What Is the DecAID Advisor?


Decayed wood elements -- snags, down wood, and live decaying trees -- are habitat for many organisms that live in terrestrial ecosystems, and contribute to other aspects of ecosystem productivity and diversity. Maintaining an adequate level and mixture of these habitat elements can be a challenging task for any forest land manager. “DecAID” is an advisory system developed from a synthesis of data and research results pertaining to forests in Oregon and Washington (Marcot et al. 2010). The DecAID Advisor is a planning tool intended to help advise and guide managers as they conserve and manage snags, partially dead trees, and down wood for biodiversity.
DecAID is an advisory tool to help managers evaluate effects of forest conditions and existing or proposed management activities on organisms that use snags and down wood. DecAID also can help managers decide on snag and down wood sizes and levels needed to help meet wildlife management objectives. It can help managers articulate those objectives in specific, quantitative terms that could be tested in the field. In this way, the name “DecAID” can be read as decayed wood advisor and management aid (“decay-aid” or "decision-aid"). The DecAID Advisor can help long-term planning, as over "decades" of time.


Wildlife species models and advisory tools related to managing decayed wood elements (principally, snags and down wood) on federal lands in the Pacific Northwest were first developed in the 1970s and 1980s (Thomas et al. 1979, Neitro et al. 1985, Marcot 1992, Raphael 1983), including some snag dynamics models (e.g., Morrison and Raphael 1993, Marcot 1992). Although these tools were based on sound empirical information and expert knowledge available at the time, the data and model structures became outdated. A considerable amount of new information about the ecology, dynamics, and management of decayed wood has become available since the 1980s. There has been an evolution from thinking of snags and down wood as just habitat structures for terrestrial vertebrates, to thinking of decaying wood in the broadest sense as an integral part of complex ecosystems and ecological processes. See the associated essay "Ecosystem Processes Related to Wood Decay".

Several key themes prevalent in recent literature include:

  • decayed wood elements consist of more than just snags and down wood, such as live trees with dead tops or stem decay
  • decayed wood provides habitat and resources for a wider array of organisms and their ecological functions than previously thought
  • “wood decay” is an ecological process important to far more organisms than just terrestrial vertebrates (“wildlife” in the traditional sense)
  • the importance of dead wood as part of carbon storage and cycling
Also, more recent field studies, particularly in eastern Oregon (e.g., Bull et al.1997), suggest that the amounts and sizes of snags selected by wildlife are far greater than those depicted by existing models.
Because of the extensive amount of new information and ideas published over the past twenty years, it was apparent that the empirical foundation of existing models needed to be updated and revised. The DecAID Advisor is being developed to fill this need.

What is DecAID?

The DecAID Advisor arose from the recognition by Pacific Northwest Region, USDA Forest Service, of the growing need to update guidelines for managing snags and down wood. It was described in the wildlife Species Habitat Project of Washington and Oregon (Johnson and O’Neil 2001). DecAID developed into a major data synthesis project under USDA Forest Service, Pacific Northwest Region, and Pacific Northwest Research Station, Portland, Oregon, with contributions of expertise from USDI Fish and Wildlife Service, and other agencies and institutions.

Modeling biological potential of primary cavity excavator birds has been used in the past (Thomas et al. 1979, Neitro et al. 1985), and we developed the DecAID Advisor to avoid some pitfalls associated with that approach. There is no direct relationship between the statistical summaries presented in DecAID and past calculations or models of biological potential. Field studies have suggested that predictions of biological potential (relative or absolute population sizes of snag-associated wildlife species) do not match research findings.

DecAID is organized around “vegetation conditions” that combine wildlife habitat type, vegetation alliance, structural condition (based on dominant tree size and canopy closure), and geographic location (subregion). Wildlife habitat types and structural conditions as used in DecAID were derived from the wildlife habitats and structural conditions defined in the Species Habitat Project (Chappell et al. 2001). In DecAID version 3.0, the structural classification was updated to reflect updated methodologies and data used to classify forest structure and successional stages (Haugo et al. 2015).

