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| Castanea: A flora of the Rock Hill Blackjacks Heritage Preserve, York County, South Carolina |
ABSTRACT
The Rock Hill Blackjacks Heritage Preserve was recently established, partly within the city limits of Rock Hill, South Carolina. It showed evidence of moderate to severe human disturbance including the effects of agriculture, forestry, construction of utility rights-of-way, and abandoned roads. Despite its small size and obvious disturbance, the preserve supported over 400 species of vascular plants. The reasons for this unusual level of diversity are not entirely clear. Geology was one factor. The preserve is underlaid by the Rock Hill gabbro/metagabbro pluton. Thin soils and seasonally wet areas provided a mixture of wetland and dryland species. It contained six distinct plant communities: 1) Bottomland Forest, 2) Gabbro Glade, 3) Montmorillonite Forest/Woodland, 4), Old Field Grassland/Shrubland, 5) Old Field Woodland, and 6) a distinct Grassland/Shrubland in the utility rights-of-way. The greatest diversity, including many tall-grass prairie species, was in the utility corridors that crisscross the preserve.
INTRODUCTION
In 1995, the establishment of the Rock Hill Blackjacks Heritage Preserve (RHBHP) protected a 117.3 hectare piece of a globally endangered community. Part of the preserve is located within the city limits of Rock Hill, South Carolina which is experiencing a rapid increase in its population because of its proximity to Charlotte, North Carolina 33 km (20 mi) to the north. The preserve is surrounded on three sides by housing developments and will likely become an island within the city. A comprehensive survey of the vascular plants within the preserve was undertaken to serve as a baseline to monitor biological changes that will inevitable take place as the result of urbanization.
A less comprehensive survey was also conducted to the east and south of the preserve because of its similar geological and botanical affinities. It is hoped that the preserve will continue to expand eastward, as it did in 2000, to include a substantial number of locally rare species that are outside the protection of the preserve. Timber harvesting, mining, herbicides, and commercial development currently threaten this area.
Human History
Before European colonization, the area that is now the preserve was part of the tribal lands of the Catawba Nation. Much of their history has been lost, but reports of early settlers indicate that there were large savannahs in the area of Rock Hill (Barden 1997, Nelson 1992). Presumably these areas were maintained by fire, both accidental and manmade. The fires were set for many reasons including hunting, wildlife management, agriculture, and defense. Europeans continued the practice of burning into the 20th century to clear land for farming and to maintain forage for game.
The land that makes up the preserve has a long history of human disturbance. Parts of the preserve have been variously used as pasture, a dumpsite, a prospective housing development, a loblolly pine plantation, and for farming, timber, and utility rights-of-way. The unusual floral diversity owes its existence in part to the failure of these enterprises, with the exception of the maintenance of the utility corridors.
Barbed wire fencing indicates that part of the land was recently used as pasture. A section of the fencing follows one of the utility rights-of-way and was probably constructed after the easement was established in 1948. Aerial photos show a steady shrinkage of the nonforested or glade-like areas, with the exception of the utility rights-of-way, from 1965 until the present (Camp 1965). This may be the result of fire suppression. In a recent experiment, trees were felled in order to create openings in the forest adjacent to a utility corridor. The oldest tree was 50 years old in 1997 (P. Johnson, unpubl. data). The diameter of this oak tree was typical of the larger trees in the preserve and dates the last timber harvest to the late 1940's. A more recent attempt at tree farming using loblolly pine has adversely affected the understory in some areas through canopy shading and thick leaf litter accumulation.
In the early 1980's, roads were cut for a proposed housing development, but the plan was abandoned at least partially because of the high water table. The field near the entrance was used as a hay field up until its purchase in 1995. Despite the protected status of the preserve, areas near its entrance road continue to be used as dumpsites.
The unusual flora was first brought to the attention of scientists in 1982 when a Duke Power employee noted vegetation native to prairies growing in a transmission right-of-way (J. Garton, pers. comm.). The first thorough survey of the region was conducted by John Nelson (1987) for the South Carolina Department of Natural Resources Heritage Trust Program. His report identified many unusual plant communities and species, including Helianthus schweinitzii which was listed as an endangered species in the Federal Register in 1991. It was this discovery that eventually led to the establishment of the preserve in 1995. Additional lands purchased in 1996 and 2000 have expanded the preserve to a total area of 117.32 hectares (~~290 acres).
