Geoscience Reference
In-Depth Information
•
Shadow: Designation used when clusters obviously contained multiple vegeta-
tion classes or landcover features which because of shadowing could not be
distinguished. Estimated forage biomass
=
0 kg/ha.
2.3.1 Habitat Carrying Capacity Estimates
Estimated forage biomass was used to estimate the nutritional carrying capacity
(NCC) value in a given successional stage at the end of the growing season. Since
much higher elevation areas are snow-covered in winter, NCC estimates focused
only on areas below approximately 823 m and assumed a season of use by BTD of
165 days (Scharpf
1985
). To compute a supportable density of deer, NCC estimates
were multiplied by the total area for each class, and then divided by the estimated
average daily needs of deer. Estimates vary widely as to daily intake rates so we
selected a middle range estimate of 1.8 kg/day (Wickstrom et al.
1984
) for 165 days
to determine the total number of deer/km
2
that the range could support for the winter
time period.
2.3.2 GIS Model Development
The resultant NCC maps were used as inputs for an ArcGIS 9.3.1 (ESRI Redlands
CA) based model that incorporated the potential negative impacts of roads
(NCC_Rd), effects of forage-cover ratios (NCC_FC), and a combination of both
on the NCC of BTD habitat. Models estimating specific road impacts on habitat use
in ungulates have largely used the densities of open roads to reduce estimates of
habitat quality or use (Witmer et al.
1985
; Wisdom et al.
1986
). In reality much of
the effect of roads actually would depend upon the spatial configuration and degree
of use (Rowland et al.
2000
). In the more open forests of the west, decreased habitat
use for elk has been reported as far as 1.8 km from roads (Rowland et al.
2000
) and
200 m for mule deer (Rost and Baily
1979
). In the denser forests of the western
Cascades, McCorquodale (
2003
) reported avoidance of roads by elk in winter of at
least 147 m, whereas Witmer and de Calesta reported a 50% decline in habitat use
by Roosevelt elk (Cervus elaphus rooseveltii) within 60 m of roads but not beyond
this distance.
Given that BTD do not range as far as elk, and that roads in this study area were
almost entirely private secondary (gravel) roads, we applied an effect distance buffer
of 100 m around roads and reduced habitat NCC by 50% to simulate reductions in
habitat use. This allowed us to incorporate both a density effect while also including
the effects of the spatial pattern of roads.
In evaluating cover effects, management recommendations call forage-to-cover
ratios of 40:60 optimal in deer and elk habitats of western Washington (Rodrick
and Milner
1991
). These ratios may overstate the importance of cover in areas
of limited winter severity, where optimal forage-to-cover ratios of 60:40 are more
likely (Brown
1961
; Thomas
1979
). For our modeling we assumed no reduction
in usability of forage where ratios fell between these values. Areas having either no
