Florida scrub
Florida scrub is a forest ecoregion found throughout Florida in the United States. It is found on coastal and inland sand ridges and is characterized by an evergreen xeromorphic plant community dominated by shrubs and dwarf oaks. Because the low-nutrient sandy soils do not retain moisture, the ecosystem is effectively an arid one. Wildfires infrequently occur in the Florida scrub. Most of the annual rainfall falls in summer.
The ecosystem is endangered by residential, commercial and agricultural development. The largest remaining block lies in and around the Ocala National Forest. Lake Wales Ridge National Wildlife Refuge also holds a high proportion of remaining scrub habitat, while the Archbold Biological Station near Lake Placid contains about of scrub habitat and sponsors biological research on it.
Definition
Florida scrub is a community of xeromorphic species living on poor, well-drained soil, including some combination of sand pines, evergreen scrub oaks, Florida rosemary, rusty lyonia, and gallberry. Depending on conditions, scrubs may be dominated by saw palmetto, rusty lyonia, and gallberry; by Florida rosemary; by sand pines; or by some combination of evergreen oaks, including sandhill oak, sand live oak, Chapman oak, and myrtle oak. Independently of the number of sand pines present in a scrub, the shrub layer is consistently dominated by myrtle oak, sandhill oak, saw palmetto, sand live oak, Chapman oak, rusty lyonia and Florida rosemary. The scrub palmetto may also be present in scrubs on the Florida peninsula. While the shrub layer is usually dominated by oaks of varying density, some scrubs have a shrub layer consisting almost entirely of rosemary. Herbert John Webber noted that the sand pine is the most noticeable plant in large and moderate sized scrubs, but stated that the scrub oaks are probably more important in maintaining the scrub community.Florida scrub is usually intimately associated with the longleaf pine sandhill or high pine ecoregion. High pinelands typically consist of longleaf pines, wiregrass, and other grasses, often with clumps or individual trees of deciduous oaks, such as turkey oak, present.
The term "scrub" may also cover plant communities in Florida called "sand pine scrub", "oak scrub", "rosemary scrub", "slash pine scrub", and "coastal scrub". Intermediate between scrub and associated habitats are "scrubby flatwoods" and "scrubby high pine". Some sub-types of Florida scrub are found outside of Florida. Myers notes that the "evergreen scrub forest" and "dune oak scrub" reported in southeastern Georgia cannot be distinguished from the oak and rosemary scrubs of Florida.
History
The Florida scrub is probably descended from the sclerophyllous Madro-Tertiary Geoflora. The geoflora spread along the Gulf of Mexico coast, and was widespread in Florida during the late Pleistocene. Near the end of the Pleistocene widespread rosemary scrubs in Florida were replaced by oak savannas, and then by sand pine scrubs. The range of scrubs in Florida contracted over the last 7,000 years as Florida's climate became more moist.Florida scrub was probably first recognized as a distinct community by Charles Vignoles in 1823, although several naturalists working in the middle of the 19th century still did not do so. Scrub is usually intimately associated with the high pine ecoregion. While the two communities occur on the same poor soil, they comprise completely different species, and the transition from one community to the other historically was abrupt. Marjorie Kinnan Rawlings described the edge of the Big Scrub as "a vast wall, keeping out the timid and the alien."
Webber states that at the end of the 19th century a natural firebreak separated scrubs from high pine land. The grasses and other herbaceous plants of high pine land diminished near the boundary with scrub, and did not quite extend to the brush layer of a scrub. The near absence of plants in the transition zone meant that any grass fire that did reach the zone did not have enough intensity to ignite the evergreen plants of the shrub layer of a scrub. Webber observed that in 1935 the natural firebreaks seemed to be weakening, which he attributed to the encroachment of human civilization on the areas where scrubs are found.
Fire
Fire in Florida scrub is infrequent but intense, characterized as "catastrophic" or "stand-replacing". Any sand pines in a scrub are killed by such fires, while shrubs burn down to the ground. Fire causes sand pine cones to open and release their seeds to replace the stand. Most shrubs regrow from their roots, while rosemary regrows from seed. As previously noted, the Florida scrub and longleaf pine sandhill communities are closely associated, growing on the same types of soil and under very similar conditions. Both communities are shaped by periodic fires, and the frequency and intensity of fires may prevent one community from replacing the other. Longleaf pine sandhill communities experience frequent, low-intensity fires that primarily burn grass and other understory plants. Those low-intensity fires do not usually invade neighboring scrub communities. When fires have been suppressed in sandhill communities, sand pines and oaks that are typical of scrub begin invading sandhill. Scrub communities typically experience fires at 15 to 100 year intervals. Fires that occur more frequently than 15 years in scrub can prevent regrowth of sand pines and shrubs, opening the area to invasion by plants of the sandhill community. Scrubby flatwoods usually burn at 5 to 20 year intervals. Long-term suppression of fire in scrubby flatwoods allows them to develop into xeric hammocks.Unlike in longleaf pine sandhills, the ground litter in scrub has a high heat of combustion, and it is not easy for fire to start or move into scrub. Fires may spread into scrub under extreme conditions, such as high wind, low humidity, and low fuel moisture. As a result of the resistance of scrub to fire, scrubs often serve as barriers to the spread of fire.
