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Xeric limestone prairies of eastern United States: review and synthesis.

Abstract
Introduction
Physiography
Bedrock Geology
Soils
Climate
Vegetation
 Ozark Plateaus
 Central Lowland
 Interior Low Plateaus
 Ridge and Valley
 Appalachian Plateaus
 Coastal Plain
Endemic Taxa and Other Noteworthy Floristic Elements
Origin Maintenance
 Physical Environment Factors and Anthropogenic Influence
 Conceptual Model
Affinities of Xeric Limestone Prairies to Other Vegetation Types
 Eastern North America
 Western North America
Direction for Future Research
Literature Cited


Introduction

Xeric limestone prairies (XLPs) (sensu Baskin et al., 1994; Baskin & Baskin, 2000) are open, nonforested areas in which herbaceous plant communities are developed on shallow, rocky soils derived from calcareous substrates including limestone, dolomite, and shale (Lawless et al., 2004) (Fig. 1). This vegetation type is characterized by dominance of [C.sub.4] perennial grasses (Baskin & Baskin, 2000; Laughlin & Uhl, 2003; Baskin & Baskin, 2004; Lawless et al., in press) and relatively high taxonomic richness of heliophytic [C.sub.3] perennial forbs, many of which are rare at the state, regional, and/or global levels (Skinner et al., 1983; White et al., 1994; Laughlin & Uhl, 2003; Lawless et al., 2004). XLPs often contain islands of woody vegetation within the grassland matrix and typically are surrounded by dry, rocky upland forests. Expansion of islands of woody vegetation and encroachment of adjacent forests into XLPs has been documented in some regions and can result in drastic reductions in the size of openings (e.g., Kimmel & Probasco, 1980; Annala & Kaputska, 1983; Laughlin, 2004).

[FIGURE 1 OMITTED]

This vegetation type is broadly distributed throughout a large portion of the eastern deciduous forest formation (sensu Braun, 1950) (Fig. 2), where it has been referred to as "glades" (Missouri, Arkansas, Illinois, Indiana, Kentucky, West Virginia, Tennessee, Georgia, and Alabama), "barrens" (Illinois, Indiana, Ohio, West Virginia, Virginia, Kentucky, Tennessee, Georgia, and Alabama), "prairies" (Alabama and Ohio), "prairie barrens" (Alabama), and "xeric limestone prairies" (Pennsylvania and Kentucky). Inconsistency in terminology used to describe XLPs throughout their broad geographic range has resulted in failure to recognize similarities among sites (Ludwig, 1999) and in confusion of these herbaceous communities with limestone cedar glades (Baskin et al., 1994; Baskin & Baskin, 1999, 2004), an herbaceous edaphic climax community in north-central southeastern United States characteristically dominated by [C.sub.4] summer annual grasses (sensu Baskin & Baskin, 1999, 2003).

[FIGURE 2 OMITTED]

Although many individual studies have been conducted on the flora, vegetation, and physical environment of XLPs in various states and physiographic regions, heretofore no attempt has been made to synthesize information on this vegetation type throughout eastern United States. Thus, the purpose of this paper is to synthesize data on the geographic/physiographic distribution, geology, soils, climate, vegetation, and endemic and other noteworthy plant taxa of XLPs in eastern United States and to provide a conceptual model of the origin, maintenance, and successional trajectories of this vegetation type. In addition, affinities of XLPs to other nonforested herbaceous vegetation types in North America are discussed. Plant nomenclature follows United States Department of Agriculture, Natural Resources Conservation Service (USDA, NRCS, 2004).

Physiography

Physiographic provinces are in accordance with Fenneman (1938, 1946 [1949]) except for the Interior Low Plateaus, which follows Quarterman and Powell (1978). Lower tiers in the hierarchy of land-form classification are in accordance with various sources (cited in Table I), and physiographic regions (sections and subsections) of the Interior Low Plateaus follow Quarterman and Powell (1978).

XLPs occur in six physiographic provinces in eastern United States: Ozark Plateaus, Central Lowland, Interior Low Plateaus, Appalachian Plateaus, Ridge and Valley, and Coastal Plain (Fig. 2; Table I). In the Ozark Plateaus, this vegetation type is distributed broadly on the Springfield and Salem plateaus and extends south into the Boston Mountains of northern Arkansas (Keeland, 1978; Kurz, 1981; Ladd & Nelson, 1982; Nelson & Ladd, 1982a, 1983; Heikens, 1991). Sites in the Central Lowland occur on the Osage Plains of west-central Missouri, on the Eastern Glaciated Plains and Lincoln Hills of east-central Missouri, and the Muscatatuck Flats and Canyons and Switzerland Hills of southeastern Indiana (Ladd & Nelson, 1982; Homoya, 1987; Maxwell, 1987). In the Interior Low Plateaus, XLPs range from the Mammoth Cave Plateau in southern Indiana to the Northeastern Blue Grass in southeastern Ohio and south to the Moulton Valley in northern Alabama (Braun, 1928; Bacone et al., 1983; Webb et al., 1997). This vegetation type is distributed over its maximum latitudinal range in the Ridge and Valley, where it extends from north-central Pennsylvania to central Alabama (DeSelm, 1993; Allison & Stevens, 2001; Laughlin, 2002). The distribution of XLPs in the Coastal Plain and Appalachian Plateaus is extremely limited; they apparently are restricted to the Southern Red Hills (Coastal Plain) in southern Alabama (Harper, 1920) and to the Marietta Plateau (Appalachian Plateaus) in eastern Ohio (Rick Gardner, personal communication).

Bedrock Geology

XLPs are developed primarily on Paleozoic calcareous substrates (Table II). In the Ozark Plateaus, this vegetation type occurs on Cambrian, Ordovician, and Mississippian limestones and dolomites; in the Central Lowland (principally) on Mississippian and Ordovician limestones; in the Interior Low Plateaus on Mississippian, Silurian, and Ordovician limestones and dolomites; and in the Ridge and Valley on limestones and dolomites of Cambrian through Devonian age. In the East Gulf Coastal Plain in Alabama, Harper (1920) described XLPs restricted to the Tertiary (Eocene) Midway Limestone. Apparently, limestone of the Pennsylvanian Conemaugh Formation is the only substrate upon which this vegetation type is developed in the (unglaciated) Allegheny Plateaus (in Belmont County, Ohio) (Rick Gardner, pers. commun.).

Soils

Soils of XLPs typically are shallow ([less than or equal to] 1.0 m, mostly [much less than] 1.0 m) and rocky, and often they are mapped as rock outcrop complexes with one or more associated soil series (Baskin et al., 1994; Lawless et al., 2004). Despite circumneutral pH values (~6.0-8.0), soil fertility is often quite low, and many researchers have documented low soil phosphorus levels (Keeland, 1978; Heikens, 1991; Ludwig, 1999; Allison & Stevens, 2001; Rhoades et al., 2004; Trammell et al., 2004; Rhoades et al., 2005). Soil moisture varies considerably throughout the growing season, characteristically decreasing from saturated conditions in early spring to xeric conditions in summer and autumn (Aldrich et al., 1982). A literature review and personal communication revealed this vegetation type to be associated with five soil orders and 37 series: 22 Alfisols, six Ultisols, five Mollisols, three Inceptisols, and one Vertisol (Table III).

The vast majority of sites occur on moderate to steep slopes, where soil erosion generally exceeds soil genesis (Ware, 2002). Therefore, topographic position and soil texture may profoundly affect the distribution of XLPs. Many of the soil series upon which sites occur are relatively fine-textured clay loams and silty clay loams, which are extremely susceptible to erosion when they occur on moderate to steep slopes. In Kentucky, many of the soil mapping units associated with XLPs are moderately to severely eroded (Lawless et al., 2004). However, other studies include only the soil series, rather than mapping units per se, upon which sites occur and thus do not include detailed descriptions of these series under localized conditions. Heikens (1991) reported only soil associations for sites in southern Illinois, but each of the five series (Alford, Hormer, Muren, Stoy, and Weir) that make up these associations are Alfisols.

Climate

Climates in which XLPs occur in eastern United States are categorized as humid subtropical and humid temperate in Koppen's classification system (Ackerman, 1941). In areas of the southeastern United States (Arkansas, Alabama, Georgia, Tennessee, Kentucky, and Virginia) and in southern portions of certain midwestern states (Missouri, Illinois, Indiana, and Ohio), this vegetation type occurs in humid subtropical climates (Cfa) with rain in all seasons and hot summers (mean temperature of warmest months [less than or equal to] 22.2[degrees]C) (Trewartha, 1968, from Ackerman, 1941). XLPs in central to northern portions of midwestern United States (Missouri, Illinois, Indiana, and Ohio) and in the Ridge and Valley of West Virginia and Pennsylvania have a humid temperate climate (Dfa) with rain in all seasons and hot summers (Trewartha, 1968, from Ackerman, 1941). The primary difference between Cfa and Dfa climatic regions is the number of inclusive months with mean monthly temperatures >10[degrees]C. Subtropical climates (Cfa) have 8 to 12 and temperate climates (Dfa) 4 to 7 inclusive months with mean monthly temperatures >10[degrees]C (Trewartha, 1968, from Ackerman, 1941).

Precipitation patterns and amounts differ considerably between XLP sites in the eastern United States and grasslands of the Central Plains and Prairies Formation (sensu Barbour & Billings, 2000). Seasonality of precipitation throughout the range of XLPs is considerably less than that in the Central Plains and Prairies Formation (Markham, 1970), where amounts of precipitation are relatively high during the growing season and significantly less during the remainder of the year (Borchert, 1950). Mean annual precipitation generally decreases from south to north in the range of XLPs, varying from about 100 cm in midwestern United States to 150 cm in southeastern United States. Conversely, mean annual precipitation in the Central Plains and Prairies Formation decreases from east to west and typically is [less than or equal to] 90 cm (National Climatic Data Center, 2001).

Vegetation

[C.sub.4] perennial grasses (Schizachyrium scoparium, Bouteloua curtipendula, Andropogon gerardii, Sorghastrum nutans, and Sporobolus clandestinus) are typically dominant in XLPs. Schizachyrium scoparium is the characteristic dominant species in XLPs in eastern United States, having the highest cover, frequency, and/or importance value in 18 of 20 studies for which quantitative vegetation data are available (Table IV). However, in some sites [C.sub.3] perennial herbs (e.g., Silphium terebinthinaceum and Monarda fistulosa var. brevis) and/or taxa in the Cyperaceae, particularly Carex spp. and Fimbristylis spp., have cover values that equal or exceed those of dominant grasses. When examined at fine spatial scales (<1.0 [m.sup.2]), the [C.sub.4] summer annual grasses Sporobolus vaginiflorus and Sporobolus neglectus are locally dominant in shallow-soil zones of some sites (Skinner, 1979; Ver Hoef et al., 1993; Baskin & Baskin, 2000; Lawless, 2005).

In any particular site, species richness of dominant and subdominant perennial graminoids is low and that of perennial [C.sub.3] forbs is relatively high. However, the forb component of XLP floras varies considerably among sites and especially across ecoregions (Lawless et al., 2004; Lawless, 2005). Therefore, vegetation of XLPs, like that of the tall grass prairie (Glenn & Collins, 1990), is characterized by a stable matrix of dominant graminoids and a forb component that varies considerably both within and among sites (Lawless, 2005).

In the following section, vegetation of XLP communities in the eastern United States will be reviewed and discussed by physiographic province. This review focuses on quantitative vegetation data, but it also includes qualitative descriptions for regions where quantitative data are not available. For quantitative studies conducted by DeSelm (1988, 1991, 1993) and DeSelm and Webb (1997), mean cover values have been recalculated to incorporate quadrats that did not include the taxon of interest (i.e., cover = 0%), because DeSelm included only quadrats in which the taxon of interest was present.

OZARK PLATEAUS

Hall (1955) compared ground-layer species composition and Juniperus virginiana seedling distribution and population structure in an old field and "glade" (= XLP) in the Missouri Botanical Garden Arboretum in the northeastern section of the Salem Plateau. In the glade, Sporobolus neglectus had the highest frequency (1.00) in 1-[m.sup.2] quadrats, followed by Carex crawei (0.90), Schizachyrium scoparium (0.80), Houstonia longifolia (0.75), Euphorbia corollata (0.60), and Rudbeckia missouriensis (0.55). Hall described glade community structure and vegetation dynamics as follows: "In a good year the clumps of Andropogon [= Schizachyrium scoparium] are larger, denser, and more frequent. The [annual] Sporobolus effectively fills in between the clumps. In a dry or unseasonable year the Andropogon may lose ground, the Sporobolus to a lesser extent, but the Bouteloua [curtipendula] may increase considerably." Of 6694 J. virginiana seedlings recorded in the glade, 99.7% were in the smallest size class ([less than or equal to] 0.46 m in height), thus indicating extremely high seedling mortality in the glade. Hall concluded, "In general, juniper population density is proportional to the degree of land abuse, so that glades may support junipers distributed as dense 'brakes,' open stands with evenly but widely spaced individuals, or scattered, clumped colonies."

