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| Big Green Mountain, a pluton in Western North Carolina's Panthertown Valley |
No, the title of this post is not a reference to some obscure, science fiction B-movie, nor is it a gripe about the naming controversy surrounding our solar system's outermost planet. This post is instead about something far more fascinating and a bit closer to home.
Scattered across the southern Appalachian Mountains and their surrounding foothills are a number of mountains which seem to stand out from the rest of their more pointed, forest-covered neighboring peaks. These summits, most famously Georgia's Stone Mountain or North Carolina's Looking Glass Rock, instead appear as squat, rounded domes of granite, most of which shows through vegetation as steep slopes of bare, exposed rock. Asheville's Looking Glass Rock, in fact, received its name due to the reflective nature of sunlight hitting the ice sheets that form on its bare granite surface in the winter.
These unique mountains, though, are far from just an oddly-shaped summit scattered haphazardly across the region. These peaks are instead referred to geologically as "plutons," or rock formations with their own curious past. Unlike most of the Appalachians, which were built from continental collisions eons ago and have been gradually worn down, plutons have actually appeared as a result of surrounding rock being slowly eroded away - literally the mountain-building process acting in reverse!
Pluton formation results initially from deposits of molten magma below the Earth's surface, typically during periods of intense geologic activity such as that found during the Appalachians' infancy. As this activity slows, magma deposits eventually cool and crystallize to form deep "bubbles" of resistant granite belowground. Then, as the eroding action of wind and water act over millennia, the less-resistant layers of rock surrounding the granite weather away, leaving portions of the hard granite projecting outward from the surface. To better understand how this process might work in an oversimplified sense, consider burying several hard, fist-size pebbles of rock in loose sand beside the ocean. As waves gradually eat away at the loose, pliable sand, you would slowly begin to the see the denser, more resistant pebbles remaining in place. This process, acted out on a much grander scale and with more complex geologic forces, helps to explain why we see huge granite monoliths like Stone Mountain projecting from the surrounding landscape today.
Rock outcrop plant communities
Plutons, however, are far from just a geological wonder. A number of wildly unique living organisms can also be found on the rock formations' surfaces. Georgia's Stone Mountain, for example, contains potholes on its summit that harbor fairy shrimp...at nearly 1,700 feet above sea level! Other plutons serve as habitat for rare plant and lichen species, some found few other places on Earth outside of these and other nearby rock outcrops in the Appalachian foothills region. To see some fantastic pictures of plant communities from plutons and other significant rock formations in southern Appalachia, check out photographer Alan Cressler's images from the region.
The plant communities on the Appalachians' rock outcrops specifically show a pattern of high endemism, a term meaning that these habitats contain a number of plant species found nowhere else other than the immediate Appalachian region. Some of these species occur in shallow depressions on the rock surfaces that slowly fill with soil, while others occur in or near pools of collected rainfall in similar depressions and surface irregularities. This unique collection of plants has led many biologists to ask an important question: why do plutons and similar outcrops contain such a high number of rare plants?
Several hypotheses have been proposed to explain this high level of endemism in Appalachian rock outcrop ecosystems, and these discussions have hinged around a central question: do rock outcrops possess a special characteristic that allows endemic plants to survive there and nowhere else, or does competition with other plants - those found outside of outcrops - serve to restrict endemics solely to their rocky islands? Answering this question is key to understanding how these plants may have evolved, since a mechanism must exist to "trap" these species on outcrops, allowing for increased rates of adaptation to the rock surfaces' unique environments.
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| A granitic dome plant community, all housed in a shallow pocket of soil atop a pluton in Western North Carolina |
Explaining outcrop endemism
In the late 1980s, biologists decided to test this question using several lines of evidence from Appalachian outcrop endemics. Specifically, they identified a single observation of these endemic plants that opened the door to more questions: plants that occur only on outcrops can also grow if planted in captivity in soil obtained far away from the outcrops themselves. This observation suggested that the physical characteristics of the outcrops themselves was not likely the culprit for why these plants are found nowhere else. So what other factors could be driving their isolation?
Further observation of rock outcrop plants showed that endemics tended to favor areas of outcrops exposed to high levels of light, while avoiding shaded portions of outcrops and surrounding forests. A closer observation of these plants yielded an even stronger relationship to light. The growth rates and rates of photosynthesis - how fast these endemic plants grow and convert light energy to sugar - were highest in high-light conditions and extraordinarily reduced in shady conditions with little direct light. Coupled with earlier observations about a lack of preference for soil, this solidified a major piece of the rock outcrop puzzle: endemic outcrop plants possess an adaptation for high-light environments, rather than a specific preference for physical conditions found only on the rocks' surface.
Even with this information, one question still remained. What process was keeping these plants from moving outward from their rocky habitats? Follow-up work examined the results of studies performed in the field and in greenhouses, in which the ability of rock outcrop endemics to compete against other native plants were examined. Once again, these results were striking. Endemic plants tended to be less tolerant of shade due to the fact that they are poor competitors for resources compared to taller plants found in surrounding forests, which quickly shade out and eliminate endemics in forested areas outside of outcrops.
Completing the puzzle
When viewed together, the above evidence creates a rough picture of how plutons and rock outcrops may have acquired their highly unique flora. Exposed rock surfaces tend to be extreme environments relative to surrounding forests, containing little to no soil depth and direct, almost continuous exposure to sunlight. Plants preferring high-light conditions could easily colonize these sites if and when sufficient soil and moisture occur, yet poor competitive abilities with neighboring forest plants would "trap" these species on their outcrop islands, in a refuge from competition.
Once trapped within these patchy habitats, evolutionary processes are free to run wild. With little migration beyond their isolated, island-like habitat, genetic frequencies can shift at random, and new mutations can arise within these outcrops and further drive selection, producing adaptations more finely-honed against the extreme pressures of outcrop life. Although there is much more to the evolution of these endemic plants than light and competition - complex relationships with pollinators and habitat variability within outcrops also exist - these basic factors show how simple adaptations and environmental patchiness can drive runaway selection. These same processes, in fact, also have a dark side. Rock outcrop plants in the Appalachians tend to be not only highly adapted but also highly threatened, all a consequence of being restricted to habitats that are small in number and even smaller in size.
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See it for yourself
- Stone Mountain Park, Georgia
- Looking Glass Rock, North Carolina
- Panthertown Valley Plutons, North Carolina (multiple peaks)
- Table Rock State Park, South Carolina
Note: Rock outcrop flora only occur in a few places on the planet, and the communities they live in can take centuries to form due to the painstakingly slow collection of soil and other resources on rock surfaces. Many species are even protected at various legislative levels across southern Appalachia. Be sure to care for these species if visiting public lands by not trampling flora on plutons and other rock outcrops across the region, never picking or disturbing native plants, and staying only on established trails atop overlooks and across outcrop hiking routes.
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Relevant sources from the scientific literature
Jerry M. Baskin, & Carol C. Baskin (1988). Endemism in Rock Outcrop Plant Communities of Unglaciated Eastern United States: An Evaluation of the Role of the Edaphic, Genetic, and Light Factors Journal of Biogeography, 15 (5/6), 829-840
Shure, D.J. 1999. Granite Outcrops of the Southeastern United States. Pp.99-116 in Savannas, Barrens, and Rock Outcrop Plant Communities of North America, R.C. Anderson, J.S. Fralish, and J.M. Baskins, eds. Cambridge University Press, New York.
