Coarse woody debris

thumb|right| Coarse woody debris in [[Białowieża Forest, Poland]]

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 in diameter.

Since the 1970s, forest managers worldwide have considered it best environmental practice 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 an important criterion for the evaluation and restoration of temperate deciduous forest. 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.

Benefits

thumb|right|Fungi sprouting from fallen log, Germany

Nutrient cycling

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 It has almost no physiologically important nutrients, so must be first enriched for consumption by transport of nutrients from outside. 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.

Habitat

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.]]

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, Turtles of many species may also use coarse woody debris for basking. Musk turtles may lay their eggs under logs near wetlands.

Soil

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.

Regional examples

thumb|right|[[Blera fallax, Belgium]]

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 the temperate deciduous forests of eastern North America, CWD provides habitat ranging from salamanders to ferns. It is an important indicator for evaluating and restoring this type of forest.