Schizachyrium scoparium had both the highest mean cover and frequency in sites sampled by Kucera and Martin (1957) in the White River region of the Salem Plateau in southwestern Missouri. Locally dominant graminoids included Andropogon gerardii, Bouteloua curtipendula, Panicum virgatum, Sorghastrum nutans, Sporobolus heterolepis, and Tridens flavus. Hedyotis nigricans, a [C.sub.3] herbaceous perennial, was the only forb with a relatively high frequency (57%). Kucera and Martin (1957) also reported increased frequencies of [C.sub.3] summer annuals (Ambrosia artemisiifolia, Croton capitatus, and Croton monanthogynus) "during the drouth years." They also noted increased densities of Juniperus virginiana in some sites, which in some instances resulted in "enclosure."

Skinner (1979) sampled vegetation associated with three threatened glade species (Penstemon cobaea var. purpureus, Centaurium texense, and Stenosiphon linifolius) using 0.01-[m.sup.2] and 0.1-[m.sup.2] quadrats centered around individuals of focal species. As noted by Baskin and Baskin (2000), "Sporobolus neglectus had the highest percent occurrence in quadrats of both sizes placed around Penstemon and Centaurium, and Schizachyrium scoparium had the highest percent occurrence in those of both sizes placed around S. linifolius, which grows in deeper soil (15.7 [+ or -] 6.1 cm) than Penstemon (10.9 [+ or -] 5.6 cm) or Centaurium (5.3 [+ or -] 2.8 cm)."

Schizachyrium scoparium had the highest mean importance value (29.2%) in 20 sites sampled by Hicks (1981) in the Hercules Glade Wilderness in the Salem Plateau. Importance values were obtained from frequency and cover data collected in fifty 20 cm x 50 cm quadrats per site. Other taxa with mean importance values [greater than or equal to] 5% included Hedyotis nigricans (10.5%), Sporobolus neglectus (9.5%), Rudbeckia missouriensis (7.3%), Panicum virgatum (6.4%), and Croton spp. (C. capitatus and C. monanthogynus) (5.1%). Woody species data were collected in one or two 15 m x 25 m macroplots per site. Juniperus virginiana had the highest mean importance value (41.2%) of all tree species, followed by Cotinus obovatus (19.2%) and Diospyros virginiana (12.5%). The mean importance value of Rhus aromatica (58.8%) exceeded those of all other shrubs and of all vines.

Ver Hoef et al. (1993) sampled 24 sites on Gasconade Dolomite and seven sites on Eminence Dolomite in the vicinities of the Current and Jack's Fork rivers in the Salem Plateau of southeastern Missouri. Mean percent cover values reported in this study actually represent average geometric means (range = 0 to 31.6), since cover class midpoints were converted to geometric midpoints prior to calculations of means. On both substrates, Schizachyrium scoparium had the highest mean cover value (31.6 on each) in 7 m x 7 m quadrats. Mean cover of Rudbeckia missouriensis (Gasconade, 31.6; Eminence, 14.1) and of the perennial [C.sub.4] grass Sporobolus clandestinus (Gasconade = 9.6; Eminence = 15.4) also was high on both substrates. Fimbristylis caroliniana had the second highest mean cover value (23.0) on Eminence Dolomite glades, followed by Hedyotis nigricans (15.4) and Liatris cylindracea (15.4). TWINSPAN analysis of cover class data collected in 7 m x 7 m quadrats identified Bouteloua curtipendula and Sorghastrum nutans as significant indicator species of Gasconade Dolomite sites. Aristida purpurascens was the only significant indicator species of Eminence Dolomite sites. Mean cover of Sporobolus vaginiflorus exceeded that of all perennial grasses in 70 cm x 70 cm quadrats sampled in the "shallow soil glade" (mean soil depth = 7.5 cm) microhabitat in the Brandewiede Hollow East A site and in the "rocky glade" (5.9 cm) and "shallow soil glade" (10.7 cm) microhabitats in the Cave Spring E site, both of which are on Gasconade Dolomite. Juniperus virginiana had the highest mean basal area on both substrates. However, woody species richness and total basal area in Gasconade Dolomite sites was significantly higher than those in Eminence Dolomite sites.

The only vegetation data for the Springfield Plateau region in Missouri were collected by George (1996). Sampling was conducted in two sites in both spring (June) and autumn (October) using 1-[m.sup.2] quadrats. Panicum virgatum had the highest mean cover in both seasons in the Cross Timbers site and in autumn in the Silver Mine Ridge Road site. Schizachyrium scoparium had the second highest cover in both seasons at the Cross Timbers site and the highest cover in spring in the Silver Mine Ridge Road site. Cover of [C.sub.4] perennial grasses increased from 10% to 70% between spring and autumn sampling. Other species with relatively high cover values in one of these sites included Rudbeckia missouriensis, Echinacea paradoxa var. paradoxa, Bouteloua curtipendula, Liatris aspera, Liatris squarrosa, Fimbristylis puberula, Hedyotis nigricans, Andropogon gerardii, and Silphium terebinthinaceum. Juniperus virginiana was the dominant woody taxon in both sites.

Of the four "glade" community types (grassland-cedar, cedar, cedar-hardwood, and hardwood) described by Keeland (1978) in the Springfield Plateau, Salem Plateau, and Boston Mountains regions of northern Arkansas, only the grassland-cedar type fits the description of XLPs. The other community types presumably represent seral stages from a grassland-cedar community type to a hardwood forest (Baskin & Baskin, 2000). In the grassland-cedar community type, Schizachyrium scoparium had the highest mean cover value (22%), followed by Desmodium rotundifolium, Sorghastrum nutans, and Desmodium paniculatum. Juniperus virginiana and Quercus stellata were the dominant overstory species, and Rhus glabra had the highest importance value of all understory species.

In the Buffalo National River in north-central Arkansas, Logan (1992) estimated abundance of species in limestone and sandstone "glades" "using a subjective five point scale," ranging from "5 [=] abundant (widespread with high cover, a dominant or near-dominant species" to "1 [=] rare (only one or two individuals)." Schizachyrium scoparium was in the highest abundance class in 18 of 20 limestone "glades." Andropogon gerardii, Sporobolus vaginiflorus, and Bouteloua curtipendula were dominant or codominant in these sites. Coreopsis tinctoria, Acalypha gracilens, Eupatorium altissimum, and Rudbeckia missouriensis were the only forbs in abundance classes 4 or 5 in three or more sites. Abundant woody species included Juniperus virginiana, Quercus marilandica, and Quercus stellata.

CENTRAL LOWLAND

Only one of five limestone "barrens" sampled by Heikens (1991) in southern Illinois is in the Central Lowland (Fults Limestone Glade, Monroe County, Illinois). The only taxon with a mean cover value [greater than or equal to] 1.0% in fifteen 50-[m.sup.2] circular plots was Schizachyrium scoparium (12.0%). Bouteloua curtipendula, Dalea purpurea, Lespedeza capitata, Rudbeckia missouriensis, Solidago nemoralis, and the site dominant, S. scoparium, were present in 100% of these quadrats. Woody taxa present in five or more of the 15 quadrats were Comus drummondii (7 quadrats) and Ceanothus americanus (6).

Community structure differed markedly among three "limestone glades" sampled by McClain and Ebinger (2002) in Calhoun County, Illinois. In Schleeper Glade, Hedyotis nigricans [percent importance value (%IV) = 35.9] and Bouteloua curtipendula (35.6) were codominant, and Croton capitatus (9.4) was the only other taxon with a %IV[greater than or equal to]5. Bouteloua curtipendula had the highest %IV (= 20.2) in Kamp's Glade, followed by Ruellia humilis (11.2), Croton capitatus (10.6), Schizachyrium scoparium (8.0), and Hedyotis nigricans (5.5). Lead Hollow Glade was the only site in which S. scoparium was dominant (%IV = 30.5). Hedyotis nigricans (%IV = 16.0), and B. curtipendula (13.1) were subdominant, and R. humilis (7.5), Eupatorium altissimum (6.0), and Croton capitatus (5.7) were the only other taxa with %IV [greater than or equal to] 5.

Vegetation data are not available for sites in the Central Lowland in Indiana. Homoya (1987) qualitatively described the vegetation of a "limestone glade" in Versailles State Park (Ripley County) as follows: "... Andropogon gerardii, Aster sagittifolius [Symphyotrichum urophyllum], Silphium trifoliatum, Physostegia virginiana, Lithospermum canescens, Kuhnia eupatorioides, Helianthus hirsutus, and Euphorbia corollata are the herbaceous species with the highest estimated importance values." Important woody species included Quercus muhlenbergii, Viburnum rufidulum, Celastrus scandens, Cercis canadensis, Ceanothus americanus, and Juniperus virginiana." In two "glades" in the Indiana Army Ammunition Plant (Clark County), Maxwell (1987) reported, "Little bluestem [Schizachyrium scoparium] dominates both glades as a xeric, bunch-grass surrounded by patches of rocky pavement." The following herbaceous species were present in both the "little bluestem glades" and the limestone cedar glades surveyed by Baskin and Baskin (1975) in nearby Bullit County, Kentucky: Ruellia humilis, Manfreda virginica, Isanthus brachiatus, Ophioglossum engelmannii, Sporobolus vaginiflorus, Heliotropium tenellum, Scutellaria parvula, Houstonia canadensis, Sisyrinchium albidum, Asclepias verticillata, and Croton monanthogynus. Juniperus virginiana also was common in the "open glades" surveyed by Maxwell (1987).

The distribution map of "limestone glades" in Missouri compiled by Ladd and Nelson (1982), shows a concentration of sites in the vicinity of the Salt, Missouri, and Mississippi rivers in the glaciated portion of the state. However, to our knowledge, vegetation data are not available for this region. Density of glades (measured as areal coverage per 7.5-minute quadrangle) in this region is low ([less than or equal to]10 acres/7.5-minute quadrangle) in comparison with the Ozark Plateaus (Ladd & Nelson, 1982). This inequity in glade development between regions (Ozark Plateaus and Central Lowland) may have prompted vegetation scientists to focus their research on the Ozarks, where glades are both more numerous and typically larger in comparison with those in the Central Lowland (Nelson & Ladd, 1982a). However, collection of vegetation data in XLPs in the glaciated portion of the Central Lowland in Missouri would fill a gap in our knowledge of this vegetation type in eastern United States.

INTERIOR LOW PLATEAUS

Four of five "limestone glades" sampled by Heikens (1991) in southern Illinois are in the Interior Low Plateaus. According to Heikens, diagnostic features of these limestone glades include: cover of "prairie species" [greater than or equal to]10%, cover of woody species < 50%, soil depth < 10 cm, and exposed rock [greater than or equal to]5%. Schizachyrium scoparium was the dominant taxon in three sites, with mean cover values ranging from 15.7% to 25.5%. Silphium terebinthinaceum (mean cover = 9.1%) and S. scoparium (7.9%) were codominant in the fourth site (Whoopie Cat Bluff). Symphyotrichum oblongifolium (mean cover = 6.9%) and Echinacea pallida (4.9%) were subdominant taxa in the Cave Creek site. Quercus muhlenbergii and Juniperus virginiana were the only additional taxa with mean cover values [greater than or equal to]5% in one or more sites. TWINSPAN analysis of "natural forest openings" sampled by Heikens (1991) identified Physostegia virginiana, Manfreda virginica, and Asclepias verticillata as significant indicator species of the limestone glade community type.

Twenty-three of 32 "limestone glades" sampled by Kurz (1981) are in extreme southern Illinois in the Interior Low Plateaus. The remaining nine sites occur "along the Mississippi River valley" (Jersey, Monroe, St. Clair, and Union counties) and "on the uplands above the Illinois River" (Calhoun and Pike counties) in the Central Lowland. For select taxa, Kurz (1981) provided frequency data averaged across all sites. However, as mentioned above, the majority of sites (71.9%) are in the Interior Low Plateaus (Shawnee Hills), and thus the cursory data provided are discussed in the context of this physiographic province. In 30 of 32 sites, Schizachyrium scoparium had the highest frequency in 0.25-[m.sup.2] circular quadrats. Bouteloua curtipendula and Sorghastrum nutans each had the highest frequency in one of the other sites. According to Kurz (1981), "Dominant species of forbs occurring in several glades were Aster oblongifolius [= Symphyotrichum oblongifolium], Echinacea pallida, Croton monanthogynus, Hedyotis nigricans, Manfreda virginica, Euphorbia corollata, Physostegia virginiana, and Brickellia eupatorioides [= Kuhnia eupatorioides]."

