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 species, (2) abundance of species present, and (3) foraging strategies L----I employed by bird species using different 10 km' habitats (hereafter, feeding guilds). We included analysis of species presence, independent of abundance, to dampen the Figure 1. Location of the study sites in east-central Minnesota. Anoka, Isanti, and Sherburne Counties effects of common or highly detectable spe are located north of the Minneapolis-St. Paul metropolitan area. The numbered circles indicate study cies, and analyzed feeding guilds to assess sites: (1) Sherburne National Wildlife Refuge; (2) Sand Dunes State Forest; (3) Cedar Creek Natural the impact of vegetation structure and bum History Area; (4) Helen Allison Savanna.
Volume 28 (4), 2008 Natural Areas Journal 331 Sherburne National Wildlife Refuge and patch of these habitat types buffered to 100 at Helen Allison Savanna, for a total of SDSF are comprised largely of prairie, m and randomly selected survey points 116 points in 2002. We visited each point marsh, willow (Salix spp.) swamp, and wet from grids laid over polygons with core twice per year and visited 97 points a total meadow interspersed with oak woodland radii > 100 m in every direction. Survey of four times over two years, and visited and dry oak savanna (Wovcha et al. 1995; points were 2: 200 m apart. 22 points twice (Table 1). Sundseth 1996) and both include areas of pine (Pinus spp.) plantation. Cedar Creek We collected data on bird species presence Bird Communities Natural History Area encompassed a large and abundance, and assigned species to variety of plant communities including feeding guilds following DeGraaf et al. We conducted 100-m fixed-radius point prairie, maple-basswood (Acer spp.-Tilia (1985). We defined abundance as the aver counts largely following the protocol used americana) forest, oak woodland, pine age maximum number of detections of each by the U.S. Fish and Wildlife Service plantation, marsh, and bogs (Haarstad and species at each point over the two years of (USFWS) in Region 3 (Upper Midwest), Delaney 1998). Helen Allison Savanna the study. We based guilds on three com which was adapted from the field method primarily was comprised of dry oak sa ponents: (1) seasonal foraging techniques, handbook of Ralph et al. (1993). The pro vanna, with some areas of marsh and wet (2) substrate, and (3) food types. We as tocol also considered the recommendations meadow (Faber-Langendoen and Davis signed species that fell into > 1 category of Drapeau et al. (1999) regarding length 1995). Most remnant savannas and savanna of 2: 1 guild components (e.g., food type and temporal spacing of repeated counts. restoration areas on the study sites were - insectivore and frugivore) to a combined Observers recorded species detections by managed with prescribed burns alone or in guild representing all feeding habits used sight and call for 10 minutes during each combination with mechanical or chemical during the breeding season (Davis et al. count. We assumed that multiple calls from treatments. Up to and including the study 2000; Table 2). The number and identity the same species were from one individual period, the savannas and restoration sites of foraging guilds is likely related to both unless calls were simultaneous, near si at Sand Dunes State Forest were managed vegetation structure and food availability, multaneous, and opposing, or if otherwise exclusively through cutting. and we included analysis of the distribu confirmed visually. We conducted surveys tion of guilds across this habitat gradient between 0400 and 1000 CST from 23 to indirectly assess the potential influence METHODS May to 1 JUly. In hot weather (generally of food availability and distribution on bird 2: 29°C), we completed surveys by 0900 community structure. We digitized aerial photographs of the four CST and we repeated surveys at each point study sites into Geographic Information 2: 5 days apart. System (GIS) files to select survey plots. Vegetation Characteristics and Burn Based on aerial photographs, we identified We conducted point counts within prai Data upland habitats with either prairie ground ries, oak savannas, and burned and intact cover or predominately oak canopy within oak woodlands. In 2001, we conducted To assess relationships between vegetative each study site. We further stratified these counts at 100 points within SNWR and structure and composition, we collected upland habitats into four habitat types as Sand Dunes State Forest. In 2002, we dis detailed vegetation data at 28 randomly follows: (1) prairie « 5% canopy cover carded one oak woodland and one burned selected non-prairie point count locations. and prairie groundcover); (2) remnant woodland point surveyed in 2001 because We adapted data collection techniques oak savanna [(hereafter savanna), 5-65% of inaccessibility and one prairie point from the Breeding Biology Research and canopy cover with no known record of because of conversion to pine plantation. Monitoring Database (BBIRD) protocol succession to oak