Southern Coastal Plain Oak Hammock

This small-patch system occurs in the Southern Coastal Plain (EPA ecoregion 75). Examples are known from some more inland portions of this region as well as the Southeastern Plain (EPA ecoregion 65) in Georgia and Alabama. Relatively dense stands of Quercus virginiana and/or Quercus geminata are diagnostic of this system. Examples often occupy locally distinct microhabitats that differ from the surrounding landscape, such as shallow depressions or slight topographic highs in a predominantly Pinus palustris -dominated landscape. Although embedded in a matrix of vegetation with extremely frequent fire regimes, patches of this system are subject to only infrequent or rare fire events. Under more frequent fire regimes, these sites would likely be occupied by Pinus palustris. It has been postulated that winter burning regimes have allowed this type to expand. A range of soil and moisture conditions may be present. More mesic examples have relatively thin soils (to 50 cm) above clay, while xeric examples occupy deep (>130 cm) well-drained sands. Dominant plants of mesic examples include Quercus virginiana and Quercus hemisphaerica, along with Diospyros virginiana. Vines including Campsis radicans and Smilax spp. dominate the sparse ground cover. In xeric examples, dominants include Quercus geminata, Pinus palustris, Quercus virginiana, Aristida beyrichiana, and Stylisma humistrata. This system is low in plant species diversity compared to most other habitats in the region.

This concept covers both xeric and more mesic types of oak domes and hammocks. In the more xeric examples (Xeric Hammock of FNAI 2010a), the canopy is more-or-less closed and dominated by Quercus geminata, although Quercus chapmanii, Quercus hemisphaerica, Quercus incana, Quercus laevis, and Quercus margarettae may also be common. An emergent canopy of pine, either Pinus clausa, Pinus elliottii, or Pinus palustris may be present (FNAI 2010a). Quercus myrtifolia may form a clonal shrub layer. Aristida beyrichiana and Stylisma humistrata may also be present. Hammocks that are intermediate in moisture status may have some live oak (Quercus virginiana) in the canopy. According to Drew et al. (1998), the dominant taxa of mesic examples are Quercus hemisphaerica, Quercus nigra, and Quercus virginiana, along with Diospyros virginiana. Campsis radicans and Smilax spp. dominate the sparse ground cover. Examples of this system are low in plant species diversity compared to other habitats in the region. Cabbage palms are a diagnostic component of examples of this system in central Florida (A. Johnson pers. comm.).

Examples are thickets or groves of Quercus species in a Pinus spp.-dominated landscape (Myers 1990). These typically occupy locally distinct microhabitats that differ from the surrounding landscape, such as shallow depressions or slight topographic highs in a predominantly Pinus palustris-dominated landscape. A range of soil and moisture conditions may be present. As currently defined, this system includes examples across a moisture gradient from mesic to xeric, ranging across parts of the southeastern coastal plains from Georgia to Mississippi. In Georgia, more mesic examples of this system have relatively thin soils (to 50 cm) above clay, while xeric examples occupy deep (>130 cm) well-drained sands (Drew et al. 1998). In Florida, the xeric hammock typically develops on excessively drained sands where fire exclusion has allowed for the establishment of an oak canopy (FNAI 2010a). This may occur naturally, when the area has isolation from, or significant barriers to, fire. This can also occur as the result of human intervention, as at old homesites where fire was excluded for many years. In these areas, xeric hammock is found as small patches within or near sandhill or scrub. Xeric hammock can also occur on high islands within flatwoods or even on a high, well-drained ridge within a floodplain. Xeric hammock can occur on barrier islands and in other coastal situations, as an advanced successional stage of coastal scrub.

Along and near the east coast of Florida, from Cape Canaveral and northward, there is more shell or humus in the sand, and a tendency to have hammocks containing Quercus virginiana with coastal strand rather than scrub; on the other hand, where there is more dry acidic sand, scrub occurs nearer the coast and Quercus geminata hammocks are found further back from the coast (A. Johnson pers. comm.).

Although embedded in a matrix of vegetation with extremely frequent fire regimes, patches of this system are subject to only infrequent or rare fire events. Under more frequent fire regimes, these sites would likely be occupied by Pinus palustris. Myers (1990) postulated that winter -burning regimes have allowed for the expansion of this type. Quercus geminata and Quercus myrtifolia are both clonal species which establish large rhizome systems capable of quickly resprouting following injury. Xeric hammocks, whether natural or anthropogenic, result from years of fire exclusion, maintained and further enhanced by incombustible oak litter and a sparsity of herbs. The thick bark of Quercus geminata makes these trees somewhat resistant to fire. Once they form a canopy that shades the understory, the trees generate a layer of leaf litter that covers open patches of sand and leads to more shaded, mesic ground conditions. The resulting shaded habitat can allow more fire-intolerant species such as Magnolia grandiflora to establish (Daubenmire 1990). Once the canopy is greater than 2 m high, even hot summer burns may not be sufficient to kill the dome, which can become established after only 7 to 16 years of fire exclusion (Guerin 1993). At that stage, oaks would only be killed through a catastrophic burn during dry conditions. Otherwise, the spread of oaks could be halted through mechanical removal or the use of herbicides if the management intent is the re-establishment of the fire-maintained community that was replaced by the xeric hammock. Xeric hammocks also form from long unburned oak scrub (Laessle 1958). There is a dynamic tension between the Quercus-dominated patches and the Pinus-dominated matrix. Oak domes are a natural part of the landscape, but can also result from human-caused fire exclusion. Near the coast, these communities are affected by salt spray (sea salt aerosol). At and near the coast, salt spray maintains the Quercus geminata at shrub height as much as does fire; one may observe a gradient of increasingly taller Quercus geminata as you move inland and the effect of salt spray becomes diminished (A. Johnson pers. comm.).

Conversion of this type has commonly come from removal of the hardwood vegetation and its replacement by residential or commercial development, or by plantations composed of Pinus species. In areas of increasing suburbanization, these plantations may subsequently be replaced by residential or commercial developments. Common stressors and threats include feral hog (Sus scrofa) rooting and livestock grazing, which are sources of soil disturbance. The spreading Quercus canopy of the xeric hammocks provides a shady refuge in otherwise open, sunny areas. As a result, hammocks have long been utilized (and disturbed) by humans seeking comfortable homesites or camping and recreation areas (FNAI 2010a). Invasive exotic plants are also a threat. Some problematic plants for this system include Dioscorea bulbifera, Eremochloa ophiuroides, Imperata cylindrica, Lespedeza bicolor, Lespedeza cuneata, Lonicera japonica, and Melinis repens (= Rhynchelytrum repens) (Brewer 2008).

This system occurs in Florida, adjacent Georgia and in very limited areas of Alabama (A. Schotz pers. comm.).