Most fires in scrub result from the spread of a fire from an adjacent plant community. Scrubs that border less flammable ecosystems are somewhat protected from the spread of fire. On the Lake Wales Ridge, scrubs often border or are surrounded by swamps, lakes, streams or bayheads. Along the coasts, scrubs are bordered by the Atlantic Ocean or Gulf of Mexico, and often by inlets, rivers, and swamps. The Big Scrub of the Ocala National Forest lies between the St. Johns and Oklawaha rivers.
The frequency of fire in scrub depends, in part, on the productivity of the soil. Sites with very poor soil tend to support rosemary scrubs, which rarely burn. The death of older rosemary bushes leaves open spots where new rosemary seedlings or sand pine seedlings may start growing. On sites with more productive soil, scrubs persist only if fires occur frequently enough. If a scrub burns more often than every 15 years, sand pines cannot reseed and the scrub becomes oak-dominated, or converts to high pine. If a scrub goes much longer than 100 years without burning, it begins to develop into a xeric woodland.
Damage from tropical cyclones may be more important than fire in maintaining the scrubs on the panhandle coast. As most of the sand pines of the panhandle produce open cones, seeds are released every year and not just after a fire, and the pines in a scrub vary in age, unlike the uniform age of the pines in a given peninsular scrub.
Recovery
Webber noted that fire in scrubs burns scrub oaks to the ground and kills other plants, including pines. Recovery of a scrub occurs because the scrub oaks have extensive root systems which survive a fire, and which quickly sprout new growth. He observed that in an area of scrub that had burned two years earlier, almost the entire area was covered by a thick high growth of scrub oaks. Many seedlings of sand pines, and of other plants typical of the scrub community, were growing under the oak canopy. The crowded conditions of the early stage of recovery leads to the death of many individual plants, and after a few years the typical scrub community is established.Rosemary scrubs regenerate after a fire from seeds that have accumulated in the soil. If rosemary scrubs burn less than ten years after a previous fire, it is unlikely that there will be sufficient rosemary seeds available to repopulate the community, and the scrub may be invaded by oaks.
Soils
The soils in Florida scrub are entisols, recently developed soils without horizons, classified as quartzipsamments. They consist almost entirely of sand, with little to no silt, clay, or organic matter. They are very well-drained, and among the least fertile soils in Florida. The soils range in color from pure white to brown, grey or yellow. The litter on the ground in scrubs produces organic acids which bleach color from sand grains, so that the darkness of soil color to a large degree correlates inversely with the length of time scrub vegetation has been growing on the soil.Endemic species
About 40 plant species, at least 40 arthropod species, and several vertebrate species are endemic to Florida scrub. As of 1990, 13 endemic plant species were listed by the U.S. as endangered or threatened, and 22 were so listed by the state of Florida.Plants
Plants that are endemic to scrub and "scrubby" communities in Florida and that are listed as endangered by the United States Fish and Wildlife Service include: Asimina tetramera, Chionanthus pygmaeus, Cladonia perforata, Conradina brevifolia, Conradina etonia, Crotalaria avonensis, Dicerandra christmanii, Dicerandra cornutissima ''Dicerandra frutescens, Dicerandra immaculata, Eryngium cuneifolium, Hypericum cumulicola, Liatris ohlingerae, Lupinus aridorum, Nolina brittoniana, Senega lewtonii , Polygonum basiramia , Polygonum dentoceras , Prunus geniculata, Warea carteri, and Pseudoziziphus celata .Plants that are endemic to scrub and "scrubby" communities in Florida and that are listed as threatened by the USFWC include: Bonamia grandiflora, Clitoria fragrans, Eriogonum longifolium var. gnaphalifolium , and Paronychia chartacea.
Plants that are endemic to scrub and "scrubby" communities in Florida and that are listed as endangered by the State of Florida include: Bonamia grandiflora, Asclepias curtissii, Chamaesyce cumulicola, Clinopodium ashei , Chrysopsis gossypina cruiseana, Chrysopsis floridana, Chrysopsis godfreyi, Chrysopsis highlandsensis, Clitoria fragrans, Dicerandra thinicola, Eriogonum longifolium var. gnaphalifolium , Euphorbia rosescens, Lechea divaricata, Lechea lakelae, Paronychia chartacea, Rhynchospora megaplumosa, and Schizachyrium niveum.
Plants that are endemic to scrub and "scrubby" communities in Florida and that are listed as threatened by the State of Florida include: Conradina grandiflora, Garberia heterophylla, Lechea cernua, and Polygonum smallianum .
Other plants that are endemic to scrub and "scrubby" communities in Florida include: Carya floridana Euphorbia c.f. floridana, Garberia heterophylla, Crocanthemum nashii , Persea borbonia humilis , Pinus clausa, Quercus inopina, Cartrema floridanum , Sabal etonia, and Sisyrinchium xerophyllum''.(