Why Down Wood Percent Cover?

For DecAID, we chose to describe amounts of down wood used by wildlife as percent cover because it is a good metric to describe the abundance of down wood as it relates to wildlife use (Carey 1995, Carey and Johnson 1995). For many wildlife species down wood provides cover. In our literature review, we found that percent cover was the most common measure used in studies showing wildlife selection of areas with higher amounts of down wood. Volume or biomass may be more relevant measures of ecosystem processes (e.g., nutrient cycling, site productivity, carbon sequestration, fuel loading), but we are opting to emphasize wildlife habitat.

Percent cover is one of the most precise and efficient means of recording down wood (Bate et al. 2002).By laying out a series of randomly placed line transects and measuring the length of the transect that intersects down wood, one can quickly and accurately determine the percent cover. The down wood falling along the transect can be categorized by diameter, species or decay class, further describing the characteristics of the down wood in the area of interest. In contrast, estimates of volume, biomass, pieces per acre, or total length require more data collection.

Although volume of down wood tends to be a measurement of down wood amount that also closely relates to wildlife use it is more complex to measure. Volume requires determining the length, the diameter of both ends of a log or use of a taper equation. Formulas are then used to calculate the volume using these variables. Taper equations will vary by species and decay class of down wood.

Number of down wood pieces per acre may offer the least amount of information when it comes to describing the amount of down wood in an area. Without knowing the length or diameters of the pieces, it has little value in describing wildlife habitat. As an example, 20 pieces of 10 foot logs 4 inches in diameter will have a much lower volume and percent cover than 20 pieces of 80 foot logs greater than 20 inches in diameter, thus resulting in different habitat conditions.

Linear feet of down wood is an often used measurement that also does not fully describe the amount of down wood in an area without knowing the diameters of the down wood being left. As an example, 500 linear feet of 4 inch diameter logs will have a much lower volume and percent cover than 500 linear feet of 20 inch diameter logs, thus resulting in different habitat conditions.

Whereas tons of logs per acre is a common measure used to describe fuel loadings, it is not as closely related to wildlife use and several assumptions and variables must be factored in to determining tons per acre. Weight of logs is based on estimated dry weight and will vary with species and decay class. An “average” species and decay class composition is usually used in tons per acre calculations based on vegetation type.

In order to compare data among studies, down wood data were converted into a common measure of percent cover. The forest inventory down wood data used in DecAID were used to calculate amount of down wood on each plot in terms of percent cover, volume, logs/ha, and linear meters. Regressions were then run on the data to calculate the relationships among 4 measures (see Table 1a-c). Correlations between percent cover and other measures were strongest with a small minimum diameter. Correlations between percent cover and logs/ha tended to be the weakest relationships. Based on the analysis, when data were reported in the literature using one of these other measures, we converted it to percent cover using the regression equations if the r² value was > 0.75.

See the “Links to Useful Information” section of the Guide to the Interpretation and Use of the DecAID Advisor for tools to help with conversions to percent cover.

Several examples of down wood of different sizes and what they represent in terms of percent cover are given in Table 2. The conversions from log size to percent cover use the formula: % cover = (((((LED+SED)/2)/12)*L)/43560)*100; where LED=large end diameter (in), SED=small end diameter (in), and L=length (ft). Small end diameter was calculated based on the "rule of thumb" of 1 inch taper per 8 feet of length (Ken Cormier, pers. comm.), and the formula: SED = LED-(L/8).

If trees are to be felled to provide down wood, or left standing to provide down wood at a later time, percent cover contribution of each tree can be estimated using taper equations. Clicking on the wildlife habitat type and subregion below will return a table with rough estimates of percent of an acre covered by different species of trees of given dbh when converted to down wood. The Flewelling Profile Model (Volume Estimator Library, Forest Vegetation Simulator) was used to determine height of a tree to a 5 inch top (12.7 cm). Total height of trees is used in the equations; this variable was estimated by averaging tree height by species and dbh class from inventory data specific to each Wildlife Habitat Type and subregion. These data were not updated with the latest inventory data for version 3.0 because the relationship between tree dbh and height should not have changed between the 2 old data and new data. The midpoint dbh for each class was input to the model as the dbh. A different version of the Flewelling Profile Model was used for each tree species.

Westside Lowland Conifer-Hardwood Forest - WA Coast

Westside Lowland Conifer-Hardwood Forest - OR Coast

Westside Lowland Conifer-Hardwood Forest - WA W Cascades

Westside Lowland Conifer-Hardwood Forest - OR W Cascades

Southwest Oregon Mixed Conifer-Hardwood Forest

Montane Mixed Conifer Forest

Eastside Mixed Conifer Forest - N Cascades/Rocky Mtns.

Eastside Mixed Conifer Forest - E Cascades/Blue Mtns.

Ponderosa Pine/Douglas-fir Forest