Considerations for Developing, Creating and Retaining Decayed Wood ElementsAugust 2002
This document contains information on considerations for developing, creating and/or retaining defective trees, snags and down wood, and considerations for dealing with hazard trees.
Bull et al. (1997) present an in-depth discussion regarding retention and selection of decayed wood elements in the Interior Columbia River Basin. Portions of the following narrative are adapted from this publication. Although tree and associated decay or pathogen species may vary among wildlife habitat types from those discussed, the ecological processes and principles outlined by the authors are generally applicable. For more specific information about forest insect and disease agents, please refer to the Pathogens and Insects sections of the individual summary narratives.
Trees and SnagsSnags used by wildlife vary considerably in terms of height, decay stage, form of decay, and tree species, thus providing a diversity of habitat features. For example, the snag that results following the death of a tree having extensive heartwood decay or a hollow interior provides habitat features that differ from a snag resulting from the death of a tree with no internal decay.
Our literature review showed wildlife using a variety of snag and tree heights, with several bat species selecting the taller snags available for roosting, especially those snags taller than the surrounding forest canopy. Thus, we recommend retaining a diversity of snag heights.
A wide variety of snag or tree species are used by wildlife species. Some species require different tree/snag species to meet different life history needs; for example, pileated woodpeckers in the Eastside Mixed Conifer habitat type select grand fir for roosting, but select against it for nesting (Bull 1987, Bull et al. 1992). Conversely, on the Olympic Peninsula (Westside Lowlands Conifer Hardwood habitat type), pileated woodpeckers select Pacific silver fir for nest trees and western redcedar for roost trees (Aubry and Raley 2002). Various tree species differ in their susceptibility and response to a myriad of decay agents present in the forest, which may affect their attractiveness to wildlife species. For example the high use of grand fir in eastside habitats may be because of the higher likelihood of grand fir to become infected with heartrot fungus (e.g. Indian paint fungus) and form a hollow bole that is needed by some species. Ponderosa pine on the other hand, has thick sapwood that readily decays, resulting in its preferred use by nesting species (Bull et al. 1997). Thus, a variety of snag and tree species should be retained in an area to help meet the full array of wildlife species needs.
Another reason to leave a variety of snag and tree species is because of the various retention rates among them. In habitats east of the Cascade crest, Douglas-fir snags, tend to remain standing much longer than ponderosa pine snags (Everett et al. 1999). West of the Cascade crest, western redcedar snags last the longest, followed by other cedars (incense, Alaska yellow, Port-Orford), and then Douglas-fir (see the Snag Dynamics document); hardwoods, Sitka spruce and true firs have the highest fall rates.
As much as possible, retain all hollow trees, snags, and down wood. Hollow structure is uncommon, and large hollow structures provide especially valuable habitat for a variety of wildlife Bull et al. (1997). Hollow structures are created in living trees by heartrot decay organisms over many years, and do not form once trees have died. Tree species that commonly form hollow interiors are grand fir, western larch, western redcedar, incense-cedar, and, in certain areas, western hemlock. Few studies have recorded wildlife use of hollow trees; in those that did, wildlife species were typically found using large grand fir east of the Cascade crest, and western redcedar west of the crest. Fungal species which cause this type of heartrot in standing large trees can be found for each habitat and structural condition under the “List of Insects and Pathogens” section in the linked tables (e.g., Eastside Mixed Conifer Forest, East Cascades/Blue Mountains, Larger Trees is linked to Table EMC_ECB.fid-5). Some root disease fungi also may cause butt rot in large living trees that can result in hollow bases (e.g., Westside Lowland Conifer-Hardwood Forest, Washington Coast, Larger Trees is linked to Table WLCH_WCO.fid-2), but these trees have a high risk of falling over due to associated root decay and weakness in the stem itself. Such trees can offer hollow log habitat, but could not reliably provide long-term standing structure. West of the Cascade crest, about half of the snags infected by root disease fell within a ten year period; only trees killed by weather factors had a higher fall down rate (see the Snag Dynamics document). Decay that occurs in the lower part of the tree is important because hollow tree butts are often large enough in diameter to accommodate larger species such as black bear (Bull et al. 2000).