Braun (1928) sampled "prairies" and "cedar barrens" on Crab Orchard Shale (= Estill Shale), Cedarville Dolomite (= Peebles Dolomite), and Monroe Dolomite (= Lilley and Bisher formations) in the Mineral Springs Region of Adams County, Ohio. She described sites developed on Crab Orchard Shale as "barren cleared slopes, everywhere showing the effects of slumping." According to Braun (1928), Juniperus virginiana, Panicum flexile, Allium cernuum, Cuphea viscosissima, Sabatia angularis, Ratibida pinnata, Monarda fistulosa, and Senna marilandica are characteristic, and Hedyotis nigricans is "particularly characteristic" of sites developed on this substrate. She reported frequency data for sites developed on Cedarville [Turkey Creek (two sites) and Cedar Fork (three sites)] and Monroe [Turkey Creek (two sites)] dolomites (Table IV, pages 427-431). Andropogon gerardii had the highest frequency (100%) in both sites on Monroe Dolomite (= Lilley and Bisher formations), followed by Silphium terebinthinaceum (90% and 82%, respectively). Schizachyrium scoparium had the highest frequency in three (Turkey Creek, site 1; Cedar Fork, sites 1 and 3) of five sites on Cedarville Dolomite. In Turkey Creek site 2, Fragaria virginiana had the highest frequency (55%), followed by Blephilia ciliata (52%), Helianthus occidentalis (50%), Lithospermum canescens (47%), Schizachyrium scoparium (43%), Andropogon gerardii (40%), and Delphinium exaltatum (40%). In Cedar Fork site 1, Manfreda virginica and Bouteloua curtipendula had the highest frequency values (90% and 78%, respectively). Remaining taxa with frequency values [greater than or equal to]50% in one or more sites include Agrimonia pubescens, Carex crawei, Leucanthemum vulgare (normative species), Comandra umbellata, Pycnanthemum tenuifolium, Ruellia humilis, Euphorbia corollata, Helianthus hirsutus, Isanthus brachiatus, Scutellaria parvula, Packera (Senecio) plattensis, Solidago nemoralis, and Sorghastrum nutans. With regard to woody species encroachment, Braun stated: "Among the tree invaders, Juniperus virginiana and Thuja occidentalis are most prominent; with these are Ostrya, Cercis, and Quercus muhlenbergii."

As is the case for the Central Lowland, vegetation data are lacking for XLPs in the Interior Low Plateaus in Indiana. Bacone et al. (1983) surveyed "glades" and "barrens" of Crawford and Perry counties and provided the following qualitative description: "On the barrens, prairie dock [Silphium terebinthinaceum] was very common, and Elliott's bluestem [Andropogon elliottii] was an important grass, in addition to big bluestem [Andropogon gerardii], little bluestem [Schizachyrium scoparium], and Indian grass [Sorghastrum nutans]." Aldrich et al. (1982) mention very little about vegetation in "limestone glades" in Harrison County, except for the following general statement: "They are dominated by prairie grasses and forbs."

Lawless et al. (in press) used a multiple-scale sampling design to identify XLP community types (0.01- to 100-[m.sup.2]) in 18 sites in the Interior Low Plateaus in Kentucky. Cluster analysis of cover class data obtained from 100-[m.sup.2] quadrats identified 12 community types. Schizachyrium scoparium had the highest mean cover value in 10 of these community types, and Andropogon gerardii and Silphium terebinthinaceum each had the highest mean cover value in one of the two remaining 100-[m.sup.2] community types. Dominant taxa (mean cover [greater than or equal to]10%) in three or more community types include S. scoparium (10 community types), Sporobolus vaginiflorus (5), Sorghastrum nutans (5), Echinacea simulata (5), Andropogon gerardii (4), and Carex crawei (3). Andropogon gerardii was subdominant (5% [less than or equal to]mean cover < 10%) in seven community types, and six taxa, including Echinacea simulata, Carex meadii, Hedyotis nigricans, Sorghastrum nutans, Juniperus virginiana, and Cercis canadensis, were subdominant in three community types. Sporobolus vaginiflorus was locally dominant in portions of many sites, as evidenced by its high percent occurrence values in numerous fine-scale community types (0.01- and 0.1-[m.sup.2]). In addition, high percent co-occurrence values of S. vaginiflorus and S. scopariumin certain fine-scale community types suggest that S. vaginiflorus is capable of coexisting with the dominant perennial grass over a range of environmental conditions. Characteristic trees and shrubs in this vegetation type in Kentucky include Juniperus virginiana, Cercis canadensis, Diospyros virginiana, Fraxinus americana, Rhamnus caroliniana, Rhus aromatica, Cornus florida, and Rosa carolina (Lawless, 2005).

Schizachyrium scoparium also is the dominant taxon in XLPs in the Interior Low Plateaus in Tennessee. DeSelm (1991) studied "small remnants of barrens" in the Central Basin, which "occur on shallow soil ... as an alternative to the cedar forest which borders glades." Schizachyrium scoparium had the highest mean cover value in each of the three sites for which data were reported: Mt. Juliet Road "glade" (mean cover = 40%); Mt. Juliet Road barren (40%); and Cedars of Lebanon glade border (76%). Sporobolus asper was codominant (mean cover = 35%) in Mt. Juliet Road barren. Although Bouteloua curtipendula was subdominant in Cedars of Lebanon glade border (mean cover = 15%), its distribution apparently was restricted within the site, as evidenced by its low frequency (15%). Hedyotis nigricans had the highest frequency (100%) and highest mean cover (19%) in the Mt. Juliet Road "glade." No other taxon had a mean cover value [greater than or equal to]5% in any site. Schizachyrium scoparium, Sporobolus vaginiflorus, and the [N.sub.2]-fixing cyanobacterium Nostoc commune had the highest frequency values in another site in Cedars of Lebanon State Forest sampled by Baskin and Baskin (1977). Schizachyrium scoparium also was the dominant taxon in three "barrens" in the Western Highland Rim (Perry and Decatur counties) sampled by DeSelm (1988). Other taxa with percent importance values [greater than or equal to]5% in one or more sites include Galactia volubilis, Opuntia humifusa, Euphorbia corollata, Sporobolus clandestinus, Solidago nemoralis, Croton monanthogynus, Fimbristylis puberula, Andropogon gerardii, and Sorghastrum nutans.

DeSelm and Webb (1997) sampled the "lower flat" (0-5% slope), "footslope" (11-20%), and "steep sideslope" (32-70%) in two grasslands in the Southern Highland Rim in northwestern Alabama. In both sites (Littleville and Cedar Creek), footslopes and steep sideslopes had the lowest mean soil depths (6.5-13.5 cm) and highest mean combined cover values of rock and gravel (24.4-86.0%), and thus portions of sites occupying these slope positions can be categorized as XLPs. In the footslope of Littleville Barren, mean total plant cover was only 46.3%, and Schizachyrium scoparium (mean cover = 5.0%) was the only taxon with a mean cover value [greater than or equal to]2.0%. Sporobolus clandestinus (mean cover = 30.2%) and Schizachyrium scoparium (16.2%) were the dominant and subdominant taxa, respectively, in the steep sideslope. Schizachyrium scoparium was the dominant taxon in both the footslope (mean cover = 18.7%) and steep sideslope (21.3%) of Cedar Creek Barren. Bouteloua curtipendula, Fimbristylis puberula, Sorghastrum nutans, and Celtis tenuifolia were the only other taxa with mean cover values >1.0% in either landform in this site.

RIDGE AND VALLEY

Bouteloua curtipendula, Danthonia spicata, and Schizachyrium scoparium "dominated in terms of abundance" in XLPs in Pennsylvania studied by Laughlin and Uhl (2003). However, quadrat sampling was not performed, and, therefore, abundance values reported in this study are semiquantitative. According to Laughlin (2002), common forbs in these sites are Anemone virginiana, Solidago nemoralis, Monarda fistulosa, Lithospermum canescens, Asclepias tuberosa, A. verticillata, A. viridiflora, Penstemon hirsutus, and Senecio obovatus, and common trees are Juniperus virginiana, Juglans nigra, Celtis occidentalis, Pinus strobus, and P. virginiana. Percentage of nonnative species in the flora of XLPs in Pennsylvania is particularly high (25% and 28% in two sites) (Laughlin, 2004). Consequently, Laughlin and Uhl (2003) concluded, "Exotic invasion is likely a significant reason for the loss of native species from limestone prairies."

Schizachyrium scoparium was the dominant taxon in 14 of 16 plots (11 sites total) sampled by Ludwig (1999) in southwestern Virginia. Andropogon gerardii was the dominant taxon in two plots. Other herbaceous taxa with cover values [greater than or equal to]5% in one or more sites include Symphyotrichum sericeum, Carex eburnea, C. hirsutella, Liatris aspera, Minuartia patula, Oxypolis rigidior, Scleria verticillata, Sorghastrum nutans, Sporobolus clandestinus, and S. vaginiflorus. In the "shrub layer" (height, 1-6 m), Cercis canadensis, Comus florida, Frangula caroliniana, Juniperus virginiana, and Quercus muhlenbergii were the only taxa with cover values [greater than or equal to]5%. In 12 of 16 plots, no taxa were present in the "tree layer" (6-10 m), and in the remaining four plots J. virginiana and/or Q. muhlenbergii were present.

Bartgis (1993) sampled "glades," "barrens," and "cedar woodlands" on Knobly and Cave mountains in West Virginia. Glades had shallower soils (mean depth, 3.9 cm) and higher cover of cobble and bedrock (mean cover, 74%) than did limestone barrens (mean soil depth, 8.9 cm; mean cover cobble/bedrock, 14%). However, perennial [C.sub.4] grasses and/or perennial [C.sub.3] herbs were the dominant taxa in both of these shallow-soil community types, and thus they can be classified as XLPs. In Knobly Mountain cedar glades, Bouteloua curtipendula (mean cover, 8%) and Solidago arguta var. harrisii (4%) were the dominant herbaceous taxa. Phlox subulata and Paronychia virginica were the only remaining herbaceous taxa with mean cover values > 1%. According to Bartgis (1993), "Bouteloua is much less important at the Cave Mountain cedar glades, where the dominants are typically Solidago arguta vat. harrisii, Monarda fistulosa var. brevis, and Paronychia virginica; Carex eburnea is locally dominant." Important woody taxa in these sites include Juniperus virginiana, Cercis canadensis, Quercus muhlenbergii, and Thuja occidentalis (Cave Mountain sites only). Total herbaceous cover was considerably higher in the Knobly Mountain limestone barrens, where B. curtipendula (mean cover, 35%), Elymus hystrix (12%), and Schizachyrium scoparium (5%) were the dominant taxa.

Eighty-six sites, including "glades" and "barrens," in the Ridge and Valley of Alabama, Georgia, Tennessee, and Virginia were studied by DeSelm (1993). He distinguished barrens from glades based upon percentage of perennial grass cover, with the former having perennial grass cover > 50% and the latter < 50%. Data were reported for three barrens, all of which are consistent with our definition of XLPs. In Crowder Cemetery Barren (Roane County, Tennessee), Andropogon gerardii (mean cover, 17.5%) and Schizachyrium scoparium (16.1%) were codominant, and Liatris cylindracea and Hypericum dolabriforme were the only other taxa with mean cover values > 1%. Schizachyrium scoparium (mean cover, 20.7%) and Bouteloua curtipendula (16.6%) were codominant in the Elementary School Barren (Catoosa County, Georgia). Subdominant taxa (5% < mean cover [less than or equal to]10%) include Panicum virgatum, Andropogon gerardii, and Hypericum dolabriforme. Schizachyrium scoparium (22.2%) and Aristida purpurascens (21.8%) were codominant in the third site (Centre, Alabama), and they were the only taxa with average cover values [greater than or equal to]1%.

