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Coarse Woody Debris: #3 Decomposition

The accumulation and decay ofcoarse woody debris (CWD) varies with forest type and age. Investigations (Thomas et al. and Maser et al. 1979) have described the trajectory of the decomposition and decay of CWD as a series of stages and conditions, each of which provides the habitat for a varying suite of organisms.

The structural aspect of the CWD habitat can best be understood in the context of the successional state of the forest surrounding it and the internal and external characteristics of the CWD itself. Log height, concentration and residence time are also of variable significance to different vertebrate groups (Graham 1925; Savely 1939). At various stages of fragmentation and decay, down and dead wood may supply elevated sites for lookouts and feeding; the trunk may provide a food source for woodpeckers and small mammals; elevated limbs may be used for perches; spaces between loose bark and the soil may be used by invertebrates for hiding and thermal cover. As the material continues to decay and soften, the same log may afford an area for storing of food by squirrels and construction of burrows by deer mice.

The decomposition processes include leaching of materials by water percolation; physical fragmentation due to gravity and water movement; biological fragmentation by plants and animals, with invertebrates probably exercising the greatest influence; transport within stream channels and along topographic gradients in the terrestrial system; collapse and settling as structural strength declines and logs are unable to support their own weight; seasoning through the decrease in moisture content, shrinkage, and formation of checks and cracks; respiration by fungi in the terrestrial system and bacteria in the aquatic systems (Crawford and Sutherland 1979 and Harmon et al. 1986); and biological transformation through the metabolism of the cell walls by microbes and invertebrates (Kaarid 1974; Swift 1977a).

The rates of snag fall, fragmentation, and mortality are influenced by species, size, microclimate and the type of mortality. Most species examined have shown a marked degree of fragmentation less than 10 years following mortality. A change in wood density associated with mineralization is caused by a combination of respiration and leaching. Mineralization rates are related to tree species and size. Log fragmentation rates also appear to follow species and size. It is not known why some species lose their bark faster than others. Log mineralization (decomposition) rates vary by species and size. The highest rates are recorded for angiosperms (flowering or hardwood). Logs that enter streams can be expected to remain intact for several decades.

Coarse woody debris is not distributed evenly across the terrestrial landscape or within stream systems. These patterns of distribution often influence decomposition rates and the role(s) that the CWD play within the ecosystem.

The next blog in this series will look at the roles of coarse woody debris in the Forest Ecosystem.