Influences of high severity fire and postfire logging on avian and small mammal communities of the Siskiyou Mountains, Oregon, USA

Joseph B. Fontaine

High severity fire is a historical and integral disturbance process in coniferous forest types. Compounded disturbances such as multiple fires or post-disturbance management activities are increasingly common, but ecological responses are not well understood and may represent novel types of disturbances. I studied bird and small mammal communities in the mixed severity fire regime of the Siskiyou Mountains of southwestern Oregon, USA, at various points in time after one or two high severity fires in and around the historic 200,000-ha Biscuit Fire. Post-disturbance time intervals included 2-4 years after a single fire, 17-18 years after a single fire, 2-3 years after a repeat fire (15 year interval between fires), and >100 years since stand-replacement fire (mature/old-growth forest). Additionally, I examined the response of these communities to postfire salvage logging of the Biscuit Fire. Avian species richness did not differ significantly among habitats. Among the recently burned habitats, bird density was highest 17-18-years after fire and lowest 2 years after a single fire. Sites 17-18 years postfire were dominated by broad-leaved shrubs. Ordination of community data revealed two distinct gradients in avian species composition, one relating to tree structure (live, dead-sound, dead-decayed) and another relating to shrub volume and height. Bird density was positively related to shrub height and volume; increases in broad-leaved plants following fire were associated with significant increases in bird density. Immediately after a single high severity fire event, small mammal communities transitioned from low abundance and high species richness to high abundance and low species richness dominated by deer mice. Partial recovery to a pre-burn state was evident 17 years after fire with wood rats being present but vole species still absent relative to unburned mature forest. Repeat fire was associated with heightened abundance of deer mice and herbaceous cover. Postfire salvage logging created a significant pulse of woody debris but no significant changes in densities or biomass of small mammals were observed. Fire effects on small mammal communities were much larger than those of postfire salvage logging in the short term. Longer term studies of changes in small mammal communities following salvage logging are needed over decades and greater time scales to fully evaluate the impacts of the management activity. To examine bird response to postfire salvage logging, we used point counts to measure changes in densities and occurrence for 17 common bird species. Response was measured at two spatial scales (20 ha and 2 ha) relative to two measures of salvage logging: proportion of surrounding area logged and logging intensity (basal area removed). The 20-ha scale comprised the logging unit as well as unit edges and surrounding unlogged areas, while the 2-ha scale comprised only the logging unit and not surrounding edges. At the 20-ha scale, we found a positive response in the density of shrub-associated species (house wren, lazuli bunting, black-headed grosbeak [scientific names given in Appendix A]) and edge-associated species (olive-sided flycatcher, yellow-rumped warbler) and little evidence of negative responses, save for a reduction in density of Hammond’s flycatcher. At the 2-ha scale, shrub-associates again responded positively but not edge-associates. Brown creeper responded negatively at the 2-ha scale and five species had suggestive negative trends but they were not significant suggesting that, except for shrub nesting species, bird use of salvage units is associated with edges and not interior portions of salvage units. The lack of a strong negative response to salvage logging of the Biscuit Fire suggests that the small logging unit sizes relative to the burn area, as well as extensive snag retention in riparian buffers, tended to retain many bird species in the burn landscape.

Influences of high severity fire and postfire logging on avian and small mammal communities of the Siskiyou Mountains, Oregon, USA

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