Hollow interiors in the basal portion of the bole sometimes may be detected by “sounding” the tree bole by striking it with the back of an axe or a sound piece of wood and listening for a sound indicative of a hollow chamber. Openings into the bole, either single large holes or multiple smaller holes, are good indicators of hollow structure. Other indicators include broken boles that are fractured in such a way so that portions of the hollow structure are evident, large-diameter broken boles with bayonet tops, large dead tops or branches, punk knots, flattened stem faces, old wounds on the bole, crooks in the bole signifying previous breakage, and the presence of fruiting bodies (conks) of heartrot fungi known to cause hollow interiors, such as Indian paint fungus or quinine fungus.
The fall rates of snags vary by tree species, tree size, and cause of death. Thus, if one is looking to create or retain long-standing snags, it is imperative to consider these parameters. Information from remeasured permanent plots (see the Snag Dynamics document) shows that over a 10-year period in western Oregon and Washington, almost half the trees (48%) killed by root disease went down, while only 4% killed by insects went down. It also shows that about a third of the trees smaller than 100-cm dbh went down, while only 4% greater than 100-cm dbh went down. In addition, the species that fell at the greatest rates were hardwoods, Sitka spruce, and true firs. The lowest fall rates were in western redcedar and other cedars (incense, Alaska yellow, and Port-Orford).
Not all trees used by "snag dependent" species are totally dead. Defective trees with deformities such as broken tops, pockets of heartrot, hollows, damaged and loose bark, dead tops, or brooms caused by mistletoe or rust can also provide important habitat for a number of species. These defective trees can substitute for some of the snags when meeting snag density and dbh guidelines.
Witches brooms caused by broom rusts may occur on spruces and true firs, while Elytroderma needle cast can cause brooms in pines. Retain these broomed trees whenever possible, as they provide valuable habitat structure. Trees with broom rust or Elytroderma needle cast are preferred for retention over those infected with dwarf mistletoes (e.g. Douglas-fir dwarf mistletoe) in places where the spread of dwarf mistletoe would be counter to management objectives. Where trees infected with broom rust or Elytroderma needle cast are absent, the habitat offered by brooms may be provided in dwarf mistletoe-infected trees by carefully selecting patches of trees that contain some trees that are infection-free in the top third of their crown and with brooms in their mid-crown that are large and dense enough to provide nesting platforms. Any trees in the patch with brooms in the top third of the crown could be removed to decrease spread and to improve productivity. It may be possible to remove the infected portion of the tree, leaving the remainder of the tree as habitat structure. Also, localized patches of infection can be isolated from the rest of the stand by creating and maintaining a buffer of non-host or unstocked area (buffer width varies with host tree species). These guidelines apply generally to all species but western larch. When selecting mistletoe-infected western larch for retention, choose older, heavily infested trees with diminished crowns and large brooms that form obvious platforms. These trees may produce fewer dwarf mistletoe seeds than similar-sized, lightly infected larch because of their relatively small, degraded crowns. In addition, it is highly likely they will die and become high-quality snags.
When other choices for long-term snag retention are available, it is advisable to avoid selecting snags and trees of susceptible species within areas infected with root disease (e.g., avoid selecting Douglas-fir in a laminated root disease pocket). The relative susceptibility of conifer species to the five most common root diseases in Oregon and Washington are shown in Table.fid-10. Trees killed by root disease typically add to the snag component for only a short time before falling over (see the Snag Dynamics document). On the other hand, they can provide opportunities for naturally created concentrations of down wood that may be valuable to certain wildlife species. Trees of species that are tolerant, resistant, or immune to the root disease present make fine candidates for long-term retention within the infected areas.
The level of several root diseases has increased over historical levels due to fire suppression and previous harvest and regeneration practices (Campbell and Liegel 1996). In many cases, losses in tree growth have been offset by increases in diversity and wildlife habitat. Small, scattered root disease centers tend to increase structural diversity and provide a continuous supply of short-term snags and down wood. They sometimes supply concentrations of jackstrawed trees. Smaller, discrete root disease centers may be contained by surrounding them with a 50-foot buffer of non-host or unstocked area. However, areas with high densities of or very large root disease pockets may prevent the attainment of long-term objectives by limiting crown cover, preventing the growth and long-term survival of desired tree species and large trees, lowering site productivity, and increasing stand susceptibility to insect attack or competition stress. Because root disease can remain viable on a site for very long periods of time, sometimes up to 80 years, encouraging occupancy of the site by non-host, resistant, or tolerant tree species during regeneration, thinning, or selective harvest activities is usually the most effective means of meeting resource objectives in problematic root disease infection areas.
