Abstract
Limited information is available on the optimum placement of designed plant communities that reference the historic Piedmont prairies of southeastern North America. Characterized as “a hybrid of horticulture and ecology,” a designed plant communities approach looks to naturally occurring plant associations for both aesthetic inspiration and guidance on ideal environmental conditions and plant composition. Designed plant communities are not intended to be facsimiles of native plant communities and are not ecological restorations. Through semi-novel plant combinations and plant selection aimed at creating vibrant multi-season bloom events for continual visual interest, these hypernatural plantings emphasize public enjoyment. Through the incorporation of the plants’ natural tendencies, however, designed plant communities can also provide ease of maintenance in tough environments and various ecological benefits. This study was conducted to identify optimum locations for the installation of hypernatural Piedmont prairie plantings in Athens-Clarke County, Georgia. Using ArcMap, the authors employed an exclusion and screening process based on the environmental factors identified in literature related to the restoration of natural Piedmont prairies, in addition to logistics related to their implementation within Athens-Clarke County (with appropriate locations limited to public or institutional properties). Appropriate variables were selected and combined to map candidate sites. The process identified 128 ha (318 ac) for hypernatural Piedmont prairie conversion in the county. Suitable locations range from dense urban areas to outlying rural parcels and included power line easements, open parklands, and roadsides. The process was designed to be easily replicated for other Piedmont locations where hypernatural Piedmont prairie plantings are desired.
This study responds to the limited number of ecological and visually stimulating planting designs in landscapes managed by municipalities, government agencies, and other institutions. Such plantings are often extremely resource intensive to establish and maintain, and many institutional landscapes have typically featured traditional lawn and bed plantings, swaths of turfgrass, or transitional patches full of aggressive invasive species. A designed plant communities approach to prairie-inspired plantings offers a way to reduce the enormous and repeating burdens of mowing, mulching, and pruning on limited maintenance budgets, while providing ample visual interest for the public as well as ecological benefits (Nassauer 1995; Rinaldi 2014; Millington 2015; Rainer and West 2015; Hitchmough 2017) (Figure 1).
The authors and other researchers are conducting ongoing trials of design of Piedmont prairie mixes for use in future public–private partnerships.
Designed plant communities are characterized as “a hybrid of horticulture and ecology,” with naturally occurring plant associations for both aesthetic inspiration and guidance on ideal environmental conditions and plant composition (Rainer and West 2015; Hitchmough 2017). Fortunately, Piedmont prairies provide a proven model for the creation of resilient, aesthetically appealing, and low-maintenance landscapes, naturally adapted to the harsh growing conditions found in human-dominated landscapes, such as institutional settings, urban streetscapes, and highway and utility corridors (Davis and others 2002; Hitchmough 2008; Diboll 2015). Research on Piedmont prairie restoration has increased in recent years, providing valuable information that landscape designers can use for optimum site selection and installation of hypernatural Piedmont prairie plantings.
Piedmont prairies were once common throughout southeastern North America. (Note that the term “prairie” has no consensus in terms of describing these environments. Some scholars refer to “grassland” or “savanna” instead. We use the term “prairie” as this is the nomenclature used by most cited works.) Sprawling out across the gentle hills of the Piedmont region in sporadic patches between forestland, these prairies once composed a vibrant and dynamic ecosystem characterized by swaths of shimmering grasses punctuated by pockets of colorful blooms set within a mostly open and fire-maintained landscape. Accounts from European colonialists of the 16th and 17th centuries provide rich descriptions of prairie landscapes (Barden 1997; Juras 1997). Famed naturalist William Bartram in particular wrote of the Piedmont’s “illumined native fields, to the utmost extent of sight” (Travels through North & South Carolina, …; 1776, p 394). Such fields could stretch up to 40 km (25 mi) wide across the landscape and as smaller scattered patches (Juras 1997). However, fire suppression and land use changes beginning in the 18th century resulted in the disappearance of the vast majority of these landscapes. Their flora was then mostly relegated to old fields and transportation rights-of-way, surviving in such marginal and full sun locations.
In recent years, various studies have provided information on identifying suitable locations for the restoration of imperiled Piedmont prairie habitat (Barden 1997; Juras 1997; Davis and others 2002; Taecker 2007; Tompkins and others 2010; Benson 2011; Szakacs 2019). These sources noted various factors that influence the presence of Piedmont prairies. Some observed the importance of soil series, soil texture, and soil nutrients, whereas others identified disturbance history, regional climate, slope, and aspect as key factors. Together, the studies show that no one variable determines the presence of Piedmont prairies, nor does it indicate proper restoration location; instead, Piedmont prairie occurrence is the result of interactions between various environmental factors and legacy land use.
