West Gulf Coast Igneous Rock Glade

This glade system is present only in Saline and Pulaski counties, Arkansas, on distinctive, massive outcrops of igneous substrate ("nepheline syenite"). Some typical dominant grasses include Aristida purpurascens, Piptochaetium avenaceum, Schizachyrium scoparium, and Sporobolus clandestinus. Other herbs may include Camassia scilloides, Clinopodium arkansanum, Delphinium carolinianum, Sabatia campestris, and Phemeranthus calycinus. Lichens are common on the rocky substrate of some examples. Some examples will have open stands of Quercus stellata, but trees may be absent. Zonal vegetation communities are present around the outcrops. Interior herbaceous-dominated zones can be mesic to wet as springs and small ephemeral streams flow across the rock outcrops and water pools in flat areas. Deeper, more heavily wooded vegetation develops along the flat or slightly sloping outcrop edges.

Some examples will have open stands of Quercus stellata, but trees may be absent. Some typical dominant grasses include Aristida purpurascens, Piptochaetium avenaceum, Schizachyrium scoparium, and Sporobolus clandestinus. Other herbs may include Camassia scilloides, Clinopodium arkansanum, Delphinium carolinianum, Sabatia campestris, and Phemeranthus calycinus (= Talinum calycinum). Lichens are common on the rocky substrate of some examples.

This ecological system is found where the igneous rock nepheline syenite occurs at or near the surface in the Upper West Gulf Coastal Plain of Arkansas. This glade system is characterized by patches of bare rock interspersed with areas of shallow soil imbedded within a matrix of deeper soil supporting forested ecosystems. Slope varies from gentle to flat. Gently sloping areas are often extremely xeric whereas flatter areas can accumulate moisture, creating seasonally wet microhabitats. Exposed bedrock may have abundant lichen and moss cover and limited vascular plants. At the edges of the rock outcrops, areas with shallow soils support grasslands with scattered stunted trees. As soils become deeper, grasslands grade into open woodlands (Witsell 2007).

This ecological system is maintained by a combination of edaphic factors and natural disturbances, including severe drought and fire (Witsell 2007). The outcrops themselves are relatively extreme environments for plant growth due to mild alkalinity, exfoliation of rock surfaces, and surface moisture and temperature fluctuations. Severe droughts kill tree saplings growing in cracks and potholes, helping to retain the open character of the glades (Quarterman et al. 1993). There is an apparent zonation or patchiness to glade/barren vegetation, with different zones that may be identified by their characteristic plant species (Quarterman et al. 1993). These zones are apparently relatively stable, with woody plant encroachment evident only in relation to the invasion of shrubs and trees into potholes or crevices where soil accumulates more rapidly.

The greatest threat to this ecological system is past and ongoing mining of the underlying bedrock. More than 85% of the system has been destroyed and only 20% of extant occurrences are under conservation ownership. Other threats include fragmentation and disruption of ecological processes, and the resulting alteration of species composition and structure. The intensity of human activity in the landscape has a proportionate impact on the ecological processes of natural ecosystems. Invasive species such as Juniperus virginiana, Ligustrum sinense, Sorghum halepense, Lespedeza cuneata, Albizia julibrissin, and Cynodon dactylon dominate some areas. Fire plays a critical role in the maintenance of this woodland-glade system, which may surround or interfinger with rocky glades. In the absence of fire and appropriate disturbance in the landscape matrix, the areas with the shallowest soils (e.g., the glades) may be the only open areas persisting in a series of woody shrub thickets. Without fire or other disturbance, Juniperus spp., Quercus spp., 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). The actual rocky or gravelly glades may not support sufficient fuel to consistently carry fire, but in the adjacent or interpenetrating perennial grasslands, occasional surface fire will retard woody plant encroachment and help maintain herbaceous diversity, as will, to an extent, grazing or mowing (Duffey et al. 1974). In addition to occasional fire, periodic drought is important in regulating woody plant encroachment. Native glade-grassland systems have evolved under a combined system of grazing, drought, and periodic fire (Duffey et al. 1974, Estes et al. 1979, Noss 2013).

Fragmentation of glades and their accompanying 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. This woodland-glade system was once more extensive on the landscape, but has 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). 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 present only in the Upper West Gulf Coastal Plain of Saline and Pulaski counties, Arkansas. It may have existed historically in Garland and Hot Spring counties (and thereby at least partly in the Ouachita region). Less than 10 occurrences of this ecological system are known to persist.