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Dead and Down Woody Material

Ronald Bartels John D. Dell Richard L. Knight Gail Schaefer

Introduction
Dead and down woody material in the form of stumps, root wads, bark, limbs, and logs, in various stages of decay, occurs in most forest ecosystems. This is especially true of the temperate and high temperate conifer forests west of the Cascade Range where highly productive forest sites are capable of producing large volumes of wood fiber. These dead and down materials have long been viewed as potential wood products that should be salvaged, as fuels that create fire hazards, as physical barriers to tree planting, and as a haven for small mammals which may impede forest regeneration. All of these are valid concerns; however, dead and down woody material serves many important functions that should be recognized. Not only is this material important in mineral cycling, nutrient mobilization, and natural forest regeneration, but it also creates a structure and diversity of habitats that are valuable to a great many wildlife species, terrestrial and aquatic.

Intensified forest management, responding to the ever-increasing demand for forest products, will have a strong influence on the amount and distribution of woody material that remains as wildlife habitat through present and future stand rotations. Leaving the perpetuation of large down material to chance will probably result in its disappearance from the managed forests of the future, along with the loss of dependent plant and wildlife species.

A thorough discussion of the ecological implications of slowly decomposing woody material in western forests was presented by Maser et al. (1979). The ecological principles and habitat relationships presented in that publication have broad application throughout western forests. A large part of the research used in developing those principles and relationships was conducted in forests west of the Cascades. Therefore, much of the information presented here has been adapted from Maser et al. (1979) and leans heavily on their work while exploring human influence on the disposition of dead and down woody materials during roading, timber harvest, reforestation, and fire management.

The Origin and Importance of Dead and Down Woody 
Material As Habitat for Wildlife

Origin of Dead and Down Woody Material
Natural tree mortality, which includes trees killed by insects, disease, or injury, provides snags to the forest environment. Snags eventually deteriorate, collapse, and become logs. Living trees that fall as a result of severe winds, landslides, and floods also are a source of logs. These logs, if not harvested, become the most significant element of the dead and down component of the forest.

Large snags and logs are integral components of old-growth stands. Both of these structural features are carried over into young stands that originate after wildfire or other natural catastrophe has removed the old-growth stand. Large snags and logs may or may not remain following timber harvest, but if planned for, both can be retained during forest management activities.

Figure 1. When they fall, trees and snags immediately enter one of the first four 
log decomposition classes (reproduced from Maser et al. 1979, fig. 44 p. 80)

The starting point for a large down log is a large living tree which sometimes passes through the snag stage before failing. After trees fall, they go through recognizable stages of deterioration. One system for classifying the stages of log decay is a five-class scheme based on easily recognized physical characteristics (fig. 1 and table 1) (Maser et al. 1979; p. 80). When dead trees fall, they enter one of the first four log-decomposition classes, depending on their condition. For example, a live tree, felled as a result of a windstorm, becomes a decomposition class 1 log, whereas the collapse of a deteriorating snag (Cline et al. 1980), creates either a decomposition-class 3 or 4 log (table 2).

The size, tree species, and condition of a log-along with moisture and temperature--determine the rate of decomposition. As a log decomposes, the plant community and life forms inhabiting it gradually change. These changes result from two processes--internal and external succession (fig. 2). Internal succession is related to the persistence of the log over time which normally is determined by the rate of decay. External succession is the change in the plant community surrounding the log.

Some species, such as alder and cottonwood, are very susceptible to decay and thus remain for a relatively short time. Conversely, some logs, such as fire-charred Douglas-fir, have persisted an estimated 470 years since fire destroyed the original stand (MacMillan et al. 1977). The length of time it takes a log of a given species and size to decompose is known as residence time.

As logs decompose they increase in moisture content and maintain a high moisture content throughout the process of decomposition. This is the basis for three ecosystem functions of down woody materials. First, many species of reptiles, amphibians, and small mammals require cool, moist microhabitats for some or all life history functions (Marcot 1979). Down logs provide suitable habitats for these functions (fig. 3). Second, logs serve as sites for nitrogen fixation by nonsymbiotic bacteria. Third, logs serve as favorable sites for regeneration of some species of tree seedlings (fig. 4).