woodland]; (3) intact We added 16 points at CCNHA and three (Martin et al. 1997) and collected vegeta- oak woodland [(hereafter oak woodland), > 65% canopy cover and :::; 1 prescribed burn during 1982-2001]; and (4) burned Table 1. Number of poiuts and mean, SE, and 95 % CI of reciprocal averaging axis-l scores for bird oak woodland [(hereafter burned wood abundances in prairie, savanna, burned woodland, and oak woodland on the Anoka Sandplain in land), areas that had succeeded to oak east-central Minnesota. Bird community data at 116 survey points were collected during May-July woodland and were currently undergoing 2002. restoration to savanna via prescribed burns] (U.S. Fish and Wildlife Service, unpubl. Bounds of 95% CI data). On the SDSF, savanna management Habitat type n x SE Lower Upper efforts excluded fire and consisted solely of mechanical removal of woody vegetation Prairie 32 266.80 18.58 228.90 304.70 - we categorized these habitats as savan Savanna 30 -31.70 7.05 -46.10 -17.30 nas, based on vegetation structure, but Burned woodland 28 -81.21 5.59 -92.70 -69.70 identified them separately in interpreting our results. We created polygons of each Oak woodland 26 -95.97 3.44 -103.00 -88.90
332 Natural Areas Journal Volume 28 (4), 2008 habitat types. Table 2. Feeding guilds· detected at 86 survey points in remnant savannas, burned woodlands, and oak woodlands on 4 study sites on the Anoka Sandplain in east-central Minnesota. All birds were assigned to guilds following DeGraaf et aJ. (1985). The first column includes only 1 species that falls To derive patch-level environmental vari into the guild represented by each row; most guilds include multiple species. When> 1 types of 2:1 ables, we used data from mUltiple points components of feeding guilds applied to a given species, the components were combined similar to within GIS habitat polygons created during Davis et al. (2000). Surveys were conducted May-July 2001 and 2002. the process of selecting survey locations. We derived estimates of patch-level vari Guild component ables opportunistically when multiple bird survey points were selected within a poly Representative species Food type Substrate Technique gon. We averaged percent canopy cover and Eastern towhee 0· G F pooled tree and shrub species richness data Brown thrasher 0 LCG F from multiple points (~ 2) within a habitat American redstart I LCA GS polygon to derive a composite measure of these variables for each patch. American robin VO GLC GF Baltimore oriole 0 UC F From records kept by land managers, we Blue jay 0 UCG F determined the timing and locations of prescribed bums at each of our study sites Downy woodpecker I B G since 1982. Through the time of the study, House wren I LC G savannas in Sand Dunes State Forest were Blue-gray gnatcatcher I UC G managed using periodic cutting rather than Cedar waxwing IF UCA GS fire, so we assigned these points 0 burns since 1982. We calculated burn frequencies Downy woodpecker IF LCB G (numberburns/yr) for each point and each Eastern bluebird IF LCG G patch over the period 1982-2001. Eastern kingbird I A S Indigo bunting 0 LC F Data Analysis Mourning dove G G G We present descriptive statistics [e.g., Mourning warbler I G G means and standard error (SE)] to sum Ovenbird MI G G marize habitat variables and compare Pileated woodpecker I B E characteristics of vegetation across habitat Red-headed woodpecker I BA GS types using I-way analysis of variance (ANOVA) and 95% confidence intervals Red-shouldered hawk C G H (CI). To assess relationships between habi tat variables, we used Pearson product-mo ment correlation. All parametric statistics a Key to abbreviations of guild components-Food type: O-omnivore; I-insectivore; V-vermivore; F-frugivore; G-granivore; C-carnivore; were derived using JMP IN 4.0.4 (SAS Institute, Inc., Cary, North Carolina: use of Substrate: LC-Iower canopy; G-ground; A-air; UC-upper canopy; B-bark; trade names does not imply endorsement Technique: F-forager; G-gleaner; S-sallier; E-excavator; H-hawker. by either the U.S. Geological Surveyor the University of Minnesota). tion data at two ll.3-m radius plots at shrub and brush heights. We also recorded We used gradient analysis to relate species each location - one centered at the point woody species richness and percent canopy assemblages to various environmental fac count location, and the second centered cover at all 119 point count locations. We tors (e.g., Blair 1996; De'ath 1999; Badgley 40 m east of that point. Within the 11.3~m measured percent cover, vegetation size and Fox 2000) and to elucidate relation radius plots, we recorded all species and and structure, and woody species richness ships between individual bird species diameter at breast height (dbh) of trees because they may be directly related to the and habitat variables (e.g., James 1971). > 8 cm dbh, number of snags, and average diversity of foraging and sheltering niches We screened all bird abundance data for canopy cover. Within a 5-m radius plot available to birds. We excluded prairie obvious inconsistencies and unexplainable centered inside each 11.3-m radius plot, we points from vegetation assessment because outliers and discarded highly improbable recorded average percent cover of shrubs, the values for most measured vegetation observations (e.g., recorded sighting of a brush (standing dead shrubs and brambles), characteristics at prairie sites would be low lazuli bunting [Passerina amoena Say] grass/herbs, bare ground, fallen logs and or zero based upon the methodology used was likely a misidentification of an in- woody debris, moss and ferns, and average for identifying study areas and stratifying
