Flora of Georgia and surrounding states
Granite Outcrop Plants
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Granite and Gneiss
Most call all of the hard, crystalline, and mostly flat rock outcrops in the Piedmont of the southeast "granite", and so I call this collection of plants. The name is mostly incorrect as the majority of these outcrops are of the metamorphic rock, gneiss. In the Georgia Piedmont, several are readily accessible: Stone Mountain, Panola Mountain, Arabia Mountain, Heggies Rock, Camp Meeting Rock, Rock and Shoals, and the St. Mary's outcrop on Kennesaw Mountain. Sandstone Beyond the Piedmont, there are a number of outcrops of sandstone that create a very similar environment and they are included here as well. Broxton Rocks in Coffee County, Georgia and the Little River Gorge in Alabama have outcrops of hard sandstone that supports some typical granite outcrop plants. |
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Limestone
Flat rock outcrops of limestone are very rare in Georgia but abundant in the Valley and Ridge and Interior Low Plateaus, especially in the Nashville Basin. Limestone can form such a smooth and hard surface that it mimics granite in its impervious and water shedding qualities. What distinguishes it from both sandstone and granite is the much greater availability of calcium and magnesium to the plants that have adapted to grow on the limestone glades. Southeastern Outcrops This hand-made map drawn by Elsie Quarterman [Professor Emeritus at Vanderbilt University plant ecologist specializing in the limestone cedar glades of Tennessee], shows the limestone, sandstone and granite outcrops that she knew of in 1993. I've found no better map to illustrate the places to look for this community. With a careful look at Quarterman's map, one can discern the differences in geology. In Georgia, the Coastal Plain outcrops are all sandstone. The Piedmont and Blue Ridge outcrops are granite and gneiss. The Valley and Ridge and Interior Low Plateaus are all limestone. A noticeable break occurs between each on this map. |
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Environmental Conditions
These outcrops share several environmental conditions that create harsh conditions for plants to survive. They include intense solar radiation, extreme and rapidly fluctuating temperatures, frequent drought, extended periods of water saturation during wet periods, extreme runoff during rains and extremely low water-holding capacity in dry weather.
While dated, these references include descriptions that accurately describe this community. Several of the names have changed *.
"Because they present habitats so drastically different than the Piedmont matrix in which they are embedded, granite outcrops have a somewhat peculiar flora, with marked adaptations to the xeric character of the environment. Slow growth rates over long periods of time (drought tolerance) and semelparity (drought evasion) are two strategies adopted by the plants to cope with the limiting abiotic conditions (Lugo and McCormick 1981). Some of the morphological adaptations facilitating the presence of species in these xeric habitats include: succulence (e.g., Talinum teretifolium), presence of spines (e.g., Opuntia humifusa) or of silky hair (e.g., Senecio tomentosus). The C, (e.g., Portulaca smallii) and CAM (e.g., Opuntia humifusa) photosynthetic pathways constitute physiological adaptations to the high temperatures and the limited water-availability (Martin et al. 1982)."
Houle, G. 1987. Vascular Plants of Arabia Mountain, Georgia. Bulletin of the Torrey Botanical Club, 114:4, 412-418..
Ecological Communities
The seminal work on granite outcrop ecology concisely describes the communities on our hard, crystalline, mostly flat rock outcrops.
"Communities in depressions, occurring as islands surrounded by naturally exposed granitic rock, were classified by the correlates of maximum soil depth and characteristic vegetation cover into: diamorpha communities, 2-6 cm; lichen-annual herb communities, 7-15 cm ; annual-perennial herb communities, 16-39 cm ; and herb-shrub communities, 40-50 cm. The total flora of the 40 island communities included 76 species, but only 39 of these occurred in five or more communities and were considered characteristic. The extreme environmental conditions of high light intensity and low soil moisture content were indicated by the fact that of the 20 plants present in over 50% of the communities, 18 were lichens, mosses, and flowering annuals and only two were perennial flowering plants. The flora of the island communities is distinct from that of border vegetation. Only three endemics are among the 39 characteristic plants. Seasonal aspects of the outcrop vegetation are striking. During the winter months, November through February, mosses and lichens are most conspicuous, but seedlings of spring-blooming annuals are also present. From late February to May there is a continuous succession of plants in flower, the majority of which complete their life cycles before the hot dry months from May to August. In September the outcrops are covered with the orange-yellow flowers of Viguera Porteri. Spring-flowering annuals germinate in October at the time summer-flowering annuals are dying. The four types of island communities described appear to represent stages in plant succession directly related to edaphic conditions. Apparently the island communities have existed long enough in geologic time for an endemic flora to have evolved. The life of any one community, however, is measured in hundreds rather than in thousands of years."
