Improving Biomass and Carbon Estimates for Coarse and Fine Woody Debris

 

Woody Detritus Density and Density Reduction Factors for Tree Species in the United States: A synthesis

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 The goal of this study was to synthesize both published and unpublished data on woody detritus density so as to improve estimates of coarse woody detritus (CWD) and fine woody detritus (FWD) biomass across the diverse forests of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

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.