Volume 28 (4), 2008 Natural Areas Journal 333 digo bunting [Passerina cyanea L.]). We and Mefford 1999) for all gradient analy savannas to 53.2 ± 11.0% in burned wood excluded species detected at < 4 points ses. In the CCAs, we standardized row and lands and 86.4 ± 1.8% in oak woodlands. from ordinations and statistical analysis column scores by centering and normal Average shrub cover was more similar because of small sample sizes. izing, in which site scores were rescaled across habitat types at 33.5 ± 7.8%, 42.1 to a mean of 0 and variance 1. We scaled ± 6.5%, and 45.3 ± 7.9% for savannas, We used canonical correspondence analysis ordination scores to optimize columns, burned woodland, and oak woodlands, (CCA), reciprocal averaging (RA), and which were bird species in the main ma respectively. We found no significant dif simple linear regression to identify the trix. In this approach, distances between ferences in shrub (F2,25 = 0.70, P = 0.507) environmental variables that best explained species (or guild) scores approximate chi or brush cover (F2,25 = 1.43, P = 0.259) bird community structure. We conducted square distances and, when environmental among habitat types. Percent grass cover gradient analysis using environmental variables are included, allow direct spatial and percent leaf litter were strongly cor characteristics within the point-count ra interpretation of relationships between the related with canopy cover (rgrass = -0.706, dius and as pooled or averaged within the two (McCune and Mefford 1999). rleaf = 0.881, Pboth ~ 0.001). larger habitat patch. We performed RA on 2002 point count data from 23 savanna, Vegetation characteristics in savannas, RESULTS burned woodland, and oak woodland points burned woodland, and oak woodlands for which we also collected point-specific were highly variable between plots at the vegetation data. We included axis-l RA Bird Communities same survey point and among pairs of plots scores for those points in simple linear within habitat types. Mean differences regressions against environmental variables We detected 78 species within 100 m of among plots within habitat types in per to determine which variables were associ survey locations during point counts - 69 cent canopy and percent brush cover were ated (P::5 0.10) with observed bird commu species in 2001 and 77 species in 2002. higher in savannas and burned woodlands nity patterns. We excluded environmental We detected 54 species and 20 feeding than in oak woodlands, but differences variables with low (::5 5%) average cover guilds at > 4 points (Table 2). When we in percent shrub cover were comparable values. When environmental variables were combined data from 2001 and 2002, the among habitat types [95% CIs (9.5, 35.3), highly correlated (Irl ~ 0.7), we included most commonly detected species in prairies (7.6, 34.2), and (9.7, 37.5) in savannas, the variable with the smaller P-value in were grasshopper sparrows (Ammodramus burned woodlands, and oak woodlands regression analyses. We used results of savannarum Gmelin), eastern meadowlarks respectively]. The range of percent shrub simple linear regressions as a basis for (Sturn ella magna L.), and field sparrows. cover extended> 50 percentage points in all CCAs at these two scales incorporating In savannas, eastern towhees, field spar three habitat types and appeared unrelated both environmental and bird community rows, and blue jays (Cyanocitta cristata L.) to canopy cover (unpubl. data). Differences variables. were the most frequently detected species, in average shrub height were similar among and in burned woodlands, red-eyed vireos habitat types, but average brush height was At the point level, we used CCA to relate (VIreo olivaceous L.), eastern wood-pewees less variable in burned woodlands (95% CI percent canopy cover and bare ground, (Contopus virens L.), and chestnut-sided of height differences [cm] between plots: canopy and shrub species richness, tree warblers (Dendroica pensylvanica L.) 7.3,20.7) than in savannas (12.1, 40.7) or density, shrub height, and bum frequency were detected most frequently. In oak oak woodlands (12.1, 39.1). at 28 points where we measured these woodland, the species we detected most characteristics to presence and abundance frequently were red-eyed vireos, ovenbirds At the patch level, oak woodlands had both of each bird species and feeding guilds (Seiurus aurocapillus L.), and eastern the highest mean woody species richness detected at those points. At