Physiognomy and Structure: Stands of this vegetation are naturally open in their degree of canopy closure (woodlands to savannas), dominated by tall evergreen needle-leaved trees that are straight, well-formed, and moderately tall. The original longleaf pine ecosystems of the southeastern United States were generally bi-layered communities with the physiognomy maintained by frequent, low-intensity surface fires that removed most small woody plants and thereby kept the canopy open (Peet 2006). These more open canopies with grass-dominated understories were more prevalent prior to the twentieth century. Without frequent fire, broad-leaved woody plants may occupy more of the cover of the stand, leading to loss of most of the biodiversity (Frost 2000, Peet 2006). Stands of sand pine scrub (G008) have an emergent overstory of Pinus clausa over xeromorphic shrub vegetation (mostly Quercus species). According to Harper (1927), "the vegetation is mostly dwarfed, gnarled and crooked, and presents a tangled, scraggly aspect."

Floristics: This woodland vegetation is dominated primarily by Pinus palustris, but stands of Pinus clausa are also included here. This vegetation covers both a large geographic range and a wide latitude of hydrological variation, from very dry to very wet. The components of the understory and ground layer will vary greatly across this range of biogeography and hydrology. The driest examples include both sand barrens and scrub dominated by Pinus palustris or Pinus clausa. The sand pine scrub (G008) consists of Pinus clausa over xeromorphic Quercus-dominated shrub vegetation, including Quercus chapmanii, Quercus geminata, Quercus inopina, and Quercus myrtifolia. The most important shrub species include Ceratiola ericoides, Lyonia ferruginea, Quercus chapmanii, Quercus geminata, Quercus inopina, Quercus myrtifolia, and Serenoa repens. The ground cover is always sparse but typically includes Andropogon floridanus, Licania michauxii, Rhynchospora megalocarpa, and a variety of lichens (Cladonia species). Xeric longleaf pine-dominated vegetation (G154) consists of open woodlands of Pinus palustris over understories of Quercus incana, Quercus laevis, and/or Quercus margarettae. Schizachyrium scoparium is an important grass throughout. Sites in North Carolina and northern South Carolina often have Aristida stricta as a dominant, and sites from southern South Carolina through much of Georgia and Florida often have Aristida beyrichiana as a dominant. West of the Apalachicola River, Aristida beyrichiana is constrained in distribution to the lowest coastal plain, dropping out in eastern-most Mississippi, though other Aristida species are important, such as Aristida purpurascens and Aristida condensata. Other frequent herbaceous plants include Chrysopsis gossypina, Pityopsis spp., Pteridium aquilinum, Rhynchosia cytisoides, Rhynchospora grayi, Sorghastrum secundum, Stipulicida setacea, and Stylisma pickeringii. Important shrubs include Gaylussacia dumosa, Licania michauxii, and Rhus copallinum. In addition, Serenoa repens may form shrubby thickets from southern Georgia across Florida and southern Alabama.

The dry-mesic loamy longleaf (G009) has irregularly scattered trees of Pinus palustris, and usually clumps of midstory Quercus spp., such as Quercus falcata, Quercus incana, Quercus margarettae, Quercus marilandica, and sometimes Quercus laevis, and a grassy understory. Low shrubs, mostly ericaceous, may be abundant. East of the Mississippi River, Aristida stricta (in North and South Carolina) or Aristida beyrichiana (from South Carolina to Mississippi) are usually the dominant or at least a characteristic herb. In central South Carolina and west of the Mississippi River, both of the Aristida species are absent and various other grass species dominate. Some typical mesic to dry-mesic herbaceous species include Andropogon ternarius, Andropogon gyrans var. gyrans, Andropogon virginicus, Panicum virgatum, Schizachyrium scoparium, Schizachyrium tenerum, Sorghastrum nutans, Sorghastrum elliottii, Sorghastrum secundum, Sporobolus clandestinus, and Sporobolus junceus.

Mesic longleaf pine flatwoods (G596) exhibit an open canopy of Pinus palustris with a grass-dominated ground layer, and a high diversity of forbs. Low shrubs, mostly ericaceous, may be abundant, such as Vaccinium spp. and Ilex spp. In addition, Serenoa repens is a characteristic species, particularly in South Carolina, Georgia, and Florida. The ground layer flora is similar to that of dry-mesic loamy longleaf (G009), but perhaps more diverse. Stands in south-central Florida contain Panicum abscissum.

