Scrub communities have numerous synonyms, many of which describe the dominant shrub of the assemblage. Oak scrub, rosemary scrub, palmetto scrub, sand pine scrub and, in some cases, coastal scrub and scrubby flatwoods are all used to describe different variations of this habitat. Differences in microclimate, topogra phy, fire regime, and stages of succession are also responsible for the variety of forms documented for this community. For this report, these assemblages will be grouped together under the generic terms "scrub" and "Florida scrub."
Florida scrub is often found within a matrix of pyrogenic plant communities, although often protected by adjacent wetlands. The surrounding landscape is often dominated by longleaf pine woodlands. Historically, abrupt ecotones were reported between the two communities where they occurred adjacent to each other. The two communities are now known to be dissimilar in composition and structure based on differences in soils and disturbance regimes; allelopathy (the ability to produce chemicals that inhibit growth of other plants) may play an important role as well (reviewed in Myers 1990).
Florida scrub is distinguished here from maritime shrub along the Atlantic coast (Gehlhausen, Harper, and Trame 1998). Maritime shrub is considered to be an intermediate stage between shoreline dunes/interdunal swales, and the more protected maritime forests. Along the Atlantic coast, maritime shrub communities are dominated by wax myrtle (Myrica cerifera) and yaupon holly (Ilex vomitoria; reviewed in Gehlhausen, Harper, and Trame 1998). On the Panhandle of Florida, this same community is characterized by Florida rosemary (Ceratiola ericoides), scrub oaks, and sand pine (Myers 1990); this community is included here as one type of Florida scrub, referred to as "coastal scrub." It differs from inland scrub in that it is often younger in origin, being partially or wholly structured by physical coastal disturbances instead of fire disturbance, and supporting fewer endemic species (USFWS 1998).
This characterization of Florida scrub excludes scrubby flatwoods, which are generally thought to be intermediate between scrub and longleaf pine woodlands. Scrubby flatwoods are similar to scrub communities because they can be dominated by evergreen, sclerophyllous shrubs. However, wiregrass and other herbaceousspecies found in pine flatwoods also occur in scrubby flatwoods (Stout and Marion 1993). The Florida Natural Areas Inventory (FNAI) and the Florida Department of Natural Resources (FDNR) (1990) recognize this as a distinct community, as it is often associated closely with mesic flatwoods, and occupies sites with a higher water table than does scrub.
This document covers the ecology of, impacts to, and management for tracts of coastal and inland Florida scrub. However, most research related to rare species conservation and fire management in Florida scrub has been conducted in inland communities, thus, management recommendations will reflect that bias. We advocate additional research into the management needs of coastal Florida scrub (FNAI 1994b).
Range
Origins and Historical Range
Scrub-like vegetation began to appear in the fossil record in the early Tertiary. This once vast ecosystem originated in the southern Rockies and northern Mexico, spreading east along the Gulf Coast to Florida (Mark Deyrup, Entomologist, Archbold Biological Station, FL, professional discussion, 14 January 1998 [hereafter referred to as M. Deyrup, 14 January 1998]; Axelrod 1958). During the late Pleistocene, the Earth's climate was cooler and dryer than it is today and scrub vegetation was probably widespread on the Florida peninsula (Myers 1990). As the Earth's climate began to warm and become more moist, much of the ancient scrub eventually became southern pine forest. Scrub-like flora still existed on the xeric soils and coastal dune ridges where drought stress and nutrient-poor soils prohibited many species from gaining a foothold (Myers 1990). Between 5,000 and 7,000 years ago, the Earth's climate became even more moist as water levels rose (Myers 1990). Electrical storms became common. Wild fires ignited by lightning, and fires intentionally set by early humans, gradually selected for fire-dependent species, which dominated the landscape upon the arrival of European settlers.
Current Distribution
Today, this rare community type is limited almost exclusively to the state of Florida, although examples of similar communities do occur elsewhere. Florida scrub still occupies excessively well-drained soils associated with ancient coastal dune systems. Coastal and inland scrub communities can be found on the peninsula, but stands also occur on the panhandle of Florida, along the Gulf coast (Myers 1990). A fewexamples exist as far west as Mobile Bay in Alabama (Stout and Marion 1993). A single stand of Florida scrub has been discovered in Mississippi, and similar communities occur in parts of southern Georgia (Stout and Marion 1993).