the patch level, wood-pewees. (4.9 canopy species, 11.7 shrub species/ we assessed the relationships between spe patch) and highest mean percent canopy cies presence, bird abundance, and feeding We detected the fewest bird species in cover (81%); savannas had the lowest guilds from 86 points within 31 patches (9 prairies (n = 45) and the most in savan mean woody species richness (3.2 canopy savanna, 11 burned woodland, and 11 oak nas (n = 64). Nine species, including four species/patch, 8.7 shrub species/patch) and woodland) and burn frequency, average sparrow species, were found exclusively percent canopy cover (33%). percent canopy cover, and total canopy and in prairies; we detected only three species shrub species richness for those patches. exclusively in savannas. Burn frequencies varied widely in savan We subjected all CCAs to 100 randomized nas and burned woodlands. In savannas, runs of Monte Carlo tests of no linear prescribed-burn frequencies during 1982- Vegetation Characteristics and Burn relationship between matrices of bird 2001 ranged from 0.0 burns/yr in Sand Data communities and environmental variables Dunes State Forest to 0.9 burns/yr at 1 (McCune and Grace 2002). We used time point in CCNHA. Bum frequencies in At the 28 non-prairie locations where of day as the random number seed. burned woodlands ranged from 0.1 burn/yr we measured vegetation, average canopy at 5 points in SNWR to 0.9 burns/yr at cover ranged from 34.8 ± 7.2% (x ± SE) in We used PC-Ord, Version 4.25 (McCune 1 point in CCNHA. At the point level,
334 Natural Areas Journal Volume 28 (4), 2008 bum frequencies were weakly negatively correlated with canopy cover (r = -0.394, Table 3. Parameter estimates of simple linear regressions of environmental variables on reciprocal P = 0.038) and tree density (r = -0.324, averaging axis-! scores of bird point-count data. Counts were conducted in 2002 at 23 points in sa P = 0.093). vannas, burned woodlands, and oak woodlands on the Anoka Sandplain in east-central Minnesota.
Relationships of Bird Communities to Environmental variables Parameter SE t-ratio P Environmental Variables estimate Bum frequency 188.39 68.88 2.74 0.012 Bird communities at prairie points were highly distinct from bird communities in all % canopy -1.94 0.33 -5.89 < 0.001 other habitat types. Reciprocal averaging % shrub -0.14 0.78 -0.18 0.856 axis-1 values for prairie survey points from % brush 0.62 0.99 0.63 0.538 2002 exhibited no overlap with points in % grass 2.03 0.48 4.20 < 0.001 other habitat types. Additionally, the range of RA axis-1 scores for prairie points was % leaf -1.96 0.45 -4.37 < 0.001 comparable to that of RA axis-1 scores % debris -0.57 2.06 -0.28 0.784 for all other points combined (Table 1). % bare 8.87 3.23 2.74 0.012 Consequently, to improve our ability to distinguish bird communities in other Av. shrub ht -1.00 0.57 -1.74 0.097 habitat types, we excluded data from prairie Average brush ht -0.35 0.57 -0.61 0.550 points from further analyses. Tree density - 8.27 1.57 -5.26 < 0.001 No. snag -4.71 4.74 -0.99 0.332 At the local, or point level, bum frequency, percent canopy cover, percent bare ground, Average. DBH 0.65 2.82 0.23 0.820 average shrub height, tree density, and No. canopy spp. -40.87 11.24 -3.63 0.002 canopy and shrub species richness were No. shrub spp. -8.99 4.74 -1.90 0.071 significantly related to bird communi ties and included in the CCAs with point count data from the same 28 points at Relationship of bird communities to Based on CCA, bird species presence which we collected local vegetation data local environmental variables was strongly associated with percent bare (Table 3). We excluded moss and fern data ground, and shrub species richness had because values at most points were small We conducted CCAs on data from 28 a weaker relationship with bird commu « 5%) or zero. Percentages of grass and points, with 50 bird species and 20 feed nity structure than did bum frequency or leaf litter were strongly correlated with ing guilds. Sample scores for observed canopy cover (Table 4). The presences of percent canopy cover and, therefore, we bird communities and those derived from red-headed woodpeckers and brown thrash excluded them from CCAs. At the patch environmental variables were highly cor ers (Toxostoma rufum L.) were strongly level, we included burn frequency, percent related (Irl = 0.891 to 0.955, Monte-Carlo associated with high bum frequencies, the canopy cover, and canopy and shrub spe randomizations: x 0.774 to 0.826, P presences of golden-winged warblers and cies richness in CCAs as environmental = = 0.01 to 0.03) (Table 4). black-and-white warblers (Mniotilta varia variables. L.) were strongly associated with shrub
Table 4. Summary statistics from canonical correspondence analyses of point-count data from 28 points in savannas, burned woodlands, and oak wood lands on the Anoka Sandplain in east-central Minnesota. Bird and vegetation data were collected May-July 200! and 2002. Environmental variables were correlated with axis-! ordination scores for each of the response variables.