Wet and mesic longleaf pine savannas and flatwoods (G190) are characterized by poorly drained, somewhat poorly drained, and seasonally saturated mineral soils, over a wide range of textures, with at least seasonally high water tables. In natural condition, canopies are open and are commonly monospecific stands of Pinus palustris or may contain other pines such as Pinus elliottii var. elliottii, Pinus serotina, or Pinus taeda. In south Florida, stands are dominated by Pinus elliottii var. densa. In high-quality stands, the ground layer contains a diverse mix of grasses, herbs, and low shrubs. Grasses naturally dominate the ground cover (Streng et al. 1993). Aristida beyrichiana or Aristida stricta often dominate within their respective ranges, but Andropogon capillipes, other Andropogon spp., Ctenium aromaticum, Muhlenbergia expansa, Schizachyrium scoparium, Sporobolus floridanus, Sporobolus pinetorum, Sporobolus teretifolius, and/or other grasses and sedges, including Rhynchospora spp. may also dominate. A great diversity of other herbs is often present, including composites, legumes, insectivorous plants, and variety of showy forbs. Some forbs may include species of Agalinis, Baptisia, Carphephorus, Crotalaria, Helianthus, Liatris, Polygala, Rhynchosia, Sarracenia, Solidago, Symphyotrichum, Tephrosia, and many others. Shrubs may include Cyrilla racemiflora, Ilex coriacea, Ilex glabra, Ilex vomitoria, Lyonia lucida, Quercus geminata, Quercus minima, Quercus pumila, Serenoa repens, Vaccinium darrowii, Vaccinium myrsinites, as well as Morella cerifera and Baccharis halimifolia in near-coastal habitats.

Dynamics:  In all of these various kinds of longleaf pine woodlands, the absence of fire for only a few years may dramatically alter the physiognomy and composition of the lower strata, with understory hardwoods and shrubs crowding out the grasses and forbs. Exposure to frequent, low-intensity surface fires is the dominant natural ecological process structuring the physiognomy of all of these savannas and woodlands, influencing the local biodiversity. In some parts of the coastal plain, this vegetation historically constituted one of the most extensive types in the region. Pinus palustris is shade-intolerant and slow to reach reproductive age but is very long-lived, with some trees in old-growth stands 200-500 years of age (West et al. 1993). Widespread alterations followed European settlement, including conversion to agriculture and reduction in fire frequency, and have produced drastic changes to this vegetation. Large remaining examples on conservation lands are managed using prescribed fire. At present, many areas have undergone long periods of time without fire, and this has resulted in greater dominance by shrubs, including Ilex glabra, Serenoa repens, and Vaccinium spp., as well as denser canopies of Pinus elliottii or Pinus taeda rather than Pinus palustris (Huffman and Judd 1998, Noel et al. 1998). Hurricanes can also play a role in the dynamics of Pinus palustris vegetation, creating openings where tree regeneration can take place. Sand pine scrub vegetation (G008) is prone to infrequent, intense fire which can crown and consume Pinus clausa trees; its serotinous cones open following fire. The common shrubs resprout after fire, and there are many herbaceous plants that benefit from fire as well. Myers (1990) indicates that much of the variability in Florida scrub is due to variation in fire-return interval which ranges from 10 to 100 years.

Frequent fire is the predominant natural disturbance of ~Dry-Mesic Loamy Longleaf Pine Woodland Group (G009)$$ and ~Mesic Longleaf Pine Flatwoods - Spodosol Woodland Group (G596)$$, and this vegetation has a natural fire-return interval of from two to three years. These fires are low to moderate in intensity, removing above-ground parts of herbs and shrubs, but having little effect on the fire-tolerant Pinus palustris trees. The vegetation recovers very quickly from fire, with live herbaceous biomass often restored in just a few weeks; many ground layer plants have their flowering triggered by burning (Platt et al. 1988a). In the absence of fire, species which are less able to withstand fire increase, including Quercus spp. and other shrubs. These are kept to low density and mostly reduced to shrub size with fire, but become tall and dense in the absence of fire. This can reduce Pinus palustris tree regeneration, and lead to a decline in ground layer density and diversity. Frequent, low-intensity fire was the dominant natural ecological force. Natural stochastic variation in fire, as well as the relative flammability of the vegetation, was presumably important in determining the relative intensity of fire and its effects on vegetation at a local scale (Beckage et al. 2009, Gagnon et al. 2010). Under natural conditions, the overstories of wet and mesic pine savannas (G190) are multi-aged (Platt et al. 1988), consisting of a fine mosaic of small even-aged groves driven by gap-phase regeneration and hurricanes that opened stands periodically (Platt and Rathbun 1993, Noel et al. 1998, Gilliam et al. 2006).

The ground cover of some wet and mesic pine savannas (G190) is being invaded by non-native plant species that include grasses (e.g., Imperata cylindrica), shrubs (e.g., Ligustrum sinense, Triadica sebifera), as well as ferns (e.g., Lygodium japonicum, Lygodium microphyllum) (Platt and Gottschalk 1991, Platt et al. 2006b, Leichty et al. 2011).