Patterns in Bird Community Structure Related to Restoration of Minnesota Dry Oak Savannas and Across a Prairie to Oak Woodland E

RESEARCH ARTICLE ABSTRACT: There is limited understanding of the influence of fire and vegetation structure on bird communities in dry oak (Quercus spp.) savannas of the Upper Midwest and whether bird communities in restored savanna habitats are similar to those in remnant savannas. During the 2001 and 2002 breeding seasons, we examined the relationship between bird communities and environmental variables, includ • ing vegetation characteristics and site prescribed-burn frequencies, across a habitat gradient in dry oak savannas in central Minnesota. The habitat gradient we studied went from: (1) prairie to (2) remnant Patterns in bird oak savanna to (3) oak woodland undergoing savanna restoration via fire or mechanical removal of woody vegetation to (4) oak woodland. We conducted fixed-radius point counts (n = 120) within habitats community structure with either prairie groundcover or predominately oak canopy. We described canopy and groundcover characteristics at a sub-sample (n = 28) of non-prairie points, and collected canopy and woody species related to restoration richness data and prescribed-bum frequencies over the past 20 years for all points. Observed bird com munities were most strongly correlated with canopy cover and bum frequency and, to a lesser extent, of Minnesota dry oak attributes of the shrub component. Most savanna points had bird communities that were distinct from those found at oak woodland or oak woodland points undergoing restoration via burning. Savanna points savannas and across similar to oak woodland points were in areas managed by periodic cutting rather than burning. Remnant savanna bird communities were more strongly associated with prescribed burning than those in other a prairie to oak habitat types, but it appeared that most oak woodlands that had undergone ~ 20 years of prescribed burning remained ecologically distinct from remnant savannas. This suggests that some savannas that woodland ecological have converted to oak woodlands may exist in an alternative, or stable, ecological state even following gradient extended restoration efforts. Index terms: alternative stable states, bird communities, fire, habitat restoration, oak savanna leakhena Au 1, 2 Minnesota Cooperative Fish and Wildlife 3 INTRODUCTION Stearns 1999). In dry oak savannas, un Research Unit, Department of Fisheries, like the more widespread mesic savannas, Wildlife, and Conservation Biology, Uni Oak (Quercus spp.) savanna was once shrubby vegetation is often interspersed versity of Minnesota, St. Paul, MN 55108 common and widespread in the Upper with prairie species in the understory Midwest, but is now one of the rarest (Will-Wolf and Stearns 1998). David E. Andersen vegetative communities (Curtis 1959; u.s. Geological Survey, Packard 1993; Anderson 1998), extending The loss of oak savannas and other distur Minnesota Cooperative Fish and Wildlife over approximately 0.02% of its pre-Euro bance-mediated habitats (e.g., prairie) has Research Unit,3 Department of Fisheries, American-settlement distribution (Nuzzo had important implications for birds and Wildlife, and Conservation Biology, Uni 1986). Savannas began to disappear after other wildlife. Changes in habitat struc versity of Minnesota, St. Paul, MN 55108 settlement in the mid- to late-1800s, and ture and foraging opportunities influence fire suppression in prairies and savannas survival and reproduction through impacts Mark Davis resulted in some areas becoming overgrown on habitat selection and niche segregation by woody species such as American hazel (Cody 1985). In a study of North American Department of Biology (CoryZus americana Walt.), smooth sumac Breeding Bird Survey (BBS) trends from Macalester College (Rhus gZabra L.), American elm (Ulmus 1966-1998, Brawn et al. (2001) found that St. Paul, MN 55105 americana L.), and choke cherry (Prunus a greater proportion of bird species found virginiana L.) (White 1986; Davis et al. in open habitats (grassland, shrub-scrub, • 1997; Bowles and McBride 1998), which and savanna) experienced significant de in turn shaded out herbaceous groundcover. creases in abundance than those associated 1 Corresponding author: Oak savannas have been further fragmented with forested habitats. Forty percent of [email protected] by conversion for agriculture and graz species associated with disturbance-medi 2 Current address: ing and by residential and commercial ated habitats declined significantly, while U.S. Fish and Wildlife Service, Division development (Bronny 1989; Bowles and only 17% increased. Many of the species of Bird Habitat Conservation, 4401 North McBride 1998). more closely associated with open habitats Fairfax Drive, MBSP 4075, Arlington, VA (lower percentages of