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Coarse Woody Debris #4: Forest Habitat

Coarse woody debris (CWD) is a critical habitat within forest ecosystems, often bridging the divide between the inert and living members of the community. Snags often are designated as hard or soft. Decayed wood serves as a growth substrate for fungi, mosses and lichens; invertebrates find that spaces under the bark serve as cover and as places for feeding. Birds use them for nesting or roosting. Cavities and loose bark serve as dens or resting or escape cover for mammals (after Thomas et al. 1979).

Autotrophs such as green algae, diatoms, blue-green algae, lichens, liverworts, mosses, clubmosses, horsetails, ferns, gymnosperms and angiosperms all use CWD as habitat. Epiphytic plants may attach themselves superficially to the surface of the CWD, send roots down into the rotting wood, or establish themselves first on the rotting wood and then over time send their roots down into the mineral soil. CWD can moderate extremes in temperature and available moisture.

These autotrophs influence the rate of decay, and the decomposition state of the wood influences the species of autotrophic populations present at any given time. Rooting plants influence fragmentation rates, both positively and negatively. They bind decayed wood together into coherent structure (Triska and Cromack 1980) and reduce the influence of erosion caused by rainfall. They also lead to enhanced rates of fragmentation by tearing woody debris apart through the growth of roots and the introduction of expansion and contraction due to temperature variation. The larger plants also attract animals that tear at the CWD to remove the plants.

Little is known about the proportion of plants that are associated with CWD versus other habitats. There are few studies concerning the use of CWD for tree recruitment in the eastern forest. Given the short unstable habitat it provides in much of the country, it is not likely to play a predominant habitat role for seedling establishment and long term success.

Fauna associated with CWD changes as the tree’s conditions goes from living, to dying, to dead and decaying. The succession is a shift throughout the cycle from host plant specificity to habitat specificity for invertebrates. At the conclusion of the cycle the decay state is more important than the species that contributed the material (Howden and Vogt 1951). Bark beetles, wood borers, termites, carpenter ants, wasps, carpenter bees, mites and true flies represent major invertebrate groups using CWD for food, shelter and breeding.

CWD provides habitat for many terrestrial vertebrates including amphibians, reptiles, birds and mammals (Thomas 1979; DeGraaf and Rudis 1983). Use of CWD is influenced by physical orientation (vertical and horizontal), size (diameter and length), decay state, species of CWD, and overall abundance (Thomas et al. 1979; Maser et al. 1979).

Standing dead trees, or snags, provide a portion of the habitat needs for many vertebrate species that can be classed as primary cavity excavators or those that occupy existing cavities. Cavities can be excavated or occur through environmental factors that lead to cracking or the formation of loose bark. Thomas et al. (1979) suggest that there is a direct relationship between the number of snags and snag-dependent wildlife in the forest.

The internal and external characteristics of the snag and the state of fragmentation and mineralization influence the use by wildlife. The size and height of snags determines which species can use them for nesting. Some wildlife will only excavate soft wood, in later stages of decay. This type of CWD are a critical habitat component for some species and are becoming increasingly rare.

The next blog post in this series will look into the recommended management of coarse woody debris.