Scrub always occupies dry, sandy, nutrient-poor soils that are found in a number of places in Florida. The range of inland peninsular scrub is generally restricted to a complex of sand ridges and ancient dunes running north and south from Clay and Putnam counties to Highlands county (Myers, 1990), with a few fragments persisting on military installations where residential development and citrus cultivation have been limited. Peninsular coastal scrub is found on both the Atlantic and Gulf coasts. Panhandle scrubs are restricted to a narrow strip along the Gulf coast and on some barrier islands (Figure 1).
Figure 1. General distribution of Florida scrub and military installations on which the community occurs.
Northernmost examples of coastal peninsular scrubs in Florida occur on the east coast in St. John's County near Durbin and on the west coast in Levy County near Cedar Key. Southernmost scrub communities in Florida once occurred on the west coast at Marco Island in Collier County and on the east coast in northern Broward County, but these have been largely lost to development and will soon be extirpated (Myers 1990).
Most remaining scrub communities range in area from 40.4 to 242 ha (Stout and Marion 1993). The largest block of Florida scrub is inland scrub; named the Big Scrub complex, it occurs in and around the Ocala National Forest in north-central Florida. Most of this 84,987-ha region is managed for the production of wood products from sand pine (Stout and Marion 1993, Myers 1990). The southern end of the Lake Wales Ridge, FL, in Polk and Highlands counties, supported another large expanse of Florida scrub that occurred in a mosaic with high pine communi ties, but more than 70 percent of the southern Lake Wales Ridge xeric uplands have been converted to other land uses (Myers 1990). Few remaining examples of scrub on the Lake Wales Ridge are larger than a few hundred acres; but the largest is 2866 acres (USFWS 1995). An estimated 64 percent of Florida scrub on Lake Wales, Lake Henry, and Winter Haven ridges of central peninsular Florida have been lost to development since the time of European settlement (Noss, LaRoe, and Scott 1992).
Occurrence on Military Installations
Florida scrub has been documented on at least eight military installations in Florida (Table 1). The quality of Florida scrub on installation lands varies with land use history.
Avon Park Air Force Range has over 5,500 acres of scrub in three areas: Bombing Range Ridge, the Osceola Plain, and the Kissimmee River Valley escarpment (TNC 1994). These areas of scrub are all intact and relatively undisturbed. A number of TES are found on Avon Park due to the relatively large fragments of scrub community still found there.
Two forms of scrub are found on Camp Blanding Training Site. A patchy canopy of sand pine over dense thickets of oaks and rusty lyonia occurs at Lowry Lake scrub and Kingsley scrub, while Blue Pond Scrub and Gidding scrub lack the sand pine overstory (FNAI and TNC 1995). Although these areas are disjunct and do not form large blocks of scrub habitat, they are still able to sustain populations of TES.
Xeric pine/oak scrub habitat occurs in a small area within the Rodman target range of Naval Air Station (NAS) Cecil Field. Although some species have not been seen in this area in the past 10 years, gopher tortoises (Gopherus polyphemus) and a number of rare scrub herbs can still be found there. Several avian TES also pass through occasionally (Lloyd Cruize, Natural Resource Manager, NAS, Cecil Field, Jacksonville, FL, professional discussion, 29 January 1998 [hereafter referred to as L. Cruize, 29 January 1998]).
Table 1. Occurrence of Florida scrub communities on military installations in the State of Florida. | |||
State |
Branch |
Installation |
Reference |
FL |
Air Force |
Avon Park Air Force Range (AFR) |
Howie (1994) |
Cape Canaveral Air Station |
FNAI (1998) | ||
Eglin Air force Base (AFB) |
FNAI (1994b) | ||
Hurlburt Field |
Labat-Anderson INC. (1994) | ||
Tyndall AFB |
FNAI (1994a) | ||
Army |
Camp Blanding |
FNAI and The Nature Conservancy (TNC; 1995) | |
Navy |
Naval Air Station (NAS) Cecil Field |
Burst (1988) | |
NAS Pensacola and Outlying Field, Bronson |
Burst (1995a) | ||
NAS Whiting Field |
Burst (1995b) | ||
NAS Jacksonville |
Environmental Service and Permitting, Inc. (1990). | ||
Eglin Air Force Base (AFB) supports remnant coastal scrub on barrier islands and on the mainland. Inland scrub occurs on the southeastern portion of the base. These include an area known as "Scrub Hill" which runs along Eglin Range Road 205. A parcel of land northwest of the intersection of Range Road 259 and Florida State Highway 98 and a patch between Eagle Creek and Little Trout Creek also support inland scrub (FNAI 1994b).