canopy cover) and 22203 In Minnesota, oak savanna is generally moderate-to-high bum frequencies have 3 Cooperators include the U.S. Geological Survey, Minnesota Department of Natural characterized by an open canopy with been declining throughout the Upper Mid Resources, University of Minnesota, The primarily bur oak (Quercus macrocarpa west, including red-headed woodpeckers Wildlife Management Institute, and the Michx.), pin oak (Q. ellipsoidalis Hill), or (MeZanerpus erythrocephaZus L.), field U.S. Fish and Wildlife Service. red oak (Q. rubra L.) overstory and grass sparrows (Spizella pusilla Wilson), eastern or forb-dominated understory (McAndrews kingbirds (Tyrannus tyrannus L.), eastern Natural Areas Journal 28:330-341 1966; Davis et al. 1997; Peterson 1998; towhees (Pipilo erythrophthaZmus L.), Leach and Givnish 1999; Will-Wolf and and golden-winged warblers (Vermivora 330 Natural Areas Journal Volume 28 (4), 2008 chrysoptera L.) (Sauer et al. 2003). Davis frequency on niche segregation. Minnesota on the Anoka Sandplain; a series et al. (2000) found that as oak woodlands of sand dunes laid down over glacial till were restored to savanna under various Because scale of study is important in approximately 5000 to 8000 years before burn regimes, there was a shift in bird understanding ecological relationships present (Wovcha et al. 1995). At the time species composition and feeding guilds (Pearman 2002), we examined how these of Euro-American settlement, this region found along the restoration gradient. In that three aspects of bird community structure was dominated by scrubby oak woodlands study, nine of the 20 species that were as were related to environmental variables with· scattered oak barrens and openings sociated with frequently burned restoration at both the local level (i.e., within ap and interspersed with marshes and swamps, sites declined in abundance in the Upper proximately 100 m or less from where prairies, and forests (Wovcha et al. 1995). Midwest over the previous 35 years. More birds were detected) and patch level (i.e., Since then, logging, agriculture, and urban recently, Brawn (2006) found that 11 of 13 within the larger habitat block where birds ization have altered the original landscape, bird species breeding in both oak savanna were detected). We examined: (1) how bird resulting in a landscape that by the mid- and oak forest habitats experienced higher community structure was related to local 1980s was dominated by cleared farmland nesting success in oak savannas. Woodland vegetation characteristics (i.e., percent (e.g., pastures) and land in agricultural habitats undergoing savanna restoration, canopy cover, tree density, snag density, production interspersed with remnant therefore, may provide important new . groundcover characteristics, and tree and patches of native habitats. We conducted habitat for some declining species (Davis shrub species richness); (2) how bird com bird surveys and vegetation assessment et al. 2000). munity structure was related to patch-level on four study sites in Sherburne, Isanti, vegetation characteristics (i.e., percent and Anoka Counties: Sherburne National Restoration approaches in oak savannas canopy cover, canopy species richness, Wildlife Refuge ('SNWR' - 12,424 ha), in the Upper Midwest include mechanical and shrub species richness); and (3) the Sand Dunes States Forest (including Uncas or chemical intervention, reintroduction influence of prescribed-burn frequency on Dunes Scientific and Natural Area; 'SDSF' of natural disturbance processes, or some bird community structure. - 4330 ha), Cedar Creek Natural History combination of these approaches. Many Area (,CCNHA' - 2200 ha), and Helen restoration efforts, such as some conducted Allison Savanna (35 ha) (Figure 1). All STUDY AREA at our study sites, focus primarily on us~ four study sites were located on the Anoka ing prescribed fire as a.means of restoring Sandplain, and were embedded in the same We conducted our study in east-central oak savannas. This approach assumes that landscape matrix. the return of disturbance and successional processes will lead to a return to histori cal (i.e., oak savanna) conditions (Suding et al. 2004). Whether these restoration approaches result in habitat conditions that approximate historical oak savannas remains unknown. Nt I To assess the influence of restoration on bird communities and the relationships be tween bird community structure and habitat Isanti condition in Upper Midwest oak savannas, we expanded on the work of Davis et al. (2000) and examined the influence of vegetative characteristics and prescribed 3 burn regimes on bird communities along a gradient from prairie to oak woodland. We studied three aspects of bird commu nities across this gradient: (1) presence of individual

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