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4.4 Rare Plant Species in the Southern Region

Plant communities, whether rare or common, are comprised of of species that share similar ecological needs and tolerances. The diversity of plant species in the South is rivaled in North America only by the California flora. This diversity is due in part to a broad array of species that are either highly localized in their distribution or are very sparsely distributed over large areas.

Two widely accepted determinants of rarity in plant species is protection under the Endangered Species Act of 1973 (ESA) or the determination of imperiled status by TNC (ABI 2001).

Within the assessment area, approximately 115 plant species are listed as either threatened or endangered under the ESA (USDI 2001). Of this number, 52 occur in Florida. Those species are clustered in the Appalachicola and Lake Wales Ridge areas. The Southern Appalachians contain the next greatest concentration of threatened and endangered plant species.

Figure 1 and Figure 2 show the distribution of rare plant taxa in the South by equal-area hexagons and Counties, respectively. These maps were derived from data held by State Heritage Programs and represent the occurrences of vascular plant species with a TNC rarity rank of G1-G2. These are species considered to be critically imperiled or imperiled (Stein and others 2000) based on the number, size, and condition of populations known to exist. The distribution of rare taxa is used here as a proxy for the distribution of plant diversity. Low diversity plant communities such as agricultural lands or beaches rarely contain uncommon taxa, whereas there is a worldwide pattern of uncommon species being associated with highly diverse plant communities. The occurrence data represented in figures 1 and figure 2 should not be interpreted as the distribution of plant species on a trajectory toward extinction. Most of the rare plants in the South (or the world for that matter) are species that are naturally rare (Rabinowitz 1981). These data are, in all likelihood, incomplete in that private lands may be under-surveyed for rare plants and some States have generally better surveys than others. However, Figure 1 and Figure 2 represent the best available data at this time and are more than adequate to elucidate the overall pattern of species diversity and rarity in the South.

These figures display three hotspots of plant diversity in the South; the southern Appalachian Mountains, the Apalachicola lowlands of the Florida panhandle, and the Lake Wales Ridge region of central Florida. The Southern Appalachians are a refuge for a wide range of species in genera with generally more northerly affinities. Many of the rare taxa in the Southern Appalachians are thought to be relicts from periods of glaciation in the distant past. The Lake Wales Ridge hotspot is a portion of Florida that was submerged during times of rising sea-levels, such as during the hypsithermal period from 8,700 to 5,000 BP. Many of the rare plants on Lake Wales ridge are thought to have been more widely distributed in the past. The Appalachicola lowlands plant diversity hotspot is more difficult to explain. Although the area has a striking diversity of habitats such as karst features, a variety of bogs, and wiregrass communities, these factors alone are unlikely to be the cause of the richest endemic flora in the South. Some scientists have suggested that some combination of habitat diversity, generally markedly low levels of soil nutrients, and a long history of frequent fires has made the area a challenge for most plant species and an opportunity for the evolution of specialized taxa.

Other areas with important levels of plant diversity in the South include the Coastal Plain, the Ozark-Ouachita Highlands, and the Cumberland Plateau.

Although most of the rare plant species in the South are species that are naturally rare, forest fragmentation and land conversion has significantly impacted the distribution and abundance of a large number of species. Other factors associated with human density such as over-harvesting and hydrologic alterations have diminished many species that were formerly common.

Many of the plant diversity hotspots represented in Figure 1 and Figure 2 occur primarily or largely on public land. This result highlights the importance of public land for the conservation or rare plants. Although not all public land management practices favor rare plants, in many places public land is the only place in which rare plant conservation is politically or economically possible.