Coarse woody debris

Coarse woody debris (CWD) or coarse woody habitat (CWH) refers to fallen dead trees and the remains of large branches on the ground in forests and in rivers or wetlands. A dead standing tree – known as a snag – 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. Coarse woody debris (CWD) or coarse woody habitat (CWH) refers to fallen dead trees and the remains of large branches on the ground in forests and in rivers or wetlands. A dead standing tree – known as a snag – 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. The amount of coarse woody debris is considered an important criterion for the evaluation and restoration of temperate deciduous forest. Coarse woody debris is also important in wetlands, particularly in deltas where woody debris accumulates. Coarse 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, 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. However CWD itself is extremely scarce in physiologically important nutrients. To fulfill nutritional requirements of consumers, CWD must be first nutritionally enriched by transport of nutrients from outside of CWD. Thus CWD is important actor contributing to soil nutrients cycles. 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. 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. 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.