Schizachyrium scoparium is also the dominant graminoid in the Ketona Dolomite "Glades" in the Ridge and Valley near its southwestern terminus, in Alabama. However, as noted by Allison and Stevens (2001), "... it usually does not achieve great density and is an aspect dominant only in late fall and winter, when the strong forb component is muted." Onosmodium decipiens and Erigeron strigosus var. dolomiticola, both Ketona Dolomite Glade endemics, were abundant at the majority of sites. Allison and Stevens (2001) also report, "Amsonia ciliata var. tenuifolia is often abundant and dense enough to be an aspect dominant in spring, and Rudbeckia triloba var. pinnatiloba is occasionally an aspect dominant in summer." Ketona Dolomite Glades also are noteworthy for supporting a number of woody taxa that typically do not occur in the remainder of the range of xeric limestone prairies, including Pinus palustris, Sabal minor, and Croton alabamensis var. alabamensis.

According to Allison and Stevens (2001), "Ketona Glades fail several to many criteria" for each of the three community types (limestone cedar glades, XLPs, and deep-soil barrens) described by Baskin et al. (1994) in the Big Barrens Region of Kentucky and Tennessee. The authors conclude that "Ketona Glades come closest to the 'xeric limestone prairie' class" but point out the following characteristics of Ketona Glades that differ from those of XLPs described by Baskin et al. (1994): (1) their occurrence on dolomite rather than limestone, (2) multiple endemic taxa, and (3) presence of two species of Leavenworthia. However, as discussed above, XLPs occur on dolomite in various regions throughout the geographic range of this community type in the eastern United States. Furthermore, XLP endemics (Table IV) occur in regions other than the Cahaba River valley in Alabama, particularly in the Ozark Plateaus in Missouri and Arkansas. In addition, Leavenworthia spp., some of which primarily are limestone cedar glade endemics (e.g., Leavenworthia alabamica, Leavenworthia exigua vat. exigua, and Leavenworthia exigua var. laciniata), occur in other XLPs in the eastern United States (Baskin & Baskin, 1977; Maxwell, 1987; DeSelm, 1988, 1991, 1993; DeSelm & Chester, 1993; Ver Hoef et al., 1993; Webb et al., 1997; Gardner & Minnie, 2004; Lawless et al., 2004).

APPALACHIAN PLATEAUS

Vegetation data are not available for XLPs in the unglaciated Allegheny Plateau, which apparently are restricted to Belmont County, Ohio (Rick Gardner, pers. commun.).

COASTAL PLAIN

Harper (1920) is the only source of information on vegetation of "limestone prairies" in the Coastal Plain, in Alabama. He provided a cursory species list and estimated "the commonest species in the areas of natural prairie, in order of abundance," the first five of which are as follows: Schizachyrium scoparium, Polygala grandiflora, Fimbristylis puberula, Hedyotis nigricans, and Prunella vulgaris. He also states, "Juniperus virginiana is practically the only tree" throughout the geographically restricted area in which "natural prairie vegetation" is developed on limestone, a very uncommon substrate in the Coastal Plain of Alabama.

Endemic Taxa and Other Noteworthy Floristic Elements

Thirteen taxa are endemic/near endemic to XLPs of eastern United States (Table V). The Cahaba River valley in the Ridge and Valley in Alabama and the Ozark Plateaus in Missouri and Arkansas are centers of endemism in this vegetation type. Allison and Stevens (2001) described eight new taxa endemic to XLPs on Ketona Dolomite rock outcrops in the Cahaba River valley in Bibb County, Alabama, including Castilleja kraliana, Coreopsis grandiflora var. inclinata, Dalea cahaba, Erigeron strigosus vat. dolomiticola, Liatris oligocephala, Onosmodium decipiens, Silphium glutinosum, and Spigelia gentianoides vat. alabamensis. Delphinium treleasei, Echinacea paradoxa var. paradoxa, and Scutellaria bushii are restricted to XLPs in the Ozark Plateaus in Missouri and Arkansas. Valerianella ozarkana occurs in the Ozark Plateaus in Missouri, Arkansas, and Oklahoma and in the Ouachita Mountains in eastern Oklahoma (USDA, NRCS, 2004; Hoagland et al., 2004), and thus should be considered an Ozark-Ouachita endemic. According to Bartgis (1993), "Monarda fistulosa vat. brevis appears to be endemic to cedar glades, limestone barrens, glade woodlands, and dry limestone cliffs of West Virginia and Virginia" (also see Kimball et al., 2001). This is the only XLP endemic outside the Ozarks Plateaus not restricted to Ketona Dolomite.

The flora of XLPs also contains taxa considered to be endemic to other vegetation types, most notably shale barrens and limestone cedar glades. Bartgis (1993) noted the occurrence of eight mid-Appalachian shale barren endemics (sensu Keener, 1983) in XLPs in West Virginia, including Calystegia spithamaea subsp, purshianus, Oenothera argillicola, Solidago arguta var. harrisii, Taenidia montana, Trifolium virginicum, Antennaria virginica, Helianthus laevigata (not an endemic), and Paronychia montana. Twelve of the 19 taxa considered to be limestone cedar glade endemics by Baskin and Baskin (1999) also occur in XLPs of eastern United States (Table VI), including Dalea gattingeri, Echinacea tennesseensis, Leavenworthia alabamica, Leavenworth& exigua var. exigua, Leavenworthia exigua var. laciniata, Leavenworthia exigua var. lutea, Leavenworthia stylosa, Lobelia appendiculata var. gattingeri, Onosmodium molle, Pediomelum subacaule, Talinum calcaricum, and Trifolium calcaricum. Interestingly, Allison and Stevens (2001) suggested Pediomelum subacaule, a [C.sub.3] perennial cryptophyte lacking long-distance dispersal mechanisms, was introduced in XLPs on Ketona Dolomite. The limestone cedar glade near-endemics Astragalus tennesseensis and Dalea foliosa (sensu Baskin & Baskin, 1999) also are known from XLPs in the Cumberland River Basin in Tennessee (Rutherford and Davidson counties) (DeSelm, 1991).

As noted by DeSelm (1991, 1993), limestone cedar glade endemic/near-endemic taxa often occur in XLPs that are adjacent or in close proximity to limestone cedar glades. Exceptions include Pediomelum subacaule and Leavenworthia exigua var. lutea in sites on Ketona Dolomite in Bibb County, Alabama, and Talinum calcaricum in an XLP in Logan County, Kentucky. In virtually all instances, occurrence of limestone cedar glade endemic/near-endemic taxa are restricted to microsites that fit the description of limestone cedar glades yet exist within the perennial grass matrix of XLPs [see Fig. 1 (middle) in Lawless et al., 2004].

Other floristic elements of XLPs also are restricted to vegetation types developed on calcareous rock outcrops. Catling et al. (1993) described a new sedge, Carex juniperorum, based on collections from alvars in the Napanee limestone plain in Ontario and from XLPs and similar habitats in southeastern Ohio (Adams County) and northeastern Kentucky (Lewis and Bath counties). Recently, Wieboldt (Belden et al., 2004) reported a new station for this species in Montgomery County, Virginia, where it occurs "in dry or seasonally wet spots in a small barren or in the thin chinquapin oak-redbud-red cedar woodland." Echinacea simulata apparently is restricted (or nearly so) to XLPs and similar habitats in Missouri, Illinois, Indiana, Kentucky, and Tennessee. However, inconsistent taxonomic treatment of this taxon (Baskin et al., 1993; Kim et al., 2004), its remarkable similarity to Echinacea pallida (Binns et al., 2002), and sympatry of Echinacea pallida and Echinacea simulata throughout much of the latter's geographic range (Missouri, Illinois, Indiana, and Tennessee) make it difficult to discern the true distribution of this species and assess its fidelity to XLPs. The winter annual Lesquerella filiformis is narrowly distributed in southwestern Missouri (Jasper, Dade, Lawrence, Greene, and Christian counties) (United States Fish and Wildlife Service [USFWS], 1988) and northwestern Arkansas (Izard and Washington counties) (Arkansas Natural Heritage Commission, 2004), where it "... occurs in shallow soils on limestone glades, outcrops in pastures and rarely in rocky open woods (USFWS, 1988)." Viola egglestonii ranges from southern Indiana (Harrison County) south to northern Alabama (Franklin County) and northern Georgia (Catoosa and Walker counties) and was once considered to be a limestone cedar glade endemic (sensu Baskin & Baskin, 1978, 1986, 1989). However, this species also occurs in XLPs and other gladelike habitats (DeSelm, 1993; Baskin & Baskin, 2003; Lawless et al., 2004), and thus it is no longer considered to be a limestone cedar glade endemic or even a near endemic (sensu Baskin & Baskin, 2003).

Floras of XLPs in the Ozark Plateaus in Missouri and Arkansas have western and southwestern affinities. Many taxa reach the eastern edge of their geographic range in XLPs in this region (Nelson & Ladd, 1982b); examples are Acacia angustissima var. hirta, Castilleja purpurea, Centaurium texense, Juniperus ashei, Marshallia caespitosa var. signata, Nemastylis nuttallii, Palafoxia callosa var. callosa, Parthenium hispidum, Penstemon arkansana, and Yucca glauca var. glauca. Evolvulus nuttallianus, Onosmodium molle var. subsetosum, Oenothera macrocarpa subsp, macrocarpa, and Solidago gattingeri occur in XLPs in the Ozarks and are absent further east with the exception of limestone cedar glades in Middle Tennessee (Baskin & Baskin, 1986; Adams et al., 2003).

Origin and Maintenance

PHYSICAL ENVIRONMENTAL FACTORS AND ANTHROPOGENIC INFLUENCE

There is no general consensus regarding the origin of XLPs. Current distribution of this vegetation type in the eastern United States likely reflects interaction of multiple factors over variable time intervals ranging from decades to millennia. DeSelm and Murdock (1993) astutely acknowledged, "The scattered occurrence of grassland and related communities [in southeastern United States] does not argue strongly for climate nor great soil group control over community distribution?' XLPs in the Ozark and mid-western regions of United States also are scattered geographically and occur on numerous soil series and orders. Shallow soils (including 37 series in five orders), calcareous bedrocks resistant to weathering, moderate to steep slopes (typically with south to west aspects), anthropogenically increased fire frequencies (native Americans and/or European settlers), land clearing for cultivation or pasturage, grazing (by both native and nonnative mammals), and erosion are factors frequently mentioned, singly or in concert, as potential sources of origin and/or maintenance for this vegetation type (Fig. 3).

[FIGURE 3 OMITTED]

Braun (1928), Steyermark (1940), and Logan (1992) suggest XLPs are a primary vegetation type. Logan (1992) states, "Glades are primary natural communities" which are "... maintained indefinitely at an early stage of succession by the substrate or by natural forces such as erosion or microclimate?' Steyermark (1940) regarded "glades" as the first two seral stages [(1) Bouteloua curtipendula-Rudbeckia missouriensis and (2) Rhus aromatica-Diospyros virginiana-Juniperus virginiana] of a six-stage primary succession culminating in an Acer saccharum-Quercus alba climatic climax (see reviews in Baskin & Baskin, 2000, and Hicks, 1981). Braun (1928) offered a compelling argument against the notion of a recent, anthropogenic origin for "extensive prairie vegetation" developed on Cedarville [= Peebles] Dolomite in the Mineral Springs region of Ohio.
 Clearing, the entrance and occupancy by a prairie community and its
 gradual elimination by forest invasion--which on the edges is
 extremely slow--would have had to take place within a period of 125
 years. It seems far more plausible that the forest is a stage in
 primary succession, representing the slow migration of the
 sub-climax prairie of ridges.


Braun (1928) goes on to state, "There is no vegetational evidence that this community ["prairie"] is not primary; and the oldest inhabitants say that there never have been anything but tall grasses and cedars in these places." Conversely, Braun (1928) refers to the "cedar barrens" developed on Crab Orchard [= Estill] Shale in this same region as "barren cleared slopes, everywhere showing the effects of slumping." Thus, she suggests an anthropogenic origin for these community types. Furthermore, Braun makes the following statement regarding the "Andropogon gerardii-Silphium [terebinthinaceum] prairies" developed on Monroe Dolomite [= Lilley and Bisher formations] in the region: "Their position in relation to cultivated fields and to cleared slopes suggests they may be secondary." Therefore, she (Braun 1928) considered XLPs developed on Peebles Dolomite to be primary and those developed on other substrates in Adams County to be secondary.