DecAID provides interpretation and advice on the roles of insects and pathogens in the creation and dynamics of dead wood, and the implications of snag and down wood management on ecosystem health, and offers mitigation considerations. It includes information and advice on relationships between forest insects and pathogens and snag and down wood management, and summarizes the occurrences of specific pathogens within various vegetation conditions.

DecAID also provides a summary of forest inventory data representing the range of “natural” (unharvested) and current conditions of snags and down wood in forests of all ownerships and disturbance histories. The DecAID Advisor presents information from research studies and inventories about range of natural conditions where available, and can be used to help identify knowledge gaps and areas of needed research.

DecAID describes fungi associated with decayed wood in Oregon and Washington, including a summary of their ecological roles, the importance of dead wood to fungi, and considerations for maintenance of fungal biodiversity. See the associated essay on "Importance of Fungi in Forest Ecosystems".

Because forest management has evolved to address forests as ecological communities and dynamic ecosystems, DecAID addresses far more than just wildlife (terrestrial vertebrate) use of snags and down wood. DecAID also includes information on invertebrate and fungal relationships to dead wood. Ecosystem management acknowledges how organisms link to their environments and how human activities influence more than just individual species. In this spirit, DecAID provides information on the array of key ecological functions and functional groups of wildlife that use snags and down wood, and can be used to describe the impact of changing snag and down wood levels on those functions and functional groups.

On What is DecAID Based?

DecAID is a summary, synthesis, and integration of published scientific literature, research data, wildlife databases, forest inventory databases, and expert judgment and experience. The information presented on wildlife species use of snags and down wood is based entirely on scientific field research and does not rely on modeling the biological potential of wildlife populations.

The information presented on ranges of snag and down wood amounts under reference and current conditions is based on forest inventories, conducted by the Forest Inventory and Analysis (FIA), USDA Forest Service, Pacific Northwest Research Station, on both federal and nonfederal lands.

See Version History for the cutoff date for incorporating papers and data sources. To select the studies to include in DecAID, we relied primarily on published literature and theses that had empirical data on wildlife use of size and amount of dead wood. Data used in the wildlife cumulative species curves were those on mean, variance, and sample size. For use in the down wood cumulative curves, the data needed to be reported in units that we could convert to percent cover, such as down wood volume. Data on down wood reported as pieces of down wood per unit area were not able to be converted to percent cover of down wood.

Components of DecAID

DecAID contains an extensive amount of information. However, it is not all-encompassing and it is important to clarify what it can and can not do.

DecAID is:

  • a thorough review of published literature and other available data on wildlife use of decayed wood elements, primarily in Oregon and Washington
  • a statistical synthesis of data showing levels of use by individual wildlife species of decayed wood elements
  • a summary of the patterns of use of decayed wood elements by wildlife species in Oregon and Washington (number of species using specific snag or down wood sizes or amounts)
  • statistical summaries of forest inventory data on snags and down wood in unharvested forests and entire landscapes across Oregon and Washington
  • a helpful tool for making informed decisions

DecAID is NOT:

  • a forest stand growth simulator
  • a snag and down wood decay simulator or recruitment model
  • a wildlife population simulator or analysis of wildlife population viability
  • a substitute for making professional decisions based on experience
Also, DecAID does not address decayed wood elements in aquatic or riparian environments, although we recognize the high value of such elements in providing habitat for associated fish, wildlife, invertebrate, and other species.

Who Could Use DecAID?

DecAID was developed with the intent for use across all land ownerships in Washington and Oregon. Federal land managers are the primary users of DecAID, and thus inventory are broken out by federal lands only as well as all land ownerships. The DecAID Advisor will be useful to a wide array of private, commercial, city, county, state, and federal land managers, as well as planners, policymakers, and researchers. Our DecAID science team (see authors) consists of wildlife biologists, research ecologists, forest inventory specialists, forest entomologists, and mycologists. The team also consulted with professionals from diverse specialties including wildlife research, fire ecology, timber management, plant pathology, silviculture, and land use planning.