Tyndall AFB harbors a number of small patches of high-quality coastal scrub on Crooked Island and on the peninsula comprising the majority of the Air Force Base itself. One mature patch of scrub is located on the southern edge of East Bay, near Strange Bayou. A number of element of occurrence records for scrub habitat on file with the FNAI occur here and on other parts of the base (FNAI 1994a).
Several fragments of coastal scrub are located around the edges of Forrest Sherman Airfield at NAS Pensacola. Another mosaic of tiny patches of scrub and scrubby flatwoods surrounded by a matrix of mesic flatwoods is located just north of Trout Point. These are areas of relatively high quality, supporting a canopy dominated bysand pine and a fair amount of lichens covering the ground. Some hurricane damage can be seen in the older trees and this has resulted in numerous gaps in the canopy (FNAI 1998).
Florida scrub is also found on Cape Canaveral Air Station. This community occupies the Welaka Sand Ridge that lies on the western edge of the Canaveral Peninsula. The canopy here is 4 to 7 m tall with a few large diameter sand pines emerging from the canopy. Another patch with a large compliment of species is located on the isthmus of the Canaveral Peninsula, east of the Banana River and north of Launch Complex 37. Other patches of coastal scrub and Oak scrub occupy relict dune ridges around the installation (FNAI 1998).
Cross Classification
Scrub is defined as a shrub community dominated by a layer of oaks or Florida rosemary, or both, and with or without a pine overstory. Open areas of sand are dispersed between the thickets. Florida scrub is synonymous with the "sand pine scrub" community of Stout and Marion (1993) and Christensen (1988), the "scrub" community described by Myers (1990) and by the Guide to the Natural Communities of Florida (FNAI and FDNR 1990), and is the same as the Florida scrub community described by the U.S. Fish and Wildlife Service (1995). Georgia's "dwarf oak-evergreen shrub forest," "evergreen scrub forest," and "evergreen scrub-lichen forest" communities (Wharton 1978) are similar to Florida scrubs without sand pine as a component of the modern flora (Stout and Marion 1993).
Abrahamson et al. (1984) divided the scrub community into two distinct types. They identified the oak understory type as a three-layered community. In this version, the scrub has a lower layer of saw palmetto (Serenoa repens) and scrub palmetto (Sabal etonia), an upper shrub layer consisting of scrub oaks, rusty lyonia (Lyonia ferruginea) and/or scrub hickory (Carya floridana), and an overstory of sand pine (Pinus clausa). Herbaceous plants tend to be scarce in this type. The rosemary type of this community is characteristic of drier ridges and knolls, with more herbaceous ground cover and open patches of sand. This version of scrub is identified by the predominance of rosemary and a pine overstory that is much more open. Other synonyms for this community include: sand scrub, rosemary scrub, and oak scrub (FNAI and FDNR 1990); in addition, slash pine scrub may be used if the overstory consists of this species of pine (Myers 1990).
Environmental Factors
Topography
Inland peninsular scrub communities occur along a complex of windswept sand ridges and ancient dune fields that once defined the coast of Florida. These make up the majority of remaining scrub habitat. Coastal peninsular scrub communities occur along the Atlantic and Gulf coasts and occupy dune systems associated with recent Pleistocene shorelines. Panhandle scrubs occur as narrow strips along the Gulf coast and its barrier islands (Myers 1990). Scrub generally occurs on areas of excessively drained upland soil that are protected from frequent fires by virtue of their position in a matrix of lakes, swamps, or other lower lying wetlands (USFWS 1995).
