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THE
MOUNTAINS: The granitic dome community
Granitic domes
The typical granitic
dome community is found mostly in the mountains but occasionally occurs
on monadnocks (mountains of resistant
rock) in the piedmont. It occurs on exposed upper slopes or ridges, usually
as steep exposures, except for small gently sloping areas at the top.
It is recognized by the smooth (exfoliating) surface of the exposed rock.
Granitic domes are sometimes called exfoliation domes because they consist
of layers of rock similar to the layers of an onion. These layers were
produced during the formation of the rock as a means of relieving pressure.
Soils generally are
shallow, and trees and shrubs are restricted to the margins or to pockets
of deeper soil, which tend to be rare because of the sparsity of fractures
or crevices. Vegetation is zoned by soil depth. Only mosses and lichens
occupy bare rock, but once a soil layer develops, a variety of vascular
plants soon invade.
Herbaceous species
that are usually only found on domes include
mountain dwarf-dandelion
(Krigia montana),
Allegheny live-for-ever (Sedum
telephioides),
cliff saxifrage (Saxifraga
michauxii),
divided-leaf ragwort (Senecio
millefolium),
and spikemosses (Selaginella
tortipila or S. rupestris).
Herbaceous species
that are also common to granitic domes include
elf orpine (Diamorpha
smallii),
smallflower phacelia (Phacelia
dubia),
eggleaf rushfoil,
several species of broomsedge,
hairy and woolly lip-fems (Cheilanthes
lanosa and C. tomentosa),
pineweed,
and Small’s ragwort.
Woody species that
occur at the edges of the dome or in mats that have deep soil include
fringe-tree,
wafer-ash (Ptelea trifoliata),
winged sumac (Rhus copallina),
eastern red cedar,
Virginia pine, and,
depending on the depth and richness of the soil, a variety of other
xerophytic to mesophytic trees.
Seepages at the upper
edges may harbor interesting species such as
round-leaf sundew
(Drosera rotundifolia),
horned bladderwort,
flatrock pimpernel, and
several species of meadow-beauty (Rhexia
mariana and/or R.
virginica).
Higher elevation granitic
domes may include such species as
sand myrtle (Leiophyllum
buxifolium),
prolific St. John's-wort (Hypericum
prolificum),
and Table Mountain pine (Pinus
pungens).
Granitic domes associated
with calcium-producing rocks are sometimes described as a basic variant
of granitic domes. These domes harbor such calcium-loving species as
yellow honeysuckle
(Lonicera flava),
southern thimbleweed (Anemone
berlandieri),
Canada columbine (Aquilegia
canadensis),
smooth indigo-bush (Amorpha
glabra),
Small’s beardtongue (Penstemon
smallii),
hairy mock-orange (Philadelphus
hirsutus),
and Seneca snakeroot (Polygala
senega).
An abundant presence
of red cedar along the outcrop margins is often a good indicator of this
variant. The adjacent woodland often is quite rich, even on the shallow
soils of the southern exposures.
A second variant of
granitic domes is the acidic cliff. The community is characterized by
sheer slopes, exposed rock, and a canopy cover that is less than 25%.
The best sites have sheer exposures of fractured, acidic rock. This variant
is common in the mountains and rare in the piedmont. In the driest, most
open sites, many species of typical granitic domes or granitic flatrock
outcrops are present, especially
broomsedges,
oat grasses,
spikemosses,
hawkweeds,
and ragworts.
On moister sites,
granitic dome species such as
mountain
dwarf-dandelion
and cliff saxifrage may
occur.
Because of the fractured
rock, crevices are common and rock-loving ferns are present, including
lipferns and
mountain spleenwort (Asplenium
montanum).
Shrubs, trees, and
other herbs are typical of the surrounding forest community; they may
be mesic, xeric, or even boggy species depending on the nature of the
adjacent forest community. Because of the open canopy, a variety of weedy
species may occur, including
fleabanes (Erigeron
spp.),
butterfly-weed (Asclepias
tuberosa subsp. tuberosa),
and Indian pink.
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