Figure 2. Logs progress through two simultaneous successional processes -- 
internal and external (reproduced from Maser et al. 1979, fig. 47, p.83)

Habitat for Wildlife
Dead and down woody materials are important components of wildlife habitats in western forests. These materials furnish cover and serve as sites for feeding, reproducing, and resting for many wildlife species (Maser et al. 1979; and figs. 5 and 6). In forests west of the Cascade crest in Oregon and Washington, 150 terrestrial wildlife species are known to utilize dead and down woody materials as either a primary or a secondary component of their habitat requirements (appendix 8). Although many more species are casual users of this material, it is not considered an important enough element to be listed as a habitat requirement. Down logs and large woody debris are also an important component of aquatic habitats in forested areas (see chapter 10 and Swanson et al. 1976).

Figure 5. A class 2 log showing some of the structural features important to wildlife 
(reproduced from Maser et al. 1979, fig. 42, p. 790).

Appendix 20 shows how logs are used by wildlife species and which elements of dead and down woody material are most important for cover, feeding, reproducing, and resting. The size and decomposition stage of the material determine its usefulness to wildlife. In general, the larger the diameter and the greater the length of a log, the more useful it is; however, small material is better than none since even small logs will provide habitat for some wildlife species (Maser et al. 1979).

Figure 6. A class 4 log showing advanced stage of decay and some of the structural
features important to wildlife (reproduced from Maser et al. 1979, fig. 42, p. 79).

As a log approaches decomposition class 3, (fig.1), the bark becomes loose and the space between it and the log provides hiding and thermal cover for wildlife (fig. 5). This condition persists through class 4. As the decomposition process continues through the class 4 and 5 stages, the log interior becomes soft enough for small mammals, such as the Pacific shrew, Trowbridge's shrew, and red-backed voles, to burrow inside. This opens the log interior to the introduction of mycorrhizal fungi. As decomposition progresses, the amount of small mammal activity alongside and within the log increases (Maser et al. 1979).

Other factors that influence wildlife use of dead and down woody material include the species composition of the plant community, the successional stage of the surrounding stand, and the existing wildlife community (Maser et al. 1979). If new habitats are created, they must be within the dispersal distance of animals residing in adjacent stands if they are to be readily reoccupied (Jones & Stokes Associates, Inc. 1980). This is also true if forest activities are such that existing wildlife species are eliminated prior to the creation of new habitats.

The persistence of large logs has special importance in providing wildlife with habitat continuity over long periods of time and through major disturbances (Franklin et al. 1981). Logs may contribute significantly to re-establishment of animal populations by providing pathways along which small mammals, such as red-backed voles and chipmunks, can venture into clearcuts and other forest openings. Large logs or scattered piles of debris can be important as cover on a site during early stages of succession, enabling wildlife to use forage areas (fig. 7).

Forest management practices, such as prescribed burning, scarification, yarding of unmerchantable materials, and herbicide treatments, create significant changes in habitat for wildlife. The impact of these changes can be reduced if suitable down material is maintained on a site through stand rotations.

Nutrient Cycling
Dead and down woody material and the wildlife that inhabit this material play an important role in the cycling of nutrients within the forest ecosystem. Large proportions of some nutrients in the forest are contained in trees and leaf litter. This is especially true for phosphorous and nitrogen and to a lesser extent for various other mineral elements. Large amounts of Nutrients are stored in branches, twigs, and foliage; smaller amounts are in the main trunk (Zinke et al; 1979).

Lichens in the canopy of old-growth forests fix significant amounts of nitrogen that ultimately become available to the entire forest through leaching, litter fall, and decomposition. Franklin et al. (1981) reported that significant epiphytic nitrogen inputs are mainly confined to oldgrowth stands.

Logs serve as storage compartments for energy and nutrients and as sites for nitrogen fixation. Logs may also provide physical stability, protecting a site from the loss of nutrients through surface erosion.

The discovery of significant bacterial nitrogen fixation in coarse woody debris is recent. Roskoski (1977) reported that greater decay and higher moisture contents were associated with a higher incidence of nitrogen fixation in woody debris. Franklin et al. (1981) reported that larger woody debris was probably more favorable for nitrogen fixation because large pieces create better moisture conditions and last longer, thereby providing a greater opportunity for inoculation by suitable bacteria. These important nutrients can then be made available to trees through association with mycorrhizal fungi that also find suitable growing conditions in large decomposing logs.

Mycorrhiza means "fungus-root" and is a symbiotic association of certain fungi with the roots of most vascular plants. Ectomycorrhiza is a type of root-fungus association necessary for survival of several families of trees including the pines, hemlocks, spruces, true firs, Douglas-fir, oaks, and alders (Maser et al. 1979).