Soils / Nutrients
One of the elements that define scrub habitat and its distribution on the landscape is the soil upon which scrub vegetation grows. Most scrub soils supporting this community are entisols, derived from quartz sand. Some scrub communities along the Atlantic Costal Ridge occur on Cocoa sand, a soil series that forms over coquina and is classified as an alfisol (Paul Schmalzer, Senior Field Ecologist, Dynamac Corporation, professional discussion, 12 May 1998 [hereafter referred to as P. Schmalzer, 12 May 1998]). They are almost devoid of silt, clay, and organic matter and are therefore nutrient-poor. No distinguishable soil horizons can be seen in the soil profile. It consists of a single, undifferentiated layer of sand ranging in color from pure white to a yellow or buff tone, depending on the age. As water percolates through the soil, organic acids are produced by the decay of forest litter. These acids chemically bleach iron oxide stains and leach organic matter out of the soil, leaving the siliceous sand its characteristic light color. The longer this bleaching action has gone on, the older the soil. Scrub communities that grow on very light or white soil are therefore older than those that grow on soils that are more yellow in color (Myers 1990).
Regardless of age, scrub soils are excessively well drained. The soil's parent material is composed of loose quartz sand. Therefore, water drains rapidly through the substrate creating very xeric conditions for which plants must be suited (FNAI and FDNR 1990). The root systems of most scrub plants are adapted to survive in the xeric conditions caused by these sandy soils. Many plants have fine, shallow roots that facilitate nutrient capture close to the surface of the ground. But they also have deep tap roots so they can access ground water at considerable depths (Myers 1990). Drought stress does not seem to be a problem for scrub-adaptedvegetation because of these deep root systems. Nutrient deficiencies are more often a problem for scrub vegetation. Because most of the nutrients in scrub ecosystems are tied up in the biomass, very little is left in the soil and available to young plants. Therefore, only plants that are hearty enough to survive under this nutrient stress will grow on scrub soils. Some scrub plants may use allelopathy to avoid competi tion for scarce nutrients (Myers 1990). These factors may explain why few exotic species have been able to establish themselves in areas of undisturbed scrub.
The greatest consequence of such poor soil conditions may be a reduction in propagation by seed in scrub species. Many species spread vegetatively by resprouting from root stocks, rhizomes, and burls protected from fires beneath the soil surface (Christensen 1988). Some species, such as sand pine and rosemary, cannot resprout in this way and must rely on seeds for regeneration. These species have adapted special mechanisms to facilitate germination in such hostile conditions.
Soil crusts may affect nutrient and water regimes in the uppermost region of the soil profile. These soil crusts consist of algae, cyanobacteria, fungi, bacteria, and the early stages of lichens and mosses. Although unseen, these micro-organisms form a fragile aggregate with the soil particles at the ground surface and contribute to the carbon content of the soil (Christine Hawkes, Graduate Student, Department of Biology, University of Pennsylvania, professional discussion, 21 May 1998 [hereafter referred to a C. Hawkes, 21 May 1998]). They may also play a role in determining the distribution and abundance of certain vascular plants on nutrient-poor scrub soils due to their suspected ability to fix nitrogen or hold water. These crusts are extremely susceptible to disturbance. Research is underway to determine exactly what role these structures play in this ecosystem (C. Hawkes, 21 May 1998).
Recently, scientists have been looking at the relationship between plants and soil microbes to see if a symbiotic relationship exists. Relationships of this nature are well studied in other ecosystems, but the micro-organisms living in scrub soils are not adequately understood. Anderson and Menges (1997) suggested that the investment in a mycorrhizal system may be disadvantageous to scrub plants if it will not increase the availability of nutrients that are limiting to the plants' growth. They found a low level of colonization by bacteria in the root systems of four scrub plants and determined that the mycorrhizal relationship between these plants and soil bacteria is not well developed; the scrub plants they studied were not enjoying an increased uptake of any nutrients that resulted from microbial activity in their root systems. These organisms may exist simply as a semi-benign parasite.
Disturbance Regime
Physical Coastal Disturbances
Coastal scrubs appear to be maintained by wind and other physical disturbances to a greater extent than inland scrubs. The scrub community is found on the oldest dune ridges along the Panhandle coast, and thus may persist for a very long time. However, as distance from the harsh coastal environment increases, abundance of sand pine also increases. Additional species begin to appear as well, including large-leaved jointweed (Polygonella macrophylla; Johnson, Muller, and Bettinger 1992), a species of concern (SOC) recognized by the U.S. Fish and Wildlife Service.
