This document outlines a process to calculate the size of the analysis area needed to appropriately compare current conditions to the DecAID inventory distribution data in post-fire project analysis. The assumption of this methodology is that the size of the analysis area needs to be large enough to reflect the size and intensity of the disturbance you are assessing. Current conditions in your analysis area should represent the variety of habitat conditions in the area in which the inventory data were collected. Larger fires and insect outbreaks can skew the current conditions, even at the scale of a 10th field HUC, to the point that this assumption is no longer valid. At a watershed scale it may appear that there is an excess of dead wood because many disturbances are as large, or larger, than the watershed scale. But, it may still be a rare occurrence at the regional or sub-regional scale, the scale at which the vegetation data were collected.
When looking at the vegetation inventory data from DecAID, we can assume that snag densities above the 80% tolerance level are likely created by stand-replacing disturbances. While not all plots in DecAID with snag densities >80% tolerance level are a result of recent stand-replacing disturbances, it can be assumed that stand-replacing disturbance creates snag densities that are >80% tolerance level from unharvested plots. Thus, a "rule of thumb" is analysis areas that have stand-replacing disturbance events composing about 20% of the area are assumed to represent the variation in snag habitat and distribution from which the vegetation inventory data were collected. However, remember the minimum size for each WHT is 12,800 acres. An analysis area of this size allows a comparison between current conditions and the vegetation inventory distribution histograms.
This analysis area should be used when doing either a Quick Assessment or Distribution Analysis for salvage sales.
Determine the number of acres in the disturbance area (e.g., fire perimeter) that currently have high snag densities (above the 80% tolerance level) due to stand replacing disturbance (e.g. high-severity fire, bug kill). The simplest method to determine this number is to tally the number of acres of high-severity disturbance from the current disturbance you are assessing. This assessment needs to be completed for each Wildlife Habitat Type. It is reasonable to assume these areas provide snag densities > 80 % tolerance level from DecAID inventory data.
For post-fire salvage assessments, the most expedient way to determine the area of high-severity disturbance is to use remote sensing maps from the current fire.
The acres of stand-replacing disturbance from the current disturbance in the disturbance area are used to calculate the appropriate size of the analysis area using a simple ratio:
(Acres current stand-replacing disturbance) / (Total acres w/in boundary of analysis area) = (20% DecAID landscape w/ high snag density) / 100
OR
(Total acres in analysis area) = (Acres current stand-replacing disturbance) / 0.20
This calculation needs to be completed for each Wildlife Habitat Type that is being considered for treatment. The analysis area needs to be large enough to encompass, at a minimum, the calculated number of acres in each habitat type so that the DecAID histograms can be appropriately used to compare the contribution of snags within the project area to the larger landscape.
Following is a step-by-step example.
In this example, the project area contains habitat in the Eastside Mixed Conifer, East Cascades/Blue Mountains (EMC_ECB) and Ponderosa Pine/Douglas-fir (PPDF) Wildlife Habitat Types. The analysis area would have to encompass an area that would cover the greatest amount of acres required for each habitat type as shown in Table 1. In this case, the analysis area would need to contain at least 79,000 acres of the EMC_ECB habitat type, and 12,800 acres of the PPDF habitat type. In this case, 3,740 acres of PPDF is below the 12,800 acres needed to use vegetation data; use the 12,800 as the minimum.
Table 1. Determination of acres needed for Analysis Area large enough to appropriately compare DecAID inventory distribution data from unharvested plots to current snag levels.
| Habitat Type | Acres high fire severity in disturbance area | Acres required in habitat type within analysis |
|---|---|---|
| EMC_ECB | 15,800 | 15,800 /.20 = 79,000 |
| PPDF | 748 | 748 / .20 = 3,740 |
| total acres | 16,548 | 82,740 |
Start out by delineating a contiguous area which encompasses the amount of area identified in Step 2 for the habitat type that encompasses the largest amount of acreage (in this case, EMC_ECB). Make the area logical in terms of watershed or administrative boundaries. One approach is to start with the 10th-field HUC watershed(s) that the disturbance occurred in. Add watersheds or 12th-field sub-watersheds along the perimeter until the area requirement is met. Once this area is delineated, make sure it also encompasses the necessary acreage for the other habitat types in the project area. If the analysis area does not encompass enough acres of these remaining habitat types, continue to expand it, as above, until it does.