Eastern Serpentine Woodland

This system consists of distinct vegetation associated with ultramafic rock substrates in the Piedmont and Blue Ridge of the eastern United States. The bedrock is serpentinite, dunite, or other ultramafic rocks. The soil has unusual and extreme chemical composition that includes strongly skewed calcium-to-magnesium ratios and often high levels of heavy metals such as chromium. Most examples are open woodlands with Pinus rigida, Pinus virginiana, and/or Quercus alba, Quercus marilandica, and Quercus stellata in the often stunted canopy. Extreme edaphic conditions lead to locally xerophytic growing conditions that contribute to relatively open canopies and a ground cover dominated by prairie grasses and a variety of forbs. Disjunct species from drier regions and some endemic plant taxa are often present. The unusual and extreme soil chemistry determines the underlying floristics and distinctive flora of the type, but fire frequency, extent, and severity determine the physiognomy of particular examples over time.

Vegetation is generally an open woodland of pines or xerophytic hardwoods. The dominant vegetation is more xerophytic and more open than the topographic setting, soil moisture, and climate would suggest, and contrasts strongly with adjacent vegetation on other kinds of rock. Pinus rigida and Pinus virginiana are frequent canopy dominants, but Quercus marilandica, Quercus alba, and Quercus stellata dominate some examples. There is generally not a well-developed understory. Shrubs may be sparse to dense. The herb layer is usually dense; grasses, including prairie elements such as Schizachyrium scoparium, Andropogon gerardii, and/or Sorghastrum nutans, usually dominate, but a number of forbs may be present. In the northern portion of this system's range in Pennsylvania and Maryland, Phlox subulata and the endemic Symphyotrichum depauperatum are characteristic; in the southern Appalachian portion of its range, Packera plattensis, Hexastylis arifolia var. ruthii, and Thalictrum macrostylum are characteristic. Often, paradoxical mixtures of xerophytic and mesophytic species are present, though the overall plant composition is characteristic of a drier setting. Disjunct species from drier regions and some endemic plant taxa are often present. There is one site where Pinus palustris occurs over serpentine (Burks Mountain, Columbus County, Georgia), but this is classed as a "Piedmont Longleaf" site.

This system occurs in a variety of topographic settings, perhaps excluding only alluvial sites. The bedrock is serpentinite, dunite, or other ultramafic rocks. The soil has unusual and extreme chemical composition that includes strongly skewed calcium-to-magnesium ratios and often high levels of heavy metals such as chromium. Owing to a high level of toxic metals and a deficiency in nutrients, serpentine outcrops are ecologically unique and provide habitat for many plant species that grow nowhere else. The soil may be shallow and rocky, or deep, and is usually very clayey. Seepage may be present locally.

Although the unique soil chemistry is the crucial determining factor for this system, fire is generally a crucial process influencing species composition and vegetation structure. The unusual and extreme soil chemistry determines the underlying floristics and distinctive flora of the type, but fire frequency, extent, and severity determine the physiognomy of particular examples over time. Without fire, vegetation can sometimes become dense enough to suppress or eliminate the distinctive herbaceous layer, as well as turning a distinctive savanna or woodland structure into dense forest. Southern pine beetle (Dendroctonus frontalis) damage is an important factor in examples dominated by Pinus species.

The most critical anthropogenic threat to native grasslands, savannas and barrens is their conversion to human-created land uses, including residential development, quarries, industrial development, infrastructure development, and others (TNC 1996c). Rocky glade areas, if present, may be the last areas to be converted to development and housing due to the unsuitability of the soil to septic tanks. Other common threats and stressors include both the removal of disturbance (e.g., fire, grazing) and the effects of inappropriate or too intensive or constant disturbance. These areas often attract off-road-vehicle use.

