Burn severity and heterogeneity mediate avian response to wildfire in a hemiboreal forest
Introduction
A recent paradigm in fire ecology identifies burn severity and burn heterogeneity as dominant drivers mediating the effects of forest fires on birds and bird communities (Fontaine and Kennedy, 2012, Hutto and Patterson, 2016, Taillie et al., 2018, Tingley et al., 2016). Studies have generally found that bird species respond uniquely to burn severity levels as opposed to simply the presence of fire. Once severity is accounted for, the response of many bird species becomes clear and often a positive relationship with fire is detected (Smucker et al., 2005). This is especially true when post-fire time series are long and the time-since-fire is taken into account (Hutto and Patterson, 2016). Based on these results and those relating to other ecosystem processes, scientists are recommending the use of severe fires and associated mixed-severity impacts as a means of maintaining biodiversity (Bond et al., 2012, DellaSala et al., 2017, Hutto et al., 2015, Kelly et al., 2017).
Mixed-severity fires create a patchwork of vegetation, comprising areas that are unburned, lightly burned and areas where complete stand turnover has occurred (DellaSala and Hanson, 2015). This landscape heterogeneity provides habitat for bird species exhibiting a range of responses to fire (Hutto et al., 2015, Hutto and Patterson, 2016). Benefits extend not only to archetypal species like Black-backed Woodpecker (Picoides arcticus; Tremblay et al., 2016), but also species that utilize residual live trees and edges like Olive-sided Flycatcher (Contopus cooperi; Baker et al., 2016) and species showing delayed responses to post-fire succession such as shrub nesters (Stephens et al., 2015).
To date, studies assessing the effects of burn severity and heterogeneity on birds have primarily been conducted in western forests (Hutto and Patterson, 2016, Kotliar et al., 2007, Smucker et al., 2005, Stephens et al., 2015, Tingley et al., 2016) as well as Appalachian forests (Greenberg et al., 2013, Rose et al., 2016, Rush et al., 2012). Although boreal and hemiboreal forests have received much attention regarding effects of fire on birds (e.g., Haney et al., 2008, Hannon and Drapeau, 2005, Niemi, 1978, Schieck and Song, 2006, Schulte and Niemi, 1998), the effects of burn severity and heterogeneity have rarely been tested across the bird community (but see Azeria et al., 2011, Knaggs, 2018).
In this study, we compare the effects of fire, burn severity and burn heterogeneity on bird species and community patterns before and after the Pagami Creek Fire (2011) in the hemiboreal forests of northern MN, USA. We used a before-after control-impact study design with point counts collected for 2–3 years pre-fire and 4–5 years post-fire at unburned controls and across a continuum of burn severities. We predicted that most individual bird species would show responses to the fire, with this effect generally depending on burn severity or burn heterogeneity, as opposed to solely the presence of fire disturbance. We expected community patterns in the highest burn severities to be most divergent from unburned controls. In addition, we compare the effects of the above parameters when bird data have been corrected with density estimates that account for detectability (e.g., Stephens et al., 2015) versus raw point-count data restricted to 100 m from observers (e.g., Smucker et al., 2005).
Section snippets
Study area
The study was conducted near the southern edge of the boreal forest in the Superior National Forest (SNF; 48°N 92°W) of northeastern Minnesota (Fig. 1). The SNF is approximately 1.6 million ha and is primarily managed for multiple uses such as timber harvest and recreation. However, it also contains a 440,000 ha wilderness area, the Boundary Waters Canoe Area Wilderness, where motorized travel is limited and timber harvesting is prohibited. Wildfire was historically the dominant disturbance
Fire effects on bird community
In total, we had 11,284 bird detections of 98 species on 655 counts from 2009 to 2016 (Appendix A). On a per-point-count level, both species richness and total abundance of birds were best predicted by the severity interaction model (S*; Table 3). The variation in the numbers of species and individual birds detected at counts varied by burn severity, with lower severity locations having similar counts to controls and higher severity locations showing lower richness and abundances (Fig. 2). This
Discussion
Wildfire has long been known to shape the ecological communities of boreal and hemiboreal forests (Heinselman, 1973, Pastor et al., 1996). Yet, the positive effects of mixed-severity fires on biodiversity, and especially avian communities, have only more recently come into focus (Hutto et al., 2015). The varying and positive effects of mixed-severity fire on bird communities have been observed in a variety of forested regions including the Pacific Northwest and Interior west (Smucker et al.,
Conclusions
We found about two thirds of bird species analyzed responded to the fire with changes in abundance. Half of these increased in burned areas, which is similar to many forested regions of North America (Kotliar et al., 2007, Smucker et al., 2005, Stephens et al., 2015, Taillie et al., 2018), though somewhat lower than others (Hutto and Patterson, 2016, Knaggs, 2018, Rose and Simons, 2016). Likewise, we also found that many of the species-specific and bird-community responses to the fire were
Acknowledgements
The authors thank Josh Bednar, Carly Lapin and Steve Kolbe for assistance with field work. Jan Green was integral to the formation of this research project. Richard Hutto, Wayne Thogmartin, Matt Etterson, Nathan Pollesch, Alexis Grinde, Katie Zlonis and two anonymous reviewers provided valuable suggestions that greatly improved the manuscript. Kim Rewinkel provided editorial support.
Funding
This work was supported by the USDA Forest Service, Northern Research Station, Rhinelander, WI. Contributions by B. Sturtevant were supported by the National Fire Plan.
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