Geology
The Rock Hill Blackjacks Heritage Preserve is located near the center of a roughly oval gabbro pluton with a surface area of 4150 hectares (Chalcraft 1970), coordinates N34 deg 54'0", W81 deg 1'10". Butler (1965, 1966) originally described this pluton, noting its comparatively high plagioclase feldspar content (up to 95%) and the extensive occurrence of outcrops and residual boulders. In the most complete survey of what is called the Rock Hill gabbro, Chalcraft (1970) mapped a large area of gabbroic anorthosite (77.5-90% calcic plagioclase) at the site where the preserve is located. An area that contains over 90% anorthosite is currently being quarried near the preserve. The plagioclase feldspar weathers to produce a whitish gray rock, while the mafic minerals (olivine, augite and hypersthene) produce contrasting darker grains (Chalcraft 1970).
At approximately the same time that Butler (1965, 1966) described the pluton, the Soil Conservation Service (Camp 1965) published a soil survey of York County that indicated stony, thin solum foams of the Iredell series covering large areas of the pluton. Iredell soils are characterized by dark, loamy A horizons and yellow, plastic B horizons. Interestingly, as early as 1848 Tuomey described the tenacious nature of the subsoil clays in the "Blackjack lands of Chester" and noted that they were often lower, flatter, and seasonably wetter than surrounding areas developed over granitic rocks (Tuomey 1848).
In a review of plutonic rocks occurring in the Carolina piedmont, McSween et al. (1991) included descriptions of all of the gabbroic plutons in North and South Carolina. Of the more than 20 gabbro bodies, the Rock Hill gabbro is distinctive for its plagioclasic-rich core and magmatic layering. Furthermore, among the many occurrences of gabbro, the Rock Hill pluton does not follow the typical pattern of deeply weathered rock with few surface exposures, but instead has abundant outcrops and large residual boulders (Butler 1965).
Thus the Rock Hill gabbro pluton is unique in both its composition and the relatively young age of its rock.
MATERIALS AND METHODS
The survey was begun in spring, 1996 and concluded in summer, 2000. Voucher specimens are located at the Winthrop University Herbarium, Rock Hill, South Carolina. In cases where a particular species was rare within the boundaries of the preserve, a specimen was collected from a location within a kilometer of the preserve. The authors are responsible for the identification of all plant specimens.
Kartesz (1999) provided the reference for the nomenclature and for the determination of whether a plant was an exotic; this checklist was also used to determine if a species was woody or herbaceous. Only species classified as strict forbs or graminoids were considered herbs. Species were further classified as prairie species and/or outcrop species. Douglas Ladd's list of the Vascular Plants of the Midwestern Tallgrass Prairie (Packard and Mutel 1997) was used to designate the prairie species and Murdy and Carter (2000) was used to designate outcrop species in Appendix A. Scientific names that differ from Radford et al. (1968) are listed as synonyms in the appendices.
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Areas found on Rock Hill Gabbro Pluton have been classified by the International Classification of Ecological Communities, sponsored by The Association for Biodiversity Information, as a "Quercus marilandica-Quercus stellata/Schizachyrium scoparium-Silphium terebinthinaceum Woodland" and are listed as critically imperiled globally (G1) (Weakley et al. 2000). This study has broken this broader community type into Gabbro Glades, Montmorillonite Forest/Woodland, and Utility Corridor Grassland/Shrubland as described below. These communities intergrade with one another to form this rare, more inclusive community.
Six plant communities were identified within the boundaries of the preserve (Table 1, Figures 1-7). Two of these communities, the Gabbro Glade and the Montmorillonite Forest/ Woodland correspond to the Piedmont Glade and Montmorillonite Forest communities described by Nelson in his 1987 survey. Montmorillonite Forests are dry, open forests with thin soils (Figure 4). The Piedmont Glade is called a Gabbro Glade in this treatment to emphasize its underlying geology. Gabbro glades are found where a sufficient amount of the underlying rock has been exposed within the Montmorillonite Forest/Woodlands to create openings in the forest canopy (Figure 3). Areas of "Upland Depression Swamp" and "a variant of the Oak-Hickory Forest" seen by Nelson (1987) are included within the Bottomland Forest community (Nelson 1986). This Bottomland community is part of the Taylor Creek watershed (Figure 2).
Two other communities, Old Field Woodland and Old Field Grassland/Shrubland were defined to indicate the sporadic disturbance that has retarded succession in many parts of the preserve (Figures 5-6). The last community type, the Utility Corridor Grassland/ Shrubland apparently requires the periodic disturbance provided by the utility maintenance crews.
