Southern Piedmont Dry Oak-Pine Forest

This system encompasses the prevailing upland forests of the southern Piedmont from Alabama north to central and southern Virginia. High-quality and historic examples are typically dominated by combinations of upland oaks, sometimes with pines as a significant component, especially in the southern portions of the region. These forests occur in a variety of habitats and, under natural conditions, were the matrix vegetation type covering most of the landscape. Much of this system is currently composed of successional forests that have arisen after repeated cutting, clearing, and cultivation of original oak-hickory and oak-hickory-pine forests. Stands of these forests are dominated by combinations of upland oaks, particularly Quercus alba, Quercus rubra, Quercus velutina, Quercus stellata, Quercus coccinea, and Quercus falcata, along with Carya glabra, Carya tomentosa, and other Carya spp. Other common tree species include Pinus taeda, Pinus echinata, Pinus virginiana, Acer rubrum, Liquidambar styraciflua, and Liriodendron tulipifera. There is considerable variation in this widespread matrix system. In particular, there are "dry-mesic" as well as "dry" components, as well as stands with codominance by Pinus echinata, and distinctive stands dominated by Quercus montana with other dry-site species on the summits of hills called monadnocks. There are particular associations that represent this variation.

Vegetation consists of forests dominated by combinations of upland oaks, particularly Quercus alba, Quercus rubra, Quercus velutina, Quercus stellata, Quercus coccinea, and Quercus falcata, along with Carya glabra, Carya tomentosa (= Carya alba), and other Carya spp. Other common tree species include Pinus taeda, Pinus echinata, Pinus virginiana, Acer rubrum, Liquidambar styraciflua, and Liriodendron tulipifera. A well-developed understory and shrub layer is generally present, with species varying with soil chemistry. The herb layer is sparse to at most moderate in density. Before natural fires were suppressed, the forests presumably had less understory and shrub component and probably a grassy herb layer. There is considerable variation in this widespread matrix system. In particular, there are "dry-mesic" as well as "dry" components, as well as stands with codominance by Pinus echinata, and distinctive stands dominated by Quercus montana (= Quercus prinus) with other dry-site species on the summits of hills called monadnocks. There are particular associations that represent this variation. There is some uncertainly about the composition and physiognomy of this system under presettlement conditions, in particular with some debate about the relative importance of Quercus spp., Carya spp., and Liriodendron tulipifera, as well as the role of Pinus echinata and its increasing abundance to the south.

This system occurs on upland ridges and upper to midslopes, occupying most of the uplands. Moisture conditions, determined by topography, are dry to dry-mesic. This system may occur on soils derived from any kind of rock type, with rock chemistry being an important determinant of variation. Soils include almost the full range of upland soils, with only the shallowest rocky soils and those with extreme clay hardpans excluded.

The Piedmont has mostly gently rolling topography ranging from 90 to 365 m (300-1200 feet) elevation. Several erosion-resistant metamorphic and igneous rock types have been left as monadnocks that stand 60 to 305 m (200-1000 feet) above the surrounding landscape. Average annual precipitation is 110-122 cm (44-48 inches). The presettlement vegetation as described by early explorers and the first settlers was a mosaic of forest and open woodland, with interspersed savannas or prairies (Lederer 1672, Logan 1859). The prairie component was located on the flat to convex and gently rolling uplands of the larger fire compartments. The largest of these in the southern part of the range was up to five miles wide without a tree or only a few blackjack oaks (Logan 1859).

This ecological system encompasses the prevailing upland forests of the southern Piedmont. High-quality and historic examples are typically dominated by combinations of upland oaks, sometimes with pines as a significant component, especially in the southern portions of the region. These forests occur in a variety of habitats and, under natural conditions, were the matrix upland vegetation type covering most of the landscape.