Other researchers also implicate agriculture as a potential source of origin and/or expansion for this vegetation type. B askin et al. (1994, 1997) attributed formation of this vegetation type in the Kentucky Karst Plain [= Pennyroyal Plain + Elizabethtown (Plain) sensu Quarterman and Powell, 1978] to the following sequence of events: clearing of marginal agricultural lands by European settlers [right arrow] cultivation and/or grazing significant erosion of the topsoil [right arrow] abandonment [right arrow] colonization of these degraded areas by XLP flora [right arrow] succession to hardwood forest in the absence of disturbance or maintenance of XLP with periodic management (i.e., disclimax) (Lawless et al., 2004). According to Rhoades et al. (2005), "Soil erosion associated with land clearing may be an alternative explanation [in contrast to geologic substrate] for the genesis of these glade openings [Crooked Creek Barrens; Lewis County, Kentucky]." DeSelm (1993) suggested land-use patterns of both native American and colonial European cultures affected the maintenance and possible expansion, respectively, of "barrens" (including XLPs) in the Ridge and Valley of Alabama, Georgia, Tennessee, and Virginia.
 Landscape disturbances caused by Native Americans, over and above
 such natural disturbances as lightning-caused fires, helped
 maintain necessary openings. European man pastured and burned these
 xeric sites and perhaps increased their area by plowing that
 resulted in soil loss.


For the Central Basin of Tennessee, DeSelm (1991) stated, the "... crescentic ring of barren-like vegetation between the glade and cedar thicket or forest" was the result of "clearing of the cedar or burning and grazing of this vegetation" and subsequent extension of perennial grasses. According to Bartgis (1993), "Most glades and barrens in West Virginia have been pastured, typically for sheep." However, he attributes the maintenance of XLPs to "periodic severe droughts," such as the one recorded in 1987 "which promote[s] prominence of Juniperus virginiana over deciduous species."

The vast majority of researchers attribute the origin and maintenance of XLPs to interaction of two or more of the following endogenous or exogenous factors: shallow soils, weathering-resistant bedrock, moderate to steep slopes (typically with south to west aspects), erosion, fire, and grazing. According to Heikens (1999), "The Ozark Plateaus supports a mosaic of prairies, forests, glades, barrens, and savannas depending on such factors as topography, bedrock, soils, fire, and native herbivores." Ver Hoef et al. (1993) stated, "Studies have indicated that bedrock type, topography, weathering, soil conditions, drought, and fire are some of the major factors influencing both the distribution of glade types and patterns within and around glades." Laughlin (2004) focused on maintenance rather than origin(s) of XLPs, yet he suggested anthropogenic fire and/or overgrazing (by nonnative ungulates) in combination with extremely xeric edaphic conditions (particularly in sites on Opequon soil series) were likely responsible for establishment of XLPs in the Ridge and Valley of Pennsylvania. Despite Logan's (1992) previously mentioned assertion that XLPs are primary communities, he also stated, "Xeric prairie and glade floras have evolved with drought and fire and need one or both to survive and compete with or displace encroaching species." According to Hicks (1981), "Prior to settlement by people of European origin, fires, either naturally occurring or set by Indians, and grazing by large herbivores such as elk and bison, now absent in the area, may have played an important role in maintaining the glade ecosystem." Kurz (1981) offers two possible explanations for the origin of sites in the glaciated portion of Illinois, both of which are plausible and not mutually exclusive.
 The slope of the glades usually faces south and it is suggested that
 the wind-blown silt [loess] was not deposited in these areas because
 of their position. Another suggestion is that the soils eroded over
 a period of time because of some disturbance leaving little loess
 covering the bedrock.


According to Aldrich et al. (1982), the persistence of limestone glades in Harrison County, Indiana, "... involves a complex interplay of factors including thin soils, a southerly aspect, seven to twenty degree slopes, xeric conditions and fire." Homoya (1994) attributes the xeric, open nature of barrens in Indiana to "... excessive drainage ...; southern or western aspects ...; excessive steepness of slope ... ; presence of a hardpan ...; presence of bedrock at or near the surface ...; and a fire regime that increases radiant heating and consumes moisture-retaining litter."

Mann et al. (1999) and Lowell and Astroth (1989) developed geographic information system models to predict locations of XLPs in the Cedar Creek Preserve in Fort Knox Military Reservation (Kentucky) and the Hercules Glade Wilderness Area in the Mark Twain National Forest (Missouri), respectively. Soil series were not a significant indicator of this habitat in Cedar Creek Preserve. However, when soil subgroup and depth ("depth of lithologic contact") were included, efficacy of the model increased considerably. Incorporation of additional soils (presence of flagstone on surface) and geologic (formation/member) data further increased predictive capability of the Mann et al. (1999) model. Presence of a mollic epipedon was determined to be the most diagnostic characteristic of "threatened calcareous habitat" in Cedar Creek Preserve. In contrast to the Mann et al. (1999) model, Lowell and Astroth (1989) found soil series (Gasconade) to be the best predictor of glade distribution in Hercules Glade Wilderness Area. Elevation and aspect of slope also were strong "controlling factors" of distribution and quality (high quality = limited redcedar invasion; low quality = extensive redcedar invasion) of XLPs in this region.

Several studies of aerial photographs obtained over relatively short chronological sequences (<50 years) suggest XLPs are quite susceptible to woody plant invasion. Laughlin (2004) reported areal reductions of 78% and 92% in Great Plains and Westfall Ridge "prairies," respectively, in the Ridge and Valley of Pennsylvania from 1949 to 1994. In limestone glades in the Ozarks of south-central Missouri, Kimmel and Probasco (1980) reported a 34% decrease in glade area with 0% to 15% woody plant cover and a 31% increase in glade area with 50% to 100% woody plant cover from 1938 to 1975. The authors attributed this phenomenon to decreased burning and grazing over the previous 30 years. Ver Hoef et al. (1993) documented average reductions in "glade" area of 14.4% and 32.4% from 1955 to 1984 on sites developed on Eminence Dolomite and Gasconade Dolomite, respectively. Rates of forest encroachment in XLPs in Hercules Glade Wilderness Area from 1938 to 1986 generally decreased when sites were burned (Lowell & Astroth, 1989). However, fire significantly increased the area of some low-quality sites (i.e., sites with significant encroachment of woody species) but failed to retard encroachment and corresponding decline in other low-quality sites.

Referring to limestone glades in Harrison County, Indiana, Aldrich et al. (1982) stated, "Aerial photographs document the continuing shrinkage of these glades, as they were nearly double their present size in the 1940's." Similarly, Bacone et al. (1983) reported, "Aerial photographs [of glades and barrens in Crawford and Perry counties, Indiana] show a remarkable decrease in size over the last forty years due to encroachment by woody species." In two "prairies" developed on Cedarville Dolomite in Adams County, Ohio, Annala et al. (1983) documented a 31% reduction in prairie areas between 1938 and 1971. The authors concluded, "The edaphic conditions of the prairie remnants [Lynx Prairie and Bohl Property] in Adams County [Ohio; Edge of Appalachia Preserve] undoubtedly have helped to retain them. Nevertheless, the successional process clearly is converting these areas to forest." In other sites in the Edge of Appalachia Preserve (The Wilderness, Buzzardroost Rock, Hanging Prairie, and Cave Hollow), Annala and Kaputska (1983) reported "... some degree of forest encroachment in all sites" from 1938 to 1971. Particularly interesting were the authors' observations that, "Many of the present-day prairie remnants have been established (or reestablished) on cultivated or pasture land," and "other areas were prairie in 1938 and indicated no evidence of agricultural disturbance."

Despite the susceptibility of many XLPs to encroachment by woody species, some sites apparently are quite stable. Although large perimeter to area ratios and patches of woody vegetation prevented accurate measurement of the area of XLPs in Kentucky (Lawless et al., 2004), comparison of aerial photographs taken of Muldraugh's Barren and Scudder Glade (Hardin County, Kentucky) in 1958 and 1988 does not show dramatic changes in these two sites. However, proximity of old homesteads (circa 18501900) to the latter and to Spalding Glade (Lame County, Kentucky) (Lane Linnenkohl, Kentucky State Nature Preserves Commission, pers. commun.) does not rule out anthropogenic disturbance (e.g., timber harvesting and grazing) as a possible cause of origin for these and other sites in Kentucky. It also is not known whether disturbance is the reason why succession is not evident in these sites between 1958 and 1988.

Because endogenous factors such as bedrock type, soil depth, and degree and aspect of slope are relatively static over geologic time periods, some researchers have focused on the frequency of disturbance events, particularly fire, as a potential source of variability in the area of XLPs in recent times. Juniperus virginiana individuals often are the source of dendrochronological data in XLPs owing to their abundance and relatively long life spans. In a study of seven live and 14 dead redcedars (J. virginiana) in Burtram Hollow "glades" (Ava Ranger District, Missouri Ozarks), Guyette and McGinnes (1982) reported, "Prior to 1870, fires, as marked by tree scars, were present somewhere on the study area (2.59 [km.sup.2]) every 3.2 years. After 1870, the frequency of scarring drops to one every 22 years." In conclusion, the authors made the following statement.
 Much of this change in fire frequency is due to fire suppression,
 possibly by early settlers, and after 1940 by the U.S. Forest
 Service. Other possible reasons for the change in fire frequency
 are the removal of a major ignition source (Osage Native Americans),
 the building of roads (firebreaks), severe overgrazing and erosion
 (reduced fuel loads).


Dendrochronological data also led Jenkins et al. (1997) to conclude that displacement of Native Americans (Cherokee) and subsequent fire suppression were largely responsible for increased woody plant density in a "savanna-glade-woodland mosaic" on Turkey Mountain in the Arkansas Ozarks. Variable fire-return intervals also were detected in the dendrochronological records obtained by Batek et al. (1999) in the Current River watershed in the Missouri Ozarks. Batek et al. made the following statement regarding the influence of Native Americans on historical fire frequencies and corresponding vegetation patterns.
 Fires exerted strong constraints on vegetation composition and
 patterns. Historical patterns of Native American occupancy in the
 region are consistent with the reconstructed vegetation and fire
 histories suggest that anthropogenic fire regimes played an
 overriding role in the development of Ozark vegetation in the
 1800's.


Although Beilmann and Brenner (1951) did not study dendrochronological records, their examination of numerous historical documents (particularly accounts of early travelers) led them to conclude, "Fire, perhaps more than any other factor, maintained the prairie and park-like aspect of the Ozarks." In a "barren-forest mosaic" in southern Indiana, Guyette et al. (2003) obtained dendrochronological results that contrast strongly with those of both Guyette and McGinnes (1982) and Batek et al. (1999). Guyette et al. (2003) summarized their findings as follows: "Fires were more frequent and occurred more regularly in the latter half of the fire chronology (1821 to 1999) where the mean fire interval was 5.3 years [as opposed to 23.0 years from 1650 to 1820] and fire intervals ranged from 1 to 40 years." Results of Guyette et al. (2003) suggest historic fire frequencies vary significantly between areas, and furthermore, that fuel loads, vegetation patterns, weather and climate, topography, and "pyro-cultural influence" profoundly affect fire-return intervals.

Boettcher and Kalisz (1991) used the mass and shape of opal phytoliths in dolomite-derived soils in an attempt to delimit the historical extent of XLPs in the E. Lucy Braun Preserve in Adams County, Ohio. Dumbbell-shaped phytoliths are characteristic of grasses, and therefore large concentrations of these structures are indicative of past and/or present grassland vegetation. Soil samples collected from XLPs and forests both had high concentrations of opal. However, dumbbell-shaped phytoliths were "infrequently encountered" in samples collected from both vegetation types, thus suggesting, "... phytolith-rich forbs as well as grasses may have been important constituents of these forest openings in the past." These data lead the authors to conclude, "... prairie and forest vegetation ... naturally alternated over time on these areas," and therefore, "... distinction between primary prairies [natural prairies] and secondary prairies [prairies formed by human disturbance of forests] has little meaning...."

CONCEPTUAL MODEL

Considerable reduction in the size of openings at many XLP sites over relatively short time periods (<50 years) support the conclusion that the overwhelming majority of them are quite susceptible to encroachment by woody species and thus are not primary. Dendrochronological studies conducted in this vegetation type suggest anthropogenically elevated fire frequencies associated with Native American cultures probably are responsible for maintenance (and possibly for the origin) of this vegetation type throughout much of the eastern United States (Fig. 3). However, Native American burning practices most likely resulted in establishment of XLPs only in areas with extremely shallow ([less than or equal to] 25 cm), rocky soils and high solar heat loads (moderate to steep slopes with south to west aspects). Prior to periodic anthropogenic disturbance, these areas would have supported relatively open-canopied, xerophytic forests that contained heliophytic herbaceous taxa characteristic of XLPs. Introduction of fire into these habitats would have decreased woody plant cover, resulting in increased insolation and thus in dominance by these heliophytic graminoids and forbs (see Jenkins & Jenkins, 1999).

