Wind River Exp. For., WA

Characteristics of Coarse Woody (CWD) and Fine Woody Detritus (FWD) in Progressive Stages of Decomposition in Forest Ecosystems of the United States, Mexico, and Russia

This web page is a part of a continuing Joint Venture Agreement between Oregon State University and the USFS Northern Research Station.

Our projects have been aimed at improving the accuracy of calculations used by the Forest Inventory and Analysis for determining the mass and carbon concentration of CWD in forest ecosystems. We recorded visual characteristics, measured bark and wood densities, and determined the carbon concentration for thousands of CWD samples. This effort provides a basis for assessment of the carbon pools associated with woody detritus in forests managed by the US Forest Service.

The CWD sampled for these projects originated from sites in the United States, Mexico, and Russia (general map of study locations). Data collection started in the early 1990's and continues to the present.
The major outputs of this project include density and density reduction factors for all tree species existing on US Forest Service land, plus carbon concentration of major species and taxa.

Project coordinators

Carbon Concentration of Standing and Downed Woody Detritus

The degree to which carbon concentration of woody detritus varies with tree taxa, decay class, tissue type, and position was examined for archived samples of 60 species collected in the Northern Hemisphere. The mean carbon concentration of all 257 samples analyzed was 49.3%, ranging from 43.4 to 56.8%. Angiosperms had a significantly lower carbon concentration than gymnosperms, with means of 47.8 and 50.6%, respectively. For whole-stems (i.e., wood and bark) the carbon concentration of gymnosperms significantly increased by ~10% with decomposition, but for angiosperms it did not. The carbon concentration of bark was higher than wood in every decay class by an average of 1.0%. A similar difference was found for standing versus downed dead wood, with the former having a higher carbon concentration than the latter. Differences between angiosperms and gymnosperms are hypothesized to be associated with the differences in initial lignin concentrations as well as subsequent decomposition by major fungal functional groups (white- versus brown-rots). The higher abundance of brown-rots in decomposing gymnosperms may lead to an increase in lignin concentrations, a compound that has higher carbon concentration than cellulose. While the data presented here can be used at various levels of resolution, a system that stratifies by general taxa (angiosperms versus gymnosperms) and decay class would likely reduce current uncertainties in dead wood carbon concentrations the most.

  return to top

CWD Density and Reduction Factors

Woody detritus or dead wood is an important part of forest ecosystems associated with many ecological benefits. As such, estimates of woody detritus biomass are highly desired for forests across the Nation. Biomass estimates are most often achieved by determining the volume of dead wood and then converting to mass by use of density values. Unfortunately, there have been few studies on how density (mass/volume) of this material changes during the decay process. The goal of this study was to synthesize both published and unpublished data on woody detritus density so as to improve estimates of coarse woody detritus (CWD) and fine woody detritus (FWD) biomass across the diverse forests of the United States. The specific objectives were: 1) estimate CWD and FWD mean density across decay classes for the tree species currently considered by the FIA inventory, 2) develop a methodology to estimate the uncertainty in these estimates, 3) provide examples how these estimates can be used, and 4) suggest future study directions to reduce uncertainty in these estimates. In the case of CWD, a total of 88 species were found to have data on densities for 5 decay classes that had been published and/or collected from North America from the boreal to the tropical zones. In general density declined as one proceeds from decay class 1 to 5, but at least five density reduction patterns were observed. For FWD our search indicated approximately 25 species had been sampled, although some of these “species” represent mixtures named after a dominant species. FWD density was a function of piece diameter and the general state of decay. We determined that by sampling representative species within a genus, the uncertainty of CWD estimates could be reduced up to 50% over not having sampled a genus. Our analysis indicated that when FWD relative density is actually measured that the uncertainty of mass is 1 to 3%. In contrast, when relative density has to be estimated, the uncertainty of FWD mass estimation ranges from 12 to 19%. Given that the decay state of FWD is rarely noted, our analysis also indicates that pulses of FWD by disturbances could add an additional uncertainty of at least 20% even when species have been sampled. We conclude that a more systematic sampling of CWD and FWD density is needed for major species if uncertainty mass estimates is to be consistently reduced to <5% nationwide.

return to top

Visual Characteristics of Decay in CWD

There are five decay classes that span a spectrum of decay from fresh mortality to nearly complete decay. Decay classes are largely qualitative based on the physical appearance and structural integrity of individual dead wood pieces.