Fire plays a critical role in the maintenance of most native grasslands. Without it, Juniperus species, Quercus species and other hardwoods quickly regenerate, shading out the herbaceous plants, and leading to a shift in species diversity from the ground layer to the upper woody strata, resulting in a net loss of species diversity (Taft et al. 1995). At sites with intermediate levels of woody encroachment, a signal of restoration potential is an inverse relationship between woody stem density and ground layer species richness (Taft 2009). In landscapes where open grassland or savanna vegetation is part of the matrix, and where woody plants have taken over areas once occupied by open grassland and savanna vegetation, the light-dependent species may only persist on the open edges (roadsides, powerlines) of forested patches (Taft 1997). In southeastern grasslands, complete transition to forest dominated vegetation can occur in one or two decades (Wiens and Dyer 1975). More information is needed about the particular appropriate ranges of fire-return times and intensities in the various systems, along with factors other than fire (e.g., soil/substrate, aspect, herbivory, hydroperiod and flooding) that help maintain grasslands and related communities. Occasional surface fire will retard woody plant encroachment and help maintain herbaceous diversity, as will, to an extent, grazing or mowing. Too intensive or frequent application of these disturbances will have deleterious effects on stand structure and species diversity. In general, mosaics of scrub and grassland, produced by light to moderate grazing (or occasional fire) will support the greatest diversity (Duffey et al. 1974). Cutting or mowing is not as favorable to plant diversity as is grazing because it is nonselective and does not result in the same kind of soil disturbance and compaction as do the hooves of grazing animals (DeSelm and Murdock 1993). Fire is a critical disturbance factor for southeastern native grasslands, but the intensity, duration, and timing of the fires are all important in their effect on the vegetation (DeSelm and Murdock 1993). In addition to occasional fire, periodic drought may also be important in regulating woody plant encroachment in native grasslands. It is believed that native grasslands have evolved under a combined system of grazing, drought, and periodic fire (Duffey et al. 1974, Estes et al. 1979, Noss 2013).

Fragmentation of native grasslands, barrens, and savannas occurs with the development of housing and industrial sites, as well as the construction of roads, which not only function as firebreaks, limiting the areas that can be burned with one ignition event, but which make it more difficult to mitigate the effects of smoke on human populations and their activities. A small isolated patch has a low probability of receiving a lightning strike frequently enough to maintain a grassland condition. In many cases, grassland systems were once extensive on the landscape, but have now been reduced to scattered and isolated remnant patches, presenting conservation and management challenges. These disturbances have had damaging effects on fragile soil profiles and plant and animal species. These combined impacts also foster a trend toward biotic homogenization, which results in the gradual replacement of ecologically distinct natural communities by those dominated by weedy generalists (McKinney and Lockwood 1999). In other cases, the grassland or glade system naturally occurs in small isolated patches occurring within an otherwise forested matrix.

Many native grassland sites, particularly the more productive ones, have been converted to plantations of exotic grasses and legumes (DeSelm and Murdock 1993). Even if not completely converted, the extirpation of native species and the concomitant spread of invasive exotic plants (particularly Ligustrum species and Lonicera species shrubs, as well as Lespedeza cuneata, Miscanthus sinensis, Microstegium vimineum, Alliaria petiolata, Ailanthus altissima, and Albizia julibrissin) will fundamentally alter the character of native grasslands, barrens, savannas, and glades. Some of these exotics are allelopathic, thereby presenting a greater threat to native species (N. Murdock pers. comm.). Opportunistic native increaser plant species (e.g., Juniperus virginiana) can also shade out light-requiring herbaceous plants (TNC 1996c).

The most significant potential climate change effects over the next 50 years include shifts to dramatically drier or moister climate regimes. A cooler and wetter regime would most likely accelerate the trend toward woody plant encroachment, removing drought as a factor in its inhibition. A moderately drier regime during the growing season could favor the characteristic native grasses and forbs, which are adapted to these conditions better than the generalists. An extremely drier regime for an extended period of time could ultimately have negative effects.

This system is widely scattered throughout the Southern and Central Appalachians and Piedmont, from Pennsylvania to North Carolina.