In more mesic habitats, XLPs probably originated from clearing of forests for agricultural purposes (grazing, row crop cultivation, or timber harvesting). Such practices likely promoted soil erosion, especially on moderate to steep slopes, resulting in abandonment of these marginal agricultural lands. The more xeric conditions of this degraded habitat provided a suitable environment for colonization by the XLP flora (Baskin et al., 1994, 1997), which undoubtedly occurred at a rapid pace in areas adjacent to XLPs or open-canopied xerophytic forests.

In the absence of management (prescribed fire, mowing, cutting, grazing), succession to forested community types occurs in XLPs. Succession converts XLPs on deeper soils to hardwood forests in periods of a century or less. In more xeric sites, Juniperus virginiana/hardwood forests are the likely outcome of succession over prolonged time periods. The rate at which XLPs revert to forested community types via succession may differ between regions as a result of climate. For instance, in habitats of comparable soil depths and plant-available water holding capacities, succession may proceed much faster in the southern Interior Low Plateaus and Ridge and Valley than in the Ozark Plateaus owing to lower growing-season precipitation in the latter.

XLPs in the Cahaba River valley in Bibb County, Alabama, and perhaps those in some sites in the Ozark Plateaus in Missouri and Arkansas may be the only examples of this vegetation type that are edaphic climaxes. Although researchers have not studied aerial photographs or dendrochronological records in the Ketona Dolomite sites, their open nature at present suggests that they are able to persist in the absence of fire. Conversely, aerial photographic and dendrochronological data available for sites in the Ozark Plateaus suggest that these sites are relatively susceptible to encroachment by woody species in the absence of periodic fire. The heliophytic taxa endemic to XLPs in both of these regions [nine of 13 differentiated at the specific (versus subspecific or varietal) level] suggests that they could have persisted over geologic time spans.

Affinities of Xeric Limestone Prairies to Other Vegetation Types

EASTERN NORTH AMERICA

A number of other vegetation types in eastern North America are remarkably similar to XLPs because of their association with calcareous rock outcrops and dominance by [C.sub.4] perennial grasses. Dry lime prairies (sensu Anderson, 1954) and hill prairies (sensu Ugarte, 1987) in upper midwestern United States and "cedar glades" (sensu Curtis, 1959) in Wisconsin are developed on shallow, rocky soils derived from limestone and dolomite (calcareous gravels in some glaciated regions). Furthermore, these vegetation types typically occur on moderate to steep slopes with south to west aspects and are susceptible to invasion by woody species, particularly Juniperus virginiana.

Dry lime prairies are concentrated in the "rugged topography of the Driftless Area of southwestern Wisconsin" and extend into northwestern Illinois, northeastern Iowa, and southeastern Minnesota (Anderson, 1954). In 41 stands sampled by Anderson, Schizachyrium scoparium had the highest average frequency (71.3%), followed by Bouteloua curtipendula (62.4%) and Andropogon gerardii (50.2%). Additional taxa with mean frequency values >25% were Symphyotrichum sericeum, Euphorbia corollata, Amorpha canescens, Solidago nemoralis, Panicum perlongum, and Dalea purpurea. Rhus glabra and Juniperus virginiana were dominant woody invaders in dry lime prairies, along with Juniperus communis.

Schizachyrium scoparium, Andropogon gerardii, Sporobolus heterolepis, and Bouteloua curtipendula had the highest total cover values in 32 hill prairies in northeast Iowa sampled by Ugarte (1987). Solidago nemoralis, Aster azureus, Amorpha canescens, and Coreopsis palmata were characteristic forbs. Interestingly, Ugarte described a Bouteloua curtipendula-Sporobolus aft. vaginiflorus community type that developed on gravelly, heavily grazed sites. Furthermore, total cover of Juniperus virginiana exceeded that of all other woody species, with the exception of Rhus glabra. Ugarte attributed "encroachment of hill prairies by woody species" to "suppression of fire and overgrazing." Portions of some hill prairies described by Evers (1955) (e.g., Sunset Trail, Bielema, and Clendenny) apparently are shallow and rocky and are dominated by C4 perennial grasses, and thus they resemble XLPs.

According to Curtis (1959), "... a steep hillside of thin loess over limestone.., or a gravelly glacial moraine" are typical habitats for "cedar glades," which he and Bray (1955) considered a type of savanna. Schizachyrium scoparium had the highest average frequency (38.0%) in this vegetation type, followed by Euphorbia corollata (27.1%), Andropogon gerardii (26.6%), Arenaria stricta (23.0%), Solidago nemoralis (20.8%), Amorpha canescens (20.8%), and Anemone cylindrica (20.1%). Juniperus virginiana is the only dominant woody taxon in this vegetation type (data of J.R. Bray from Curtis, 1959).

"Calcareous prairies" in the Kisatchie National Forest in Louisiana, described by MacRoberts and MacRoberts (1993), are difficult to characterize owing to the complex geology in this region and lack of information on vegetation and soil depth. "Calcareous concretions" were reported in the soil surface in both sites. However, rock outcrops in proximity to these sites are primarily sandstone and support glade communities. The calcareous prairies "... are on or near the summits of hills ...," where soil depth is considerably greater than in adjacent sandstone glade communities (Michael MacRoberts, pers. commun.). Composites, grasses, and legumes are dominant taxa in these prairies, and both sites currently are being invaded by woody species (Crataegus spp., Rhus copallina, Viburnum dentatum, Comus drummondii, Diospyros virginiana, and Prunus spp.) from calcareous forests in the region. Both the calcareous prairies and sandstone glades are embedded within locally dominant longleaf pine savanna vegetation, and MacRoberts and MacRoberts (1993) attribute woody encroachment in the calcareous prairies to decreased fire frequency. Because of the uncertainty about whether these areas fit the definition of XLP, we have not included them on the distribution map (Fig. 2) of XLPs in eastern United States.

In eastern North America, alvars are restricted to the Great Lakes Region, where they are developed on Ordovician limestones and dolomites (Catling & Brownell, 1999). Catling and Brownell describe two principal alvar types: shoreline and plateau. They acknowledge disparate origins for these two community types in the following statement: "Lack of tree cover on shoreline alvars may be largely a result of flooding and erosion, whereas fire may be the most important factor in limiting woody cover on plateau alvars." Alvar grasslands most closely resemble XLPs and occur in both of these habitats. In contrast to alvar pavement community types, alvar grasslands typically occur in soils with depth >2 cm. Xeric grasslands are dominated by C3 (e.g., Carex scirpoidea, Danthonia spicata, and Poa compressa) perennial graminoids and are extremely variable with regard to forb composition (Catling & Brownell, 1999).

WESTERN NORTH AMERICA

In west-central Kansas, "very dry," "dry," and "dry mesic" prairies on rocky shallow soils (Mollisols) over limestone are floristically (at species or genus level) and vegetationally similar to the XLPs in eastern United States (Hulett & Tomanek, 1969; Heitschmidt et al., 1970; Hladek et al., 1972). Schizachyrium scoparium was the dominant species in remnant stands on very dry limy sites and was nearly as important as Andropogon gerardii in stands on dry sites (Hladek et al., 1972). Order of importance on dry mesic sites was Andropogon gerardii > Bouteloua curtipendula > Schizachyrium scoparium. Important forbs (all perennials) in these limestone prairies include Ambrosia psilostachya, Aster oblongifolius, Dalea purpurea, Echinacea angustifolia, Houstonia angustifolia, Lesquerella ovalifolia, Malvastrum coccineum, Oenothera serrulata, Paronychia jamesii, Psoralidium tenuifolium, Scutellaria resinosa, Solidago rigida, Tetraneuris stenophylla, Thelesperma gracile, and Tragia ramosa (Hulett & Tomanek, 1969; Heitschmidt et al., 1970; Hladek et al., 1972). Plant taxa endemic, or nearly so, to dry limestone prairies of the Mixed Grass Association of the southern Great Plains (southern Nebraska to northern Texas) include Aster fendleri, Clematis freemontii, Oenothera macrocarpa var. oklahomensis, O. freemontii, Phlox oklahomensis, Scutellaria resinosa, and Tomanthera densiflora (Great Plains Flora Association, 1977; 1986).

Mean annual precipitation in XLPs in eastern United States ranges from approximately 100 cm in Missouri (Missouri Climate Center, 2004) to approximately 150 cm in Alabama (Southeast Regional Climate Center, 2004). These precipitation regimes generally support forest vegetation in eastern United States (Braun, 1950), and thus the existence of nonforested herbaceous communities in this region (including XLPs) is not the result of low precipitation. Rather, these grasslands and rock outcrop communities are either edaphic climaxes (probably sites on Ketona Dolomite in the Ridge and Valley in Alabama and perhaps some sites in the Ozark Plateaus in Missouri and Arkansas in the case of XLPs), or they are the result of disturbance, either anthropogenic (vast majority of XLPs) or natural (e.g., scouring action of water bodies in riparian communities such as shoreline alvars and cobble bars).

Borchert (1950) attributed both the origin and maintenance of grasslands in the Central Plains and Prairies Formation to low mean annual precipitation and high seasonality in precipitation. Other researchers (e.g., Saner, 1950; Anderson, 1982; Axelrod, 1985) present cogent arguments for the combined effects of climate, fire, and grazing in establishment and maintenance of these grasslands. However, the rocky, shallow-soil grasslands in western and west-central Kansas (to which XLPs bear the closest resemblance) are an extraordinary case. In these sites, unlike the majority of prairies in central North America developed on deep, fertile Mollisols (Weaver, 1954), edaphic conditions are an important factor limiting recruitment, survival, and reproduction of woody taxa. The combination of these edaphic conditions with the low mean annual precipitation in this region (55 to 65 cm; Hladek et al., 1972; Heitschmidt et al., 1970) suggests that climatic and edaphic factors interact to produce the xeric conditions responsible for the absence or limited occurrence of woody taxa in these grasslands. Furthermore, low total plant cover (-10-25%; Heitschmidt et al., 1970; Hladek et al., 1972) in these rocky prairies makes it unlikely that they are even capable of supporting a fire.

Directions for Future Research

Additional studies are needed to further our understanding of the flora, vegetation, physical environmental characteristics, origin, and maintenance of XLPs in eastern United States. Phytogeographical analysis of the collective flora of this vegetation type in eastern United States would provide insight into the origins of the XLP flora and afford an opportunity for floristic comparisons among regions. Vegetation data are lacking for many regions, including the Central Lowland in Missouri and Indiana, the Interior Low Plateaus in Indiana, the Appalachian Plateaus in Ohio, and the Ridge and Valley in Bibb County, Alabama. Additional dendrochronological studies are necessary for a more complete evaluation of the historical role of fire in the origin and maintenance of XLPs. Additional soil chemical and physical analyses in XLPs and adjacent habitats could provide insight into the relative importance of edaphic conditions in curbing encroachment of woody taxa into XLPs. Analyses of soil profiles in XLPs may allow inferences about the factors (i.e., erosion and lack of profile development) responsible for current edaphic conditions. Aerial photographic studies, particularly for sites in southeastern United States, would allow further assessment of XLP stability under a variety of topographic, edaphic, and climatic conditions. Further comparisons (via floristic, vegetation, and physical environmental data) of XLPs, "cedar glades" in Wisconsin (sensu Curtis, 1959), and dry lime prairies (sensu Anderson, 1954) and loess hill prairies (sensu Ugarte, 1987) in the upper Midwest would facilitate development of a comprehensive classification system for xeric grasslands of eastern United States.

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Jenkins, S. E., R. P. Guyette & A. J. Rebertus. 1997. Vegetation-site relationships and fire history of a savanna-glade woodland mosaic in the Ozarks. Pp. 184-199 in S. G. Pallardy, R. A. Celich, H. G. Garrett & P. S. Johnson (eds.), Proceedings of the eleventh central hardwood conference, Univ. of Missouri, Columbia. USFS NCES GTR-NC 188.

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Keener, C. 1983. Distribution and biohistory of the endemic flora of the mid-Appalachian shale barrens. Bot. Rev. 49: 65-115.

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Kucera, C. L. & S.C. Martin. 1957. Vegetation and soil relationships in the glade region of the southwestern Missouri Ozarks. Ecology 38: 285-291.

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--, J. M. Baskin & C.C. Buskin. 2004. The floristic ecology of xeric limestone prairies in Kentucky, and a comparison to limestone cedar glades and deep-soil barrens. Sida 21: 1055-1079.

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Woodward, S. L. & R. L. Hoffman. 1991. The nature of Virginia. Pp. 23-48 + plates 1-24 in Virginia's endangered species, Coord. K. Terwillinger. The McDonald and Woodward Publ. Co., Blacksburg, Virginia. Figure 3 is a map of the physiographic provinces of Virginia drafted by S.L. Woodward (pets. commun, with J. M. Baskin).