Fire Regime
The fire regime is determined by the frequency of fires in an ecosystem (fire return interval), the intensity of the fire, its type and the spacial pattern of the burn area (Christensen 1985; Parker 1989). These factors interact with each other to exert selective pressure on the reproductive strategies in plants, but can also act independently to influence the quality, quantity, and timing of reproduction in plants (Ostertag and Menges 1994).
Most scrub plants fall into one of four guilds, each characterized by a certain reproductive strategy: resprouters (mainly woody shrubs), resprouters and seeders (small statured shrubs, palms, and herbaceous perennials), obligate seeders (mainly herbs), and resprouters and clonal spreaders (Menges and Kohfeldt 1995). Each of these strategies has advantages and disadvantages in reproduction but each attempts to maximize species survivability according to a specific fire regime.
Seeders typically do not survive fire and must rely on either a persistent seed bank or a method for distant dispersal to persist in a patch. Herbaceous plants that rely on seed regeneration may be most susceptible to localized extinction from fire return intervals that are either too short or too long (Quintana-Ascencio and Menges 1996). If fire returns too frequently, individuals may not have time to reach reproductive maturity; if fire return intervals are too long, many herbaceous plants will be outcompeted by dominant shrubs and/ or trees. For these species, successful dispersal between habitat patches may be important for long-term persistence (Quintana-Ascencio and Menges 1996).
Resprouters have an advantage in that they can recover very quickly once a fire has passed, but dispersal to other patches may be limited. Those species that reproduceby both seeding and resprouting practice a bet-hedging strategy. They are ensuring their survival no matter what the fire return interval.
Unlike surrounding longleaf pine woodlands, scrubs are characterized by long periods between fires. The natural fire return interval for inland scrub may be every 30 to 60 years (Christensen 1988). Myers (1990) states that typical natural fire intervals may be every 10 to 60 years. Because scrubs lack much of the dried grasses, fine textured pine needles, and other flashy fuels that ignite and burn easily, they usually have a high heat of ignition and are difficult to burn. This is evident in the long period of time between fires. The poor productivity of scrub soils accounts for the slow accumulation of leaf litter and duff. This fuel builds up at a faster rate in more productive communities (like longleaf pine woodlands) and results in more frequent fires. In general, scrub acts as a barrier to the spread of fire due to its inability to burn. When these areas do burn, the fires are almost always catastrophic because they result in a crown fire that kills the pine overstory outright and burns back the oak shrub layer to ground level (Christensen 1988). Conflagrations result when high fuel levels are combined with severe burning conditions: high temperatures, low humidity, low fuel moisture, and sustained winds. Yet even under these conditions, fires usually start in adjacent plant communities and spread through the scrub, leaving a patchy burn area in which some places are completely charred while others remain untouched by the fire.
Anthropocentric Disturbances
When fires are suppressed, the Florida scrub community may be converted to a pioneer xeric hammock, characterized by tree-sized oaks and a loss of many scrub species. The pioneer hammock community can be restored to a more diverse scrub assemblage through growing season fires to top-kill the oaks (USFWS 1998).
If a scrub community is cleared, as for agriculture, and then abandoned, a "pioneer scrub" may develop. This community is characterized by the most invasive weed and scrub species, and may become dominated by oaks. It is unknown if such a site can be restored to a high-quality Florida scrub community (USFWS 1998).
Physiognomy and Structure
The Florida scrub community is easy to recognize throughout its range because the shrub layer is usually composed of the same few species (see Biological Composition, below). The shrub layer may be dominated by oak species, or may consist of nearly pure stands of rosemary. Sand pine may occur in dense stands, as scatteredindividuals, or may be absent altogether; sometimes it is replaced by slash pine (Pinus elliottii; Myers 1990). Community structure can vary following disturbance by fire or harvesting. On more productive sites that have not burned for several decades, a well-defined layer of litter and duff usually occurs. Bare patches of sand are common and the ground cover is generally sparse. Ground cover density is often inversely proportional to the density of sand pine and shrubs (Myers 1990).