That said, given their rarity in the landscape today, research has yet to develop a set of specific criteria for what constitutes a “Piedmont prairie.” And while studies have identified associated flora and some specific environmental characteristics, no one definition has yet been established (Szakacs 2019). In terms of flora, Piedmont prairies comprise a number of plant species, with a majority belonging to Poaceae, Asteraceae, and Fabaceae families (Davis and others 2002). Grasses (Poaceae) include splitbeard bluestem (Andropogon ternarius Michx.), Indiangrass (Sorghastrum nutans (L.) Nash), and purpletop tridens (Tridens flavus (L.) Hitchc. var. flavus) (Juras 1997; Davis and others 2002). Forbs (Asteraceae) include Rudbeckia or orange coneflower (Rudbeckia fulgida Aiton) and blackeyed Susan (R. hirta L.), and various species of goldenrod (Solidago L.) (Davis and others 2002). Some sites also contain rare or endangered species from the Asteraceae family, such as smooth purple coneflower (Echinacea laevigata (C.L. Boynt. & Beadle) S.F. Blake), Schweinitz’s sunflower (Helianthus schweinitzii Torr. & A. Gray), and Georgia aster (Symphyotrichum georgianum (Alexander) G.L. Nesom) (Smith 2008). The floristic diversity of Piedmont prairies is not limited to these few species, however, as a prominent study of 6 remnant Piedmont prairies revealed 277 associated plant species (Davis and others 2002).
While the soils of the Piedmont appear to influence the occurrence of Piedmont prairies, the exact required soil conditions are not universal, with remnant prairie sites varying “widely in [soil] series, texture, parent material, and drainage” (Benson 2011). Of the 6 soil orders recognized in the USDA Soil Taxonomy, 2 are associated with prairie ecosystems: alfisols and ultisols. (The majority of the soils in our study area are ultisols, with no alfisols present. Therefore, soil order was not used as a criterion for the site selection process to follow. Still, a brief discussion of soil is provided here for context.)
A study focused on North Carolina restoration sites identified alfisols as a prime determinant for Piedmont prairie location (Benson 2011). Another study, which focused on the federally endangered prairie remnant Helianthus schweinitzii, found that “sixty-nine percent of the Helianthus schweinitzii element occurrences [were] found in ultisols” (Smith 2008: 28). Some studies have recognized that soil order may not be the most effective marker for Piedmont prairies and have instead sampled soil for “effective cation exchange capacity (ECEC) and clay percentage rather than soil type” (Taecker 2007). It is possible that “soil surface texture,” such as coarse, gravelly sand, or eroded topsoil, “may be more important than the specific soil series for Piedmont prairie occurrence” (Benson 2011). As soil surface texture is influenced by the effects of weather and climate, the slope and aspect characteristics of known Piedmont prairies can provide additional insight into their preferred environment.
With rare exceptions, Piedmont prairies are unlikely to form in areas where water is prone to stand (Tompkins and others 2010). The gently rolling hills of the Piedmont shed enough water so as not to become inundated and are thus dry enough for fire to occur. Further, they are not steep enough to experience extreme erosion of soils (barring disturbance). Along these lines, James Benson found that slope position was a primary factor in Piedmont prairie occurrence (2011). Benson’s study generally places Piedmont prairies on slopes less than 15% and at the mid-slope position.
This “not-too-dry-but-not-too-wet” position was further refined by identifying that southern aspects promoted Piedmont prairie occurrence (Benson 2011). The southern aspect results in greater solar exposure, meaning a hotter and drier site. Although examples of Piedmont prairies do exist on lower, wetter slope positions, in general, much remnant prairie flora is found on exposed and elevated locations.
Disturbance of the landscape can be considered one of the most significant determinants of Piedmont prairies. Historically, fire served as the disturbance mechanism and was essential to the creation and maintenance of Piedmont prairies (Davis and others 2002; Benson 2011). In the Southeast, the successional process results in a forested site. The fires would help reduce woody plant competition and promote the growth of grasses and other disturbance-tolerant species. Without fire disturbance, very few open spaces would occur in the landscape. Sources have documented that Native Americans of the Southeast took advantage of lightning-caused fires and also burned grassland to encourage the grassland growth that attracted herbivores such as bison (Bison bison L. [Bovidae]) and deer (Cervus L. [Cervidae]) (Barden 1997; Juras 1997; ITIS 2022).
In the Southeast today, however, fire suppression disrupts the disturbance effect that fire historically had. Aside from prescribed burns, fire has been mostly extinguished from the modern landscape. Regular disturbance continues, however, through agricultural activity, logging, mowing, grazing, and other similar means. Benson writes that all sites in his study “had been disturbed in some manner,” and though fire activity was nonexistent in these study sites, it was replaced by mechanical means of disturbance (2011). Other researchers have noted that remnant Piedmont prairie sites are found in heavily disturbed areas such as railroad and utility rights-of-way (Taecker 2007). In fact, Piedmont prairie restoration sites have been selected for their history of disturbance (Tompkins and Bridges 2013). Disturbance in combination with clay soils, solar exposure, and wind pressure make Piedmont prairies high-stress environments.
