West Gulf Coast Sandstone Barrens

This system is confined to the Catahoula geologic formation of eastern Texas and western Louisiana. It includes a vegetational mosaic ranging from herbaceous-dominated areas on shallow soil and exposed sandstone to deeper soils with open woodland vegetation. Woodlands include a post oak-dominated overstory grading into longleaf pine-dominated areas. Seasonal droughtiness, shallow soils, aluminum toxicity, and periodic fires are important factors that influence the composition and structure of this system. Vegetation associated with thin soils over the tuffaceous sandstone of the Catahoula Formation is primarily herbaceous. But where the soil is deeper, or fire is excluded for long periods, it can display significant woody cover, with usually stunted representatives of species such as Pinus palustris, Pinus taeda, Pinus echinata, Quercus stellata, Quercus marilandica, and Carya texana dominating the canopy. Shrubs may form a patchy, discontinuous layer. Open sites may have significant herbaceous cover, usually dominated by graminoid species.

Vegetation associated with thin soils over the tuffaceous sandstone of the Catahoula Formation is primarily herbaceous. But where the soil is deeper, or fire is excluded for long periods, it can display significant woody cover, with usually stunted representatives of species such as Pinus palustris, Pinus taeda, Pinus echinata, Quercus stellata, Quercus marilandica, and Carya texana dominating the canopy. Shrubs may form a patchy, discontinuous layer with species such as Ilex vomitoria, Morella cerifera, Vaccinium arboreum, Forestiera ligustrina, Gelsemium sempervirens, and Crataegus spp. commonly encountered. Maintenance of fire in the landscape will reduce woody cover in these sites, with herbaceous-dominated sites displaying increased species richness. On open sites, there may be exposed patches of bedrock or mineral soils, or areas of patchy cover of foliose and/or fruticose lichens. Open sites may have significant herbaceous cover, usually dominated by graminoid species such as Schizachyrium scoparium, Sporobolus clandestinus, Sporobolus silveanus, Schizachyrium tenerum, Tridens strictus, Scleria spp., and/or Aristida spp. Forbs, including Bigelowia nuttallii, Plantago spp., Minuartia drummondii, Chaetopappa asteroides, Lechea san-sabeana, Sabatia campestris, Croton michauxii, Croton monanthogynus, Krameria lanceolata, Selaginella arenicola ssp. riddellii, Phemeranthus parviflorus, and a variety of other herbaceous species, may also be present. Lack of fire tends to lead to closing of the woody canopy and a reduction in diversity in the herbaceous layer. More wooded sites may have an herbaceous cover that contains species such as Chasmanthium sessiliflorum, Ranunculus fascicularis, and Piptochaetium avenaceum (Elliott 2011). Undisturbed examples are dominated by Bigelowia nuttallii, Aristida longespica, Schizachyrium scoparium, Croton michauxii (= Crotonopsis linearis), and Sporobolus silveanus (Marietta and Nixon 1984). Woodlands include a Quercus stellata-dominated overstory grading into Pinus palustris-dominated areas.

The habitat of this system includes shallow soil and exposed sandstone, which tend to an herbaceous-dominated vegetation expression, as well as zones of deeper soils with open woodland vegetation. Examples of this system are restricted to surface outcrops of the Oligocene Catahoula geologic formation, an often tuffaceous sandstone. Sites are generally level to gently undulating (but sometimes steep), with surface or near-surface exposure of the underlying sandstone bedrock. Soils are shallow loams, such as Browndell-Rock outcrop. Soils may contain montmorillonitic clays. These thin soils can be extremely xeric during dry periods, but can also be saturated during wetter months (Elliott 2011).

Seasonal droughtiness, shallow soils, aluminum toxicity, and periodic fires are important factors that influence the maintenance of this system as one with primarily herbaceous composition and structure. This ecological system is maintained by a combination of edaphic factors and natural disturbances including severe drought and fire. 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 primary threats to this system are conversion and degradation of abiotic and biotic components through fire suppression, tree farming, recreational vehicle use, and livestock. These incompatible land uses result in an increase in woody cover, invasive species (e.g., Sorghum halepense), and erosion and loss of soil. 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. 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 virginiana, Pinus taeda, 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). 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 endemic to areas where sandstones of the Catahoula Formation occur near and at the surface in western Louisiana and eastern Texas. Sandstone glades are estimated to have historically covered less than 2000 acres in Louisiana and today 50-75% of that historic distribution is thought to remain (Smith 1993).