Life on the Rock

 

A granite rock outcrop showing a variety of plant types during an April visit. 
Clover Community Park, Clover, South Carolina.

The granite rock outcrops of the Southeastern Piedmont host an impressive array of rare and beautiful plants.  Spring is the best season the enjoy these unique ecosystems and experience this wonderful flora. 

 

A large solution pool with Diamorpha smallii, Minutartia sp.
and Isoetes sp. 40 Acre Rock, Lancaster County, South Carolina.

Granite rock outcrops are ancient in origin.  Hundreds of millions of years ago the area that is now the Piedmont experienced extreme vulcanism. In some cases, the magma never reached the surface but cooled and crystalized underground to form granite.  Tectonic uplift and erosion exposed portions of this granite.  In some places the outcrops are mountains rising above the surrounding country.  Stone Mountain, Georgia is the most dramatic of these.  In other places, the outcrop is a flat rock surrounded by forest. 

Prickly Pear Cactus (Opuntia humifusa) is adapted to dry conditions
and thrives on the Piedmont granite rock outcrops.
Dunn's Mountain Park, Rowan County, North Carolina.

Outcrops are impermeable stone and the plants that grow on them are adapted for a desert-like life.  Rainfall is typically the only source of water for the plants that call outcrops home.  Many of these plants reproduce quickly before the water on the outcrop dries up.

 

Peppered Rock-shield Lichen (Xanthoparmelia conspera) on granite.

Clover Community Park, Clover, Lancaster County, South Carolina.  

The first plants to colonize outcrops are lichens and mosses.  These hardy pioneers grow directly on the rock surface.  Lichens and mosses release acids that start converting stone to soil. Grimmia laevigata, Dry Rock Moss, is one of the species that live on bare rock.  When dry, this moss looks almost black but greens up when wet.  The action of erosion and pioneer plants make small depressions, called solution pools.  Over time, soil accumulates and herbaceous plants can take root.  When plants in the pools die they add organic matter to the thin, rocky soil.  As more soil builds up, hardy trees like pines and junipers colonize the outcrop. 

Light gray Reindeer lichen Cladonia sp. growing among various mosses.
Dunn's Mountain Park, Rowan County, North Carolina.

A single red plant of Elf Orpine, the black moss
Grimmia laevigata and the lichen Cladonia sp. grow at
Dunn's Mountain Park, Rowan County, North Carolina. 

In early April, granite rock outcrops in erupt with color.  Three tiny plants, Diamorpha smallii (Elf Orpine),  Minuartia uniflora (Piedmont Sandwort) and Minuartia glabra (Appalachian Stitchwort) burst out red and white in the solution pools.  Diamorpha smallii stands about three inches tall.  It makes bright red, succulent leaves that store water, which is in short supply on the rock.  Elf Orpine has flowers with four white petals and each petal is decorated with a red spot.   Diamorpha smallii is endemic to the Southeastern United States and limited to the rock outcrop habitat. 

 

A solution pool filled with Elf Orpine (Diamorpha smallii).
40 Acre Rock, Lancaster County, South Carolina.

Closeup of Diamorpha smallii.
Clover Community Park, Clover, South Carolina.

Piedmont Sandwort and Appalachian Stitchwort are also granite rock outcrop specialists. Appalachian Stitchwort is about twice as tall as Piedmont Sandwort and both have green leaves and five-petaled, white flowers.  Diamorpha and the two species of Minuartia often occur in the same solution pools and light it up with red and white.  

Piedmont Sandwort (Minuartia uniflora) flowering in a small solution pool.
40 Acre Rock, Lancaster County, South Carolina. 

Appalachian Stitchwort and the moss Grimmia laevegata.
40 Acre Rock, Lancaster County, South Carolina. 

Woolly Ragwort (Packera tomentosa) is a member of the sunflower family and grows mainly in the sandy soil of the Coastal Plain of the Southeast.  When it does occur on the Piedmont it is on rock outcrops.  This attractive plant has hairy, green leaves and an inflorescence of yellow ray and disk flowers.  Wooly Ragwort is much taller than Elf Orpine and Piedmont Sandwort that grow in the same area.  

Wooly Ragwort (Packera tomentosa) flowering in a 
bed of the moss (Polytrichum sp.) 
Clover Community Park, Clover, South Carolina.

False Garlic (Nothoscordum bivalve) is a relative of onion and garlic.  Like Wooly Ragwort, it is common on the Coastal Plain but also grows on Piedmont granite rock outcrops.  False Garlic makes star-shaped, white flowers and lacks the strong odor onion or garlic.    

False Garlic (Nothoscordum bivalve).
40 Acre Rock, Lancaster County, South Carolina.

Quillworts are members of the genus Isoetes, and dominate some of the solution pools on outcrops.  Quillworts look like grass in the shallow water but are more closely related to ferns.  Isoetes makes no flowers or fruits but reproduces by spores produced at the base of its leaves.  Identification of Quillworts is difficult because different species can hybridize and spontaneously double their chromosome number. 

 

Quillworts (Isoetes sp.) growing in a solution pool.  
Clover Community Park, Clover, South Carolina.

A single plant of Isoetes sp. This plant was collected 
by a researcher studying this rare plant.
40 Acre Rock, Lancaster County, South Carolina.

Granite rock outcrops have been used and abused by people for centuries.  Outcrops are natural sites to quarry the granite for construction and headstones.  Some flat outcrops have been turned into parking areas.  Recreation is also a threat to the rare and threated species that call outcrops home.  All-terrain vehicle drivers, motorcyclists, bikers and hikers plow through solution pools killing plants.  Broken glass litters some outcrops and graffiti is an issue in many areas.  Despite these problems, granite rock outcrops, particularly in spring, can be magical places. 

Diamorpha smallii flowering in a field of shattered glass.
Clover Community Park, Clover, South Carolina.