Steps in Estimating Dead Wood Mass

Estimating the mass of decomposing wood involves several steps. Ideally one would weigh the mass of woody debris in an area, but realistically this would be time consuming and physically difficult. The first step is to measure the density of the wood to be inventoried by sampling wood and bark in a range of decay states. The second step is to determine the volume of the woody detritus in the inventory area by decay classes. The third step is to convert the volume to mass using the decay class-specific density values. Finally, total mass can be converted to specific elemental stores by knowing the concentration of elements such as carbon and nitrogen. Unfortunately, volume inventories are often conducted without knowledge of how density or nutrient contents change with decay class. While some substitution of these conversion factors is inevitable, it has been common to use western U.S. conifer values for many temperate biomes and in some cases tropical ones. This introduces substantial uncertainty in woody detritus mass and other related estimates (e.g., carbon stocks, fuel loadings, smoke emissions).

Assessing the State of Decay of Woody Detritus

Decay classes are a subjective way to break the decomposition process, which is a continuum, into recognizable stages. While continuous methods using techniques such as ordination exist (Harmon et al. 1987), they are very awkward to implement in the field. Classes can be defined by the presence of criteria that tend to be associated with certain decay classes. However, there can be a great deal of variation within and between pieces of decomposing wood, and this may cause misclassification of some individuals. By averaging over a large number of pieces these errors tend to cancel out.

The number of decay classes used varies from study to study. For CWD, as many as 13 and as few as two classes have been used in the past (Harmon et al. 1986). One of the more common systems developed in the Pacific Northwest uses five decay classes (Sollins 1982). For FWD, a single average number is often used, although a separation into undecayed or fresh versus decayed has also been employed especially after recent disturbance (Brown 1974). The density of decayed FWD tends to be highly variable and it is highly dependent on whether pieces are added in a pulse or continuously. When added as a pulse, many FWD pieces have similar densities that decrease through time. When continuously added the average density of FWD is likely to be fairly consistent, however, there is a great range in densities depending on when a particular piece was added.

It has been demonstrated that wood density generally declines as decay advances with some exceptions. For CWD, some species have very decay resistant heartwood (e.g., black locust, Robina pseudoacacia). If the outer decayed sapwood and bark layers disappears via respiration or fragmentation and the residual heartwood has little decay, then more advanced decay classes can theoretically have an increase in density. The same can be true for FWD such as branches, which often lose decomposed outer wood as decomposition proceeds. This often results in undecayed, resin-impregnated core of wood being left; as the proportion of this resin-impregnated wood increases the density can increase.

Density Determination

Density is expressed as a dry mass divided by green volume in most cases, although density can be determined in alternative ways. For example, recently resistance drill systems have been used, although these are usually calibrated to traditional methods. For FWD it is typical to collect a random sample within an area or along transects. For CWD a number of logs, snags, or stumps of each decay class are located and then subsampled with a chainsaw to remove cross-sections along the stem. Alternatively a coring device is used to remove samples, although this can only be used for relatively sound wood. Key characteristics such as presence of leaves, twigs, branches, bark, cross-sectional shape, wood hardness, and strength are typically recorded. Volume is determined either by displacement in water or a particulate solid (e.g., millet seed), taking a known volume using a core, or by measuring external dimensions. While determining volume using water displacement is probably the most accurate method for solid samples, it is relatively slow and works best for small volumes. However, it is very difficult to use this method for highly decomposed wood and bark, which means either decay classes with this material are not included or only the more solid pieces are examined leading to an uncertainty or outright bias for more advanced stages of decay. Coring suffers from the same problem regarding the extent of decay present. While volumes determined by external dimensions are less precise, these measurements can be taken quite rapidly in the field on very large volumes. This eliminates bias and leads to a better averaging of density within a piece.

Mass Determination

Mass of samples is typically determined by drying in an oven, often at temperatures ranging between 55 and 75 C until mass remains constant. This can take weeks for even small cross-sections. To speed up drying times, smaller subsamples are often used. This entails weighing the entire sample and then subsampling for moisture determination. The ratio of oven dried mass to the fresh mass of the subsample is used to convert the fresh mass to dry mass of the entire sample.

As noted above, for FWD decay classes are not usually noted, although green versus decayed material generally are often recorded separately. Subsamples are sometimes taken to determine density for a particular ecosystem, but this is rarely done and the default values presented in Brown (1974) are often used. When sampled, volume is determined by measuring dimensions of pieces or by volume displacement. Mass is determined via oven drying using similar approaches to that of CWD.