A brief natural history of Piedmont Granitic Flatrocks, and a visit to one in Virginia

by Drew Chaney

April 25, 2022

The Piedmont Granitic Flatrock is an ecosystem endemic to the Piedmont region of the southeastern United States, found from Alabama north to south-central Virginia. It occurs on flat or gently sloped outcrops of granite and similar rocks, usually near streams which have eroded the rock bare over time. These outcrops contain multiple microhabitats, each with their own set of species adapted to their conditions.

Bare rock is colonized by lichens (Xanthoparmelia spp., Cladonia spp, etc.). Mats of mosses (Grimmia laevigata, Polytrichum spp.) form ideal rooting spots for herbaceous species like common buttonweed (Hexasepalum teres), poverty oatgrass (Danthonia spicata), little bluestem (Schizachyrium scoparium), fork-tip three-awn grass (Aristida dichotoma), Appalachian sandwort (Minuartia glabra), granite stonecrop (Sedum pusillum), roundleaf fameflower (Phemeranthus teretifolius), prickly-pears (Opuntia spp.), glade rushfoil (Croton willdenowii), broomsedge (Andropogon virginicus), granite flatsedge (Cyperus granitophilus), granite whitlow-grass (Draba aprica), Small’s purslane (Portulaca smallii), pineweed (Hypericum gentianoides), and common hairsedge (Bulbostylis capillaris). Areas where soil builds up deeper allow woody plants to root, the most typical being loblolly pine (Pinus taeda), eastern redcedar (Juniperus virginiana) winged elm, (Ulmus alata), and Virginia pine (Pinus virginiana). Another feature of these outcrops are depressions in which water forms ephemeral pools. These serve as microhabitats for many flatrock endemic plant species, of which more will be discussed below. They are also ideal breeding locations for many amphibian and insect species. 

Small’s stonecrop (Diamorpha smallii)

Granitic flatrock endemic species

Flatrock landscape with solution pit

There are several species that are endemic or near-endemic to granitic flatrocks in the southeastern Piedmont. Many of these are restricted to a microhabitat called solution pits, seasonally inundated depressions in the rock formed by carbonic weathering by water, lichens and plants. Small’s stonecrop (Diamorpha smallii), pool-sprite (Gratiola amphiantha), and multiple quillworts (Isoetes melanospora, I. piedmontana, and I. tegetiformans) are species restricted entirely (or nearly so in Diamorpha) to this microhabitat, while granite-loving flatsedge (Cyperus granitophilus), Small’s purslane (Portulaca smallii), granite stonecrop (Sedum pusillum), and granite whitlow-grass (Draba aprica) are more widespread, occurring in moss mats in both dry and damp areas of flatrocks. 

In Virginia, examples of this community are found in Brunswick, Dinwiddie, Lunenburg, Mecklenburg, Nottoway, Powhatan, and Prince Edward counties in the south-central and southeastern Piedmont. They occur on outcrops of granite, granodiorite, granitic gneiss, and similar rocks, usually along streams. Only three of the endemic species to this community occur in Virginia, Small’s stonecrop (Diamorpha smallii), Small’s purslane (Portulaca smallii), and granite flatsedge (Cyperus granitophilus). In addition, the only Virginia locality for the Piedmont quillwort (Isoetes piedmontana) is a granitic flatrock in Powhatan County. 

Distribution of granitic flatrocks in the Virginia Piedmont

This unique community faces multiple threats. Non-native invasive species of course are one of the major problems these often small ecosystems face, as they can shade out and outcompete native heliophytes. Quarrying activity and roads have impacted multiple of Virginia’s examples, and Dinwiddie County’s only station is located between a road and an agricultural field, and has suffered from trash dumping. Other human activities like four-wheeling pose a significant threat as well. 

Diamorpha with broken glass
Mary Jane keys a plant

On April 15, 2022, part-time CUH staff member and Professor of Biology at Mary Baldwin University, Mary Jane Epps and I ventured down to Brunswick County, VA to visit one of Virginia’s finest examples of the Piedmont Granitic Flatrock community. This site is owned by the Nature Conservancy and is the only site in Virginia for Small’s stonecrop (Diamorpha smallii). When we arrived, we quickly spotted it growing in mats of Grimmia on the edge of the exposed granite, and in round depressions on the exposed rock face. It immediately stood out amongst the other vegetation due to its striking red coloration and succulent foliage, an adaptation to the hot and dry microclimate it inhabits.

It is a winter annual, germinating in fall and overwintering as a tiny rosette before flowering in spring and dying after the seed ripens in summer, thus avoiding the harshest part of the year. (Temperatures on the surface of granitic flatrocks in Georgia have been measured as high as 140° F in the summer!). The delicate, four-petaled white flowers had not opened yet on the date we visited, but seeing this unique endemic growing in the gravelly solution pits (depressions formed by carbonic acid weathering by plants, lichens, and water flow) that are its home was very exciting nonetheless. Other solution pits were completely full of water and had many tadpoles in them.

Tadpoles and oak leaf in a solution pit

Around the edges of the exposed rock, Appalachian sandwort (Minuartia glabra), tiny bluets (Houstonia pusilla) and Virginia dwarf-dandelion (Krigia virginica) formed diminutive mats of color in beds of moss.  A few perennial species adapted to harsh dry conditions were scattered amongst them, the most abundant being southeastern prickly-pear (Opuntia mesacantha), broomsedge (Andropogon virginicus) and roundleaf fameflower (Phemeranthus teretifolius).

Small black blueberry (Vaccinium tenellum)

Unfortunately, the invasive Chinese privet (Ligustrum sinense) was abundant in the shrubby thickets around the flatrocks, and a private driveway crossed over one of the outcrops, rendering a portion of the granite almost completely free of mosses and lichens. Venturing deeper into the piney woods, we explored another more secluded barren, hidden away from areas of human activity. Similar species were found on the rock’s various microhabitats, but the woodland around the barren was the real highlight. Stunted loblolly pines (Pinus taeda), eastern redcedars (Juniperus virginiana), hickories (Carya spp.) and post oak (Quercus stellata) formed a low, open canopy reminiscent of the oak/pinyon/juniper woodlands of the Southwest. Fringetree (Chionanthus virginicus), and small black blueberry (Vaccinium tenellum) were blooming in the understory, and the ground was covered by thick, crunchy mats of reindeer lichens (Cladonia spp.) and haircap mosses (Polytrichum spp.)

Eastern fence lizards scurried under fallen branches, and sawfly larvae munched away at loblolly needles. After poking around the woodlands for a bit, we returned to the car to begin the long drive back home, satisfied with the day’s excursion. 

Endemic: a species restricted to a certain area where it is native

Heliophyte: a species adapted to sunlight