Avian community responses to burn severity at locations with historically different fire regimes

Background/Question/Methods

Wildfire is an important driver of forest bird communities in western North America. An ecological understanding of species and community responses to fire is needed to effectively manage fire and post-fire forested habitats. Having experienced different fire regimes over their evolutionary histories, birds may respond variably in different regions with implications for forest management. Specifically, communities in southwestern forests that historically experienced high-frequency, low-severity fire regimes may respond more negatively to high-severity fire than in more northern forests that evolved under mixed-severity regimes. We analyzed and compared avian relationships with burn severity following wildfire in two forests, one in Idaho representing forests shaped by mixed-severity fire regimes, and another in Arizona representing a higher frequency, low-severity regime. We conducted point-count surveys in both forests for 3 years immediately following wildfire (Idaho n = 303 point × year surveys [2008–2010]; Arizona n = 447 [1997–1999]). We quantified burn severity using the remotely sensed delta normalized burn ratio metric (ΔNBR). We analyzed avian occupancy relationships with burn severity using community occupancy models fitted within a Bayesian hierarchical framework.

Results/Conclusions

Occupancy patterns were largely consistent with species’ life histories. Cavity nesters and aerial insectivores (Mountain Bluebird [Sialia currucoides; detected on n = 37 and 7 point × year occasions in Idaho and Arizona, respectively], House Wren [Troglodytes aedon; n = 15 and 6], Olive-sided Flycatcher [Contopus cooperi; n = 15 and 37]) occupied more severely burned sites, consistent with expected increases in nesting substrate and foraging opportunities for these species. In contrast, canopy-nesting foliage gleaners and pine-seed consumers (Clark’s Nutcracker [Nucifraga Columbiana; n = 50 and 17], Townsend’s Warbler [Setophaga townsendi; n = 133 in Idaho only], Pygmy Nuthatch [Sitta pygmaea; n = 116 in Arizona only]) occupied low-severity sites. In Idaho, more species responded positively (4 species with relationships whose 95% Bayesian credible intervals did not overlap zero) than negatively (0 species) to fire, in contrast with Arizona (6 positive versus 12 negative relationships). Consistent with expected community responses under different fire regimes, species richness tended to increase in Idaho but decrease in Arizona with increasing burn severity. Our results suggest suppression of severe fire would benefit more southwestern bird species. Nevertheless, fire-associates occurred in both study areas, suggesting some mixed-severity fires are needed to support all avian species.