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 (about 135 cm) 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 20 km2 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", "coastal scrub", and "scrubby flatwoods". Some sub-types of Florida scrub are found outside of Florida. Myers notes that the "evergreen scrub forest" and "dune oak scrub" reported in southestern 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 (consisting almost completely of silica sand), they comprise completely different species, and the transition from one community to the other historically (prior to human intervention to suppress wildfire) 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 (high pine) 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 (typically one to ten year intevals), 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 (oaks, rosemary, etc.), 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 ot 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 (sometimes called "rosemary balds"), 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 (every 15 to 100 years). 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 developing 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 2 to 3 ft high growth of scrub oaks. Many seedlings of sand pines (6 to 8 in high), 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 (thinning out) 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.



Vertebrates
Notable vertebrates of the Florida scrub include the Florida scrub jay, the endemic Florida mouse, sand skink (Neoseps reynoldsi), bluetail mole skink (Plestiodon egregius lividus), Florida scrub lizard, Florida worm lizard (Rhineura floridana), and the gopher tortoise which is an important keystone species.

Arthropods
Arthropods known to be endemic to Florida scrub include:

Millipedes and spiders
The Florida scrub millipedes are three species endemic to Florida, Floridobolus floydi, F. orini, and F. penneri. F. penneri, the oldest described species, has been found only in scrubs on the Lake Wales Ridge in Polk and Highlands counties. F. orini has been found only in the Big Scrub in Ocala National Forest in Marion County, and F. floydi only in scrubs on the southern part of the Brooksville Ridge in Citrus, Hernando, and Pasco counties.

The red widow spider has been found in scrubs on the southern Lake Worth Ridge and in scrubs on the Atlantic Coastal Ridge, but may have a wider distribution. Its webs are commonly found on the leafs of scrub palmettos. Zelotes ocala has been found only in scrubs on the Marion Uplands in Alachua, Marion and Putnam counties, and on the southern Lake Wales Ridge in Highlands County. Zelotes florodes has been found only on the Lake Wales Ridge in Polk and Highlands counties. The Mccrone's burrowing wolf spider has two subspecies: Geolycosa xera xera has been found on the Marion Uplands in Lake, Orange, Seminole, and Volusia counties and on the Lake Wales Ridge, while Geolycosa xera archboldi has been found only on the southern part of the Lake Wales Ridge.

The wolf spider Schizocosa ceratiola (commonly called Lycosa ceratiola in scientific literature) has been found on the Mount Dora Ridge in Lake County, on Atlantic Coastal Ridge in Martin and Palm Beach counties, and on the Lake Wales Ridge in Highlands County. Another wolf spider, Hogna pseudoceratiola (often called Lycosa pseudoceratiola in scientific literature) has been found on the Atlantic Coastal Ridge in Miami-Dade, Palm Beach, Martin, and Indian River counties. The rosemary wolf spider has been found in rosemary scrubs on the Marion Uplands in Putnam County. The Lake Placid funnel wolf spider has been found on the southern Lake Wales Ridge in Highlands County.

Cockroachs, grasshoppers, and lice
The Florida sand cockroach has been found in coastal scrub in Pinellas County, on the Lakeland Ridge in Polk County, and on the Lake Wales Ridge in Polk and Highlands County. The rosemary bird grasshopper has been found in Florida scrubs on the Orlando Ridge in Orange County, the Mount Dora Ridge in Lake and Orange counties, on the Atlantic Coastal Ridge in Martin county, and on the southern Lake Wales Ridge in Highlands County. The east coast scrub grasshopper has been found only on the Atlantic Coastal Ridge in Palm Beach County. The broad cercus scrub grasshopper is primarily found in Florida scrub, but also occurs in pine sandhill communities. It has been found on the Mount Dora and Orlando ridges in Orange County and the southern Lake Wales Ridge in Highlands County. The tequesta scrub grasshopper has been found on the Orlando Ridge in Orange and Seminole counties, on the Mount Dora Ridge in Orange County, and on the southern Lake Wales Ridge in Highlands County. The bird louse Brueelia deficiens is found in Florida only on the Florida scrub jay, and so is endemic to the same locations as the scrub jay. B. deficiens has also been reported as occurring on other corvids in the western United States and Canada.