The Piedmont Monadnock Forest (Schafale and Weakley 1990) is included within this broad type. Stands are dominated by Quercus montana, and occur mainly on resistant ridges (monadnocks) over felsic rocks of the Piedmont, including quartzite, rhyolite, and pyrophyllite. Soils are well-drained, acidic and nutrient-poor. Lightning strikes and high winds are common in these exposed locations (Schafale and Weakley 1990).

In successional forests recovering from clearcutting or cultivation, Pinus taeda, Pinus echinata, and/or Pinus virginiana typically dominate for a number of decades, with Quercus spp., Carya spp., and other hardwoods gradually invading the understory.

Fire was probably an important natural disturbance in this system, affecting vegetation structure and composition of the lower strata. It may have been important in favoring oaks and pines over other trees. Fires were likely almost always low-intensity surface fires. Native American burning was also important in the Piedmont (Cowell 1998). These forests appear to occur naturally as predominantly old-growth, with canopy dynamics dominated by gap-phase regeneration. Small to medium-sized canopy gaps created by wind are the primary natural disturbance at present, and probably were in the past as well. Fire likely created some small to medium-sized gaps in the past also, and likely caused all canopy gaps to persist longer. The dominant tree species are capable of living for several centuries.

Fire and grazing are possibly the most important natural processes affecting the floristic composition and vegetation structure of this system (Landfire 2007a). The presence of frequent (2-5 years) surface fire is important in order to support the reproduction of Pinus echinata and the development of diverse herbaceous understories. Pinus echinata is a shade-intolerant species and does not compete and regenerate well when fire is absent. Where fire occurs at an appropriate frequency, the stand may develop a relatively pure canopy of Pinus echinata, typified by a very open woodland structure with scattered overstory trees and an herbaceous-dominated understory (Landfire 2007a).

The frequency of fire is variable across the landscape to create a mosaic of vegetation. However, most agree that the fire-return interval was relatively short. Fire may have been as frequent as every two to three years. Brewer (2001) compared the current tree species composition to bearing tree records in the upper coastal plain of northern Mississippi and found that Pinus echinata and more fire-tolerant species such as Quercus velutina and Quercus stellata were prevalent on the landscape, indicating a greater fire frequency. Without a short fire-return interval, community succession tends to favor upland mixed pine-xeric hardwood forests or hardwood-dominated forests. Landers (1989) inferred a fire-return interval of 10 times per century for pure stands of Pinus echinata.

Lightning fires occurred primarily during the spring dry season (April and May) with a second peak of Native American burning during the fall (October and November). Occasionally, during extensive droughts, mixed-severity or stand-replacement fires did occur, especially on drier pine-dominated sites. Local thunderstorms and outbreaks of southern pine beetle (Dendroctonus frontalis) created gaps on a small but continual basis. More extensive regional disturbances included tropical storms during the growing season, ice storms during winter, and tornadoes throughout the year (Landfire 2007a).

Land clearing is a threat. This is mainly for development or conversion to plantation forestry, but in the past was primarily for farming. Successional forests (which include novel ecosystems) occur on formerly farmed sites. Many have lost significant topsoil when farmed, mostly prior to the 1930s. Most of the characteristic dominant hardwoods (primarily Quercus species and Carya species) are only moderately tolerant of shade. In recent years, more shade-tolerant species appear to be increasing in many of these forests, particularly Acer rubrum (McDonald et al. 2002). This may be a result of loss of regular fire in the system. Invasive exotic plant species such Ligustrum sinense, Lonicera japonica, and Microstegium vimineum will also increase under these conditions, and represent a threat. Loss of predators (bobcat, mountain lions, wolves) has led to increases in deer and rabbits, herbivores which have caused overbrowsing of herbaceous flora and decline of many plant species (Taverna et al. 2005). Fragmentation is also a threat, as it leads to an increase in white-tailed deer, an important browser on herbaceous plants.

This system ranges throughout the Piedmont from Alabama to Virginia. In Virginia, it is primarily central and southern, but extends into a narrow portion of northern Virginia in the Piedmont ecoregion.