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Distribution, Habitat Characteristics, and New County-level Records of Baccharis halimifolia L. on a Portion of its Present US Range Boundary
Gary N. Ervin

Southeastern Naturalist, Volume 8, Number 2 (2009): 293–304

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2009 SOUTHEASTERN NATURALIST 8(2):293–304 Distribution, Habitat Characteristics, and New County-level Records of Baccharis halimifolia L. on a Portion of its Present US Range Boundary Gary N. Ervin* Abstract - Baccharis halimifolia (Eastern Baccharis, Silverling, Groundsel-bush, or Salt-bush) (Asteraceae), a weedy shrub native to the US Gulf and Atlantic Coastal Plains, is believed to be expanding its distribution throughout much of its native range and in other regions of the globe to which it has been introduced (e.g., Australia and Mediterranean Europe). The present survey represents an effort to document the current distribution of this species across a portion of its apparent interior range limit in the south-central United States and to record habitat associations of B. halimifolia within this region. Data support previous research suggesting that B. halimifolia associates with various forms of canopy-removing anthropogenic disturbance, and that the limit of this species’ present distribution in the mid-South lies primarily along the southern half of counties in Tennessee. However, habitat associations did not appear to vary across the study area, suggesting potential for further expansion into humanaltered habitats throughout Tennessee and possibly further northward. Introduction Baccharis halimifolia L. (Asteraceae), commonly known as Eastern Baccharis, Silverling, Groundsel-bush, or Salt-bush, is considered native to the Atlantic and Gulf Coast states of the US from Texas to Massachusetts (USDA NRCS 2007, Weakley 2007), where it is recognized as a common species in upland fringes of coastal saline marshes and back dune habitats (Krischik and Denno 1990). Baccharis halimifolia is a dioecious, woody perennial that can reach heights of 4 to 6 m, with heights in the range of 2 to 3 m being more typical (Gilman 1999, Nesom 2001). This species exhibits a semi-deciduous growth habit in the northernmost portions of its North American range, but it may retain its leaves year-round throughout most of its global distribution (Krischik and Denno 1990, Westman et al. 1975). Baccharis halimifolia also exhibits adaptations to a wide range of soil and environmental conditions including a soil pH range from 3.6 to 9.0, tolerance to a range of soil nutrient concentrations (nitrogen: 560 to 5500 ppm; phosphorus: 4 to 73 ppm), the ability to survive periodic fl ooding and drought, and basal resprouting following fire damage (Westman et al. 1975). Despite the apparently broad environmental tolerances of B. halimifolia, seed production in this species has been demonstrated to decline significantly under shaded conditions, one of many indications of this species’ adaptation to disturbed habitats (Panetta 1977, 1979a). *Department of Biological Sciences, Mississippi State University, MS 39762; gervin@biology.msstate.edu. 294 Southeastern Naturalist Vol. 8, No. 2 Wind-pollinated fl owers of B. halimifolia are produced during autumn, followed by copious production of small, plumed, wind-dispersed achenes (Krischik and Denno 1990). Seed mass has been reported at approximately 0.1 mg seed-1 (air-dry mass), and as many as 1.5 x 106 achenes may be produced per plant, with the highest rates of seed production occurring in the absence of shade (Panetta 1977, Westman et al. 1975). Available data suggest that in the absence of seed burial, germination follows shortly after the early winter fruit dispersal. Seedlings then are thought to be capable of winter establishment, when most neighboring species lay dormant and shading effects are minimized (Panetta 1977, 1979b). It has been reported for some time that B. halimifolia is capable of establishing in interior regions of the southeastern United States, particularly in disturbed habitats such as fallow fields and hedgerows, as well as inland saline soils (Krischik and Denno 1990). Areas where B. halimifolia has been reported include the interior regions of the coastal plains (Duncan 1954, Krischik and Denno 1990), as well as the Piedmont, Ridge and Valley, Interior Low Plateau, and possibly even in the foothills of the Blue Ridge and Cumberland Plateau (Estes 2004, 2005; Weakley 2007). Radford et al. (1968) indicated that B. halimifolia is thought to be native, and historically endemic, to the outer coastal plain, but they acknowledged that it had a widespread distribution at the time of publication of their fl ora. Duncan (1954) reported that B. halimifolia increased its distribution substantially during the first half of the 20th century, and was considered in 1954 to be a weed “of great importance” in Georgia (USA). Outside its native range, B. halimifolia has become an invasive weed in Australia, France, Spain, and Abkhazia on the eastern coast of the Black Sea in eastern Europe (Westman et al. 1975). In its introduced range, B. halimifolia is considered especially problematic in pasturelands because of the noxious chemistry of the plant’s foliage that is believed to protect the plant against herbivory and may be toxic to cattle (Boldt 1989, Kraft and Denno 1982, Nesom 2001). In the south-central US, this species is now known from the southern half of Arkansas, throughout Louisiana, four counties in western central Alabama, the two coastal Alabama counties (Baldwin and Mobile), the southern two-thirds of Mississippi, and four counties in the northern third of that state (USDA NRCS 2007). Baccharis halimifolia recently was reported from two counties in central Tennessee (Estes 2004, 2005), and it presently is known to occur in only seven counties in western and middle Tennessee (Tennessee Vascular Plants Database November 2007). However, Estes (2005) indicated that this Baccharis species should be considered non-native in Tennessee and ought to be expected in association with human-disturbed habitats across the southern half of that state. That Estes expected B. halimifolia could be encountered in disturbed habitats of southern Tennessee, at the presumed northern extent of the species’ range, is not surprising. Many examples exist of species spreading outside their historic range in association with human disturbance of natural 2009 G.N. Ervin 295 habitats (Christen and Matlack 2006, Harrison et al. 2002, Shuster et al. 2005). It is widely accepted that changes in the structure and/or function of natural ecosystems can provide opportunities for invasion and human-assisted redistribution of species across vast distances (Hobbs 2000, Vitousek et al. 1997). Examples of such ecosystem change are increased availability of canopy openings, increased frequency of edge habitat, and provision of dispersal corridors, such as highways and power line and pipeline rights-of-way that aid in the spread and establishment of invasive plants (Jules et al. 2002, Rouget and Richardson 2003, With 2002). Given the natural-history traits of B. halimifolia, as described above, these factors seem likely to infl uence its distribution and spread into new habitats within and beyond its historic native US range. Thus, the objectives of this study were to (1) document the present distribution of B. halimifolia across a portion of its range in northern Mississippi and western Tennessee, and (2) to evaluate the degree to which this species is associated with human disturbance and human-altered landscapes within the study region. Methods Study area Roadside surveys were conducted for Baccharis halimifolia in 21 counties of West Tennessee (all counties between the Tennessee and Mississippi rivers) and in 17 counties of northeastern Mississippi from Oktibbeha County (location of Mississippi State University) to the TN–MS border to the north and the AL–MS border to the east (Fig. 1). This region was expected to provide a gradient of B. halimifolia density, from the known very high densities encountered in Oktibbeha and Webster Counties of MS (G.N. Ervin, pers. observ.) into the northernmost counties of western Tennessee, where B. halimifolia was expected to become absent or of very low frequency (Estes 2005). Survey methods Survey routes (Fig. 1) were selected a priori by using a road atlas to determine the most efficient routes by which all counties in the selected survey area could be visited. These routes consisted primarily of state and federal highways, as those routes constituted the most direct means of transecting multiple counties in the study area and were readily available as georeferenced data layers. Data layers for the highways were obtained from the Mississippi Automated Resource Information System (MARIS; http://www. maris.state.ms.us/) for Mississippi highways or the National Map Seamless Data Server (US Geological Survey, EROS Data Center, Sioux Falls, SD) for highways in Tennessee (Bureau of Transportation Statistics [BTS] Roads data). For Tennessee routes, the data obtained included all BTS recognized roads within the map server download window (including all of Tennessee and portions of adjacent states), so Tennessee state and federal highways were extracted from those data to make route determination more efficient. The 296 Southeastern Naturalist Vol. 8, No. 2 selected routes were digitized in