SOLUTION TO THE PROBLEM
Suitability Criteria
Athens-Clarke County, Georgia, was the area of study for the screening process. The county is located within the Southern Outer Piedmont ecoregion and is characterized by a development pattern that features a highly urbanized core surrounded by bands of suburban development and agricultural/green space land uses. The county is located in USDA climate zone 8a (USDA 2012).
Based on existing research on Piedmont prairies, 4 environmental and locational criteria were determined applicable for the analysis.
Slope: This study therefore sets a range for analysis to include slopes between 3% and 20% grade. This range eliminates steeper slopes and bottomlands, which are not generally associated with Piedmont prairie occurrence.
Aspect: Piedmont prairie sites tend to occur on locations between eastern and western aspects, with southerly and southeasterly aspects most strongly associated with Piedmont prairies. This analysis, therefore, included only south, southeast, and southwest aspects.
Land Cover: The practical installation of hypernatural Piedmont prairies necessitates evaluating existing land cover to eliminate areas that would generally be too resource intensive to convert, such as woodlands. This analysis thus analyzed the land cover of Athens-Clarke County, Georgia, using the 2011 National Land Cover Database (USGS 2014). The authors excluded all but 5 of the 16 land cover classes, limiting it to the following: Barren land, Developed Open Space, Hay/Pasture, Herbaceous, and Scrub/Shrub. These classes represent the most applicable sites for installations. While Scrub/Shrub contains trees and other woody vegetation, it may constitute only 20% of the site. Therefore, this land cover class was used in this analysis.
Ownership: The creation and management of urban greenspace is often headed by institutions, such as universities, and by governmental agencies, such as leisure services and departments of transportation. Therefore, Athens-Clarke County’s land ownership was classified into 3 groupings: Athens-Clarke County (ACC)–owned lands, Georgia Department of Transportation (GDOT)–owned lands, and state-owned University of Georgia (UGA)–owned lands. All other commercial and residential properties were excluded from this analysis.
Soil order was also excluded from the analysis given the homogeneity of soils in the study area. Additionally, this analysis assumed that the 5 land cover classes selected as criterion were the most likely to experience periodic or routine disturbance, and thus disturbance itself was not an included criterion.
Method
Data for the site selection process were sourced from the following: Athens-Clarke County Government for parcel ownership (ACC 2016); the Multi-Resolution Land Characteristics Consortium’s National Land Cover Database for land cover (USGS 2014); the Atlanta Regional Commission for county boundaries (ARC 2011); and the US Geologic Survey for elevation modeling (USGS 2003).
We utilized ArcMap to process the data (Figure 2):
The flowchart depicts the process of data refinement for an output of optimum locations. Illustration credit: Sean Dunlap
We filtered land cover data to include only targeted types (Barren land, Developed Open Space, Hay/Pasture, Herbaceous, and Scrub/Shrub) (Figure 3). We screened ownership and land cover data to include only public parcels (GDOT, UGA, ACC) (Figure 4).
We restricted slope to a 3–20% grade (Figure 5).
We performed an aspect geoprocessing operation to identify the targeted aspect class (southeast, south, southwest) areas (Figure 6).
We compiled the refined criteria output (land cover, ownership, slope, aspect) into one data layer using the intersect operation, without any weighted values added to the criteria. This approach resulted in a final output of optimum locations (Figure 7).
Land cover in Athens-Clarke County, Georgia. Despite the urbanization, significant amounts of open space, both within and outside the developed core, are promising locations for design application.
This ownership map reveals the extent of public and institutional land in Athens-Clarke County, Georgia, and makes clear the potential for the creation of pollinator corridors on these lands.
This analysis highlights slopes between 3% and 20% grade. This range has been identified as prime occurrence areas of Piedmont prairies.
Slopes with aspects ranging from east to west appear to be highly conducive to Piedmont prairie occurrence, with aspects of southeast to southwest as the most conducive.
Optimal areas: This map depicts the final output of intersecting 3% to 20% slopes, southeast to southwest aspects, public and institutional lands, and targeted land cover classes (Barren, Developed Open Space, Hay/Pasture, Herbaceous, and Scrub/Shrub). While seemingly scarce, this analysis identifies 128 ha (318 ac) of prime area for hypernatural Piedmont prairie installation. The identified locations range from within dense urban areas to within outlying rural parcels.