We intend DecAID to be used broadly by land planners, timber consultants, and forest managers. We hope the DecAID Advisor serves as a template for use in other geographic areas, given similar literature reviews, research data, and summaries of expert judgments.

How Can DecAID Be Used?

DecAID presents information on wildlife use of snag diameter, snag density, down wood diameter, and down wood percent cover, and on reference conditions (unharvested) and current (all) conditions of snag density and down wood percent cover by diameter classes. The information is presented at three statistical tolerance levels, 30%, 50%, and 80% tolerance levels. Minimum and maximum values are also presented. Additional available data on dead wood species, decay condition, etc. are summarized but not analyzed statistically.

DecAID allows the user to specify a vegetation condition, and to:

  • view a synthesis of empirical data on species use of wood decay elements in Washington and Oregon;
  • determine which selected species would be associated with specific sizes or amounts of snags or down wood at various statistical levels;
  • determine the sizes or amounts of snags or down wood to meet specified species objectives;
  • view a narrative interpretation of these data along with the literature sources;
  • view summaries of the range of snag and down wood levels in unharvested forest, representing reference conditions, and across all current forest conditions;
  • determine effects of present or expected stand conditions on species using wood decay elements;
  • determine implications of insect and pathogen activity on the creation and management of snags and down wood for wildlife habitat;
  • view advice on the roles of insects and pathogens in creation and dynamics of snags and down wood; and
  • determine implications of snag and down wood levels on managing for overall forest ecosystem health.

Paying Attention to Scales of Space and Time

A critical consideration in use of DecAID is that of scales of space and time. DecAID will be best applied at scales of subwatersheds, watersheds, subbasins, physiographic provinces, or large administrative units such as Ranger Districts, National Forests, or BLM Districts.

DecAID is not intended to predict occurrence of species at the scale of individual forest stands or specific locations – there are far too many other factors influencing the presence or absence of organisms at that scale than could be reasonably depicted here. DecAID is not intended to predict the specific species composition in a given geographic area. Instead, it is intended to be used to help advise on broad, cumulative patterns of species occurrence and distributions in generally-described habitat types and structural conditions. Validation of other wildlife-habitat relationships databases (for example, Dedon et al. 1986, Fielding and Haworth 1995, Raphael and Marcot 1986) suggests that any information of this type would likely err on the side of commission, that is, including species that may not really occur on any one site, particularly with smaller geographic areas such as an individual forest stand. DecAID is intended to be a broader planning aid than a species- or stand-specific prediction tool.

Because DecAID is not a time-dynamic simulator (such as Marcot 1992 for snags; Mellen and Ager 2002 for snags and down wood; and Bragg 2000, for down wood in riparian systems), it does not account for potential temporal changes in vegetation and other environmental conditions, species functional and numerical responses, population dynamics and demographics, fire likelihoods, and ecosystem health. DecAID could be consulted to review potential conditions at specific points in time and for a specific set of conditions, but dynamic changes in forest and landscape conditions would have to be modeled or evaluated outside the confines of the DecAID Advisor.

Regarding the use of inventory data to represent reference and current conditions, the dead wood estimates must be interpreted in light of the inherent scale imposed by the inventory designs. Each observation (CCplot) that entered our summaries was an individual condition class from a field plot. Because the plots sampled an area that is smaller than a typical forest stand, the plot-level observations should not be thought of as representing stand-level conditions. Rather, our summaries describe the aggregate properties of the variability of dead wood on multiple CCplots that sample a given wildlife habitat. We believe it is reasonable to apply distributional information about dead wood that is based on many inventory CCplots in a given vegetation condition to a management “unit” at the scale of a landscape or sub-watershed.

The distribution and estimated variation of dead wood within each wildlife habitat is the result of the interaction between plot size and the spatial pattern of dead wood. Smaller plot sizes would result in greater variability, since smaller plots are more likely to sample dense clumps of dead wood or fall in gaps where no dead wood exists.