Axis-1 summary Correlation with axis 1 Response Eigen- % variance Bum % canopy % bare Average Tree No. canopy No. shrub variable value explained frequency cover ground shrub height density species species Abundances 0.253 14.0 - 0.811 0.720 -0.371 0.517 0.708 0.627 0.472 Presence 0.159 10.5 - 0.709 0.794 - 0.397 0.454 0.759 0.645 0.425 Feeding 0.101 15.8 - 0.741 0.750 - 0.375 0.511 0.751 0.597 0.451 guilds
Volume 28 (4), 2008 Natural Areas Journal 335 height, and the presence of eastern phoe eastern phoebes and black-throated green frequencies (Figure 2d). bes (Sayornis phoebe Latham) was most warblers (Alpha-code 'BTNW' in Figure 2; strongly associated with greater canopy Dendroica virens Gmelin) were associated Bird community structure at burned wood cover (Figure 2c). most strongly with high levels of canopy land points overlapped with that found in Abundance of individual bird species was cover and tree density (Figure 2b). oak woodland and savanna points. Lark most strongly related to burn frequency and sparrows (Chondestes grammacus Say), percent canopy cover. Although included in Dominant feeding guilds shifted with Baltimore orioles (Icterus galbula L.), the CCA, percent bare ground had a rela changes in canopy cover and bum frequen brown thrashers, and field sparrows were tively weak relationship to bird abundance. cy (Figure 2d). Insectivory and air sallying more strongly associated with savannas The differentiation of bird communities at were associated with high levels of canopy than with burned woodlands (Figures 2b savanna points from those at burned wood cover, and omnivory was strongly related and 2c), and we detected these species land and oak woodland points appeared to prescribed burning. Low-to-moderate more frequently in savannas than in burned more strongly related to bum frequency burn frequencies were associated with woodlands. All of these species were om and shrub height than to percent canopy omnivore lower canopy foragers and upper nivorous, and most (with the exception of cover or canopy species richness (Figure canopy gleaner/foragers, while omnivore Baltimore orioles) fed on lower canopy or 2a). Red-headed woodpeckers, brown lower canopy gleaner/foragers and ground ground substrates. Many species, including thrashers, and field sparrows were strongly foragers and insectivore/frugivores were black-capped chickadees (Poecile atrica associated with high bum frequency, while associated with moderate-to-high burn pillus L.), red-eyed vireos, and American redstarts (Setophaga ruticilla L.) were strongly associated with both burned woodland and oak woodlands. a. Relationship of bird communities to Burned patch environmental variables Woodland Results of our patch-level analyses were 8207*' 85 similar to those at the point level. Bird species presence, abundance, and feed S43* * ing guilds were strongly associated with canopy species richness and less strongly associated with shrub species richness. Bird community CCA scores were significantly ~26Axis 1 correlated with linear combination scores of environmental variables (Irl = 0.820 to 0.918, Monte-Carlo randomizations: x = Burnfreq Oak 0.456 to 0.559, P = 0.01). Associations be tween species presence and feeding guilds S55*' and environmental variables were similar to those found for bird species abundances, although the strengths of these associations Savanna varied. The strengths of the associations of burn frequency and percent canopy with bird abundance were similar, whereas species presence and feeding guilds had slightly stronger associations with percent canopy cover (Table 5). CCA scores for S201 bird communities in burned woodlands * overlapped heavily with those both in oak woodlands and in savannas (Figure 3a). Figure 2. Biplots of canonical correspondence analysis for bird abundances (a and b), presence (c), and Insectivory, upper canopy gleaning, and feeding guilds (d) from 28 survey points on the Anoka Sandplain in east-central Minnesota. Points are marked by an asterisk, and species or feeding guilds are marked with a diamond. Point labels with'S' air sallying were predominant in areas of indicate savanna points, 'B' indicate burned woodland points, and '0' indicate oak woodland points. moderate-to-high percent canopy cover Some labels were removed or shortened to improve clarity. Environmental variables include: Bllrnfreq whereas omnivory and lower canopy or - prescribed burn frequency over the period 1982-2001; % can - % canopy cover; % bare - % bare ground foraging were predominant in areas ground; # tree - average density of trees; shr_ht - average shrub height; # can - tree species richness; # undergoing prescribed bums (Figure 3b). shrub - shrub species richness. Bird and vegetation data were collected May-July 2001 and 2002.