PATRICK J. LAWLESS, (1) JERRY M. BASKIN, (1) AND CAROL C. BASKIN (2)

(1) Department of Biology University of Kentucky Lexington, Kentucky 40506-0225, U.S.A.

(2) Department of Plant and Soil Sciences University of Kentucky Lexington, Kentucky 40546-0312, U.S.A.
Table I
Summary of the physiographic distribution of xeric limestone prairies
in eastern United States organized by state and physiographic province
(sensu Fenneman, 1938, [1946] 1949)

 Physiographic/natural
State Physiographic province region

Alabama Interior Low Plateaus Little Mountain
 Interior Low Plateaus Moulton Valley
 Interior Low Plateaus Southern Highland Rim
 Ridge and Valley Cahaba Valley
 Coastal Plain Fall Line Hills
 Coastal Plain Southern Red Hills

Arkansas Ozark Plateaus Springfield Plateau
 Ozark Plateaus Salem Plateau
 Ozark Plateaus Boston Mountains

Georgia Ridge and Valley Chickamauga Valley


Illinois Central Lowland Chicago Lake Plain
 Central Lowland Western Grand Prairie
 Central Lowland Glaciated Middle
 Mississippi Border
 Ozark Plateaus Northern Section of Ozark
 Division
 Ozark Plateaus Southern Section of Ozark
 Division
 Interior Low Plateaus Lesser Shawnee Hills

Indiana Central Lowland Muscatatuck Flats and
 Canyons
 Central Lowland Switzerland Hills
 Interior Low Plateaus Mitchell Plain
 Interior Low Plateaus Mammoth Cave Plateau

Kentucky Interior Low Plateaus Marion [Area]
 Interior Low Plateaus Mammoth Cave Plateau
 Interior Low Plateaus Ohio River Hills and
 Lowlands
 Interior Low Plateaus Pennyroyal Plain
 Interior Low Plateaus Knobstone Escarpment &
 Knobs
 Interior Low Plateaus Outer Blue Grass

Missouri Central Lowland Osage Plains
 Central Lowland Eastern Glaciated Plains
 Central Lowland Lincoln Hills
 Ozark Plateaus Springfield Plateau
 Ozark Plateaus Upper Ozark
 Ozark Plateaus St. Francois Mountains
 Ozark Plateaus Elk River
 Ozark Plateaus White River
 Ozark Plateaus Lower Ozark
 Ozark Plateaus Ozark Border

Ohio Interior Low Plateaus Northeastern Blue Grass
 Appalachian Plateaus Marietta Plateau

Pennsylvania Ridge and Valley Appalachian Mountain

Tennessee Interior Low Plateaus Western Higland Rim
 Interior Low Plateaus Cumberland River Basin
 Ridge and Valley -- (a)

Virginia Ridge and Valley Allegheny Mountains

West Virginia Ridge and Valley -- (a)

State Physiographic reference Site reference(s)

Alabama Quarterman & Powell, 1978 DeSelm & Webb, 1997;
 Webb et al., 1997
 Quarterman & Powell, 1978 DeSelm, 1997; Webb et
 al., 1997
 Quarterman & Powell, 1978 Webb et al., 1997
 Sapp & Emplaincourt, 1975 Allison & Stevens, 2001
 Sapp & Emplaincourt, 1975 Webb et al., 1997
 Sapp & Emplaincourt, 1975 Harper, 1920

Arkansas Fenneman, 1938, Keeland, 1978
 [1946] 1949
 Fenneman, 1938, Keeland, 1978;
 [1946] 1949 Logan, 1992
 Fenneman, 1938, Keeland, 1978
 [1946] 1949

Georgia Clark & Zisa, 1976 DeSelm, 1993

Illinois Schwegman, 1973 Kurz, 1981
 Schwegman, 1973 Kurz, 1981; McClain &
 Ebinger, 2002
 Schwegman, 1973 Kurz, 1981
 Schwegman, 1973 Kurz, 1981; Heikens &
 Robertson, 1995
 Schwegman, 1973 Kurz, 1981; Heikens &
 Robertson, 1995
 Schwegman, 1973 Kurz, 1981; Heikens &
 Robertson, 1995

Indiana Homoya et al., 1984 Maxwell, 1987
 Homoya et al., 1984 Homoya, 1987
 Quarterman & Powell, 1978 Aldrich et al., 1982
 Quarterman & Powell, 1978 Bacone et al., 1983

Kentucky Quarterman & Powell, 1978 Lawless et al., 2004
 Quarterman & Powell, 1978 Lawless et al., 2004
 Quarterman & Powell, 1978 Lawless et al., 2004
 Fenneman, 1938, Lawless et al., 2004
 [1946] 1949
 Quarterman & Powell, 1978 Lawless et al., 2004;
 Rhoades et al., 2004
 Quarterman & Powell, 1978 Lawless et al., 2004;
 Rhoades et al., 2005

Missouri Thom & Wilson, 1980 Ladd & Nelson, 1982
 Thom & Wilson, 1980 Ladd & Nelson, 1982
 Thom & Wilson, 1980 Ladd & Nelson, 1982
 Thom & Wilson, 1980 Ladd & Nelson, 1982
 Thom & Wilson, 1980 Ladd & Nelson, 1982
 Thom & Wilson, 1980 Ladd & Nelson, 1982
 Thom & Wilson, 1980 Ladd & Nelson, 1982
 Thom & Wilson, 1980 Ladd & Nelson, 1982
 Thom & Wilson, 1980 Ladd & Nelson, 1982
 Thom & Wilson, 1980 Ladd & Nelson, 1982

Ohio Quarterman & Powell, 1978 Braun, 1928
 Brockman, 1998 Rick Gardner, pers.
 commun.

Pennsylvania Berg et al., 1989 Laughlin & Uhl, 2003

Tennessee Quarterman & Powell, 1978 DeSelm & Chester, 1993
 Quarterman & Powell, 1978 Baskin & Baskin, 1977;
 -- (a) DeSelm, 1993

Virginia Woodward & Hoffman, 1991 Ludwig, 1999

West Virginia -- (a) Bartgis, 1993

(a) Not reported.

Table II
Summary of the geologic substrates upon which xeric limestone prairies
occur in eastern United States. Formations and members are organized
by physiographic province and geologic system(s), respectively

Geologic
formation/member System(s) State(s) Reference(s)

Ozark Plateaus
St. Louis Limestone Mississippian Illinois Heikens, 1991
Salem Limestone Mississippian Illinois Heikens, 1991
Burlington Limestone Mississippian Missouri Nelson & Ladd,
 1983
Keokuk Limestone Mississippian Missouri Nelson & Ladd,
 1983
Boone Formation Mississippian Arkansas Theo Witsell,
 (limestone) pers. commun.
Pitkin Limestone Mississippian Arkansas Theo Witsell,
 pers. commun.
Kimswick Formation Ordovician Missouri George, 1996;
 (limestone) Arkansas Theo Witsell,
 pers. commun.
Plattin Formation Ordovician Missouri George, 1996;
 (limestone) Arkansas Logan, 1992
Joachim Dolomite Ordovician Missouri Erickson et
 al., 1942;
 Arkansas Theo Witsell,
 pers. commun.
Everton Formation Ordovician Missouri Erickson et
 (dolomite) al., 1942;
 Arkansas Theo Witsell,
 pers. commun.
Powell Dolomite Ordovician Missouri Erickson et
 al., 1942;
 Arkansas Theo Witsell,
 pers. commun.
Cotter Dolomite Ordovician Missouri Erickson et
 al., 1942;
 Arkansas Theo Witsell,
 pers. commun.
Jefferson City Ordovician Missouri Erickson et
 Dolomite al., 1942;
 Arkansas Theo Witsell,
 pers. commun.
Gasconade Dolomite Ordovician Missouri Erickson et
 al., 1942;
 Ver Hoef et
 al., 1993
Eminence Dolomite Cambrian Missouri Ver Hoef et
 al., 1993
Potosi Dolomite Cambrian Missouri Ver Hoef et
 al., 1993
Derby-Doerun Dolomite Cambrian Missouri Ladd & Nelson,
 1982
Bonneterre Formation Cambrian Missouri Ladd & Nelson,
 (dolomite) 1982

Central Lowland
Keokuk Limestone Mississippian Missouri Ladd & Nelson,
 1982
Burlington Limestone Mississippian Missouri Ladd & Nelson,
 1982
Chouteau Limestone Mississippian Illinois Willman et al.,
 1967
Salem Limestone Mississippian Illinois Willman et al.,
 1967
St. Louis Limestone Mississippian Illinois Willman et al.,
 1967
Ste. Genevieve Mississippian Illinois Willman et al.,
 Limestone 1967
Kimswick Formation Ordovician Missouri Ladd & Nelson,
 (limestone) 1982
Plattin Formation Ordovician Missouri Ladd & Nelson,
 (limestone) 1982
Dillsboro Formation Ordovician Indiana Homoya, 1987
 (limestone and
 shale)
Whitewater Formation Ordovician Indiana Homoya, 1987
 (limestone and
 shale)

Interior Low Plateaus
Frailey Shale Mississippian Illinois Heikens, 1991
Haney Limestone Mississippian Illinois Heikens, 1991
Renault Limestone Mississippian Illinois Heikens, 1991
Bethel Limestone Mississippian Illinois Heikens, 1991
Ridenhoen Formation Mississippian Illinois Heikens, 1991
 (limestone)
Salem Limestone Mississippian Kentucky Lawless et al.,
 2004
Glen Dean Limestone Mississippian Kentucky Lawless et al.,
 2004
Reelsville Limestone Mississippian Kentucky Lawless et al.,
 2004
Beech Creek Limestone Mississippian Kentucky Lawless et al.,
 2004
Girkin Limestone Mississippian Kentucky Lawless et al.,
 2004
Paint Creek Limestone Mississippian Kentucky Lawless et al.,
 2004
Warsaw Limestone Mississippian Tennessee DeSelm &
 Chester, 1993
Fort Payne Formation Mississippian Tennessee DeSelm &
 (limestone) Chester, 1993
Tuscumbia Limestone Mississippian Alabama Webb et al.,
 1997
Bangor Limestone Mississippian Alabama Webb et al.,
 1997
Louisville Limestone Silurian Kentucky Lawless et al.,
 2004
Peebles Dolomite Silurian Ohio Swinford, 1985;
 Gardner &
 Minnie, 2004
Lilley Formation Silurian Ohio Swinford, 1985;
 (dolomite) Gardner &
 Minnie, 2004
Bisher Formation Silurian Ohio Swinford, 1985;
 (dolomite) Gardner &
 Minnie, 2004
Estill Shale Silurian Ohio Swinford, 1985;
 Gardner &
 Minnie, 2004
Upper Part of Crab Silurian Kentucky Lawless et al.,
 Orchard Formation/ 2004
 Lower Part of Crab
 Orchard and
 Brassfield
 Formation (clay
 shale, limestone,
 and dolomite)
Laurel Formation Silurian Kentucky Rhoades et al.,
 (shale and 2004
 dolomite)
Lebanon Limestone Ordovician Tennessee DeSelm 1988,
 1991
Ridley Limestone Ordovician Tennessee DeSelm 1988,
 1991

Ridge and Valley
Keyser Formation/ Devonian, Pennsylvania Steve Grund &
 Tonoloway Formation Silurian John Kunsman,
 (undivided) pers. commun.
 (limestone)
Onondoga Formation/ Devonian Pennsylvania John Kunsman,
 Old Fort Formation pers. commun.
 (undivided)
 (limestone)
Chambersburg Ordovician Pennsylvania John Kunsman,
 Formation pers.
 (limestone)
Coburn Formation Ordovician Pennsylvania John Kunsman,
 through Nealmont pers. commun.
 Formation
 (undivided)
 (limestone)
Reedsville Formation Ordovician Pennsylvania John Kunsman,
 (limestone) pers. commun.
Tytoona Cave Ordovician Pennsylvania Steve Grand,
 (limestone) pers. commun.
Benner Formation/ Ordovician Pennsylvania Steve Grund &
 Loysburg Formation John Kunsman,
 (undivided) pers. commun.
 (limestone)
Bellefonte Formation/ Ordovician Pennsylvania Steve Grund,
 Axemann Formation pers. commun
 (undivided)
 (limestone and
 dolomite)
Tonoloway Limestone Ordovician West Virginia Bartgis, 1993
Helderberg Limestone Ordovician West Virginia Bartgis, 1993
Ben Hur Limestone Ordovician Virginia Ludwig, 1999
Knox Group Dolomite Ordovician Virginia DeSelm, 1993
Chickamauga Limestone Ordovician Alabama, DeSelm, 1993;
 Georgia, DeSelm et
 Tennessee, al., 1969;
 DeSelm, 1989
 Virginia Ludwig, 1999
Conasauga Formation Ordovician Alabama DeSelm, 1993
 (limestone)
Mines Limestone Cambrian Pennsylvania Steve Grund,
 pers. Commun.
Ketona Formation Cambrian Alabama Allison &
 (dolomite) Stevens, 2001
Rome Formation Cambrian Virginia Ludwig, 1999
 (limestone)
Honaker Formation Cambrian Virginia Ludwig, 1999
 (limestone and
 dolomite)

Coastal Plain
Midway Limestone Tertiary Alabama Harper, 1920
 (Eocene)

Appalachian Plateaus
Conemaugh Formation Pennsylvanian Ohio Rick Gardner,
 (limestone) pers. commun.

