DecAID Post-Fire Snag Analysis – Sunflower Fire
Disturbance is a common occurrence in forested landscapes. Disturbances such as wildfire, blowdown,
and bark beetle attacks can create small patches of standing and down dead trees. These pulses of high
amounts of dead wood are favored by a variety of snag and down wood associated wildlife species,
including black-backed woodpecker, American three-toed woodpecker, and Lewis’ woodpecker.
Inventory data in the DecAID Advisor suggests that historic snag densities may have been far more
variable than is prescribed by current Forest Plan standards with areas ranging from no snags to snag
densities in excess of 40 snags per acre. The wildlife data in DecAID suggest that pulses of high snag
densities are important for some species (e.g., black-backed woodpeckers), and that such snag pulses
should be maintained across the landscape at a level that reflects natural or reference conditions.
A snag analysis (based on species and inventory/reference information) is conducted post-fire to
determine whether or not snag pulses are within the natural or reference range for snag distributions and
whether they should be maintained to provide valuable wildlife habitat and contribute to the viability of
species dependent on these ephemeral/temporary habitats. DecAID provides a framework for conducting
this analysis. DecAID is not a model or a wildlife population simulator. Data provided in DecAID
allows the user to relate the abundance of deadwood habitat for both snags and down logs to the
frequency of occurrence of selected wildlife species that require deadwood habitat for some part of their
life cycle. This data is presented at 30%, 50%, and 80% tolerance levels. These levels are derived from
and combined for one or more published studies of the species in question. Tolerance intervals are
estimates of the percent of all individuals in the population that are within some specified range of values.
The 80% tolerance level indicates that 80% of the individuals in the population have a value for the
parameter of interest (e.g. snag density) between zero and the value for the 80% tolerance level. For
example, an 80% tolerance level for snag density (snags/acre) means that 80% of all individuals observed
of some species used snag densities less than or equal to some specific snag density level; the remaining
20% of all individuals would use snag densities higher than the 80% level.
For the Sunflower Analysis:
1. First step was determining the size of the analysis area. Determined that the Wall watershed was
the appropriate scale, as it met DecAID size requirements for each habitat type (Ponderosa
Pine/Douglas-Fir and Eastside Mixed Conifer/Eastern Cascades and Blue Mountains). GNN is
also only valid at the watershed or larger scale.
2. Next was determining the pre-fire condition of snags in the analysis area. My analysis used
gradient nearest neighbor (GNN) data produced by the Regional Office. The data used did not
account for road impacts to snag densities; as a result, actual snag density distributions would be
slightly lower due to the tendency for those areas adjacent to open roads to have lower snag
densities due to a combination of hazard and danger tree felling, firewood cutting, and other
factors. Raster data was masked based on the Forest Boundary and attribute data exported to
excel to allow more streamlined analysis of data. Data was sorted according to watershed, habitat
type, size class (open, small/medium, and large), and snag density group. Acreage was calculated
based on counts of raster data; data was then weighted for the 3 size classes for each habitat type
and snag size class (>10 and >20 inches) to determine the current (pre-fire) snag density
distribution in the analysis area.
3. The next step was determining the 80% Tolerance Level, as described in the process outlined in
DecAID. Inventory data (80% tolerance level) was weighted for all 3 size classes for each
DecAID habitat type in the analysis area (Ponderosa Pine/Douglas-Fir and Eastside Mixed
Conifer/Eastern Cascades and Blue Mountains). Fire Regime Condition Class data was used to
determine the proportion of each habitat type in each size class under the reference condition for
the weighting process. DecAID information (Figures EMC_ECB_O.inv-14, EMC_ECB_S.inv14, ECM_ECB_L.inv-14, PPDF_O.inv-14, PPDF_S.inv-14 and PPDF_L.inv-14) was examined
to determine the proportion of each habitat type expected to occur (based on inventory data) in
high density snag patches (fire mortality, bug kill, etc.). Generally, for a quick assessment, 20%
of the landscape would have been expected to exceed the 80% Tolerance Level for snags in the
PPDF and EMC habitat types under the reference condition; some habitat types and size classes
would be expected to have slightly less or slightly more than 20% of the landscape with snag
densities in excess of the 80% tolerance level.
4. Next step was updating the existing condition for fire effects and other actions, activities, or
events that would increase or decrease the proportion of the analysis area that has snag densities
in excess of the 80% tolerance level. For this analysis, these events included fire impacts, insect
and disease activity, hazard/danger tree felling associated with the fire area, and fuels treatment
activities. As the GNN data is based on a snapshot taking place approximately September 1,
2012, all of these activities occurring after this date were accounted for…see the analysis
document. So as not to double count acres, only those areas that were below the 80% tolerance
level that moved above the 80% tolerance level were considered a net increase…for example, any
acres that were already above the 80% tolerance level that were impacted by high or moderate
severity fire were not added in post-fire, as they were accounted for in the existing condition.
5. Final step is to determine what the net impact of the fire and other actions, activities, and events
that have created or reduced acres with snag densities in excess of the 80% tolerance level is and
determine what the new existing condition (acres and proportion) for the analysis area is. This
new existing condition was then compared to what would have been expected historically/under
the reference condition to determine whether a surplus of acres is present in one or more (or
none) of the represented DecAID habitat types. A surplus of acres with snag densities in excess
of the 80% tolerance level would indicate that salvage may be warranted, where appropriate
given management area and other Forest Plan constraints. Where there is a deficit, salvage has
the potential to affect the availability of habitat for and potentially the viability of wildlife species
dependent on this habitat type.