Black-backed woodpecker occupancy in burned and beetle-killed forests: Disturbance agent matters

• Published in: Forest ecology and management 2020-01, Vol.455, p.117694, Article 117694
• Main Authors: Tingley, Morgan W., Stillman, Andrew N., Wilkerson, Robert L., Sawyer, Sarah C., Siegel, Rodney B.
• Format: Article
• Language: English
• Subjects: Bark beetle; California; Drought; Habitat; Occupancy; Picoides arcticus; Wildfire
• ISSN: 0378-1127; 1872-7042
• DOI: 10.1016/j.foreco.2019.117694

•Occupancy was compared across 75 burned and 113 beetle-killed forest stands.•Woodpeckers were much more likely to be in forests killed by fire than by beetles.•Results controlled for snag density, a relative measure of disturbance severity.•Fire and bark beetle outbreaks do not create equivalent habitat for this species. In the western United States, the black-backed woodpecker (Picoides arcticus) is a “snag specialist”, found predominantly in burned montane forests. While fire is a key disturbance agent in this system, recently, unprecedented large tracts of drought-stressed forest in the Sierra Nevada and Southern Cascades of California have succumbed to bark beetle outbreaks. Although this tree mortality could potentially be a boon for snag-dependent species, it is unclear whether the resulting snag forests provide sufficiently high-quality habitat for black-backed woodpeckers and other wildlife that are regionally associated with burned forests. We tested for differences in black-backed woodpecker occupancy between fire- and beetle-killed forests, and whether key environmental relationships driving woodpecker occupancy differed between stands affected by the two disturbance agents. Between 2016 and 2018, we surveyed for black-backed woodpeckers during 4448 surveys at 75 burned and 113 beetle-killed forest stands throughout the black-backed woodpecker’s range in California, detecting at least one black-backed woodpecker on 448 surveys (16.2%) in burned forests and 115 surveys (6.8%) in beetle-killed forests. Controlling for a suite of environmental variables that can affect habitat quality, the odds of black-backed woodpeckers occurring in burned forests were predicted to be 12.6 times higher than in beetle-killed forest. Occupancy declined with time-since-disturbance in fire-killed but not beetle-killed forests, but occupancy increased similarly with snag density resulting from either disturbance agent. Across our broad study region, black-backed woodpeckers were more likely to occur in burned forests at higher latitudes and elevations; these patterns were even stronger in beetle-killed forests, where we found woodpeckers only at the more northerly and higher elevation sites. Our results demonstrate that for this disturbed-habitat specialist, disturbance agent matters; black-backed woodpeckers do not use habitat created by bark beetle outbreaks as readily as habitat created by fire. Given the likely increased magnitude and extent of bark beetle outbre