Xeric hardpan forest

Distinguishing Features

Xeric hardpan forest can be distinguished from other piedmont forests by its fairly open canopy of stunted trees dominated by blackjack oak (Quercus marilandica) and post oak (Q. stellata).

Introduction

Xeric hardpan forest occurs on upland flats and gentle slopes of the piedmont region where a clay hardpan or shallow rock layer impedes water movement and root growth. The scattered boulders and shallow rocks associated with this community are usually gabbro, a dark gray to black rock that contains calcium-rich feldspars that weather to form a nearly basic (circumneutral) soil that is dark in color, unlike the typical red clay soils of the piedmont. The clay hardpan or rock near the surface of the forest floor results in moisture conditions so variable that the soil can be a gummy paste during wet periods and a brick-like substrate during dry periods. In spite of high amounts of magnesium, calcium, and iron, the soil is fairly infertile. With its shallow, nutrient-poor soil and highly variable moisture conditions, xeric hardpan forest is unproductive and even mature stands are typically stunted. A local name for xeric hardpan forest is "blackjack lands," reflecting the common occurrence of blackjack oak.

Vegetation

The canopy is dominated by blackjack oak and post oak. Other canopy species include Carolina shagbark hickory (Carya carolinae-septentrionalis), pignut hickory (C. glabra), American ash (Fraxinus americana), Virginia pine (Pinus virginiana), shortleaf pine (P. echinata), and other oaks such as white oak (Q. alba) and willow oak (Q. phellos). Due to the stunted nature of the forest, it can be difficult to separate the overstory from the understory. Eastern red cedar (Juniperus virgini-ana) and eastern redbud (Cercis canadensis) are dominant subcanopy trees. Other understory species include winged elm (Ulmus alata), persimmon (Diospyros vir-giniana), and fringetree (Chionanthus virginicus). The shrub layer is often sparse and includes species such as blackhaw (Viburnumprunifolium), sparkleberry (Vac-cinium arboreum), deerberry (V. stamineum), Saint Andrew's cross (Hypericum hypericoides), and fragrant sumac (Rhus aromatica).

On shallow soils associated with exposed bedrock, or where the canopy is kept open by artificial means, the herbaceous layer is often abundant and diverse. In these prairie-like habitats, grasses such as poverty grass (Danthonia spicata) and various species of broomsedge (Andropogon spp.) are dominant. Other interesting and unusual herbs include curlyheads (Clematis ochroleuca), butterfly weed (Asclepias tuberosa), rattlesnake master (Eryngium yuccifolium), and prairie dock (Silphium terebinthinaceum). While a number of these species have midwestern prairie affinities (discussed below), others are primarily associated with granitic outcrops, including erect dayflower (Commelina erecta) and Appalachian rock pink (Phemeranthus teretifolius). Small depressions fill with water in winter and dry out in summer. In these ephemeral pools, wetland species such as quillwort (Isoetes spp.) occur adjacent to drought-adapted species. Variability in factors such as soil moisture, soil depth, amount of sunlight, and disturbance history result in a mosaic of environments that contribute to a rich diversity of herbaceous plants.

Seasonal Aspects

A strong sweet fragrance in spring may indicate that fringetree, autumn olive (Elaeagnus umbellata), or Japanese honeysuckle (Lonicera japonica) is in bloom. Most fragrant in evening, their odor attracts evening flying moths that function as pollinators. Other spring-flowering plants include the solitary nodding flowers of curlyheads and the bicolored pea-like flowers of goat's rue (Tephrosia virginiana). From late spring through summer, numerous butterflies, bees, and other insects harvest nectar from the orange-red flowers of butterfly weed. The bright yellow bee-pollinated flowers of sundrops (Oenothera fruticosa) have a similarly long flowering season. The showy pink to purple flowers of purple gerardia (Agalinis purpurea) and the greenish white heads of rattlesnake master bloom in summer, as do the yellow composite flowers of roughleaf sunflower (Helianthus strumosus), Schweinitz's sunflower (H. schweinitzii), and prairie dock. Some sunflowers track the sun during the day, facing east in the morning and west in the afternoon.

The conspicuous reddish-orange fruits of Carolina rose (Rosa carolina), the bluish berry-like cones of eastern red cedar (Juniperus virginiana), and the long, flat seedpods of eastern redbud (Cercis canadensis) persist well into winter. In contrast, birds and mammals often eat the fleshy, sweet fruits of lowbush blueberry (V. pallidum) soon after ripening.

Distribution

Xeric hardpan forest is a relatively rare community restricted to small, widely scattered sites in the piedmont region from Virginia south to Georgia.

Dynamics

Historically, fires played an important role in maintaining this community. With frequently dry conditions and a grassy herbaceous layer, such sites were susceptible to fire. The routine use of fire by Native Americans, coupled with periodic lightning-induced fires, reduced the growth and reproduction of woody plants, resulting in an open canopy of widely spaced trees and a conspicuously dense herbaceous layer. Over the last century, fire suppression has promoted the growth and regeneration of trees such that xeric hardpan forest today typically has a more closed canopy and a less developed herbaceous layer than in the past.