Categorizing natural plant communities was difficult because of the lack of historical records and because there are so few plant communities that remain intact. Developing a lexicon of disturbed communities is even more daunting because it must include the various stages of succession. Classifying the Utility Corridor Grassland/Shrubland was particularly difficult because of its emergent character. This relatively recent assemblage of plants bore a striking resemblance to the tallgrass prairies of the Midwest and was composed of a subset of species found in the other five communities, especially the Gabbro Glades and the Old Field Grassland/Shrubland (Figure 7). There is a historical issue as to whether these 'prairies' existed before the advent of human disturbance, and if present, how extensive were they? The name selected for this community was chosen to draw attention to a widespread practice of land management that has received little attention in the literature (but see Cameron et al. 1997).
The authors independently assigned each plant to one or several of the six communities. Most of the placement conflicts were resolved by adding a plant to a community from which it had been excluded by one of the authors.
The communities are dynamic and are not as sharply defined as the map indicates. The borders of the glades were determined from a recent aerial photo. Small glades were obscured by the surrounding Montmorillonite forest and are not shown on the community map. Older photos show that the larger glades have decreased in area since the 1960's, perhaps because of fire suppression and a decrease in logging activity. The Oldfield Grassland is rapidly succeeding into Oldfield Woodland with more trees.
A preliminary survey was also conducted of the undeveloped area surrounding the preserve but was restricted to species on the Rock Hill Gabbro Pluton. The area surveyed was bounded by Highway 21 to the North, Highway 901 to the Southwest, SSR 245 to the East and Interstate I-77 to Southeast. All species not found on the preserve are listed in Appendix B. This list is incomplete, but is included in order to place the preserve within a broader, well-defined geological context.
RESULTS
A total of 468 species were identified. Most of them (410) were found somewhere on preserve (Appendix A). The Utility Corridor Grassland/Shrubland community had the most species (58.8%) followed by Old Field Grassland/Shrubland (46.1%), Gabbro Glade (32.2%), Bottomland Forest (28.5%), Oldfield Woodland (19.5%), and Montmorillonite Forest/Woodland (17.6%) (Table 2). There were approximately 3.95 km of Utility Corridors about 10 m wide within the preserve (0.395 h) making it by far the smallest of the six communities.
Overall there were relatively few exotic species (36) comprising 8.8% of the flora. Not surprisingly, most of these introduced species were found in the Oldfield Grassland/Shrubland bordering a road, which partially explains its diversity (Table 2). The Gabbro Glades were also species rich despite being smaller in area than the Oldfield Grassland/Shrubland and having only one exotic species. Note however, that no single glade contained all 132 species. Rather, each had its own unique composition of species.
Diversity in the three tree-dominated communities-Bottomland Forest, Oldfield Woodland, and Montmorillonite Forest(Woodland-followed a water gradient with the wettest, the Bottomland Forest, having the most species despite its small size. The Gabbro Glades contained within the species-poor Montmorillonite Forest, were an exception to this pattern (Table 2).
Slightly over half of the species were from five families. The Asteraceae had the most representatives. Graminoids (Poaceae, Cyperaceae and Juncaceae) were collectively the most abundant plant form (95 species), accounting for 23.2% of the total number of species.
Over half the species of the Oldfield Woodland were woody whereas roughly one quarter of the species in the Bottomland Forest were woody. The understory of the Bottomland Forest was more diverse than that of the Oldfield Woodland. Unexpectedly, the Oldfield Woodland contained more woody species than the Bottomland Forest. The Oldfield Woodland contained 27 woody species that were not found in the Bottomland Forest. Likewise, the Bottomland Forest supported 17 woody species that were not found in the Oldfield Woodland. The two communities had 15 species in common. Most of the woody species (75.9%) found in the Montmorillonite Forest/Woodland were also found in the Oldfield Woodland community. Overall, the preserve was dominated by herbaceous species, establishing its savannah-like character.
Gabbro Rock outcrops were scattered throughout the preserve but were not mapped because of their small area. Some species were confined to depressions in exposed gabbro rock while others, typical of outcrops, were less limited in their range. Thirty of the 80 outcrop species in Murdy and Carter (2000) were found on the preserve (Appendix A). Four others were found outside the preserve on the surrounding gabbroic pluton.
