Introduction and Objectives

Introduction

Importance of Woody Detritus

Woody detritus or dead wood is an important part of forest ecosystems. Although woody detritus is associated with many ecological benefits (plant and animal habitat, carbon, nutrient and water storage, as well as soil formation) the magnitude of these benefits all vary with the species of log, the environment (i.e., climate, soil characteristics, topography), and the amount of dead wood on a site (Franklin et al. 1987, Harmon et al. 1986, Triska and Cromack 1980). The same is true for possible negative aspects of this material such as a source of fuel influencing fire behavior and smoke emissions. Therefore, this material is increasingly being included in ecological studies and environmental assessments with the number of publications on this subject rapidly growing.

Woody detritus takes many forms. It may range from sound to highly decayed materials. Woody detritus is present as roots, stumps, branches (including attached dead branches), standing dead (i.e., snags), downed material, and as buried wood in litter, duff, and mineral soil. Very few inventories measure all these forms and size classes--- snags and downed material being the most commonly measured. For example, the Forest Inventory and Analysis (FIA) program of the USDA Forest Service conducts a national inventory of woody material including standing dead, FWD (fine woody detritus), and CWD (coarse woody detritus) (for more details see Woodall and Williams 2005). FWD with the exception of roots is typically less than 7.6 to 10 cm, the former size limit being based on lag-times of fuels (i.e., the time to equilibrate with the atmosphere in terms of moisture content; see Burgan 1988). For woody roots the size break is usually 2 mm which is based on conventions on the maximum size of live fine roots. CWD exceeds these diameters but also typically must exceed a length of 1 m. It can either be standing (e.g., snags) or downed (e.g., logs).

Measurements of woody detritus abundance include the number of pieces per area, volume, mass, and cover (typically either basal area for snags or projected cover for logs). Many ecosystem functions, such as a fuel for fire, are dependent on mass. There is a basic problem, however, because the mass of woody detritus is not easy to directly measure unless the material is very small. Volume is easier to measure and the majority of methods, be they transect- or plot-based ones, measure piece dimensions to estimate volume (Brown 1974, Harmon and Sexton 1996). Mass can be derived from volume if the density (i.e., mass/volume) of this material is known. A similar problem exists with live trees as these are also rarely weighed directly. Rather, equations to predict volume from more easily measured dimensions, such as diameter at breast height and/or height, are established. Volume can then be converted to mass using the density of wood and bark. Unfortunately, estimation of woody detritus amounts is not as straightforward as for live trees. Woody detritus is often irregular in shape relative to live trees and therefore volume is estimated directly for each piece by techniques such as line intercepts or dimensional measures in plots. Moreover, rather than a single density value per species, the density of woody detritus pieces also depends on the extent of decomposition. This change can be dealt with by recognizing various decay classes of material and determining the density of each decay class. When decay class-specific density and elemental concentrations are coupled with the volume in each decay class, one can estimate the mass, carbon, and nutrient stores of woody detritus.

The downed woody material (DWM) sampling design of the FIA program uses line-intersect transects to estimate the volume of downed CWD and FWD, but not their biomass (Woodall and Williams 2005). 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) and 2) a decay reduction factor that accounts for the decline in density as this form of detritus decomposes. Unfortunately, there are very few published values of the decay class-specific density or decay reduction factors; these have typically been only available for a few western tree species (Sollins et al. 1987). The little work that has been done has shown that species have different patterns of density decline (Yatskov et al. 1992). Thus, uncertainty in biomass estimates can be introduced if the incorrect pattern is assumed for a species that has not been studied. FIA’s own simulation studies have shown that a 15 percent variation in biomass conversion constants could affect a plot’s total DWM biomass estimate by 5 percent (Woodall and Lutes 2004). However, since FIA uses decay reduction factors based on a few western tree species for the entire nation, conversion constants may vary in excess of 50 percent causing substantial error in biomass estimates.

Given the fact woody detritus inventories are rapidly becoming a common component of forest resource inventories around the world, there is great need to refine the constants used in biomass estimation procedures for both CWD and FWD. While the current datasets on changes in wood density with decomposition are far from ideal, a substantial amount of published and unpublished data exists. Therefore, the goal of this study was to synthesize both published and unpublished data on woody detritus density and carbon content to improve estimates of CWD and FWD biomass and carbon across the diverse forests of the United States.

1) estimate CWD and FWD density across decay classes for the tree species currently considered by the FIA inventory


		Improving Biomass and Carbon Estimates for Coarse and Fine Woody Debris

Link to Appendices, Tables and Figures