Retain a range of hard to soft snags because different wildlife species use different decay stages to meet different life history needs. For example, Aubry and Raley (2002) found strong differences in decay characteristics between nest trees and roost trees used by pileated woodpeckers, with nest snags tending to be harder than roost snags. In addition, hard snags left now may become soft snags in the future, acting as a recruitment source for highly decayed snags in the future. Because of different decay rates among tree species, leaving a mix of species will also help retain a mix of decay stages at some future point in time. However, in the drier habitat types one needs to consider that snags with visible external, exposed decay are less likely to survive a fire event, thus affecting their longevity; if these are desired for longer term retention, they should be retained in the moister sites on the landscape such as north slopes, riparian areas, draws, etc.
When decayed wood elements are deficient, managers commonly attempt to create them from sound trees using various methods such as topping, girdling, or felling. Sound trees that are killed by mechanical methods, insect attack, or some tree pathogens (pathogens, which cause necrosis or death of living tissue, are distinguished from decay fungi, which cause decay of non-living tissue without affecting living tissue), tend to decay from the outside in, and will never form a decayed hollow structure. Also, Schreiber (1987) noted that “green snags” or snags recently created form green trees will not provide for short-term nest requirements for cavity-nesting birds. When selecting live trees for retention or snag creation, look for trees that have visible indicators of heartwood decay (see information on indicators of hollow trees found earlier in this section). Trees having heavy mistletoe infections, especially stem infections, often make good candidates for snags because decay is frequently associated with stem infection areas, they often possess unique structures useful to wildlife, and removal of their infection potential is often beneficial from the standpoint of long-term stand management.
Some attempts have been made to create snags using bark beetle attractant pheromones, but the method remains largely experimental. Pheromones present some advantages over mechanical methods such as topping or girdling in that they are less expensive, safer to administer, and mimic a natural process. In addition, Ohmann showed that in western Oregon and Washington trees killed by insects tend to remain standing longer than those killed by other vectors (see the Snag Dynamics document). Snags created using beetle attractant pheromones would provide good foraging habitat for woodpeckers. Success of pheromones in creating snags, however, has been highly variable, ranging from largely unsuccessful to excessive mortality. Many variables, which are difficult to quantify or predict, influence the effectiveness of this method. For example, existing beetle population levels must be high enough so that sufficient numbers of beetles to overcome a healthy tree’s natural defenses may be attracted to the baits. Generally speaking, it is easier to kill a tree using bark beetle attractant pheromones east of the Cascade crest than is it on the west side; however, the risk of killing more trees than planned is also greater on the east side.
Living trees having internal decay or hollow interiors offer long-lasting nesting and roosting habitat, relative to dead trees that may fall sooner. In addition, they often are safer than snags to retain during logging operations. Methods for creating cavity nesting habitat in living trees involve injuring trees in ways that provide entry courts for heartwood decay fungi. Common techniques revolve around partial crown removal, which may be accomplished by mid-crown topping, girdling, or removing forks. Partial crown removal, however, does not consistently produce toprot that eventually provides a decayed or hollow interior, and will not provide internal stem decay in lower portions of the bole.
Artificial inoculation of trees with heartwood decay fungi provides a more reliable means of creating internal decay in sound, living trees. Artificial inoculation may be accomplished using cultured wooden dowels, inoculated in the lab with locally-selected strains of stem decay fungi, which are subsequently inserted by tree climbers into pre-drilled holes in living tree boles (Parks and Hildebrand In prep). The entire process of fungal material collection, isolation, culture, and inoculation requires at least eight months. This technique may be useful for habitat restoration in stands developed using traditional forestry practices that maximized tree growth while minimizing defect.
Down woodBecause all down wood is generated from a live tree or snag, planning for down wood features needs to be done during the planning process to create and retain snags and trees. As much as possible, retain all hollow down wood as well as the hollow trees and snags that will become hollow logs upon falling. Hollow structure is uncommon, and large hollow structures provide especially valuable habitat for a variety of wildlife. Hollow logs can only be created as living trees infected by heartrot fungi, and do not form once the tree has died. Tree species that commonly form hollow interiors are grand fir, western larch, western redcedar, incense-cedar, and in certain areas, western hemlock.
Once on the forest floor, different tree species decay at different rates. For example, in western Oregon and Washington, Sollins et al. (1987) found Douglas-fir logs that had persisted up to 200 years but did not find any western hemlock or western redcedar logs older than 100 years on the ground. Thus, when considering long-term down wood retention levels, the species of log must be taken into account.
