Distribution and Composition of Florida Scrub

Florida Scrub Description and Extent

Florida scrub is shrub-dominated vegetation growing on xeric sands in Florida and neighboring states (U.S. Fish and Wildlife Service 1999). Many of these areas are relict beach ridges and bars (Stout and Marion 1993), with scrub vegetation having variable but continuous presence for at least 50,000 years (Watts and Hansen 1994). Major areas of Florida scrub occur in north-central Florida on the Ocala National Forest, in south-central Florida on the Lake Wales Ridge, along the Atlantic Coastal Ridge, on the Brooksville Ridge in west central Florida, and near the Gulf of Mexico in the Florida Panhandle. Florida scrub is known for its high level of endemism, with the major hotspot on the Lake Wales Ridge. The Lake Wales Ridge is one of the longest (160 km) and certainly the oldest (several million years) ridge in Florida (Watts and Hansen 1994). However, some plants and insects are endemic to other ridges. To the north and west, Florida scrub grades into shrub-dominated communities in coastal Georgia and Alabama.

About 80-90% of Florida scrub has been converted to agricultural and urban uses, and this loss has been especially acute near the Orlando and Tampa areas. The main threats to Florida scrub include habitat loss, habitat fragmentation, fire suppression, and trampling of lichens and soil crusts (Menges 1999). Road building and other soil disturbances encourage invasion of exotic species (Greenberg et al. 1997).

Climate and Soils

The climate of Florida is humid and largely subtropical, with many hot summer days and very rare freezing weather. Rainfall exceeds 120 cm, with most falling as convectional or tropical storms in the summer (from June through September). Although precipitation exceeds potential evapotranspiration (Fernald and Purdum 1992), dry periods during winter and spring can stress plants (Menges and Gallo 1991). Winter fogs are important in providing water to plants (Menges 1994). The warm dry month of May is particularly conducive to wildfires.

Florida scrub is found on distinct sandy soils. Most scrub soils are Entisols derived from quartz (Kalisz and Stone 1984, Schmalzer et al. 1999). These soils are very low in nutrients and are excessively well drained. Although the soil surface is not always far from the water table by non-Florida standards, they are elevated relative to soils of neighboring plant communities such as flatwoods and hammocks. The level of the surficial water table drops during the winter but can come within a meter of the surface of some scrubby flatwoods soils during wet summers. The xeric, nutrient-poor nature of the soil is reflected in the xeromorphic adaptations of scrub organisms, despite annual rainfall of ca. 50 inches.

Scrub soils are acid and infertile (Kalisz and Stone 1984), although the younger coastal soils have higher pH and soil nutrient levels (Schmalzer and Hinkle 1996, Schmalzer et al. 1999). Various types of coastal scrub vary in soil characteristics (Schmalzer et al. 2001) but this type of information is lacking for interior scrub sites. Tissue nutrient concentrations vary little with time-since-fire. Scrub soils include those with yellow (reddish) or white layers just beneath the surface. Sandhill vegetation occurs on yellow/red soils as well, and one study found few differences in soil properties between sandhill and scrub on yellow sands (Kalisz and Stone 1984). Florida scrub soils are much poorer than soils supporting California chaparral (Carrington and Keeley 1999). Florida scrub plants can extend roots well beyond their aboveground spread (Hunter and Menges 2002) and are capable of acquiring nutrients at these greater distances (Hawkes and Casper 2002). Cryptobiotic soil crusts fix nitrogen, which can be passed to vascular plants (Hawkes 2003).

Major Species and Compositional Variation

Florida scrub is often a shrubland, although some areas are dominated by sand pine in the tree layer. Oaks are the major species, especially Q. myrtifolia, Q. inopina, Q. chapmanii, and Q. geminata. Florida rosemary (Ceratiola ericoides) dominates some areas and may be mixed with oaks. Dwarf palmettos (Serenoa repens, Sabal etonia) are a consistent element. Other common shrub genera (at least in some areas) include Lyonia, Vaccinium, Befaria, Osmanthus, Palafoxia, Sideroxylon, Ilex, and Persea. These shrubs vary widely in form and life history. The families Fagaceae and Ericaceae are dominant.