Commonly Associated Plant Communities
Florida scrub occurs in close association with longleaf pine-turkey oak sandhills, scrubby flatwoods, coastal strands, and xeric hardwood forest (FNAI and FDNR 1990). These ecosystems typically have a greater fire frequency than scrub. Once established, the fuel load and environmental conditions within each community tend to perpetuate that community type until some disturbance or alteration in fire frequency causes the system to undergo succession.
Successional Relationships
In the complete absence of fire, scrub communities may succeed to xeric hardwoods (Christensen 1988, Myers 1990, Stout and Marion 1993). Frequent fire may lead to the disappearance of sand pine and the development of an oak-dominated scrub community (Stout and Marion 1993) or a longleaf pine sandhill (Myers 1990). Figure 2 illustrates one model for the influence of fire and soil drainage on vegetation dynamics in xeric Florida uplands (Menges and Hawkes 1998).
Biological Composition
The scrub community is composed of a relatively small number of species. The vegetation is typically dominated by a dense layer of evergreen, or nearly evergreen, shrubs. Over 90 percent of the shrub layer in Florida scrubs consists of the same six species in the same order of abundance: myrtle oak (Quercus myrtifolia), scrub oak (Q. inopina), saw palmetto, sand live oak (Quercus geminata), Chapman's oak (Q. champanii), rusty lyonia, and Florida rosemary.
A closed or open-canopied overstory of sand pine may or may not exist (Stout and Marion 1993). Ocala sand pine (Pinus clausa var. clausa), one of the two genetic races of sand pine, has evolved serotinous cones that regenerate themselves in the harsh conditions of Florida scrub (Christensen 1988). When a fire burns thiscommunity, nutrients (particularly phosphorus) previously tied up in the biomass are released and quickly taken up by new seedlings in what Anderson and Menges (1997) term a "nutrient pulse." The cones remain closed on the tree until the heat of a passing fire causes them to open, thus allowing them to take advantage of the newly fertilized soil. This race of sand pine prevails in areas that are subject to fire and tends to grow in even-aged stands. The other race of sand pine, Choctawatchee sand pine (Pinus clausa var. immuginata), bears open, non-serotinous cones and does not depend on fires to re-seed itself (Christensen 1988). This is an adaptation to locales unlikely to burn within one generation. These trees predominate in panhandle scrubs and in certain coastal scrubs found on peninsular Florida along the Gulf Coast where the natural fire interval was even longer than that for inland peninsular scrub. This variety of sand pine is not found on the Atlantic coast. Coastal scrub tends to be more dependent on mechanical disturbances (windthrow and washout for example) than peninsular scrub. Selection pressures in coastal ecosystems would favor trees that were more adapted to mechanical disturbance than to fire. Choctawatchee sand pines grow in uneven-aged stands as a result of their reseeding strategy. The two varieties may grow together in one stand or may form pure stands of each. When they do grow in pure stands, as many as 20 percent of the closed-cone trees may produce some open cones (Myers 1990). These trees are ensuring they will regenerate whether a fire passes through in their lifetime or not.
Bare patches of sand are common and the ground cover is generally sparse. Ground lichens (Cladonia and Cladina spp.; Myers 1990) typically dominate, but some herbaceous plants do grow on scrub soils. Depending on location, these can include: gopher apple (Licania michauxii), beakrush (Rhynchospora megalocarpa), milk peas (Galactia spp.), beardgrass (Andropogon floridanum), and lichens (Myers 1990). The following species may also occur in the ground layer: Florida alicia (Champannia floridana), hemlock rosette grass (Panicum patentifolium), scrub balm (Dicerandra frutescens), wedge-leaved snakeroot (Eryngium cuneifolium; federally listed as endangered), Ashe's calamint (Calamintha ashei), Florida gayfeather (Liatris ohlingerae), and paper-like nailwort (Paronychia chartacea; federally listed as threatened; Stout and Marion 1993).
Legend: FF = frequent fires (1-10 years) MF = moderately frequent fires (5-20 years)
IF = infrequent fires (15-100 years) NF = no fires within 100 years
WF = winter fires PR = pine removal
F = fire reintroduced regime
Figure 2. A model for the influence of fire and soil drainage on vegetation dynamics in xeric Florida landscapes. Arrows indicate hypothesized transitions between communities under indicated fire. These transitions do not alter the soil drainage characteristics at the site. (Figure courtesy of the Ecological Society of America.)