Wildlife species used logs that were in a variety of decay stages. Most studies which recorded wildlife selection of different log decay stages found that wildlife selected for moderate to soft decay stages of logs. Softer logs also support high densities of mycorrhizal fungi (Harvey et al. 1987). However, a few species select for harder logs. In addition, hard logs left now will become the soft logs of the future, acting as a recruitment source for highly decayed logs. Because of different decay rates among tree species, leaving a mix of log species will also help retain a mix of decay stages at some future point in time. Decayed down wood will burn readily in a fire; these logs will need to be protected if they are to be maintained in wildlife habitat types with a frequent fire return interval; they should be managed for in less fire-prone areas such as north slopes and bottoms of draws in areas with low- to moderate-frequency, moderate- to high-severity fire regimes.
In the few studies where wildlife selection for length of down wood was recorded, they were selecting for longer pieces of wood. Large logs are better at regulating temperatures and moisture than smaller logs, and thus provide important habitat for Plethodontid amphibians and other species sensitive to temperature and moisture (Kluber et al. 2009). We recommend leaving longer logs on the landscape to meet wildlife needs. However, other down wood features need to be retained as well, such as appropriate species and decay classes, and hollow structures or larger diameters.
A key consideration when creating or retaining freshly killed down material is the susceptibility of the down trees to bark beetle attacks. It is important to note that some bark beetles which cause tree mortality will only attack standing live trees (e.g., mountain pine beetle, western pine beetle) while other species also will attack fresh down material (i.e., Douglas-fir beetle, spruce beetle). The amount of time that fresh down material is attractive to bark beetles usually does not extend beyond one year and varies with the species of bark beetle, the size of the material, the time of year it is placed on the ground, and the amount of solar radiation it receives. When a standing tree or piece of fresh down material is successfully attacked, adult beetles burrow beneath the bark to the phloem and excavate and lay eggs in tunnels called egg galleries. Most beetle attacks occur in the spring and early summer months, although a few species having multiple or overlapping generations (e.g., western pine beetle) may continue attacks into early fall. After hatching, the developing larvae burrow and feed in the phloem outward from the egg galleries and eventually pupate. Adults then emerge from the pupae, burrow through the bark, and fly to another tree to repeat the cycle. The risk of additional damage to other trees occurs during the flight period of the emerging brood. In the Pacific Northwest, development in a tree or log from the time of initial attack by adult beetles to emergence of their brood usually is completed within a period of one year or less. Therefore, trees and down wood colonized by bark beetles may pose risk of additional damage to other trees during the year following initial attack, but generally do not perpetuate any risk after one year.
Stands predominated by mature spruce may occur in some riparian areas east of the Cascade crest. If freshly felled or windthrown, large Engelmann spruce trees are considered for retention in such stands, it should be realized that even relatively low retention levels may significantly increase risk of spruce beetle infestations in standing live spruce trees. Most spruce beetle epidemics originate in fresh windthrown trees (Holsten et al. 1999), and outbreaks in pure or nearly pure spruce stands at times have resulted in mortality of nearly all of the live spruce trees down to 30 cm (12 in) in diameter (Schmidt and Frye 1997). Ultimate stand effects are determined by outbreak severity and initial stand conditions such as species composition, age and size of spruce trees (spruce beetles prefer to attack large, older trees), and density. Severe spruce beetle outbreaks modify pure or nearly pure mature spruce stands by reducing average age and density, and by lowering the average diameter and height of the stand to that of the surviving trees, which are usually the understory trees. When beetle activity is less severe, or when outbreaks occur in mixed stands where spruce is less abundant or scattered, spruce beetle will tend to create small- to medium-size mortality patches, reducing the spruce component and creating canopy gaps as it preferentially kills the larger spruce trees. West of the Cascade crest, the risk of significant mortality occurring in Sitka spruce as a result of spruce beetle activity is considered to be very low.