336 Natural Areas Journal Volume 28 (4), 2008 Bird communities at most savanna points N o Golden-winged were more strongly associated with varying b. Eastern en warbler frequencies of prescribed burning than with kingbird ~ canopy cover or woody species richness. 0 0 At savanna points, bird communities were NOCA generally more consistently associated with RSHA YWAR 0 0 prescribed burns than bird communities in 0 SOSP Blue-winged warbler either burned woodland or oak woodland. The few exceptions were bird communities Lark sparrow at points S5, SIS, and S32 (Figure 3a), o which were located in Sand Dunes State Eastern bluebird 0 ~HCOI WITU shr_ht Forest. The axis-l ordination values for COYEd\ 0 .? Black-and-white those savanna points were more similar AM GO 0 CSWA oYTVI ° warbler to oak woodland and burned woodland o 00 .00 EATOo RBGR BGGN Axis 1 points than to other savanna points at sites outside of SDSF (Figure 3a). This is notable Field BAOR °EAWP # shrub o 0 0 AMRE because savanna conditions at SDSF were sparrow ~REVI o <:- 0 $SCTA~# 'tOVEN maintained through mechanical manage GCFL. YBCU ~can<> oLEFL ment techniques and not fire. Burnfreq "(# tree,pBTNW o Brown % can Eastern DISCUSSION thrasher DOWO phoebe <> Red-headed o Remnant and restored dry oak savannas woodpecker <>WBNU support many bird species that recently have been declining in the Upper Midwest. Of 2: 14 species detected more frequently Figure 2 (b). in savannas than in other habitat types, BBS data from 1980-2002 indicated that 2: Eastern <> Golden-winged 6 of these species experienced significant declines in the region (USFWS Region 3), c. kingbird BWWA warbler NOC Ao 0 ·0 and 2: 4 species experienced significant in RSHAo oSOSP shr ht creases (Sauer et al. 2003). Several species o YWAR /-0 Black-and-white of special concern within USFWS Region Lark sparrow.:) H0'g'R / warbler 3 (U.S. Fish and Wildlife Service 2002), % bare 0 <> YTVI # shrub including blue-winged warblers (Vermi /: BTNW vora pinus L.), golden-winged warblers, EABLO CSWA AMRE <> and red-headed woodpeckers, were most Axis 1 #can abundant in savannas. In our study, dry oak <>LEFL savannas in central Minnesota supported # tree high bird species richness (including sev eral species of special concern) compared to prairie and woodland habitats, and oriole % can (> bird community structure across a habitat Eastern gradient that included dry oak savannas phoebe o was most strongly associated with burn Brown NOFL¢- frequency, canopy cover, and attributes of thrasher o the shrub· component. Our results further DOWO suggest that management of these habitats through cutting rather than fire may not result in restored oak savannas that sup N port bird communities similar to those in Red-headed en remnant savanna habitats. <> woodpecker ~. We detected the lowest bird species rich ness in prairie habitats, and the highest in Figure 2 (c).
Volume 28 (4),2008 Natural Areas Journal 337 means rather than burning did not result in bird communities similar to those in oak d. savannas. The reasons for these differences N are not clear. CJ) ~ Unlike patterns in species presence and abundance, the diversity of bird foraging guilds was not highest in savannas com pared to oak woodlands and burned oak woodlands. Numbers of feeding guilds <> IBGAS among the three habitat types were similar, although there was a notable shift in feed ing guilds represented along the gradient from oak woodland to savanna. In areas with heavier canopy cover that lacked prescribed fire, guilds comprised of insec tivores were predominant. In areas with lower canopy cover and moderate-to-high frequencies of prescribed burning, most species were omnivorous feeders. There Axis 1 also appeared to be a slight shift from up per canopy and air foraging guilds in oak woodlands to lower canopy and ground foraging guilds in savannas. These results are consistent with findings by Davis et al. (2000), who reported that generalist and lower canopy foraging increased as oak woodlands were restored to savanna using prescribed burns.
Figure 2 (d). At both point- and patch-level spatial scales, percent canopy cover and burn fre quencies appeared to be the most important savannas, consistent with the expectation habitat gradient that included oak savan variables related to bird community struc that a-diversity, or the number of species nas in Indiana, and concluded that birds ture. Tree and shrub species richness and in a given habitat, is directly related to experience savannas and woodlands more shrub height also were related significantly structural diversity (Cody 1985). Savannas as ecotones than as habitats distinct from to bird communities (Table 3), although are more structurally diverse than either forests or grasslands. Our results based on shrub cover was not related to observed prairies or oak woodlands because they bird species presence and abundance during bird communities, perhaps because shrub include features of both habitat types, and the breeding season were also consistent cover was generally high in all habitat may represent an ecotone, rather than a bi with the notion that oak savannas func types except prairie. Overall, our axis-l orne (Temple 1998). Grundel and Pavlovic tion as an ecotone with respect to birds, CCA vegetation models explained from (2007) studied bird communities across a except that restoration through mechanical 8.8-15.8% of variation (Tables 4 and 5)
Table 5. Summary statistics from canonical correspondence analyses of point-count data from 86 points located in 31 patches of savanna, burned wood land, and oak woodland on the Anoka Sandplain in east-central Minnesota. Point counts were conducted May-July 2001 and 2002; vegetation data were collected May-July 2001 and 2002. Environmental variables were correlated with axis-I ordination scores for each of the response variables.