Beetles
The Highlands tiger beetle Cicindela highlandensis has been found only on the southern Lake Wales Ridge. Another tiger beetle, C. scabrosa, has been found on the Crescent City Ridge in Putnam County, the Atlantic Coastal Ridge in Broward County, and the Lake Wales Ridge in Highlands County. The scarab beetles Serica frosti and Anomala eximia have been found only at the Archbold Biological Station in Highlands County. The June beetle Phyllophaga elizoria has been found on the Atlantic Coastal Ridge in Indian River County and the Lake Wales Ridge in Highlands County. Other scarab beetles of the Phyllophaga genus include P. elongata, found on the Lake Wales Ridge in Polk and Highlands counties, and in Hillsborough, Lake, Levy, and Marion counties; and P. panorpa and P. okeechobeea, found on the Lake Wales Ridge. The flower chafer Trigonopeltastes floridanus has been found on the Lake Wales Ridge in Highlands County, the Orlando Ridge in Orange County, the Atlantic Coastal Ridge in Indian River County, the Mount Dora Ridge in Marion County, and the Northern Highlands in Alachua County. The earth-boring scarab beetle Mycotrupes pedester has been found in Charlotte, DeSoto, and Lee counties. Young's deep digger scarab has been found on the Mount Dora Ridge in Marion and Putnum counties.

At least two dung beetles are endemic to the Florida scrub: Onthophagus aciculatulus has been found only at the Archbold Biological Station, and Ataenius saramari has been found on the Mount Dora Ridge and Sumter Uplands in Marion County, on the Atlantic Coastal Ridge in Martin and St. Lucie counties, near St. Cloud, and on the Lake Wales Ridge in Highlands County. An undescribed dung beetle of the Psammodius genus has been found at the Archbold Biological Station. Aethecerinus hornii has been found in Lee County and on the Lake Wales Ridge in Highlands County. The firefly Pleotomodes needhami has been found only at the Archbold Biological Station. The firefly Lucidota luteicollis has been found on the Lake Wales Ridge in Highlands and Polk counties, the Mount Dora Ridge in Marion County, and the Brooksville Ridge in Levy County.

Others
Nemomydas melanopogon, a Mydas fly, has been found on the Mount Dora Ridge in Lake County, the Crescent City Ridge in Putnam County, and the Lake Wales Ridge in Highlands and Pol counties. Another Mydas fly, Nemomydas lara, has been found on the Mount Dora Ridge in Lake and Marion counties and the Orlando Ridge in Orange County. The flea Polygenis floridanus is found only on the Florida mouse, which is endemic to Florida scrub. The geometer moth Nemouria outina has been found on the Lake Wales Ridge in Highlands County, the Brooksville Ridge in Hernando County, and the Orlando Ridge in Orange County.

The wasp Dasymutilla archboldi has been found on the Lake Wales Ridge in Hignlands and Polk counties. The wasp Photomorphus archboldi has been found on the Lake Wales Ridge in Highlands County, and the Sumter Upland and the Mount Dora Ridge in Marion County. The ant Dorymyrmex elegans has been found on the Lake Wales Ridge in Highlands County. The ant Dorymyrmex flavopectus has been found on the Lake Wales Ridge in Highlands County and the Mount Dora Ridge in Lake and Marion counties. The ant Odontomachus clarus has been found on the Lake Wales Ridge in Highlands and Polk counties. O. clarus has also been reported from Mexico and the southwestern U.S. The bee Dialictus placidensis has been found near Lake Placid and near Oneco.

Distribution
Florida scrubs are found in three distinct areas in Florida: on a series of ridges running along the center of the peninsula (inland peninsula), along both coasts of the peninsula, and the panhandle coast. Historically, the largest areas of scrub in Florida were in the Big Scrub (also known as the Etonia scrub), which is about 40 mi long and 15 to 20 mi wide, in and about the Ocala National Forest, and a mosaic of patches of scrub along the Lake Wales Ridge. High pine habitat occurs as islands in a sea of scrub in the Big Scrub, but in the rest of Florida scrub habitat occurs as islands as small as 50 ft in diameter in an expanse of high pinelands.

Inland peninsula
Florida peninsula inland scrub is the plant community for which this ecoregion is named. Clumps of sand pines (Pinus clausa) constitute the canopy. Common plant species include sand live oak (Quercus geminata), myrtle oak (Q. myrtifolia), sandhill oak (Q. inopina), Chapman oak (Q. chapmanii), Florida rosemary (Ceratiola ericoides), scrub palmetto (Sabal etonia), saw palmetto (Serenoa repens), garberia (Garberia heterophylla), fetterbush lyonia (Lyonia lucida), rusty staggerbush (L. ferruginea), cup lichens (Cladonia spp.), scrub holly (Ilex opaca var. arenicola), American olive (Osmanthus americanus var. megacarpus), flag-pawpaw (Asimina obovata), silk bay (Persea humilis), Adam's needle (Yucca filamentosa), and eastern prickly pear (Opuntia humifusa). As of 1990, about 70% of the scrub on the Lake Wales Ridge had been lost to citrus groves and housing developments.

Peninsula coasts
Scrub communities on the peninsula coasts are found on dune systems associated with the Pamlico and Silver Bluff terraces of the Pleistocene period. Peninsular coastal scrubs are found on the east (Atlantic) coast of Florida as far north as St. Johns County, and were formerly found as far south as northern Broward County, although the southernmost scrubs on the east coast have been lost to urban development. On the west (Gulf) coast, scrubs are found as far north as near the Cedar Keys, and were formerly found as far south as Marco Island, but the scrubs on that island have also been lost to urban development. Extensive loss of scrub habitat has occurred throughout Florida. By one estimate, almost 70% of the scrub habitat in northern Brevard County was lost between 1943 and 1991.

