Background: Low-severity prescribed fire is a tool used for reducing fuel loads on public lands, particularly in dry conifer forests of the western United States characterized by historically mixed- and low-severity fire regimes. Understanding the ecological effects of prescribed fire treatments is important for predicting the impacts of these management actions on wildlife communities. But few studies have estimated small landbird responses to forest treatments at spatial scales relevant to their ecology or have examined potential differences in treatment effects applied within historically mixed- vs. low-severity fire regimes. Therefore, we evaluated prescribed fire treatment effects and relationships with burn severity for avian communities in dry conifer forests dominated by ponderosa pine (Pinus ponderosa) located on seven National Forests in the interior western United States. We surveyed birds for 1–4 years and 1–3 years before and after prescribed fire treatments at mixed- and low-severity fire regime locations, respectively, following a before-after, control-impact study design – 8 paired control-treatment units in mixed-severity locations (16 total study units with 320 survey points) and 4 paired control-treatment units in low-severity locations (8 total study units with 278 survey points). Using a Bayesian hierarchical multi-species occupancy model, we analyzed occupancy patterns for 95 species.
Results: We found 33 species with statistically supported treatment effects and/or burn severity relationships primarily in mixed-severity locations. The data supported positive treatment effects at mixed severity locations for 9 species (American Robin [Turdus migratorius], Western Bluebird [Sialia mexicana], Hairy Woodpecker [Dryobates villosus], Black-backed Woodpecker [Picoides arcticus], American Three-toed Woodpecker [Picoides dorsalis], House Wren [Troglodytes aedon], Dusky Flycatcher [Empidonax oberholseri], Western Wood-peewee [Contopus sordidulus], Gray Flycatcher[Empidonax wrightii]), whose occupancy shifted towards more severely burned points after treatment, and a negative effect for one species (Ruby-crowned Kinglet [Corthylio calendula]), whose occupancy shifted away from burned points. At low severity locations, only two species exhibited treatment effects, both negative (Red-faced Warbler [Cardellina rubrifrons], and Lark Sparrow [Chondestes grammacus]). We also found supported occupancy relationships with burn severity post-treatment (i.e., regardless of species distribution before treatment) for 29 species, most of which were consistent with their life histories (e.g., patterns of positive relationships for cavity-nesting, bark insectivores and negative relationships for open-nesting, foliage insectivores). Stronger responses to prescribed fire treatments at mixed-severity locations were unexpected because prescribed fire applications are more similar to historical wildfires characterizing low-severity fire regimes.
Conclusions: Bird populations in historically low-severity locations may be relatively unresponsive to prescribed fire because fire there is typically more frequent, expected, and regular. By comparison, fire events are relatively rare historically in mixed severity locations, potentially eliciting more responses to an infrequent opportunity, even by species that are strongly associated with recently burned forests by wildfire. Our results suggest that fire management activities intended to reduce fuels and lower the risk of high-severity wildfire can also be effective in creating habitat for some fire specialists at least in the short term.