Axis-l summary Correlation with axis 1 Response Eigenvalue % variance Burn frequency % canopy No. canopy species No. shrub species Abundances 0.242 10.9 0.847 - 0.849 -0.747 - 0.428 Presence 0.154 8.8 0.775 - 0.871 -0.780 - 0.386 Feeding guilds 0.073 11.4 0.759 - 0.903 - 0.685 - 0.354
338 Natural Areas Journal Volume 28 (4),2008 tinuous or linear change in structure from oak woodland to savanna (Eiswerth and a. Haney 2001; Muradian 2001) or whether 825 burned woodlands, like oak woodlands * and savannas, are a stable ecological S45 state (Muradian 2001). Because many oak S43 *' woodlands undergoing restoration have S5 different species composition, structural * *S34 Oak S32 *' S37 characteristics, and ecosystem processes Woodland than remnant savannas, introduction of *' Savanna prescribed burns may create feedbacks that make woodlands resistant to savanna restoration (Suding et al. 2004). Axis 1 To successfully transform burned wood lands out of a self-reinforcing ecological state, savanna restoration efforts may have to reach or exceed some unknown ecologi cal threshold. Muradian (2001) defined an ecological threshold as that critical value 823 of an independent variable in an ecologi cal system that triggers a sudden change *' of that system from one stable state to another. The relative paucity of omnivores N S204 and ground-foragers in burned woodlands C/) S201 * compared to savannas suggests that this ~ * 8204* change may involve a shift in groundcover characteristics that would accommodate Figure 3. Biplots of canonical correspondence analysis for bird abundances (a) and feeding guilds (b) species that feed at lower structural levels from 86 survey points within 31 habitat patches on the Anoka Sandplain in east-central Minnesota. and on a wide variety of food types. Points are marked by an asterisk, and feeding guilds are marked with a diamond. Point labels with 'S' indicate savanna, 'B' indicate burned woodland, and '0' indicate oak woodland. Some labels were removed to improve clarity. Environmental variables include: Bumfreq - prescribed burn frequency Management Implications over period 1982-2001; % call - % canopy cover; # callspp - tree species richness; # slzrspp - shrub species richness. Bird community data were collected May-July 2001 and 2002. Vegetation data were collected May-July 2000-2002. In our study, bird communities along a prairie to oak woodland gradient were in bird community stmcture, suggesting turally similar between savannas managed most strongly related to a combination that bird communities were quite variable with cutting and those managed with fire, of prescribed burning regime and percent across the habitat gradient we studied, and the introduction of fire into an area may canopy cover. Woody species richness and that other factors we did not measure (e.g., influence habitat enough to significantly shrub and groundcover characteristics also food availability and distribution) likely change bird community structure. The bird influenced these communities. Remnant influenced community structure. community could be responding to differ savanna sites that had been managed with ences in plant species composition (which cutting alone appeared to support bird com Bird community structure in remnant we did not assess) or other biotic or abiotic munities more similar to those found in savannas appeared to be most strongly factors influenced by management. woodland habitats than did other savanna telated to burn frequency and less strongly points. However, restoration of oale savanna related to shrub height and woody spe At only a few burned woodland points (e.g., from oak woodland conducted using only cies richness. However, at savanna points B25, B31, CCB3) was bird community prescribed burns over a ~ 20 year period managed with periodic cutting but not fire, structure similar to bird community struc did not necessarily result in habitats that bird communities appeared more similar ture at most remnant savanna points (Figure supported bird communities similar to those in remnant savannas. The prospect to bird communities in oak woodlands 2a), suggesting that despite ~ 20 years of than to those at other savanna points. We prescribed burning, most burned wood that ecological thresholds or some other could not directly relate these results to lands remained ecologically distinct from nonlinear processes exist in these habitats any specific differences in vegetation, remnant savannas. It is unclear whether (Eiswerth and Haney 2001) suggests that it which suggests that although canopy and savanna restoration in these woodlands is may not always be possible to switch from groundcover characteristics may be struc- producing what could be described as con- oak woodland to savanna by merely rein-
Volume 28 (4), 2008 Natural Areas Journal 339 ACKNOWLEDGMENTS
