now use the term coarse woody habitat (CWH) instead of CWD, the reason being that CWD also stands for chronic wasting disease.
From the 1970s, woodland managers worldwide have been encouraged to allow dead trees and woody debris to remain in woodlands which then becomes part of the chain of decay and renewal of, sometimes, ancient forests and helps them regenerate in a more organic way.
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[edit] Location of CWD
Natural tree mortality and decay, as well as sometimes catastrophic events such as fires and storms are at the basis of all CWD. The prehistoric forest, with its dead trees and woody remains lying where they fell and new vegetation appearing from beneath the debris, constitutes the ideal woodland in terms of recycling and regeneration. In healthy wild forests in the temperate zone 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.
[edit] CWD's Role
CWD as a source of recycled materials not only provides the nutrients for the regeneration of many plant species in the form of carbon constituents necessary for new plant life but it also serves as the microhabitats for many plant and animal species. 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.
[edit] The animals that live on coarse woody debris
All saproxylic (deadwood-dependent) organisms such as Fungi (for example, liquicolous basidiomycetes in southern woods of North America), truffles, mosses, and in the animal kingdom, invertebrates, as well as birds and small mammals, are able to find their highly specific needs provided for.
40 % of all forest fauna is dependent on this type of woodland organic system. Studies in western North America showed that only five per cent by volume in living trees consisted of fungi and nitrogen-fixing bacteria, whereas in dead standing and fallen trees it was as high as forty per cent by volume.
One third of all woodland birds live in the cavities of dead tree trunks. Great Spotted Woodpeckers (Dendrocopos major), Crested Tits (Parus cristatus), as well as owls live in dead trees, and grouse shelter behind woody debris.
Fallen debris and trees in streams, too, play a substantial role when they provide shelter for fish and mammals; trap floating leaves into dams; and reduce the flow of the water, thus diminishing soil erosion.
[edit] Local variations of the effects of CWD
Local variations result not only in the different effects CWD has on an ecosystem but in how they manifest themselves in the varying wildlife populations that continue to thrive in these CWD areas although they have become rare in cultivated ecosystems.
In the "Trees for Life" experiment in Glen Affric in Scotland it was found that the black tinderfungus beetle (Bolitothorus reticulatus) is dependent on dead wood and among dead Scots Pines the pine hoverfly (Blera fallax) is at home.
In certain subtropical areas such as Australia where fire constitutes a main hazard in woods, 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.
The same applies to areas which are subject to violent winds or storms where invertebrates lodge in the wooden debris for safety. Whatever the climatic conditions, all kinds of animals can utilise CWD for cover and seclusion for feeding, mating and rearing their young.
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 accessing and damaging young trees.
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