Mulvania observes that the scrubs on the coasts, and particularly those on the east coast, appear to be more recent in origin than scrubs of the interior. The soils on which coastal scrubs are found appear to be recently formed, containing a large proportion of shells, while shells have disappeared from the older soils on which interior scrubs grow. Mulvania also notes that the vegetation of coastal scrubs is less uniform than that of interior scrubs.

Panhandle coast
Scrub communities on the panhandle coast are also found on Pamlico and Silver Bluff dune systems, as well as on recent dune systems along the barrier islands of the panhandle from the Ochlockonee River to Gulf State Park in southern Alabama. The easternmost scrubs of the panhandle coast, just west of the Ochlockonee River, are 200 km from the westernmost scrubs of the peninsula, near Cedar Key. The sand pines of the Florida peninsula and of the panhandle coast differ enough to be classified into two varieties or races, Pinus clausa var. clausa (Ocala), on the peninsula, and Pinus clausa var. immuginata (Choctawhatchee), along the panhandle coast. While most panhandle sand pines (var. immuginata) have non-serotinious, or open, cones, most of the peninsular sand pines (var. clausa) have serotinous, or closed, cones. The scrub palmetto, which may be present in scrubs on the Florida peninsula, does not occur in panhandle coastal scrubs.

Type by dominant flora
Various authors classify Florida scrub into several phases or types, including coastal scrub, oak scrub, oak-hickory scrub, oak-palmetto scrub, rosemary scrub or bald, sand pine scrub, and scrubby flatwoods.

Coastal scrub
Coastal scrub is a habitat on barrier islands where oaks, in particular, a coastal form of southern live oak, are dominant, and including saw palmetto, wax myrtle, tough buckthorn, rapanea, and Florida privet.

Oak scrub
Oak scrub is a habitat dominated by a shrub layer of evergreen scrub oaks, but without a tree canopy. It may also include saw palmetto, rosemary, shrub hickory, and rusty lyonia shrubs.

Oak-hickory scrub
Oak-hickory scrubs are dominated by oaks and scrub hickorys, with a scattering of sand pines and slash pines.

Oak-saw palmetto scrub
Scrubs without a pine tree canopy may fall on a gradient, with such scrubs on drier soil dominated by oaks, and on wetter soils dominated by saw palmetto. Oak-saw palmetto scrubs are intermediate on that gradient. They are dominated by scrub oaks, saw palmetto, and several plants in the Ericaceae family, including rusty staggerbush, poor grub, feterbuch lyonia, and tarflower.

Rosemary scrub or bald
Rosemary scrubs or balds occur in the driest, and least often burned, portions of scrubs, and are often found on the crest of a hill surrounded by other phases of Florida scrub or other xeric upland habitats. They usually have fewer species present than do scrubs dominated by sand pines or oaks. The exclusion of fire from scrub areas has allowed other species to encroach on rosemary balds so that they have become oak scrubs and, eventually, xeric oak hammocks. The dominant species in rosemary scrubs are Florida rosemary and sand pine.

Sand pine scrub
Scrubs with a scattered to dense canopy of sand pines are known as "sand pine scrubs". Sand pine scrubs have a shrub layer consisting either of evergreen oaks, saw palmetto, rusty lyonia, and other species, often including rosemary, or one dominated by rosemary. The World Wildlife Federation formerly classified "sand pine scrub" as a temperate coniferous forest ecoregion.

Scrubby flatwoods
Scrubby flatwoods has been defined as scrubs without a sand pine canopy that are dominated by saw palmettos, evergreen oaks, rusty lyonia, or Florida rosemary. Scrubby flatwoods are an ecotone, intermediate between sand pine scrubs and flatwoods, sitting higher and with better drainage than flatwoods, but lower and with poorer drainage than sand pine scrubs. The Scrubby flatwoods have more scrub oaks in the shrub layer, and less coverage in the herb layer than flatwoods have. Scrubby flatwoods occur widely in Florida, but are more common in the southern part of the peninsula, where they are subject to a subtropical climate. Associations resembling scrubby flatwoods occur along the Gulf coast into Alabama and Mississippi, and into Georgia and South Carolina on the Atlantic coast.

Trees in scrubby flatwoods may include South Florida slash pine, sand pine, and longleaf pine. The shrub layer includes scrub oaks, and saw palmetto. The scrub palmetto is also a common component of scrubby flatwoods in central Florida. Ground cover includes wiregrass and forbs, lichens, and spike moss. Scrubby flatwoods from which fire is excluded are eventually invaded by sand pine and associated scrub shrubs.