The following describes the general qualitative changes observed as logs decompose. While not all species move through this exact progression, many do; major exceptions are noted. Decay Class 1 logs are the least decomposed, with most having leaves still attached and all having intact bark, fine twigs and branches. Logs originating from cutting may not have branches and twigs, but the cuts appear fresh and have not yet turned gray due to sun bleaching. Decay Class 2 logs are ones starting to decompose, leaves largely are absent, and many of the fine twigs have fallen off the larger branches. Bark is typically loose, but only starting to fall off the log. An exception would be for the genera Betula and Prunus, which tend to retain bark throughout decomposition. For all species there is evidence the surface layers of the wood are decomposing, but the inner, central region of the wood is undecayed unless previously infected with heart rots. For logs originating from cutting, the ends are gray from sun bleaching. Decay Class 3 logs have only a few large branches remaining, often in the form of stubs, the bark is falling off in large patches, and evidence of sloughing of sapwood is also evident. The outer wood is easily crushed by hand, although the inner portions can appear completely sound. Despite the large amount of decay, Decay Class 3 logs are able to support their own weight along most of their length. For certain genera with decay resistant heartwoods, such as Calocedrus, Quercus and Thuja, decayed sapwood may fall off to the extent that relatively sound heartwood may form the outer surface. Decay Class 4 logs can not support their own weight and most of their length conforms to the contours of the underlying ground. Although circular cross-sections can remain, much of the log forms an elliptical cross-section. Branches, if present, are short stubs, which move when pulled. This indicates decay has spread to the innermost portions of the log and has weakened the wood considerably. Bark, if present, is in small loose patches on the log and found in piles alongside or under the log. In the case of the genera Betula and Prunus the bark is loosely surrounding the inner, highly decomposed wood. Decay Class 5 logs are the most decomposed, of elliptical shape (the long axis is often many times that of the short axis) and are beginning to be incorporated into the forest floor. The wood is extremely decayed, usually in the form of cubical brown rot that can be easily crushed by hand. Bark is not evident from the surface (except for the genera Betula and Prunus), and in most cases underlies the extremely decomposed wood.

Density of Decomposing CWD

A total of 88 species were found to have data on CWD decay class densities that had been published and/or collected from North America from the boreal to the tropical zones. The majority of species (53) were from either boreal or temperate ecosystems. A total of 49 genera had some data about CWD decay class density, the majority of which were represented by one species. Of the species compiled, 60 were hardwood and 28 were softwood species.

Examining all the species that have been sampled reveals wood density of decay classes declines as expected, but that it is highly variable (Figure 1). Much of this variation is associated with variation in the initial density, which ranges from 0.25 to 0.95 g cm^-3. Expressed as a percent, minimum and maximum initial densities are 52% lower and 95% higher than the mean value, respectively. The relative variation appears to increase until decay class 3 and then declines for decay classes 4 and 5. This pattern may be caused by decomposition and fact that below a certain density wood lacks the strength to hold together.

Comparison of hardwood and softwood mean density indicates that softwoods do have a slightly lower density than hardwoods for decay classes 1 to 3 (Figure 2). However, by decay class 4 the mean densities of the two groups are quite similar. Hardwood density, at least for undecayed wood and decay classes 1 to 3 appear is more variable than softwood density. The variation, expressed as a range, within these groups is much higher than the variation between these two groups. This classification would therefore not seem to be a useful way to stratify unsampled species, although the uncertainty for softwood density is considerably lower than that for hardwoods.

CWD Density Reduction Patterns

When all species that have been sampled are considered, there is a clear decrease in relative density as CWD decay class advances (Figure 3). Although the mean relative density exhibits a steady decline, the maximum and minimum relative density can be as much as 60% higher or 40% lower than the mean, respectively. The greatest differences between the minimum and maximum appear in decay classes 2 and 3, but even for decay class 1 there is considerable variation. Despite the high level of variation observed, relative density is less variable than absolute density. This indicates that removing the effect of initial density can help reduce uncertainty when estimating density of unsampled species.

As with absolute density, dividing the species into hardwoods and softwoods does not appear to help reduce uncertainty in estimating relative density. Although the mean relative density of hardwoods is slightly lower than that of softwoods (Figure 4), these differences are very small compared to the range in values observed. For example, the relative density of decay class 3 hardwoods is 0.1 relative density units lower than that for softwoods. However, this is approximately one-third the difference between the minimum and the mean for hardwoods. The range for softwoods is lower than that for hardwoods, but as with absolute density it completely overlaps that of hardwoods. This indicates that separating species by hardwood versus softwood classes will not substantially decrease the uncertainty in prediction of mean relative density. However, separation into these two classes might reduce uncertainty below using the maximum and minimum of all species because the uncertainty range for softwoods is lower than that for hardwoods.