For standing dead trees the five decay classes are defined as:

Class 1: All limbs and branches are present; the top of the crown is still present; all bark remains; sapwood is intact, with minimal decay; heartwood is sound and hard;
Class 2: There are few limbs and no fine branches; the top may be broken; a variable amount of bark remains; sapwood is sloughing with advanced decay; heartwood is sound at base but beginning to decay in the outer part
of the upper bole;
Class 3: Only limb stubs exist; the top is broken; a variable amount of bark remains; sapwood is sloughing; heartwood has advanced decay in upper bole and is beginning at the base;
Class 4: Few or no limb stubs remain; the top is broken; a variable amount of bark remains; sapwood is sloughing; heartwood has advanced decay at the base and is sloughing in the upper bole;
Class 5: No evidence of branches remains; the top is broken; <20 percent of the bark remains; sapwood is gone; heartwood is sloughing throughout.

For downed dead trees the five decay classes are defined as:

Class 1: Sound, freshly fallen, intact logs with no rot, no conks present indicating a lack of decay, original color of wood, no invading roots, fine twigs attached with tight bark;
Class 2: Sound log sapwood partly soft but can’t be pulled apart by hand, original color of wood, no invading roots, many fine twigs are gone and remaining fine twigs have peeling bark;
Class 3: Heartwood is still sound with piece supporting its own weight, sapwood can be pulled apart by hand or is missing, wood color is reddish-brown or original color, roots may be invading sapwood, only branch stubs are remaining which can’t be pulled out of log;
Class 4: Heartwood is rotten with piece unable to support own weight, rotten portions of piece are soft and/or blocky in appearance, a metal pin can be pushed into heartwood, wood color is reddish or light brown, invading roots may be found throughout the log, branch stubs can be pulled out;
Class 5: There is no remaining structural integrity to the piece with a lack of shape as rot spreads out across ground, rotten texture is soft and can become dry powder when dry, wood color is red-brown to dark brown, invading roots are present throughout, branch stubs and pitch pockets have usually rotten down.

return to top

Images of decay classes with description from Russia

  • Birch (Betula pendula and Betula costata)
  • Kedr (Pinus koraiensis and Pinus sibirica)
  • Larch (Larix daurica and Larix sibirica)
  • Pine (Pinus silvestris)
  • Spruce (Picea abies, Picea obovata, and Picea ajanensis)

return to top

Methods used in data collection

return to top

Fine Woody Debris


 Accurate estimation of fine woody debris (FWD) population attributes (e.g., biomass) is a critical component of nationwide efforts to quantify carbon stocks and wildfire hazards. FWD can be defined as any piece of dead woody debris greater than 0.5 cm and less than 10 cm in diameter and typically represents material such as tree and shrub branches and roots (Woodall and Monleon, 2008). FWD can be branches suspended in standing live or dead trees, attached to a fallen dead tree, freshly fallen onto the forest floor from live or dead trees or in may become incorporated into the duff, or found within the soil (i.e., roots). As with the larger coarse woody debris (CWD; dead woody debris with a diameter greater than 10 cm), FWD offers many benefits to the forest ecosystem such as storing carbon and nitrogen, as habitat for smaller vertebrates, and as a habitat and food source for invertebrates, and fungi. In some forests, though, FWD is a contributing source of fuel for catastrophic wildfires. Given its critical ecological niche across the U.S., refining techniques for FWD biomass estimation is paramount.

Fine woody debris is inventoried nationwide by U.S. Department of Agriculture’s Forest Inventory and Analysis (FIA) program in order to determine FWD mass within particular forest conditions or domains (e.g, an entire state or forest type). Current field data procedures consist of tallying FWD by size class along line intersect sampling transects within FIA plots (for details refer to Woodall and Monleon, 2008). These counts of individual FWD pieces are then summed and an estimate of volume is determined using geometric mean diameters. Biomass conversion constants published in the literature are currently utilized to convert volume to biomass. Two constants are used: 1) the initial density (i.e., specific gravity in g/cm3) and 2) a decay reduction factor that accounts for the decline in density as this form of detritus decomposes (Harmon et al 2008).

In this study we determined which types of data field crews could collect that would potentially decrease the uncertainty in mass determination to such an extent that it would make extra time spent by the field crew worthwhile.

return to top

Links to related research


return to top


[To ASC]