Above the shrubs can be a canopy of pines (Pinus clausa, P. elliottii var. densa, P. palustris). However, Florida scrub can have a scant or absent canopy of pines. Because pines are quite characteristic of other ecosystems that abut Florida scrub (notably sandhill and flatwoods) and because pine distribution changes temporally (especially compared to most of the shrub species), characterizations of Florida scrub as a forest or savanna are somewhat misleading. For this reason, I recommend avoiding the term “sand pine scrub” as a general term for Florida scrub as this is only one variant, although it does occupy a considerable area.

In some areas, a subcanopy of hardwoods 3-10 meters tall can form above a shrub layer. This may reflect fire suppression or a history of low intensity or winter fires. The evergreen oaks mentioned earlier can attain the subcanopy (especially Q. myrtifolia and Q. geminata). On yellow sands, the deciduous trees Q. laevis, Q. incana, and Carya floridana can be prominent (the first two species are more characteristic of sandhill vegetation, however).

Herbaceous plants are secondary in most types of scrub, although some occur in gaps of various types of scrub. Gaps are most prevalent in rosemary scrub and gap specialists are prominent in rosemary scrub (e.g., Polygonella basiramia; Hawkes and Menges 1995, Eryngium cuneifolium, Menges and Kimmich 1996). Joining the herbs are some gap-specialist subshrubs, e.g., Dicerandra christmanii and D. frutescens on yellow sands (Menges et al. 1999). In one type of oak-dominated scrub (scrubby flatwoods), gaps are more ephemeral (Young and Menges 1999) but still important to many species. The interaction of fire and gaps explains much variation in Florida scrub vegetation (Menges and Hawkes 1998, Quintana-Ascencio and Menges 2000). Gaps are obvious aboveground, but belowground gaps are smaller from those aboveground. The dominant shrubs that form the matrix in Florida scrub have massive rhizome and root systems that store carbon from many years of photosynthesis (Langley et al. 2002). Belowground gaps among these shrubs are smaller than aboveground gaps, e.g. Ceratiola ericoides (Florida rosemary) (Hunter and Menges 2002). Even herbaceous plants are able to take up nutrients at distances much greater than their aboveground extent (Hawkes and Casper 2002). The extent to which gaps are openings aboveground vs. belowground may also affect species composition (Petru and Menges 2003).

Florida scrub is characterized by vegetation with xeromorphic attributes including deep roots, small evergreen leaves often with thick waxy cuticles, hairs, or inrolled shapes (to conserve water, presumably), and high allocation to belowground structures (Johnson et al. 1986, Langley et al. 2002). Uptake of water and nutrients in scrub oaks occurs from deep soil horizons (Johnson et al. 2003). Many animals avoid desication through behavior, including burrowing in the loose sand.

Florida scrub has moderate variation in species composition depending on soil type and geographic position. It also varies along an elevation gradient from xeric to less xeric based on distance to the water table (Abrahamson et al. 1984, Schmalzer and Hinkle 1992a). Although many classifications of Florida scrub exist (e.g. Abrahamson et al. 1984, Schmalzer et al. 1999), I will describe variants from Menges (1999). These range from relatively open stands dominated by Florida rosemary (rosemary scrub), various sorts of dense oak-dominated scrub (with sand pines: sand pine scrub; with mainly oaks: scrubby flatwoods; with substantial palmetto in coastal areas: oak-palmetto scrub (Schmalzer and Hinkle 1992a,b, Schmalzer 2003)); on yellow sand with scrub hickory (Carya floridana) (oak-hickory scrub or southern ridge sandhill; hickory phase of Abrahamson et al. 1984), and scrub overgrown into a forest (xeric hammock; Myers and White 1987). Also sharing xeric soils is sandhill vegetation, dominated by pines, grasses and herbs but with few shrubs. All these types of vegetation grade into one another, although regions of relatively rapid vegetation change (ecotones) can be delimited (Boughton et al in preparation).

Vegetation composition among different ridges in Florida can be inferred by comparisons of published studies (e.g. Laessle 1958, Myers 1985, Schmalzer and Hinkle 1992a, Menges et al. 1993, Greenberg 2003) but there has been little integration of these datasets. Across Florida scrub sites, some species nearly always occur (e.g., Quercus geminata, Serenoa repens), but some species that are common in some areas are much sparser or absent in others (e.g., Quercus inopina, Osmanthus megacarpa). Scrub-like communities in Alabama and Georgia have some species in common with typical Florida scrub, but often in unusual combinations (Menges 1999).

See: Endemism in Florida Scrub.