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Abstract

Effects of Wildfire on Breeding Bird Communities in Coniferous Forests of Northeastern Oregon

by R. Sallabanks (Sustainable Ecosystems Institute)

EXECUTIVE SUMMARY

Breeding bird communities were studied in 1995 in mixed conifer forests of northeastern Oregon one year after the Twin Lakes Fire burned approximately 22,000 acres on the Wallowa-Whitman National Forest. Twenty-five study plots (homogeneously burned forest patches, at least 40 acres in size) were established in one of four experimental treatments:

1. unburned forest (five replicates)
2. lightly burned forest (five replicates)
3. moderately burned forest (five replicates)
4. heavily burned forest (ten replicates)

For each plot, relative avian abundance was measured using standardized point count censusing techniques, and vegetation was sampled. Each of four point count stations per plot was visited three times during the breeding season.

Changes in the vegetation structure of the burned forest were profound. As the degree of burn increased, snag density increased, live tree density decreased, crown closure decreased, understory structure decreased, live ground cover decreased, and percent bare ground increased. This alteration of habitat was correlated with several changes in avian community composition.

In general, the Twin Lakes Fire was found to cause small declines in overall avian abundance and species richness with these measures decreasing as the degree of burn increased. Two avian guilds (foliage feeders and overstory nesters) and five species (Golden-crowned Kinglet, Mountain Chickadee, Swainson's Thrush, Townsend's Warbler, and Varied Thrush) appeared to be negatively affected by the fire. However, one avian guild (aerial feeders) and two species (American Robin and Mountain Bluebird) were found to be positively correlated with degree of burn. The Pine Siskin was the most abundant species detected in the study area. Significant bird-habitat relationships (habitat functions) were derived for 17 species of bird to identify the specific habitat variables that were correlated with a particular bird species' abundance. These functions helped explain why certain bird species responded as they did to the effects of the fire and provide forest managers with information on which habitat variables are important for which bird species. Snags, for example, accounted for 56% of the variation in the abundance of Mountain Bluebirds and therefore help to explain why this species was so common in heavily burned plots where snag densities were highest. Cavities and cavity-nesters did not differ among treatments, but they are expected to show stronger responses in subsequent years.

These results are useful to forest managers because they provide information on which bird species are likely to be present given different burn conditions. Low intensity ground fires where tree mortality is minimal and understory vegetation remains intact may cause few changes in the composition of avian communities. As fuel loads accumulate, fires increase in intensity, and forests burn more heavily, however, certain bird species will disappear. The first to disappear will be species that are dependent upon dense foliage for gleaning insects and overstory cover for nesting (e.g., Golden-crowned Kinglet, Mountain Chickadee, Swainson's Thrush, Townsend's Warbler, and Varied Thrush). Conversely, if managers wish to encourage aerial feeding birds (e.g., flycatchers, swallows, and the Mountain Bluebird), or ground-feeding species that prefer little or no understory (such as the American Robin or Cassin's Finch), then fires that burn a forest more heavily are required. Active management for cavity nesters might include prescribed fire to create more snags, but the response of these birds species may be slow and may not reach the desired maximum until 15-20 years post-fire. Three studies that have addressed the implications of salvage cutting of burned forest on avian communities are discussed.