b. CGHo We thank the U.S. Fish and Wildlife Ser vice, Division of Migratory Birds, and IFLCGG o Sherburne National Wildlife Refuge for their financial support and collaboration, Axis 1 with special thanks to J. Holler, S. Lewis, and C. Blair. The Minnesota Department of Natural Resources, The Nature Con servancy, and the University of Minnesota graciously allowed access to Sand Dunes State Forest, Helen Allison Savanna, and Cedar Creek Natural History Area study % can sites, and C. Lehman provided advice and Insectivore technical support. Leakhena Au was sup ported on a National Science Foundation Graduate Research Fellowship during this project, and the U.S. Geological Survey Minnesota Cooperative Fish and Wildlife Burnfreq Research Unit provided in-kind and lo gistical support. E. Norland, J. Kapoor, J. Montagna, E. Jacobsen and others assisted in collecting and entering field data. We N thankJ. Loegering, two anonymous review CJ) ers, and J. Bruggink for their comments on IBGAS previous drafts of this manuscript. ~ <}
Figure 3 (b). Leakhena Au is a Fish and Wildlife Biolo gist with the u.s. Fish and Wildlife Service. She is currently a grant administrator with troducing fire, but as observed in western depending on the bird species of interest, the Division of Bird Habitat Conservation grasslands (Muradian 2001), some savanna concurrent management for multiple spe in Arlington, VA. Her program focus is restorations may also require chemical cies may be possible. protection and restoration of habitat for or mechanical interventions. Factors op wetland-dependent bird species throughout erating at a broader spatial context (e.g., Finally, our results highlight the complex the United States and Canada. proximity to habitats that could serve as a ity of restoration and assessing ecological source of immigrants) may also influence condition of dry oak savannas in the Up David E. Andersen is the Leader of the bird community structure and the range of per Midwest. We used bird community U.S. Geological Survey Minnesota Coop potential future conditions. structure to assess ecological condition erative Fish and Wildlife Research Unit of oak savannas and whether restoration in the Department of Fisheries, Wildlife, Our results also suggest how management produced savannas that supported bird and Conservation Biology at the Univer for individual species could be enhanced communities similar to those in remnant sity of Minnesota. His research focuses by understanding how closely associated oak savannas, which provided insight that on population ecology, habitat relations, target species are with specific environ looking at vegetation structure alone did and management of birds, including for mental conditions. For instance, red-headed not (e.g., differences in bird communi est-nesting songbirds, raptors, waterfowl, woodpeckers were very strongly associated ties between habitats managed with fire and upland game birds. with high bum frequencies and were only versus those managed with mechanical weakly associated with other assessed manipulation). However, other factors Mark Davis is De Witt Wallace Professor environmental variables. Red-headed likely influence bird community structure ofBiology at Macalester College, St. Paul, woodpeckers were most weakly associated in remnant and restored oak savannas (e.g., MN. His teaching and research interests with shrub height. In contrast, golden food availability and the landscape context include both plant and animal ecology. winged warblers were most strongly of individual sites), and these need to be Current research interests include tree associated with shrub height. Managers better understood to effectively manage grass interactions along the prairie-forest may manipulate habitat characteristics to these rare habitats. border in North America and biological enhance conditions for target species, and invasions.
340 Natural Areas Journal Volume 28 (4), 2008 LITERATURE CITED Drapeau, P., A. Leduc, and R McNeil. 1999. Peterson, D.W. 1998. Fire effects on oak Refining the use of point counts at the scale savanna and woodland vegetation in Min Anderson, RC. 1998: Overview of midwestern of individual points in studies of bird-habi nesota. Ph.D. diss., University of Minnesota oak savanna. Transactions of the Wisconsin tat relationships. Journal of Avian Biology Graduate School, Minneapolis. Academy of Sciences, Arts and Letters 30:367-382. Ralph, C.J., G.R. Geupel, P. Pyle, T.E. Mar 86:1-18. Eiswerth, M.E., andlC. Haney. 2001. Maximiz tin, and D.F. DeSante. 1993. Handbook of Badgley, C., and D.L. Fox. 2000. Ecological ing conserved biodiversity: why ecosystem field methods for monitoring land birds. biogeography of NorthAmeric an mammals: indicators and thresholds matter. Ecological General Technical Report PSW-GTR-144, species density and ecological structure in Economics 32:259-274. U.S. Department of Agriculture, Forest relation to environmental gradients. Journal Faber-Langendoen, D., and M.A. Davis. 1995. Service, Pacific Southwest Research Station of Biogeography 27:1437-1467. Effects of fire frequency on tree canopy cover Albany, Calif. ' Blair, R.B. 1996. Land use and avian species at Allison Savanna, eastcentral Minnesota, Sauer, J.R., lE. Hines, and 1 Fallon. 2003. diversity along an urban gradient. Ecological USA. Natural Areas Journal 15:319-328. The North American Breeding Bird Survey, Applications 6:506-519. Grundel, R., andN.B. Pavlovic. 2007. Distinc results and analysis 1966-2002. Version 2003.1, U.S. Geological Survey Patuxent Bowles, M.L., and lL. McBride. 1998. Vegeta- tiveness, use, and value of midwestern oak Wildlife Research Center, Laurel, Mary land, . tion composition, structure, and chronologi savannas and woodlands as avian habitats. USA. Available online
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