The 2006 Warm Fire: effects on habitat and prey species of the northern goshawk

Abstract

Reductions in the frequency of fire in Southwestern ponderosa pine and mixed-conifer forests since initiation of forest management early in 20th century changed the composition and structure of the forest habitats of the northern goshawks (Accipiter gentilis), a food-limited species of conservation concern, and the birds and mammals it feeds on. A conservation strategy for the species in the goshawk's food web in these forest types recommends restoring the mix of predator and prey habitats that historically characterized these forests--characteristics that were sustained by frequent low-severity surface fire. Thus, the effects of fire severities (high- and low-severity) and lack of fire on today's habitats may influence the abundances of bird and mammal goshawk prey. . The 2006 Warm Fire burned 235 km² of ponderosa pine and mixed-conifer forest on the North Kaibab Ranger District in northern Arizona in late June and early July 2006. Forest habitat metrics such as live tree and snag densities, cone production, canopy and ground cover estimates were collected from 2007-2010 on 60 0.5km transects to compare the effects of high- and low-severity fire and no fire on a suite of 13 important prey species of the goshawk. I describe habitat changes resulting from the different fire severities in ponderosa pine and mixed conifer forests. I estimated abundances for 13 bird and mammal prey species in forests burned by different fire severities and tested predictive models designed to gain an understanding which habitat characteristics, affected by fire, best predicted individual bird and mammal abundances. Red squirrels and golden-mantled ground squirrels showed the most sensitivity to fire, while chipmunks were evenly distributed across fire severities. Hairy woodpeckers and northern flickers, in contrast, benefitted from high-severity fire, due to increased snags, cavity nesting opportunities, and foraging. American robins and Steller's jays were also evenly distributed across fire severities. High-severity fire had a significant impact on forest ecosystems. Changes in forest structure were found to be detrimental to some species while creating short- and possibly intermediate-term benefits for others. Lowering the risk of high-severity fire by restoring composition and structure should also protect the habitats of goshawk and the prey species most sensitive to fire. The effects of low-severity fire were mixed, suggesting that it may be possible to return fire to ecosystem without a significant impact to many birds and mammals in the short term. Frequent low-severity fire could also help to maintain sub-canopy and understory grass / shrub openings by removing excess growth and ground debris.