The Wildlife Professional - Spring 2010 2009

evolved. In cooler, lowland areas, fuel treatments can focus on reducing the smaller surface and understory ladder fuels, while leaving high levels of overstory canopy cover as well as large logs for resting, prey habitat, and maintaining microclimate conditions. In contrast, on upslope and more southwestern aspects, forests can be treated to produce more open, fire-resistant conditions, with selective tree removal to create larger gaps between trees. A landscape treated in these ways would have a range of habitats for different species, mimicking the historic forest variability produced by low-intensity fire (see diagram below).

To test these ideas we analyzed the Dinkey Creek and Big Creek mixed-conifer watersheds in the Sierra Nevada, identified as preferred habitat for fishers and California spotted owls (Strix occidentalis occidentalis). We divided the landscape into nine topographic categories: three aspects (northeast, southwest, and neutral) crossed with three slope positions (riparian/ lower slope, mid-slope, and ridge top), and calculated the percentage of the total watershed in each category. We then compared the proportion of owl nests and fisher resting sites in each area. Both species had significantly higher-than-expected use of the more mesic, high canopy cover areas (in riparian/ lower slopes and northeast aspects) and lower-than-expected use of open ridge tops. Such information can guide fuels management in various topographic regions for the benefit of at-risk species.

Mature fir intermediate fir Small fir Mature sugar pine intermediate pine Mature Jeffrey pine Black oak Hardwood Small pine
Relative soil moisture
Dry
Wet
Variable forest structures that reflect historic patterns can benefit the wildlife that evolved
in those forests. Historically, in mixed-conifer forests (as shown in this schematic), tree
density and the percentage of fire-sensitive fir and cedar increased on northern-facing, flatter
slopes, while steeper southwest-facing slopes and ridge tops had the lowest stem density
and greatest percentage of fire-resistant pine. Forest managers can promote these variable
structures by using fuel and harvest treatments that differ depending on topographic features
such as slope position and aspect.

Strategic Treatments

Reducing surface and ladder fuels achieves the greatest reduction in fire severity. In the western U.S., thinning can remove fire-sensitive tree species such as firs and cedar, and leave more fire-resistant pines. When thinning overstory trees, the goal is to leave openings and tree groups rather than a regular spacing of the remaining or “leave” trees. This pattern, which is found in most forests with active fire regimes, creates habitat heterogeneity and still provides breaks in the forest canopy to reduce crown fire spread.

Local stand conditions will often determine what size tree groups and gaps can be created. High canopy-cover areas are usually defined by groups of larger trees. Gaps can be located in areas with thinner soils or lower productivity since these areas historically supported lower tree densities and fuel loads. In the forest matrix between tree groups and gaps, frequent-fire forests generally consisted of widely spaced, large trees, most of which were pines. The relative proportion of these conditions—low density, dispersed large trees, and large and small gaps and tree groups—and their composition vary depending on forest conditions and topographic position.

Thinning larger, overstory trees can have ecological benefits under certain conditions. In drier, upslope areas, for example, thinning larger fire-sensitive trees can reduce moisture stress in the leave trees, reducing large-scale mortality risk from bark beetles and increasing forest resilience to fire. Given the deficit of large trees in many managed forests, however, their removal should be balanced against the need for large trees and snags.

Preserving Key Habitat Forest managers must determine how to provide the right combination of variable forest conditions and high canopy cover, old-forest sites to maintain or increase threatened and endangered species populations across a forested landscape. A few key considerations:

Know a species’ needs. Conserving wildlife habitat requires providing specific stand structures associated with preferred use sites—for nests, dens, and resting—as well as managing the whole landscape to support foraging and movement. Some sensitive species like the spotted owl prefer old-forest conditions that, because of fire suppression, now have high surface and ladder fuel loads.