ArcGIS 9.0 (Environmental Systems Research Institute, Inc.), converted to an ArcGIS shapefile, and transferred to an HP iPAQ HX 2110, running Windows MobileTM 2003 second edition, version 4.21.1088. Navigation along the routes was performed with the assistance of Farm Works Site Mate version 11.40 (CTN Data Service, Inc.) geographic Figure 1. Map of the study area. Locator map in upper left shows the study area relative to the remainder of Tennessee and Mississippi. Survey routes are indicated by the dark line passing through each county or by mapped Baccharis halimifolia patches. As indicated, counties are shaded based on the frequency of observation of B. halimifolia patches along the survey route (patches per 20 km of survey). 2009 G.N. Ervin 297 information system (GIS) software and a Holux compact fl ash-card global positioning system (GPS) unit, model GR-271. Where new highway construction had occurred since assembly of the transportation GIS data layer, digitized routes were corrected after the surveys by visual inspection and comparison with B. halimifolia locations and an independent land-cover data layer (National Land Cover Dataset 2001 [NLCD 2001], downloaded from the Multi-Resolution Land Characteristics Consortium: www.mrlc.gov). The corrected routes are depicted in Figure 1. Data handling within ArcGIS was performed in the Albers map projection (USA Contiguous Albers Equal Area Conic, USGS version) and the 1983 North American Datum geographic coordinate system (NAD 1983). However, data collection in the field was performed in the 1984 World Geodetic System datum (WGS 1984), and data were re-projected to NAD 1983 as necessary within ArcGIS. Rate of travel along routes, including time for stops to record data and collect voucher specimens, ranged from approximately 24 km h-1 (15 mi h-1) to 77 km h-1 (48 mi h-1), with a mean of about 59 km h-1 (37 mi h-1). This rate of travel was infl uenced to a large extent by the class of highway and density of B. halimifolia along the survey route, with fastest travel in the northernmost counties of the survey area. The total length of the survey routes was 792 km (495 mi) in Mississippi and 1190 km (744 mi) in Tennessee, with a total of 1982 km (1239 mi) surveyed. A mean (± 1 SE) of 50 ± 3 km (31 ± 2 mi) were driven in each county. Farms Works Site Mate also was used as the primary means of data entry for each logged occurrence of B. halimifolia along the routes. For each patch observed during this survey, six attributes were recorded (Table 1). Most patches recorded (≈95%) were estimated to lie within about 30 m of the road shoulder (i.e., within the right-of-way [ROW] proper); thus, all patches Table 1. Attributes recorded for each Baccharis halimifolia patch observed during this study. Attribute Possible levels Patch size Up to: 5 individuals, 10 indiv., 5 m diameter, 10 m dia., 25 m dia., 50 m dia., 100 m dia., 200 m dia., >200 m dia., and one patch >2000 m Patch density Low, medium, high Land coverA Wetland-woody, wetland-herbaceous, mixed forest, evergreen, grassland, shrub-scrub, pasture, cultivated, developed Habitat type (as a modifier of land cover) Natural, riparian, fallow (herbaceous), (fallow) shrub, managed, forest edge, fencerow, pasture, right-of-way, other Type of disturbance (where evident) None, construction (included new highway construction), soil disturbance (including vegetation), vegetation disturbance only Canopy presence Canopy, edge, open ALand cover is based on the Anderson Level II land-cover classes used in the NLCD 2001 Land Cover Data (Homer et al. 2004). The developed-open, low-, medium-, and high-intensity classes were collapsed to a single developed class for this study. 298 Southeastern Naturalist Vol. 8, No. 2 were inherently associated with disturbance, and results must be interpreted in that context. Land cover as recorded was based on the Anderson Level II land-cover classes (Homer et al. 2004) for the larger area (≈30-m x 30-m “plots”) in which each patch occurred. Habitat type, as used in this survey, was intended as a modifier of land cover to indicate the specific location of the patch itself. Because of the largely descriptive nature of this study, there was little need or opportunity for statistical analyses of the data collected. However, chi-squared tests were conducted to determine quantitatively whether the observed distribution of B. halimifolia patches among land-cover categories differed from what would have been expected by chance alone. The relative percentages of each of fifteen Anderson Level II land-cover classes was determined within a 50-m buffer on each side of the route driven in these surveys. Those land-cover data, used to derive expected frequencies for