Although the estimates of amounts of dead wood are from plots measured at a single point in time, the current conditions express events that have occurred over the past decades to centuries.

Availability, Progress, and Related References

The DecAID project is an ongoing, dynamic process. The DecAID Advisor is available as a Web site operating as an interactive program. A companion web site is available to help use and interpret the data in DecAID in on-the-ground implementation of projects (http://www.fs.fed.us/r6/nr/wildlife/decaid-guide/index.shtml).

We have presented work to date in other publications. An overview of DecAID was provided in Mellen et al. (2002). The wildlife component of DecAID was described in Marcot et al. (2002) and the ecological functions of wildlife pertaining to decayed wood were presented by Marcot (2002) and Marcot and Vander Heyden (2001). The forest inventory summaries were described in Ohmann and Waddell (2002). The statistical basis for the meta-analysis in DecAID is provided in Marcot et al. (2010).

Literature Cited

Bragg, D. C. 2000. Simulating catastrophic and individualistic large woody debris recruitment for a small riparian system. Ecology 81(5):1383-1394.

Bull, E. L., C. G. Parks, and T. R. Torgersen. 1997. Trees and logs important to wildlife in the interior Columbia River basin. Gen. Tech. Rep. PNW-GTR-391. USDA Forest Service, Portland OR. 55 pp. http://www.fs.fed.us/pnw/publications/pnw_gtr391/

Chappell, C. B., R. C. Crawford, C. Barrett, J. Kagan, D. H. Johnson, M. O'Mealy, G. A. Green, H. L. Ferguson, W. D. Edge, E. L. Greda, and T. A. O'Neil. 2001. Wildlife habitats: descriptions, status, trends, and system dynamics. Pp. 22-114 in: D. H. Johnson and T. A. O'Neil, editors. Wildlife-habitat relationships in Oregon and Washington. Oregon State University Press, Corvallis OR.

Dedon, M. F., S. A. Laymon, and R. H. Barrett. 1986. Evaluating models of wildlife-habitat relationships of birds in black oak and mixed-conifer habitats. Pp. 115-120 in: J. Verner, M. L. Morrison, and C. J. Ralph, ed. Wildlife 2000: modeling habitat relationships of terrestrial vertebrates. Univ. Wisconsin Press, Madison, WI.

Fielding, A. H., and P. F. Haworth. 1995. Testing the generality of bird-habitat models. Cons. Biol. 9(6):1466-1481.

Haugo, Ryan, Chris Zanger, Tom DeMeo, Chris Ringo, Ayn Shlisky, Kori Blankenship, Mike Simpson, Kim Mellen-McLean, Jane Kertis, Mark Stern. 2015. A new approach to evaluate forest structure restoration needs across Oregon and Washington, USA. Forest Ecology and Management 335:37-50.

Johnson, D., and T. O'Neil, editors. 2001. Wildlife-habitat relationships in Oregon and Washington. Oregon State University Press, Corvallis OR.

Marcot, B. G. 1992. Snag Recruitment Simulator, Rel. 3.1 [computer program], USDA Forest Service, Pacific Northwest Region, Portland OR.

Marcot, B. G. 2002. An ecological functional basis for managing decaying wood for wildlife. pp. 895-910 In: Laudenslayer, William F., Jr.; Valentine, Brad; Weatherspoon, C. Philip; Lisle, Thomas E., technical coordinators. Proceedings of the symposium on the ecology and management of dead wood in western forests. 1999 November 2-4; Reno, NV. Gen. Tech. Rep. PSW-GTR-181. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture. http://www.fs.fed.us/psw/publications/documents/gtr-181/068_Marcot.pdf

Marcot, B.G., K. Mellen, S.A. Livingston, and C. Odgen. 2002. The DecAID Advisory Model: Wildlife Component. pp 561-590. In: Laudenslayer, William F., Jr.; Valentine, Brad; Weatherspoon, C. Philip; Lisle, Thomas E., technical coordinators. Proceedings of the symposium on the ecology and management of dead wood in western forests. 1999 November 2-4; Reno, NV. Gen. Tech. Rep. PSW-GTR-181. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture. http://www.fs.fed.us/psw/publications/documents/gtr-181/044_MarcotDecAid.pdf