In recent years, he role of mycorrhiza in plant nutrition has been widely recognized (Maser et al. 1978b). The fungi penetrate tiny, nonwoody rootlets of host plants to form a balanced mycorrhizal coupling with no harm to the roots. The host provides photosynthetic products to the fungus that in turn absorbs mineral elements from the soil and makes them available to the host. Each depends on the other. Because the majority of mycorrhiza-forming fungi depend on host roots for survival, spores must be deposited on or within soil where host roots will be available to establish new colonies (Maser et al. 1978b).

Mycorrhiza-forming fungi that produce aboveground fruiting bodies and release their spores into the air are called epigeous fungi. Although spores from epigeous fungi can be moved long distances many of them may be deposited where no host roots are available. Species that produce hypogeous (below ground) fruiting bodies have a more specialized means of spore dispersal. Fruiting bodies mature below the ground and are eaten by small animals. All tissues of a fruiting body are digested except the spores, many of which remain viable after being passed through the animal's digestive tract. Animals defecate these spores, usually on or within soil. The spores are washed into soil by precipitation and are thus strategically placed for contact with host plant rootlets (Trappe and Maser 1978). Stomach content analysis, in which the kinds of fungi actually eaten by small mammals were identified, confirmed that hypogeous species predominate in the fungal portion of the diets (Maser et al. 1978a).

Chipmunks inhabit all forest successional stages from the Cascade Range west to the coast (Gashwiler 1959, 1970) and are known to feed extensively on a wide variety of hypogeous fungi. They are considered major dispersers of mycorrhiza-forming fungal spores (Maser et al. 1978b, McIntyre 1980). These chipmunks are capable of traveling relatively long distances in a short time and regularly visit clearcuts from surrounding cover (Gashwiler 1959, Maser et al. 1978a). Appropriate fungi are ingested in standing timber, and the spores are subsequently defecated in clearcuts where the symbiotic association with rootlets of new trees can begin. Large logs serve several important functions during the dispersal of hypogeous mycorrhiza-forming fungi. They serve as home sites and travel lanes, as well as supplying cover for the small mammals that are the primary dispersers of these fungi. Also, the decomposed logs provide suitable sites for re-establishment of colonies of hypogeous fungi (Maser et al. 1978b).

California western red-backed voles normally disappear from clearcuts within a year after logging and burning (Gashwiler 1959,1970). It is hypothesized that they disappear because they no longer have their specialized food supply-hypogeous ectomycorrhiza-forming fungi, which do not fruit without their coniferous host (Maser et al. 1978a, 19780. If large logs are present, voles generally start to reinvade a stand at about the time understory vegetation is being shaded out. The decaying logs provide a site in which the mycorrhizzal fungi fruit which in turn provides food for the voles. The above sequence can take place within 20 years but may require 40 years or more. Again, logs are a necessary habitat component. The stand must also be within the dispersing distance of voles from adjacent suitable habitat.

According to Zinke et al. (1979), demands for increased utilization of logging slash will cause a significant drain of nutrients from the forest. With intensified timber harvesting, more attention should be given to replenishing site fertility and to the role that dead and down woody material plays in this restoration.

Dead and Down Woody Material in Unmanaged Stands

In an unmanaged stand, logs are recruited to the forest floor by the fall of either living or dead trees (Maser et al. 1979). Large volumes of coarse woody debris are characteristic of our unmanaged forest ecosystems. Large down logs can be the dominant feature of old-growth forests, and in numbers, volume, and weight of organic matter, they are an important component (Franklin et al. 1981).

In studying a small 25-acre western Oregon watershed covered with oldgrowth Douglas-fir/western hemlock forest, Grier and Logan (1977) found that down logs averaged 85 tons per acre. Amounts within the watershed varied greatly; the lightest weights (25 tons/ acre) occurring on a dry ridgetop and the heaviest (259 tons/acre) on a lower slope, streamside area. Losses by downslope transfer had occurred on the ridgetop, and substantial amounts of debris had accumulated on the lower slope. Franklin et al. (1981) reported that the weight of down logs from nine oldgrowth stands west of the Cascade Range in Oregon and Washington averaged 53 tons per acre. They also found, however, that there was only a loose correlation between the age of the stand and the weights of down logs. Some natural, young Douglas-fir stands had accumulations of down logs as large as those found in old growth. This was primarily material carried over as snags and logs from earlier stands.

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