The preserve is home to 172 species that have been categorized as North American prairie species (Packard and Mutel 1997). If other prairie species found on the Rock Hill Gabbro Pluton are included, the total number increases to 188 (Appendix B). The majority of these prairie species (132) were found in the three Utility Corridors that transected the other communities (Table 2). This blending of communities produced areas that have been described as Piedmont Prairies (Barden 1997).
This emerging assemblage is of recent origin. The oldest right-of-way on the preserve is the overhead power line, dating back to 1936. The next oldest is the telephone right-of-way (1947) followed by the sewer right-of-way (1983). Floristically the telephone right-of-way was the most diverse followed by the more recent sewer line and last by the older distribution line. Apparently, factors other than the duration of disturbance are involved in the appearance of these communities. This conclusion is supported by the fact that a recent (1972) transmission power line near the preserve supports a more developed, prairie-like community than the (1947) telephone right-of-way on the preserve.
DISCUSSION
There are three defining characteristics of the Rock Hill Blackjack Oaks Preserve: 1) unusual geology, 2) high floristic diversity, and 3) location in a rapidly developing urban environment. The first two are probably related, whereas the third threatens its integrity.
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The preserve is underlain by the Rock Hill gabbro pluton. The pluton is unusual because of the extent of gabbro rock that is exposed, and the presence of an anorthosite-rich area within the gabbro. On nearby gabbro plutons such as the Ogden pluton the gabbro has been eroded to form flat depressions of Iredell soil. The Rock Hill pluton has more exposed rock in the form of residual boulders and flatrock outcrops. The most concentrated anorthosite is outside the preserve. Its associated soil is even more basic than the soil formed from gabbro parent material (pH ~~ 5.3) and was probably the epicenter of the unusual floristic diversity found in the area.
The Iredell soil found throughout the preserve is shallow. Pools form during the spring and winter when the soil becomes a gray mud that restricts the use of heavy machinery; by summer it dries brick hard. This combination of physical characteristics has limited agriculture on the pluton and has undoubtedly contributed to its preservation. The prevalence of these conditions also helps to explain the sympatry of wetland and drought tolerant-species. The soil is relatively infertile although it contains comparatively high concentrations of Ca, Mg, and Fe (P. Johnson, unpub. data). The low nutrient availability may allow stress-tolerant species to coexist with more ruderal species (Grime 1977).
To some extent the plant diversity can be explained by the presence of a variety of community types within a small area. Even within communities, conditions of hydrology, soil depth, and level of disturbance vary, forming a mosaic of environments. The wet conditions along the tributaries of Taylor Creek support a limited Bottomland Forest in addition to a number of wetland species in a wetland outside the preserve (Appendix B). The relatively low number of woody species in this community can be attributed to recent forestry practices that have clearcut all the way to the streambed.
The Gabbro Glades were the least disturbed communities, having escaped the repeated timber harvests of the surrounding forest communities. They are similar to the Cedar glades first described by LeGrand (1988) in North Carolina. Each glade has its own unique set of species, although Silphium terebinthinaceum Jacq. var. terebinthinaceum is a reliable indicator species (Figure 3). Many species typical found on granitic outcrops are present where the gabbro rock is exposed.
The Gabbro Glades, because of their Iredell soil, are also comparable to sites studied in North Carolina (Batson 1952, Dayton 1966). One striking difference, however, is that Schizachyrium scoparium (Michx.) Nash var. scoparium is not a dominant grass in the preserve. Andropogon virginicus L., Saccharum brevibarbe (Michx.) Pers, var. contortum (Ell.) R. Webster, Sorghastrum nutans (L.) Nash and even Tridens strictus (Nutt.) Nash are much more abundant. This may be because Batson restricted his study sites to areas without streams or "extensive rock outcrops" in North Carolina.
Although the "Quercus marilandica-Quercus stellata/Schizachyrium scopariumSilphium terebinthinaceum Woodland" assemblage (Weakley et al. 2000) is endemic to North and South Carolina, to our knowledge it is best developed on the Rock Hill Gabbro Pluton. It has many similarities to the Blackbelt prairies and the barrens of Tennessee and Kentucky. Like them, it is unlikely that these communities were ever contiguous with the central prairies of North America (Baskin et al. 1999) despite the many species they have in common.
