Ouachita Mountain Flint Rock Glade

This system represents a mosaic of glades and woodlands found on novaculite geology in the central Ouachita Mountains of western Arkansas and adjacent Oklahoma. Novaculite is a weakly metamorphosed rock of sedimentary origin that is primarily composed of microcrystalline quartz and chalcedony. Examples of this system generally occupy ridgetops at 450-640 m (1476-2100 feet) elevation. They are a mosaic of small woodlands scattered on ridges and upper slopes with outcrops and patches of talus scattered throughout. Some woodland or forest patches may appear as almost linear strips interspersed with grassy openings. Wooded patches have a variable, often patchy, structure with some areas of dense canopy interspersed with more open canopies and open grassy patches. In general, the grassy openings occur on shallow soils with exposed bedrock, while the woodlands occur on somewhat deeper soils. In all cases, these are fairly extreme growing conditions due to droughty, rocky soils.

Several distinct communities may be recognized at a local scale within this system. Open habitats may be characterized by sparse tree cover of dwarfed (1-3 m) Quercus marilandica var. ashei, which can sometimes occur in clumps. Herbaceous cover is 100%, except where bare rock is exposed or on talus. Lichens cover 40-70% of the exposed rock surface. Open community components of this system grade into more densely wooded types, with a variable structure, dominated by Quercus stellata, Ulmus alata, Quercus marilandica, Juniperus virginiana var. virginiana, Pinus echinata, and Carya texana. More submesic areas have Quercus rubra-dominated woodlands with Carya texana that may approach a forest physiognomy.

The novaculite formation is of Devonian and Mississippian age and consists of novaculite interbedded with some shale, ranging in thickness from about 76 to 275 m (250-900 feet) (Arkansas Geological Commission 2001, Babcock et al. 2001). Examples of this system are found on ridgetops and south-facing sideslopes over fractured outcrops of novaculite, a hard, siliceous, weakly metamorphosed rock of sedimentary origin as a bedded, virtually pure silica chert deposited under geosynclinal conditions. It is primarily composed of microcrystalline quartz and chalcedony. The Arkansas Novaculite formation is of Devonian and Mississippian age and consists of novaculite interbedded with some shale. These glade openings can range in size from small (less than one hectare) to larger, often linear formations covering as much as 40 hectares.

The structure of this system is thought to be controlled by edaphic factors, along with a combination of periodic fire and severe drought. Many existing overstory trees have multiple stems indicating past die-back due to severe drought of decades-long intervals. Summer leaf loss is common and snags extant. Minor droughts cause extensive die-backs in smaller stems and appear to maintain shrubby conditions in places and limit the abundance and distribution of shortleaf pine. Historically, fire is thought to have played a more important role than today in maintaining the open canopy. A lack of fire presumably decreases the extent of the glade openings and allows a change in structure through increased coverage by shrubs and trees. The trees, when present, are dwarfed and often multi-stemmed from drought die-back. Summer leaf loss is common.

Large areas of this system are in public ownership in steep, inaccessible locations. The most critical threat is continued lack of fire, followed by conversion to human-created land uses, including ridgetop roads, communication towers, residential development, quarries, and others (TNC 1996c). Rocky glades and outcrops may be the last areas to be converted to development and housing due to the unsuitability of the soil to septic tanks.

Fire plays a critical role in the maintenance of most native grasslands, which may surround or interfinger with rocky glades. In the absence of fire and appropriate disturbance in the landscape matrix, the areas with the most shallow 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 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). More information is needed about the particular appropriate ranges of fire-return times and intensities in the various systems. 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 may also be important in regulating woody plant encroachment into native grasslands. It is believed that these 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. In many cases, these glade-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 and/or glade system naturally occurs in small isolated patches occurring within an otherwise forested matrix.

The spread of invasive exotic plants (particularly Ligustrum species and Lonicera species shrubs, as well as Lespedeza cuneata and Albizia julibrissin) will fundamentally alter the character of glades and their accompanying native grasslands. Some of these exotics are allelopathic, thereby presenting a greater threat to native species (N. Murdock pers. comm.).

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.

This system is endemic to the central Ouachita Mountains in Arkansas and adjacent Oklahoma.