High levels of recently downed Douglas-fir (either through felling, windthrow or stem breakage) increase the risk of Douglas-fir beetle infestations in standing, live, mature Douglas-fir trees. Fresh down wood is very attractive to Douglas-fir beetles, and nearby trees are often killed by beetles attracted the newly killed boles, as well as by the broods that emerge from the colonized material one year later. Like spruce beetle, the Douglas-fir beetle prefers to attack large trees. In Washington and Oregon, forests containing a high proportion of Douglas-fir are fairly widespread, and Douglas-fir beetle outbreaks typically affect a more extensive area than do spruce beetle outbreaks. When thinking about retaining fresh, large Douglas-fir logs, consider the current level of Douglas-fir beetle populations in the general region, the stand attributes of the area of concern (proportion and size of Douglas-fir), the level of stress that the stand is currently experiencing (e.g. drought, off-site seed source, overcrowding in eastside stands, etc.), management objectives, and potential consequences. East of the Cascades Crest, effects of Douglas-fir beetle outbreaks generally are similar to those previously discussed for spruce beetle, except that in pure or nearly pure Douglas-fir stands, intense Douglas-fir beetle activity sometimes may cause mortality of nearly all of the Douglas-fir down to 10 inches in diameter (Wright and Lejeune 1967). Abundant new snags are created in these areas, but the accompanying losses of existing cover and large stand structure also may degrade the quality of habitat available for some wildlife species, negatively affect water quality, or increase risk of wildfires for many decades. In mature Douglas-fir forests west of the Cascade crest, Douglas-fir beetle typically creates scattered canopy gaps and scattered patches of forest with reduced crown cover, resulting in the release of understory trees and shrubs.
In many instances Douglas-fir beetle activity may be considered beneficial to wildlife, but in some cases the associated mortality is not desirable. In the latter case prompt removal of windthrown, broken, and standing trees that currently contain bark beetles, before the spring following the initial attack, has been the usual recommendation to prevent or reduce the severity of an outbreak (e.g., for a winter storm event that occurs in January 2002, removal of infested material would be completed prior to brood emergence and flight in Spring 2003). In some situations, when valuable resources are at risk and it is neither desirable nor feasible to remove all of the infested trees, the Douglas-fir beetle anti-aggregation pheromone, MCH, may be deployed to protect standing Douglas-fir trees in designated areas from attack. MCH treatment is appropriate for any stand where Douglas-fir beetle-caused tree mortality is expected to be high enough to significantly impact resource management objectives (Ross et. al. 2001). MCH also may be used in fresh blowdown patches to prevent infestations and subsequent population increases of Douglas-fir beetle. Traps with attractant pheromones (Ross and Daterman 1997) are sometimes used in combination with MCH to further reduce beetle populations and to enhance protection of treated areas.
When creating large-diameter down wood in mixed-species stands containing a high percentage of Douglas-fir or eastside Engelmann spruce, selecting a variety of the available tree species for placement on the ground may help to mitigate risk of adverse effects caused by bark beetles. Guidelines (Hostetler and Ross 1996) for reducing the risk of adverse impacts of Douglas-fir beetle have been developed for Westside situations. Techniques include restricting falling operations to certain months of the year so that abundant favorable beetle habitat is not present during the main Douglas-fir beetle flight period, and staggering down wood input over several successive years to restrict available beetle habitat. As in windthrow situations, it is often possible to mitigate potential adverse effects of large down wood creation by using MCH, the Douglas-fir beetle’s anti-aggregating pheromone, alone or in combination with aggregating pheromones in trapping systems to reduce or prevent attacks in the down material and in nearby standing trees.
When considering the creation of wildlife habitat using slash piles, it should be realized that slash piles of fresh pine material in sapling and pole-size pine stands pose risk of pine engraver and California five-spined ips outbreaks. Tree mortality that is clumped in distribution and sometimes considerable may result. Outbreaks of these beetles may be prevented by properly timing thinning and harvest activities and by scattering all slash greater than three inches in diameter in canopy openings.
Hazard treesSnags or living trees that are located on developed sites within striking distance of a target, such as a camping pad, restroom facility, or picnic table, and that have advanced stem decay or are infected with root disease pathogens, pose a very real risk for human safety. Such trees are considered hazard trees within the context of these locations, and are highly likely to be removed for reasons of public safety. Complete removal, however, is not always necessary. Topping trees or snags low enough so that they can no longer reach a target may provide some habitat while effectively mitigating hazard, and may also provide opportunities for public interpretation.
Higher levels of planned retention usually are achieved when Occupational Safety and Health Administration (OSHA) guidelines are taken into consideration while designating wildlife trees in harvest areas. A greater number of wildlife trees are likely to be retained during logging operations when they are located away from designated landing sites, skid trails, or cable corridors. Designating clumps containing snags and green replacement trees for retention as opposed to scattered individual trees may also result in greater retention, in addition to potentially providing better habitat.
ContactsWhen developing plans for selecting, creating, or retaining dead tree elements on the landscape, it may be desirable to contact your local forest entomologist or forest pathologist to obtain additional advice or information regarding the insects or pathogens and their effects.