Burbanck, M.P. & R.G. Platt. 1964. Granite Outcrop Communities of the Piedmont Plateau in Georgia. Ecology, 45:2, 292-306.
*Talinum teretifolium = Phemeranthus teretifolius
Senecio tomentosus = Packera tomentosa
Viguera Porteri = Helianthus porteri
A more contemporary treatment considers the Burbanck & Platt communities as "successional soil communities" and classifies the habitat into marginal (edge of the rock outcrop next to the adjacent forest); exposed rock, and vernal (temporary) pools.
Edwards, L., J. Ambrose & L.K. Kirkman. 2013. The Natural Communities of Georgia. The University of Georgia Press, Athens.
These outcrops share several environmental conditions that create harsh conditions for plants to survive. They include intense solar radiation, extreme and rapidly fluctuating temperatures, frequent drought, extended periods of water saturation during wet periods, extreme runoff during rains and extremely low water-holding capacity in dry weather.
While dated, these references include descriptions that accurately describe this community. Several of the names have changed *.
"Because they present habitats so drastically different than the Piedmont matrix in which they are embedded, granite outcrops have a somewhat peculiar flora, with marked adaptations to the xeric character of the environment. Slow growth rates over long periods of time (drought tolerance) and semelparity (drought evasion) are two strategies adopted by the plants to cope with the limiting abiotic conditions (Lugo and McCormick 1981). Some of the morphological adaptations facilitating the presence of species in these xeric habitats include: succulence (e.g., Talinum teretifolium), presence of spines (e.g., Opuntia humifusa) or of silky hair (e.g., Senecio tomentosus). The C, (e.g., Portulaca smallii) and CAM (e.g., Opuntia humifusa) photosynthetic pathways constitute physiological adaptations to the high temperatures and the limited water-availability (Martin et al. 1982)."
Houle, G. 1987. Vascular Plants of Arabia Mountain, Georgia. Bulletin of the Torrey Botanical Club, 114:4, 412-418..
Ecological Communities
The seminal work on granite outcrop ecology concisely describes the communities on our hard, crystalline, mostly flat rock outcrops.
"Communities in depressions, occurring as islands surrounded by naturally exposed granitic rock, were classified by the correlates of maximum soil depth and characteristic vegetation cover into: diamorpha communities, 2-6 cm; lichen-annual herb communities, 7-15 cm ; annual-perennial herb communities, 16-39 cm ; and herb-shrub communities, 40-50 cm. The total flora of the 40 island communities included 76 species, but only 39 of these occurred in five or more communities and were considered characteristic. The extreme environmental conditions of high light intensity and low soil moisture content were indicated by the fact that of the 20 plants present in over 50% of the communities, 18 were lichens, mosses, and flowering annuals and only two were perennial flowering plants. The flora of the island communities is distinct from that of border vegetation. Only three endemics are among the 39 characteristic plants. Seasonal aspects of the outcrop vegetation are striking. During the winter months, November through February, mosses and lichens are most conspicuous, but seedlings of spring-blooming annuals are also present. From late February to May there is a continuous succession of plants in flower, the majority of which complete their life cycles before the hot dry months from May to August. In September the outcrops are covered with the orange-yellow flowers of Viguera Porteri. Spring-flowering annuals germinate in October at the time summer-flowering annuals are dying. The four types of island communities described appear to represent stages in plant succession directly related to edaphic conditions. Apparently the island communities have existed long enough in geologic time for an endemic flora to have evolved. The life of any one community, however, is measured in hundreds rather than in thousands of years."
Burbanck, M.P. & R.G. Platt. 1964. Granite Outcrop Communities of the Piedmont Plateau in Georgia. Ecology, 45:2, 292-306.
*Talinum teretifolium = Phemeranthus teretifolius
Senecio tomentosus = Packera tomentosa
Viguera Porteri = Helianthus porteri
A more contemporary treatment considers the Burbanck & Platt communities as "successional soil communities" and classifies the habitat into marginal (edge of the rock outcrop next to the adjacent forest); exposed rock, and vernal (temporary) pools.
Edwards, L., J. Ambrose & L.K. Kirkman. 2013. The Natural Communities of Georgia. The University of Georgia Press, Athens.