B. halimifolia patch distribution, were extracted from the NLCD 2001 dataset in ArcGIS. Expected patch frequency (number of patches per land-cover class) was calculated as the proportional representation of each land-cover class within the buffer multiplied by the total number of patches. The chi-square test statistic then was calculated in the standard fashion, as Σ (O - E)2 ÷ E, where O = observed number of patches per land-cover class, and E = expected number of patches per land-cover class. An additional hard copy datasheet was maintained with data on patch size, habitat, and observational notes for each observed B. halimifolia patch. One voucher specimen was collected for each county in which B. halimifolia was recorded and deposited in the Mississippi State University herbarium (MISSA; G.N. Ervin collection numbers 258 to 285), including Lowndes County, which lies just east of Oktibbeha County, the southernmost county in the survey (Fig. 1). One duplicate voucher was collected for each county where B. halimifolia was recorded in Tennessee, and those have been transferred to the University of Tennessee Herbarium (TENN) in Knoxville, TN, along with data on each patch of B. halimifolia recorded in that state. Results and Discussion Six hundred thirty-five patches of Baccharis halimifolia were found during this study. Fifty-two of these patches were observed in ten counties of West Tennessee, providing a substantial extension of the recorded distribution of B. halimifolia in the study area (Table 2). In addition to seven new county records in Tennessee, nine counties in Mississippi were identified that appear to be lacking collections of this species, based on inquiries with state herbaria (Table 2). The nine new Mississippi counties contained 82 patches of B. halimifolia, with only 28 of these occurring in the five counties bordering Tennessee. Counties were grouped by patch frequency (patches per 20 km of survey route) for evaluation of habitat characteristics across the study area (Figs. 1 and 2). Sub-regions of the study area were referred to as having low (up to 2009 G.N. Ervin 299 2.01 patches per 20 km), medium (2.02 to 7.0 patches per 20 km), or high (more than 7.0 patches per 20 km) frequency of B. halimifolia patches. Because of the low number of patches in the “low frequency” group, data were calculated as percent of all patches within each of the three frequency groupings (nine counties each) for graphical display, instead of simply numbers of patches (Fig. 2). With few exceptions, B. halimifolia showed strikingly similar patterns of association with habitat characteristics across the study area (Fig. 2). Most patches were small (a few individuals) and of low density, aside from the “high frequency” counties, where B. halimifolia was found almost equally in patches of five or fewer individuals (30%) and patches of 50 m to 100 m diameter (24%). As expected, B. halimifolia tended towards disturbed habitats, with most patches occurring in association with highway or power line right-of-way, managed lands (largely pine plantations), and forest edges. All these areas tended to have low degrees of canopy development, as indicated by data on canopy development at patch locations and the high prevalence of vegetation disturbance around observed patches. Land-cover classification tended to support the association with disturbance as well, with a third or more of the patches (30 to 45%) in areas of developed land-cover and another 20 to 30% associated with scrub-shrub cover, which most frequently was harvested timber land. Very few patches were associated with wetland areas and natural habitats, and most of those were in the northern region of the study area (low patch-frequency group). Chi-squared analyses indicated that the distribution of B. halimifolia differed from what one would expect by chance, based on the proportional cover of land use along the survey route. Because of the low numbers of patches in some groups, even in the cumulative data set for the entire study area, four land-cover categories (open water, barren, grassland/herbaceous, and emergent herbaceous wetland Anderson Level II categories) were lumped for the chi-squared analysis. Additionally, all developed land cover was lumped in the road surveys, so the same procedure was followed for this Table 2. Counties believed to represent new records in the distribution of Baccharis halimifolia. This list is based on inquiries with major herbaria in Mississippi (MISS, MISSA, MMNS, SWSL, USM), the herbarium of the University of Tennessee (TENN; online database), and information contained in Estes (2005). State Counties with presumably new recordsA Mississippi