Results
We identified 128 ha (318 ac) for hypernatural Piedmont prairie conversion in Athens-Clarke County. Suitable locations range from dense urban areas to outlying rural parcels and included power line easements, open parklands, and roadsides. To illustrate, several locations are highlighted here, following the approach that the highest priority areas to convert to hypernatural Piedmont prairies should be those with the most visibility; for example, they fall within existing park lands or are part of a potential corridor network.
Dudley Park, a highly visited urban public space on the edge of downtown Athens, was identified as an optimum location (Figure 8). City parks are ideal for hypernatural Piedmont prairie designs as municipalities already allocate resources for their upkeep, and they are publicly accessible. At parks such as this, converting underutilized swaths of turfgrass that are currently serving no recreational function to vibrant, designed plant communities would provide many benefits, ranging from public education to gasoline use reduction.
This area in and around Dudley Park, part of the greenway network in Athens-Clarke County, Georgia, features suitable locations for the installation of Piedmont prairie–inspired designs. Prime areas include along a utility easement and within open and underutilized lawn areas.
Results also illustrate the potential for a hypernatural Piedmont prairie corridor to be installed in the rights-of-way and on University of Georgia property along Milledge Avenue (Figure 9). Discontinuous spaces like this can be “stitched” together to create prairie corridors, filling gaps in the landscape matrix while providing benefits to both wildlife and people (Schulte and others 2017). This area would be an ideal location for a pollinator corridor as it incorporates UGA research lands and highly visited sites (a sports stadium, UGArden student farm, and student housing) along an already scenic roadway. Other areas that are suitable that may produce additional benefits include the ACC landfill (Figure 10), which already hosts a wide variety of wildlife, and the recently acquired Tallassee Tract (Figure 11), itself known for its significant ecological value.
Milledge Avenue, a major roadway in Athens-Clarke County, Georgia, contains numerous suitable installation locations flanking each side of the roadway; with much of that land owned by the University of Georgia, the corridor has potential to serve as a pollinator pathway research project.
The Athens-Clarke County landfill may seem like an undesirable place for the installation of hypernatural planting designs, but what better place to beautify? The site is also known in the community for its wildlife viewing opportunities; such plantings would only bolster this reputation.
The Tallassee Tract was recently acquired by the Athens-Clarke County Unified Government. It features more than 120 ha (300 ac) of valuable ecological habitat, and in particular, for butterflies. The area is mostly forested, but along the utility rights-of-way are ample spaces for habitat-enriching planting designs that can be appreciated by humans and wildlife.
POTENTIAL APPLICATION
Results of the screening process indicate promising potential for installations of hypernatural Piedmont prairies in a wide variety of locations within the constraints of this model. Though this analysis pertains to a specific county, its methodology is intended to be readily replicated for other locations. The methodology provides a general framework for site selection in landscapes across the Piedmont.
We designed the technique used in this analysis to be a coarse screening or rapid assessment of potential optimum areas. The inherent limitations of this type of analysis became apparent with the generated results. The limiting factor in this case appears to be the land cover data. When overlaid atop a high-resolution aerial image, some areas classified as one of the types selected for analysis (Barren land, Developed Open Space, Hay/Pasture, Herbaceous, and Scrub/Shrub) were instead characterized by forested land cover. This limitation is attributable to the 30-m (98-ft) scale of the data: The resolution is generally too low for small sites. Data are generated using LANDSAT satellites, which do not provide high resolution imaging. Even with this limitation, the land cover data appeared correct for a large amount of the study area. Locally sourced LiDAR data may help when a finer level of detail is needed for site selection.
Future research may broaden to include private properties. In addition to increasing the amount of acreage in optimum condition for planting, the inclusion of private properties would allow for modeling ecological connectivity, such as pollinator corridors, between parcels. This next step would also generate the possibility of forming a public–private partnership program that may, for example, aim to reduce mowing and increase landscape biodiversity. Community-oriented landscape design practices can offer both beautiful and beneficial plantings across the Piedmont (Figure 12).
Researchers at the University of Georgia are working with the Georgia Department of Transportation and The Ray (a living lab and innovative movement to build net-zero highways) to trial hypernatural seed mixes for roadside plantings such as the one shown here.
ACKNOWLEDGMENTS
The authors acknowledge some of the partners, collaborators, and funders of a broader designed Piedmont prairie project, which included this research as well as a study of establishment in a trial setting. As such, we acknowledge the University of Georgia Office of Sustainability for research funding; Heather Alley and Jennifer Ceska of the Mimsie Lanier Center at the Georgia State Botanical Garden for guidance and seed donation; JoHannah Biang and David Berle and UGArden for the planting trial location and continued maintenance.
Footnotes
Photos by Brad Davis
Map credits: Athens-Clarke County; Atlanta Regional Commission; US Geological Survey
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