Piedmont prairie remnants. Prairies are dominated by grasses and other herbaceous plants, rather than by trees and shrubs. While generally associated with Midwestern states such as Kansas and Illinois, prairies were once widely scattered throughout the piedmont (as well as in the black belt region from eastern Alabama to northern Mississippi). Evidence for historic piedmont prairies comes from literary accounts of European explorers (such as John Bartram) as well as early maps of the region. Historic piedmont prairies were kept open largely because of the frequent use of fire by Native Americans to improve game hunting. Because of fire suppression and the extirpation of large grazing animals such as elk and bison, the piedmont prairie ecosystem has largely disappeared from the southeastern landscape. Not all is lost, however, as remnants of the prairie flora persist in open areas of xeric hardpan forest, in disturbed areas such as roadsides and unsprayed power line corridors, and in various preserves. For example, the Rock Hill Blackjack Heritage Preserve of South Carolina harbors more than 172 North American prairie species, most of which occur in open, grassy areas adjacent to xeric hardwood forest. Among the many interesting prairie species that persist in piedmont habitats are obedient plant (Physostegia virginiana), prairie dock, Indian-grass (Sorghastrum nutans), and rattlesnake master.

Conservation Aspects

The unusual geology and soils associated with this community occupy a very small percentage of the piedmont landscape. As a result, good examples of the community are rare. The species-rich herbaceous layer of high-quality sites includes a number of rare plants, even some federally listed (endangered) species such as smooth coneflower (Echinacea laevigata), Schweinitz's sunflower, Carolina prairie-trefoil (Lotus helleri), and Georgia aster (Symphyotrichumgeorgianum). One of the biggest threats to xeric hardpan forest is fire exclusion, which has resulted in an increase in the density of trees and a reduction in the abundance and diversity of herbaceous species. Sites are also vulnerable to development, logging, and rock quarries. Given its unusual substrate, and its diverse and unusual flora, more attention should be given to preserving and restoring examples of this threatened community.

SUGGESTED READING

Barden, L. S. 1997. Historic Prairies in the Piedmont of North and South Carolina, USA.

Natural Areas Journal 149:149-52. Nelson, J. 1992. The Vanishing Blackjacks. South Carolina Wildlife 39:34-39. Schmidt, J. M., and J. A. Barnwell. 2002. A Flora of the Rock Hill Blackjacks Preserve, York County, South Carolina. Castanea 67:247-79.

XERIC HARDPAN FOREST: CHARACTERISTIC PLANTS

Trees

ABUNDANT SPECIES

Cercis canadensis Juniperus virginiana Quercus marilandica Quercus stellata

Eastern redbud Eastern red cedar Blackjack oak Post oak

OCCASIONAL TO LOCALLY ABUNDANT SPECIES

Carolina shagbark hickory tCarya carolinae-septentrionalis Carya glabra Chionanthus virginicus Diospyros virginiana Fraxinus americana Liquidambar styraciflua Pinus echinata Pinus virginiana Quercus alba tQuercus phellos Ulmus alata

Shrubs

Pignut hickory

Fringetree, Old man's beard

Persimmon

White ash

Sweetgum

Shortleaf pine

Virginia pine

White oak

Willow oak

pp. 229-30

ABUNDANT SPECIES

Vaccinium pallidum Vaccinium stamineun tViburnum prunifolium tViburnum rafinesquianum

Lowbush blueberry Deerberry Blackhaw Downy arrowwood

OCCASIONAL TO LOCALLY ABUNDANT SPECIES

'Hypericum hypericoides 'Rhus aromatica Rosa carolina Vaccinium arboreum

Herbs

Saint Andrew's cross Fragrant sumac Carolina rose Sparkleberry p. 318 p. 319

ABUNDANT SPECIES

'Danthonia spicata Schizachyrium scoparium

Poverty grass Little bluestem

OCCASIONAL TO LOCALLY ABUNDANT SPECIES

Agalinis purpurea Andropogon virginicus Asclepias tuberosa fClematis ochroleuca

Purple gerardia Broomsedge, Broomstraw Butterfly weed Curlyheads pp. 436-37

1—

Commelina erecta

Erect dayflower

p. 354

LU

Coreopsis major

Whorled coreopsis

PP- 355-56

O

Eryngium yuccifolium

Rattlesnake master

P. 370

Z

Helianthus strumosus

Roughleaf sunflower

pp. 382-83

< CL

'Liatris pilosa

Shaggy blazing star

O

Oenothera fruticosa

Sundrops

pp. 410-11

<

Polygonatum biflorum

Solomon's seal

pp. 428-29

tj

Rudbeckia hirta

Black-eyed Susan

pp. 432-33

Sorghastrum nutans

Indiangrass

p. 444

X

Tephrosia virginiana

Goat's rue

pp. 449-50

1-

Invasive Exotic Plants

0 s

Elaeagnus umbellata

Autumn olive

pp. 282-83

< £3

'Lespedeza bicolor

Shrubby lespedeza

CL

Lonicera japonica

Japanese honeysuckle

pp. 294-95

LU ^

Microstegium vimineum

Japanese stiltgrass

pp. 403-4

1—

Rare Plants

LU

Echinacea laevigata

Smooth coneflower

p. 367

;z

Helianthus schweinitzii

Schweinitz's sunflower

p. 382

0

'Lotus helleri

Carolina prairie-trefoil

CL

Silphium terebinthinaceum

Prairie dock

1-z

fSymphyotrichum georgianum

Georgia aster

<

t = plant not included in the species profiles (Part IV)

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