AlcornB, BentonB, ItawambaB, Lee, MarshallB, PrentissB, TippahB, TishomingoB, UnionB Tennessee Dyer, Fayette, Hardeman, Haywood, McNairy, Shelby, Tipton AAlthough not in the official survey area, Baccharis halimifolia also was collected in Lowndes County, MS, which lies immediately to the east of the southernmost county in Figure 1. A less thorough count of patches in Lowndes County yielded 37 patches along a 45-km route (16.4 patches per 20 km), with patches ranging in size from a single plant to greater than 200 m diameter. These were located in virtually all of the land-cover types included in the survey. BEight of the nine new counties in MS either border TN or border a county that does. That is, those eight are within the northernmost two tiers of MS counties. 300 Southeastern Naturalist Vol. 8, No. 2 Figure 2. Comparison of Baccharis halimifolia habitat characteristics in counties with low, medium, and high patch frequencies. Category labels are described in Table 1. 2009 G.N. Ervin 301 quantitative analysis; all four levels of developed land cover (open and low-, medium-, and high-intensity developed spaces) were lumped for analysis. The results indicated that the observed distribution of B. halimifolia differed significantly from expected in each of the three sub-regions of the study area (χ2 LOW = 47.6; χ2 MED = 213; χ2 HIGH = 1160; df = 8 and P < 0.001 for all). Among the nine land-cover categories used in these analyses, scrub/shrub Figure 3. Baccharis halimifolia distribution in the United States (upper), relative to USDA Plant Hardiness Zones ( l o w e r ) . County distribution is based on data collected in the present study, supplemented with county record data from USDA PLANTS database and the U n i v e r s i t y of Tennessee herbarium online database. Plant Hardiness Zone map, 2001 edition, courtesy of the US National Herbarium, USDA. Values given for each zone (lower map) are the mean annual minimum temperatures (ºC). 302 Southeastern Naturalist Vol. 8, No. 2 land cover consistently had more than the expected number of patches, and developed land cover was occupied less frequently than expected. Thus, it seems B. halimifolia was observed most frequently in areas that have been disturbed but where some secondary succession had occurred. That there were few qualitative differences in habitat associations of Baccharis halimifolia across the study area suggests that this species is not limited by habitat availability across its present range in Mississippi and Tennessee. This distribution may be due, in large part, to its tendency of inhabiting human disturbed habitats, exemplified by the dense cluster of large patches in Decatur County, TN (northeastern most county occupied by B. halimifolia in Fig. 1). All fourteen of these patches fell within an area of relatively new four-lane divided highway construction (Tennessee State Highway 69), with varying levels of new construction and mowing disturbance. These results, in combination with the experimentally demonstrated broad habitat tolerances of B. halimifolia (Panetta 1977, 1979a; Westman et al. 1975), suggest strongly that climate may be the most infl uential variable in determining potential range expansion within the present US distribution of Baccharis halimifolia. Currently, B. halimifolia is recorded from areas of Massachusetts, Rhode Island, Connecticut, New York, New Jersey, and Pennsylvania within USDA Plant Hardiness Zones 6a and 6b (Fig. 3; Cathey 1990, USDA NRCS 2007). In Tennessee, B. halimifolia presently is known from at least one location in Plant Hardiness Zone 6a, in Rutherford County (northernmost county in the center of Tennessee; Fig. 3). A nearby county with patches of B. halimifolia–Maury–County, lies approximately equally in zones 7b and 6a, but all Tennessee patches identified in the present study fell within zone 7b. If the climate throughout zone six in the US (yellow zones) were suitable for growth and reproduction of B. halimifolia, this species could be expected eventually to expand its range throughout Tennessee and Kentucky, and possibly into states farther north, taking advantage of suitable habitats in areas from the Ohio River Basin to the Gulf and Atlantic Coasts (Fig. 3). Acknowledgments This work was supported by funding from the US Geological Survey Biological Resources Discipline (04HQAG0135), US Department of Agriculture (2007-55320- 17847), and the Mississippi Computational Biology Consortium (NSF EPSCoR #EPS-0556308). Assistance in data collection was provided by T. Ervin and G.L. Ervin. Chris Holly, Lucas Majure, and two anonymous reviewers provided helpful critique of an earlier version of this paper. Christopher Brooks provided statistical advice. 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