Marcot, Bruce G., Janet L. Ohmann, Kim Mellen-McLean, and Karen L. Waddell. 2010. Synthesis of regional wildlife and vegetation field studies to guide management of standing and down trees. Forest Science 56(4):391-404. http://www.fs.fed.us/r6/nr/wildlife/decaid/documents/marcot_etal_2010_DecAID.pdf

Marcot, B. G., and M. Vander Heyden. 2001. Key ecological functions of wildlife species. Pp. 168-186 in: D. H. Johnson and T. A. O'Neil, ed. Wildlife-habitat relationships in Oregon and Washington. Oregon State University Press, Corvallis OR.

Mellen, K., and A. Ager. 2002. A coarse wood dynamics model for the western Cascades. pp 503-516. In: Laudenslayer, William F., Jr.; Valentine, Brad; Weatherspoon, C. Philip; Lisle, Thomas E., technical coordinators. Proceedings of the symposium on the ecology and management of dead wood in western forests. 1999 November 2-4; Reno, NV. Gen. Tech. Rep. PSW-GTR-181. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture. http://www.fs.fed.us/psw/publications/documents/gtr-181/040_MellAger.pdf

Mellen, K., B.G. Marcot, J.L. Ohmann, K.L. Waddell, E.A. Willhite, B.B. Hostetler, S.A. Livingston, and C. Ogden. 2002. DecAID: A decaying wood advisory model for Oregon and Washington. pp 527-534. In: Laudenslayer, William F., Jr.; Valentine, Brad; Weatherspoon, C. Philip; Lisle, Thomas E., technical coordinators. Proceedings of the symposium on the ecology and management of dead wood in western forests. 1999 November 2-4; Reno, NV. Gen. Tech. Rep. PSW-GTR-181. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture. http://www.fs.fed.us/psw/publications/documents/gtr-181/042_MellenDec.pdf

Morrison, M. L., and M. G. Raphael. 1993. Modeling the dynamics of snags. Ecol. Applic. 3(2):322-330.

Neitro, W. A., V. W. Binkley, S. P. Cline, R. W. Mannan, B. G. Marcot, D. Taylor, and F. F. Wagner. 1985. Snags (wildlife trees). Pp. 129-169 in: E. R. Brown, ed. Management of wildlife and fish habitats in forests of western Oregon and Washington. Part 1 - chapter narratives. USDA Forest Service, Portland OR.

Ohmann, J. L., and K. L. Waddell. 2002. Regional patterns of dead wood in forested habitats of Oregon and Washington. Pp.535-560. In: P. J. Shea and W. Laudenslayer (Ed.). Proceedings of a conference on The Ecology and Management of Dead Wood in Western Forests, Reno, Nevada. USDA Forest Service, Pacific Southwest Research Station General Technical Report PSW-GTR-181.

Raphael, M. G. 1983. Cavity-nesting bird response to declining snags on a burned forest: a simulation model. (pp. 211-215) In: J. W. Davis, G. A. Goodwin, and R. A. Ockenfels (Ed.). Snag habitat management: proceedings of the symposium. USDA Forest Service General Technical Report RM-99. Flagstaff AZ.

Raphael, M. G., and B. G. Marcot. 1986. Validation of a wildlife-habitat relationships model: vertebrates in a Douglas-fir sere. Pp. 129-138 in: J. Verner, M. L. Morrison, and C. J. Ralph, ed. Wildlife 2000: Modeling habitat relationships of terrestrial vertebrates. University of Wisconsin Press, Madison, WI. 470 pp.

Thomas, J. W., R. G. Anderson, C. Maser, and E. L. Bull. 1979. Snags. Pp. 60-77 in: J. W. Thomas, ed. Wildlife habitats in managed forests the Blue Mountains of Oregon and Washington. Agriculture Handbook No. 553. USDA Forest Service, Portland OR.

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