Table III
Summary of the soil series upon which xeric limestone prairies occur
in eastern United States. Series are organized alphabetically by
order. Physiographic province(s) and state(s) in which each series
occurs are provided along with reference(s)

Soil series Subgroup Physiographic province

Alfisols
Beasley Typic Hapludalfs Interior Low Plateaus
Bratton Typic Hapludalfs Interior Low Plateaus
Caneyville Typic Hapludalfs Interior Low Plateaus
Colbert Vertic Hapludalfs Ridge and Valley
 Interior Low Plateaus
 Interior Low Plateaus
Conasauga Oxyaquic Hapludalfs Ridge and Valley
Crider Typic Paleudalfs Central Lowland
Cumberland Rhodic Paleudalfs Interior Low Plateaus
Eden Typic Hapludalfs Central Lowland
Elba Typic Hapludalfs Appalachian Plateaus
Fredonia Typic Hapludalfs Interior Low Plateaus
Hagerstown Typic Hapludalfs Interior Low Plateaus
 Interior Low Plateaus
 Ridge and Valley
Lenberg Ultic Hapludalfs Interior Low Plateaus
Lowell Typic Hapludalfs Appalachian Plateaus
Opequon Lithic Hapludalfs Interior Low Plateaus
 Ridge and Valley
Rosine Ultic Hapludalfs Interior Low Plateaus
Seaton Typic Hapludalfs Central Lowland
Shrouts Typic Hapludalfs Interior Low Plateaus
Stookey Typic Hapludalfs Interior Low Plateaus
 Ozark Plateaus
Talbott Typic Hapludalfs Interior Low Plateaus
Vertrees Typic Paleudalfs Interior Low Plateaus
Wellston Ultic Hapludalfs Interior Low Plateaus
Zanesville Oxyaquic Fragiudalfs Interior Low Plateaus

Ultisols
Bodine Typic Paleudults Interior Low Plateaus
Clarksville Typic Paleudults Ozark Plateaus
Elliber Typic Hapludults Ridge and Valley
Gilpin Typic Hapludults Interior Low Plateaus
Lebanon Typic Fragiudults Ozark Plateaus
Nixa Glossic Fragiudults Ozark Plateaus

Mollisols
Barfield Lithic Hapludolls Interior Low Plateaus
Corydon Lithic Argiudolls Interior Low Plateaus
Gasconade Lithic Hapludolls Ozark Plateaus
 Ozark Plateaus
Gladeville Lithic Rendolls Interior Low Plateaus
 Ridge and Valley
Sogn Lithic Haplustolls Central Lowland

Inceptisols
Garmon Dystric Eutrudepts Interior Low Plateaus
Muskingum Typic Dystrudepts Interior Low Plateaus
Sulphura Typic Dystrudepts Interior Low Plateaus

Vertisols
Houston Oxyaquic Hapluderts Coastal Plain

Soil series State Reference

Alfisols
Beasley Kentucky Lawless et al., 2004;
 Rhoades et al., 2004
Bratton Ohio Gardner & Minnie, 2004
Caneyville Kentucky Lawless et al., 2004;
 Rhoades et al., 2004;
 Mann et al., 1999
Colbert Alabama, Georgia, DeSelm, 1993
 Tennessee, Virginia
 Alabama Webb et al., 1997
 Kentucky Lawless et al., 2004
Conasauga Alabama, Georgia, DeSelm, 1993
 Tennessee, Virginia
Crider Indiana Maxwell, 1987
Cumberland Kentucky Lawless et al., 2004
Eden Indiana Homoya, 1987
Elba Ohio Rick Gardner, pers.
 commun.
Fredonia Kentucky Lawless et al., 2004
Hagerstown Indiana Bacone et al., 1983
 Kentucky Lawless et al., 2004;
 Mann et al., 1999
 Pennsylvania Laughlin & Uhl, 2003
Lenberg Kentucky Lawless et al., 2004
Lowell Ohio Rick Gardner, pers.
 commun.
Opequon Ohio Gardner & Minnie, 2004
 Pennsylvania Laughlin & Uhl, 2003
Rosine Kentucky Lawless et al., 2004
Seaton Illinois McClain & Ebinger, 2002
Shrouts Kentucky Lawless et al., 2004
Stookey Illinois Heikens, 1991
 Illinois Heikens, 1991
Talbott Tennessee DeSelm, 1991
Vertrees Kentucky Lawless et al., 2004
Wellston Indiana Bacone et al., 1983
Zanesville Indiana Bacone et al., 1983

Ultisols
Bodine Tennessee DeSelm & Chester, 1993
Clarksville Missouri Skinner et al., 1983
Elliber Pennsylvania Laughlin & Uhl, 2003
Gilpin Indiana Bacone et al., 1983
 Kentucky Lawless et al., 2004
Lebanon Missouri Skinner et al., 1983
Nixa Missouri Skinner et al., 1983

Mollisols
Barfield Tennessee DeSelm, 1991
Corydon Indiana Homoya, 1994
 Kentucky Lawless et al., 2004;
 Mann et al., 1999
Gasconade Missouri Hicks, 1981; George, 1996
 Arkansas Keeland, 1978
Gladeville Tennessee DeSelm, 1991
 Tennessee Mann et al., 1999
Sogn Illinois McClain & Ebinger, 2002

Inceptisols
Garmon Kentucky Lawless et al., 2004;
 Mann et al., 1999
Muskingum Indiana Bacone et al., 1983
Sulphura Tennessee DeSelm & Chester, 1993

Vertisols
Houston Alabama Harper, 1920

Table IV
Summary of dominant taxa in xeric limestone prairies of eastern
United States. Data (quantitative and qualitative) are organized
by physiographic province (sensu Fenneman 1938. 1946 [1949])

Physiographic
province Dominant taxon Reference Data

Ozark
Plateaus
 Sporobolus Hall, 1955 frequency
 neglectus
 Schizachyrium Kucera & mean cover
 scoparium Martin, 1957
 Schizachyrium Keeland, 1978 mean cover
 scoparium
 Schizachyrium Skinner, 1979 mean frequency
 scoparium (a)
 Sporobolus Skinner, 1979 mean frequency
 neglectus (b)
 Schizachyrium Hicks, 1981 mean importance
 scoparium value
 Schizachyrium Logan, 1992 semi-
 scoparium quantitative
 Schizachyrium Ver Hoef et geometric
 scoparium al., 1993 mean of
 cover-class
 midpoint
 Panicum virgatum George, 1996 mean cover
 Schizachyrium George, 1996 mean cover
 scoparium

Central
Lowland
 Schizachyrium Maxwell, 1987 qualitative
 scoparium
 Schizachyrium Heikens, 1991 mean cover
 scoparium
 Schizachyrium McClain & mean cover
 scoparium Ebinger, 2002
 Bouteloua McClain & mean cover
 curtipenduda Ebinger, 2002
 Hedyotis McClain & mean cover
 nigricans Ebinger, 2002

Interior Low
Plateaus
 Schizachyrium Braun, 1928 mean frequency
 scoparium
 Andropogon Braun, 1928 mean frequency
 gerardii
 Schizachyrium Baskin & frequency
 scoparium Baskin, 1977
 Schizachyrium Kurz, 1981 mean frequency
 scoparium (c)
 Schizachyrium DeSelm, 1988 importance
 scoparium value
 Schizachyrium Heikens, 1991 mean cover
 scoparium
 Silphium Heikens, 1991 mean cover
 terebinthinaceum
 Schizachyrium DeSelm, 1991 mean cover
 scoparium
 Schizachyrium DeSelm & Webb, mean cover
 scoparium 1997
 Sporobolus DeSelm & Webb, mean cover
 clandestinus 1997
 Schizachyrium Lawless et al., mean cover
 scoparium in press
 Andropogon Lawless et al., mean cover
 gerardii in press
 Silphium Lawless et al., mean cover
 terebinthinaceum in press

Ridge and
Valley
 Bouteloua Bartgis, 1993 mean cover
 curtipendula
 Solidago arguta Bartgis, 1993 qualitative
 var. harrisii
 Monarda fistulosa Bartgis, 1993 qualitative
 var. brevis
 Paronychia Bartgis, 1993 qualitative
 virginica
 Schizachyrium DeSelm, 1993 mean cover
 scoparium
 Andropogon DeSelm, 1993 mean cover
 gerardii
 Schizachyrium Ludwig, 1999 mean cover
 scoparium
 Andropogon Ludwig, 1999 mean cover
 gerardii
 Schizachyrium Allison & qualitative
 scoparium Stevens, 2001
 Bouteloua Laughlin & Uhl, semi-
 curtipendula 2003 quantitative

Coastal Plain
 Schizachyrium Harper, 1920 qualitative
 scoparium

(a) Based on frequency data collected in 0.01- and 0.1-[m.sup.2]
quadrats centered around the rare focal species Stenosiphon
linifolius.

(b) Based on frequency data collected in 0.01- and 0.1-[m.sup.2]
quadrats centered around the rare focal species Penstemon cobaea
var. purpureus and Centaurium texense.

(c) Tweny-three of 32 sample sites were located in the Interior
Low Plateaus and nine in the Ozark Plateaus; however, data were
not stratified by site.

Table V
Summary of the geographic distribution of 13 taxa endemic, or
nearly so, to xeric limestone prairies of the eastern United States

Kev: OZMO = Ozark Plateaus of Missouri; OZAR = Ozark Plateaus of
Arkansas; RVWV = Ridge and Valley of West Virginia; RVVA = Ridge
and Valley of Virginia; RVAL = Ridge and Valley of Alabama

Endemic taxa OZMO OZAR RVWV RVVA RVAL

Castilleja kraliana x
Coreopsis grandiflora var. inclinata x
Dalea cahaba x
Delphinium treleasei x x
Echinacea paradoxa var. paradoxa x x
Erigeron strigosus var. dolomiticola x
Liatris oligocephala x
Monarda fistulosa subsp. brevis x x
Onosmodium decipiens x
Scutellaria bushii x x
Silphium glutinosum x
Spigelia gentianoides x
tialerianella ozarkana x x

Total 5 5 1 1 8

Table VI
Distribution of limestone cedar glade endemic/near-endemic taxa in
xeric limestone prairies of eastern United States

Limestone Cedar Glade XLP
endemic taxa distribution Reference

Astragalus tennesseensis * Tennessee DeSelm, 1991
Dalea foliosa * Tennessee DeSelm, 1991
Dalea gattingeri Alabama DeSelm, 1993
 Georgia DeSelm, 1993
 Tennessee DeSelm, 1993
Echinacea tennesseensis Tennessee DeSelm, 1991
Leavenworthia alabamica Alabama Webb et al., 1997
Leavenworthia exigua var. Georgia DeSelm, 1993
 exigua Tennessee DeSelm & Chester, 1993
Leavenworthia exigea var. Kentucky Lawless et al., 2004
 laciniata
Leavenworthia exigua var. Alabama Allison & Stevens, 2001
 lutea
Leavenworthia stylosa Tennessee Baskin & Baskin, 1977
Lobelia appendiculata var. Tennessee Baskin & Baskin, 1977;
 gattingeri DeSelm, 1991
Onosmodium molle Tennessee DeSelm, 1991
Pediomelum subacaule Alabama Allison & Stevens, 2001
 Georgia DeSelm, 1993
 Tennessee DeSelm, 1991; 1993
Talinum calcaricum Kentucky Lawless et al., 2004
Trifolium calcaricum Virginia Collins & Wieboldt, 1992;
 Ludwig, 1999

* Near endemic.
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Author:Lawless, Patrick J.; Baskin, Jerry M.; Baskin, Carol C.
Publication:The Botanical Review
Geographic Code:1USA
Date:Jul 1, 2006
Words:17318
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