Evidently the prairie-like patches found on the preserve are of recent origin and exist only in the Utility Corridors. Their rapid development may be of interest to community ecologists. There is no way to tell whether these patches are representative of the far more extensive grasslands described by Bartram (1791) and others for the pre-settlement regions of the Southeast, but it may be our best approximation of their original condition if the fires set by native Americans produced an effect similar to the mowing currently used to maintain the Utility Corridors.
The only endemic species to the prairies of the Piedmont is Helianthus schweinitzii Torr. & Gray. The prairies of the central plains likewise contain few endemics. However, the isolated cedar barrens of Tennessee, that are floristically similar to the Utility Corridor community, contain many endemic species (Baskin and Baskin 1999). Most of the species that are listed as prairie species exist in small numbers in the other communities found on the preserve. Apparently all that was required to establish this prairie-like community, in this geological context, was a seed source and a recurrent form of disturbance to suppress the woody vegetation. At the Blackjack Preserve, the prairie species were saved through an accident of geology in Gabbro Glades that have historically withstood development. The corridors linking these propagule sources were provided by the utility rights-of-way.
The variety of plant species along the utility rights-of way is exceptional even to a casual observer. Over 200 species of plants were identified from a 50 x 60 m research site and its seedbank (P. Johnson, unpubl. data). The surveys revealed seven dicot genera that had five or more representatives (Eupatorium, Helianthus, Liatris, Solidago, Hypericum, Desmodium, and Quercus) indicating an unusual diversity within genera. There were four state (Carex meadii Dewey, Parthenium auriculatum Britt., Ruellia humilis Nutt., and Sporobolus composites (Poir.) Merr. var. composites) over 40 York County records and at least 20 plants listed by the South Carolina Heritage Trust as requiring attention at the state level (Schmidt 2000).
The Rock Hill Blackjack Oaks Preserve is destined to become an island in a rapidly developing urban environment. What impact this change will have is open to conjecture but at least two species, Ranunculus fascicularis Muhl. ex Bigelow, Isoetes sp. and Alopecurus carolinianus Walt. have not been observed in several years. The first two Bottomland forest plants may have suffered from alterations in the flood cycle of the Taylor Creek watershed, while the latter grass species is probably an intermittent waif. The preserve continues to be used as a dumpsite and in 1997 the overhead distribution line was treated with a broadcast herbicide. The area to the northwest of the preserve is currently undergoing commercial development.
Species outside the confines of the preserve face an uncertain future. A rock quarry has partially obliterated the largest glade on the gabbro pluton. The increasing size of the slag pile poses an ongoing threat to the richest remaining prairie site. The transmission right-ofway that harbors a portion of this site is currently (2000) being used as a staging area for a logging operation. A highway that transects the pluton is being expanded and the area to the south of this highway is now a housing development.
Transplanting rare species into the preserve from neighboring unprotected areas may offer the only chance for their survival. The preserve is already being used as a mitigation site for Helianthus schweinitzii populations whose habitats were destroyed by industrial development. Mixing disparate populations in this way is controversial because it threatens the genetic integrity of the existing population.
Despite these problems there is interest in continuing to expand the preserve. Verbal agreements have been made with two of the transecting utilities to ban the use of herbicides along their rights-of-way. The establishment of the preserve and the fact that small glades were enough to preserve relic populations of the native flora offers hope for their continued survival.
ACKNOWLEDGMENTS
Thanks are extended to Dick Houk, who played a crucial role in establishing the preserve; Jim Sorrow, South Carolina Department of Natural Resources, for permission and encouragement to conduct this survey; Paul Johnson, graduate student, who collected many of the specimens; Dr. Gerald Long for assistance in the identification of the Poaceae species; Dr. Patrick McMillan for assistance in the identification of the Cyperaceae and Paula Mitchell for editing the manuscript.
LITERATURE CITED
BARDEN, L.S. 1997. Historic prairies in the Piedmont of North and South Carolina. Nat. Areas J. 17: 149-152.
BARTRAm, W. 1791. Travels through North and South Carolina, Georgia, East and West Florida, the Cherokee Country, the extensive territories of the Muscogulges or Creek Confederacy, and the country of the Chactaws in William Bartram Travels and other writings. 1996. The Library of America Volume 84.
BASKIN, J.M. and C.C. BASKIN. 1999. Cedar glades of the Southeastern United States. p. 206-219. In: Anderson, R.C., J.S. Fralish, and J.M. Baskin (eds.). Savannas, barrens, and rock outcrop communities of North America. Cambridge University Press, New York, New York.
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