Burn severity and heterogeneity mediate avian response to wildfire in a hemiboreal forest

Recent studies have shown the importance of accounting for burn severity when assessing the effects of forest fires on avian communities. We add to this growing literature with one of the first studies to assess these effects in boreal and hemiboreal regions of North America. We conducted point counts in control and treatment areas for 2–3 years before (2009–2011) and 4–5 years after (2012–2016) the Pagami Creek Fire in northern Minnesota, USA. Our primary objectives were to (1) assess the effects of burn status, burn severity and burn heterogeneity on avian communities and patterns of abundance in individual bird species and (2) compare these results when counts were either restricted to 100 m count radii or corrected for potential detection heterogeneity. Species richness and total abundance of birds were best described by a burn severity by year interaction, with lower-severity burns having similar patterns to control areas and higher-severity burns showing reduced abundance and richness following the fire. Nevertheless, values associated with higher-severity burns returned to near-control levels after five years. Of 43 species analyzed, 27 showed detectable responses to the fire through shifts in abundance. Nearly equal proportions of these species had generally positive versus negative effects of the fire. Sixteen (59%) of these fire responses were mediated by burn severity, while four (15%) and seven (26%) were best predicted by burn heterogeneity or burn status, respectively. Of the species whose response to the fire was mediated by burn severity, the majority (81%) had decreased abundance as burn severity increased. However, all species responding to burn heterogeneity or burn status showed positive relationships to these metrics. Results were similar between analytical methods that adjusted for species detectability vs those that did not, with 78% of species having the same interpretation of fire effects. Burn severity and heterogeneity, in addition to the sole distinction of whether an area was burned, dictate the response of birds to forest fire in hemiboreal forests. Mixed-severity fires, which include a range of burn severities and associated vegetation change, will likely benefit the most bird species in this region. When possible, management should focus on retaining this spectrum of post-fire conditions, though of primary importance is the inclusion of wildfire in forest planning and conservation efforts.