For federal lands contact information for the USDA Forest Service, Forest Health Protection, Regional staff in Portland, OR, and the Service Center staffs in La Grande, Bend, Medford, and Sandy, OR, and Wenatchee, WA, are available at: USDA Forest Service, Pacific Northwest Region, Forest Health Protection
For State or private lands in Oregon contact information is available at: web site temporarily down
Oregon Department of Forestry, Forest Health Management
For State or private lands in Washington contact:
Washington State Department of Natural Resources, Forest Health
In addition, annual aerial insect detection survey data is available, and that of the previous five years should be obtained. These data, which include current tree mortality, are available for all forest ownerships. They can indicate whether sufficient snags already exist in an area, as well as provide trends for mortality, allowing project funds to be reallocated from areas with recent high levels of natural mortality, to snag-deficient areas having low levels of natural mortality. Annual aerial survey map data of insect activity in Oregon and Washington are available online at http://www.fs.fed.us/r6/nr/fid/data.shtml.
ReferencesAubry, Keith B, and Catherine M. Raley. 2002. Selection of nest and roost trees by pileated woodpeckers in coastal forests of Washington. J. Wildl. Manage. 66:392-406.
Bull, Evelyn L. 1987. Ecology of the pileated woodpecker in northeastern Oregon. J. Wildl. Manage. 51(2):472-481.
Bull, Evelyn L., Catherine G. Parks, and Torolf R. Torgersen. 1997. Trees and logs important to wildlife in the interior Columbia River Basin. USDA Forest Service, PNW Research Station, Portland, OR. PNW-GTR-391. 55 pp. http://www.fs.fed.us/pnw/publications/pnw_gtr391/
Bull, Evelyn L., Richard S. Holthausen, and Mark G. Henjum. 1992. Roost trees used by pileated woodpeckers in northeastern Oregon. J. Wildl. Manage. 56:786-793.
Campbell, S., and L. Liegel, tech cords. 1996. Disturbance and forest health in Oregon and Washington. Gen. Tech. Rep. PNW-GTR-381. Portland, OR: USDA Forest Service, Pacific Northwest Region; Oregon Department of Forestry, Washington Dept. of Natural Resources. 105p.
Harvey, A.E., M.F. Jurgensen, M.J. Larsen, and R.T. Graham. 1987. Decaying organic materials and soil quality in the Inland Northwest: a management opportunity. USDA Forest Service, Intermountain Res. Station, GTR-INT-225, Ogden, UT. 14 pp.
Holsten, E.H., R.W. Their, A.S. Munson, and K.E. Gibson. 1999. The spruce beetle. USDA Forest Service, Forest Insect and Disease Leaflet 127. 11p.
Hostetler, B.B., and D.W. Ross. 1996. Generation of coarse woody debris and guidelines for reducing the risk of adverse impacts by Douglas-fir beetle. Unpublished paper. 6p.
Parks, C., and Hildebrand, D. In prep. Experimental techniques for inoculation of trees with stem decay to create wildlife habitat -- laboratory and field procedures. Draft Handbook. 24p.
Ross, D.W., and G.E. Daterman 1997. Using pheromone-baited traps to control the amount and distribution of tree mortality during outbreaks of the Douglas-fir beetle. Forest Science 43(1): 65-70.
Ross, D.W., K.E. Gibson, and G.E. Daterman 2001. Using MCH to protect trees and stands from Douglas-fir beetle infestation. USDA Forest Service, FHTET-2001-09. 11p.
Schmidt, J.M., and R.H. Frye 1977. Spruce beetle in the Rockies. USDA Forest Service, Gen. Tech. Rep. RM-49. 38pp.
Schreiber, Barry. 1987. Diurnal bird use of snags on clearcuts in central coastal Oregon. M.S. Thesis, Oregon State Univ., Corvallis. 63 pp.
Sollins, P., S.P. Cline, T. Verhoeven, D. Sachs, and G. Spycher. 1987. Patterns of log decay in old-growth Douglas-fir forests. Can. J. For. Res. 17:1585-1595.
Wright, K.H., and R.R. Lejeune 1967. Douglas-fir beetle, Dendroctonus pseudotsugae Hopk. In: Davidson, A.G. and R.M. Prentice, eds. Important forest insects and diseases of mutual concern to Canada, the United States, and Mexico. Ottawa, ON: Canada Department of Forestry and Rural Development. pp. 16-20.