Ecosystems: Florida Scrub (Part 1 of 2) By Cathy Vogelsong Reprinted from The Understory, Oct.-Nov. 2004 (My article is based on the classic Ecosystems of Florida by Myers and ,Jewel, and a seminar given by Mark Deyrup, Senior Research Biologist at Archbold Biological Station. Mr. Deyrup very generously edited my paper, and many of his comments are included here in italics.) Part 1: The xeric scrub, found in "islands" along Florida's central ridge, dates back 10 million years to the early Pleistocene era. It also occurs in some other areas of the state, and there is a bit in Alabama and Georgia. The largest contiguous scrub areas are now in the Ocala National Forest. It is the most ancient of Florida's ecosystems. In south Florida, these high dunes may have been the only land above sea level, but in some areas of north Florida were not flooded. Flooding, due mostly to sea level changes during the Pleistocene, occurred, not during the glaciations, but during the interglacial intervals. The most extensive flooding may have been about 135,000 years ago. During the glaciations, the amount of water bound up in the ice lowered the sea level, so that the peninsula was actually larger. It appears that scrub habitat expanded and contracted, depending on the climate and the sea level. This is why the distribution of scrub organisms is complicated, because some were better than others at withstanding reduction in habitat area, or at recolonizing dry areas. As a result of both age and isolation, many species found in scrub are Florida endemics. Relative to other habitats in the U.S., especially the East, the percentage and absolute number of species restricted to scrub habitat is remarkably high. Scrub was originally an extensive ecosystem but was already rare before man because of the gradual change to a cooler, moister climate. The remaining patches of ancient scrub are located in Ocala National Forest and the Lake Wales Ridge. Additional scrub habitat is also found in coastal ridges (old dunes) in both the Panhandle and the peninsula. This coastal scrub, while much newer, is classified as scrub because of shared plant species and soil characteristics. Life in a scrub is heavily dependent upon abiotic factors, chiefly fire and soil. Technically, scrub soils are entisols (soils with little or no horizontal development), derived from quartz sand, and originating as water or wind deposited beaches and ridges. Sand is the basis of scrub. This is silica sand, well aerated, with no binding elements. Organic material oxidizes easily, so the scrub sand is unusually sterile. The sands vary from loose white "sugar" sand to more compact yellow sand. At least on the interior ridges, the yellow sands seem older in origin, and the white sands are in areas that were probably reworked by the sea during periods of flooding in the Pleistocene. Even in periods of high rainfall, the water percolates rapidly through silica sand to a depth of fifteen feet, so these are extremely well-drained soils. Drought stress must occur, since many plants, especially small ones, die if the winter dry season is prolonged through the warmer months of April and May. I think that drought stress is seasonal. The dwarfed, gnarled shrubby appearance of the scrub may be due in part to low nutrients, since scrub soil is almost devoid of clay and organic matter. But the most direct factor that keeps the scrub low is fire. When fire is excluded for 40 or 50 years, scrub develops into a xeric forest, as we have seen here on the [Archbold] Station. Many of the open site specialists, including some rare and endangered species, appear to vanish from this forest. These days, such species sometimes hang on along the open edges of sand roads, and may be able to move back into a patch of scrub after a fire. Most plant species in scrub are evergreen. This is probably an adaptation to retain nutrients, since the nutrients in litter are rapidly leached out of the sand, and so are lost to the system. Scrub plants have many adaptations to the desert-like conditions (both nutrient and water) that they live in. These include the stunted habit of the plants, and development of very deep taproots, and/or a disproportionately large system of shallow hair roots. The scrub palmetto has no above-ground trunk. Shrubs develop multiple trunks and are highly tolerant of shifting sands burying their bases. Leaves may be small, tough, and hairy with a thick cuticle and curled edges. Some plants orient their leaves vertically, so that the sun does not strike the leaf surface directly during the hottest part of the day. Plants are often thorny or high in aromatic oils, which may be defenses against some herbivores. Scrub vegetation can very greatly, but keeps a consistent appearance. The only tall canopy tree found in scrub is pine, chiefly the sand pine (Pinus clausa), which may be dense, sparse, or lacking entirely. In some places there may be slash or longleaf pine. This depends largely on fire frequency and the distance to the water table. The dominant shrubby layer (to 30') may also be dense and impenetrable, or open with visible patches of sand. Ninety percent of the shrubs are the same six species--myrtle/scrub oak, sand live oak, Chapman's oak, saw palmetto, rusty lyonia and Florida rosemary--with the latter dominating a subgroup called rosemary scrub. Garberia, silkbay, wild olive, scrub palm and hog plum are also common. The saw palmetto and yaupon holly are characteristic of coastal scrub.