At least 18 temperate genera have been sampled, about half of which contain multiple species. Several patterns of relative density decline are evident in the mean for each genus, which suggests that perhaps stratifying by genus would reduce uncertainty (Figure 5). However, plotting the minimum and maximum relative density for well sampled genera indicates there is considerable variation (as much as 0.2 relative density units) within each genus (Figure 6). This variation is quite high relative to the typical standard error of the mean for a species, which generally ranges from 0.02 to 0.05 relative density units. This indicates that while genus may used to predict unsampled species, the uncertainty would be as much as 10-fold higher than for species that have been sampled.

Examination of the mean of each species indicates a number of repeated patterns to relative density declines (Figure 7). The simplest pattern, a steady decline in density (hereafter referred to as the S pattern), was observed for 15 species (Figure 7a). In this case, density appeared to decline by approximately an equal amount between decay classes 1 to 4 and then to a lesser degree from decay class 4 to 5. The most common pattern, a lag followed by steady decline (the LS pattern), occurred in 30 species (Figure 7b). In this pattern decay class 1 is similar in density to fresh wood and then there is a steady decline in density. A lag that lasted into decay 2 was evident in 5 species, and this was typically followed by a steady decline of density in decay classes 3 to 5 (the SLS pattern; Figure 7c). For 6 species, it appears that a steady decline is followed by an asymptote in decay classes 4 and 5 (the A pattern; Figure 7d). This pattern is the one followed by Douglas-fir, the most common species used to represent density reduction patterns. The most complex pattern involved an initial decline in density to decay class 1, followed by a mid-plateau, and finally a steady decline in density (the MP pattern; Figure 7e). This pattern was followed by 7 species.

Understanding the causes of these patterns might be helpful in predicting unsampled species. Unfortunately, we can only hypothesize about possible controls of these patterns.
It is possible that the lags we observed in the LS and SLS patterns are associated with the progression of exterior indicators of decay versus the loss of wood via decomposition. In species or environments in which the exterior indicators develop quickly relative to the rate of wood decomposition then the interior wood does not have “enough time” to lose density. Thus, a “lag” in density decline appears. Conversely, if exterior indicators take longer to develop then the wood can decompose to a greater degree and a steady density decline appears. This difference might not be caused by the species per se, but the environment. In cold and wet environments bole decomposition rates can be very slow due to waterlogging (Harmon et al 1986). The same limitation would not be evident for leaves, twigs, and branches; therefore their decomposition would proceed ahead of the bole’s decomposition. The cause of the asymptotic pattern (A) might be related to the presence of a relatively small resistant core of wood, although this is unlikely in the case of Douglas-fir. The most complex pattern, the MP, is possibly associated with a proportionally large amount of highly resistant wood. This would be typified by western redcedar. In this case sapwood decomposition is quite rapid leading to a decline to decay class 1. The remaining heartwood, is however, very resistant to decay and although certain indicators of decay class 3 develop (e.g., the sloughing of sapwood) the underlying wood remains relatively sound. Eventually this wood begins to decompose and weaken leading to the characteristics typical of decay class 4.

Although these density reduction patterns are useful for descriptive purposes, at this time they can not be used for predictive purposes. First, as discussed above, the mechanisms causing the differences are not known. Second, determining the degree to which these patterns are statistically different is problematical. In some cases, patterns only might differ for one decay class (e.g., S versus the LS patterns) and therefore for most decay classes they are statistically the same. In others cases the mean relative density of two patterns are quite different, yet the range for both patterns is highly overlapping. Third, while it is logical to hypothesize density reduction patterns would be similar within a genus. Unfortunately we saw little evidence to support that hypothesis in the genera that had been well sampled.

Fine Woody Debris Density

Relatively few species have had FWD density determined. Our literature search indicated approximately 25 species have been sampled, although some of these “species” represent mixtures named after a dominant species. The majority of studies appear to have been from northern regions.

The density of undecayed or green FWD decreased as diameter increased (Figure 8). For example, in the smallest size class (0-8 mm diameter) the mean density of undecayed FWD was 0.61 g cm^-3. In contrast, for the largest size class (25 to 76 mm diameter) the mean undecayed density was 0.50 g cm^-3. Regardless of diameter undecayed density was highly variable with values ranging between 0.1 and 0.95 g cm^-3. Given the few species in which undecayed FWD has been determined, the ratio of undecayed branch to bole density is a useful variable from which to predict the undecayed FWD density of species that have not yet been studied. These ratios also decline as diameter size class increases, reflecting the decrease in FWD density as the size class increases (Figure 9).

Density of decayed FWD also decreases with diameter size class (Figure 10) from 0.49 to 0.41 g cm^-3 from the smallest to largest size classes. Relative density of FWD as indicated by ratios of decayed to undecayed pieces increases as size class increases (Figure 11). This indicates that large diameter pieces are less decomposed than smaller ones. Specifically, the largest pieces have a mean relative density of 0.88, whereas the smallest ones have a mean relative density of 0.61. This might be caused by the increased surface area to volume ratio of the smaller pieces which might allow faster colonization by agents of decomposition.