Fire Regime of Florida Scrub

Historic Fire Regime

Florida scrub has occupied xeric ridges for at least 50,000 years, during which time the extent of xeric elements has fluctuated considerably (Watts and Hansen 1994). Presumably, the frequency of fire has varied as well, especially in response to species that might increase the amount of fine fuels such as pines and grasses. Without a studied direct record of past fires in fire scars, peat, or pollen, our understanding of the historic fire regime is based on current fire behavior, recent fire history, and inferences from the life history of Florida scrub organisms.

Most pre-European-settlement fires were likely caused by lightning. Lightning is very frequent in central Florida (Fernald and Purdum 1992) and even today, initiates many fires in Florida. Lightning occurs primarily in summer, and the intersection of the end of the dry season with the beginning of the lightning season creates the peak fire season in May.

Native Americans likely also played a role in recent centuries, particularly in initiating fires outside the primary lightning season. Robbins and Myers (1992) compiled literature from various places in North America and inferred that the original Native Americans deliberately used fire for a number of purposes, and that their use could have increased fire frequencies outside the lightning season. By the 1700s, Seminole Indians had moved into Florida and did use fire extensively. However, Native American populations have never been great in interior Florida and their impact was probably never primary. Early European settlers burned primarily to improve forage for cattle and did so during the winter dry season (Robbins and Myers 1992). Currently, accidental, prescribed, and lightning fires are all important in Florida and in Florida scrub (but see: Human Alterations of Fire Regimes in Florida Scrub).

Fire season reflects ignition sources, with a presettlement regime dominated by lightning fires having a peak in May and a trough in mid-winter, and regimes featuring Native Americans or European settlers including a wide variety of ignition times (Robbins and Myers 1992). Summer (June-September) lightning ignitions are common, but these storms occur during the rainy season and less area is burned than in fires ignited during drier times.

Fire Return Intervals

No direct fire history studies can pinpoint fire return intervals for Florida scrub. Fire frequencies are inferred by reference to the life history of component species, fire movement patterns through areas with different vegetation structure, and vegetation regrowth between fires. Fire return intervals listed in Menges (1999) range from 5-20 years for oak dominated scrub types to 15-100 years for rosemary scrub. Similar intervals are listed by other authors (e.g., Myers 1985). Fires in Florida scrub tend to be less frequent than in adjoining vegetation (e.g. pine flatwoods) that have more fine fuels (Breininger et al. 2002).

Explicit population viability analyses of population growth and extinction of scrub endemics in relation to fire return intervals can be used as a guideline. Optimal fire return intervals range from < 5 to 20 years for Eriogonum longifolium var. gnaphalifolium (Satterthwaite et al. 2002), < 50 for Hypericum cumulicola (Quintana-Ascencio et al. 2003); < 15 years for Eryngium cuneifolium (Menges and Quintana-Ascencio 2004), and 6-10 years for Dicerandra frutescens (Menges et al. in preparation). Florida scrub-jays are thought to require fire return intervals of about 8-15 years on the Lake Wales Ridge (Woolfenden and Fitzpatrick 1996). Breininger and Carter (2002) suggest that Florida scrub-jays could survive well in a landscape burned every 20 years, if it was burned by extensive fires that would affect fire-resistant vegetation patches.

Other disturbances besides fire may be important in coastal scrubs on or near barrier islands. Wind and storm disturbances may affect vegetation dynamics (Johnson 1997) and the demography of individual species (Gibson and Menges 1994). Nevertheless, even barrier islands may experience frequent fires (Huffman et al. 2004).

Fires more frequent than every 10 years have the potential to divert scrub toward sandhill communities on yellow sands (Myers 1985) and can eliminate Florida rosemary from xeric scrub on white sand (Menges, personal observation). Very infrequent fire can potentially convert scrub to hammocks (forests), especially in northern Florida or where seed sources of mesic oaks are nearby (Myers and White 1987). In south-central Florida, fire suppression in scrub leads to slow structural development toward xeric hammock on yellow sands but with little species compositional turnover (Givens et al. 1984, Menges et al. 1993).

Fire Intensity and Consumption

Florida scrub can be classified as a high intensity fire system. Like other well-studied shrublands (e.g., California chaparral, bushlands in Australia), fires move through the shrub canopy. Green leaves on palmettos, oaks, and other shrubs are usually consumed or at least scorched. Ground vegetation and litter (and sometimes duff) tend to be consumed as well. Pine trees often torch but crown density is too low to support active crown fires, although passive crown fires can occur during spring droughts on hot days with high winds and low humidity (Hough 1973).

Nonetheless, there is great variation in fire intensity in Florida scrub at both large and small scales (Outcalt and Greenberg 1998, Wally et al. submitted). Unburned patches of varying size are typical of Florida scrub burns. Lightly burned patches with only the litter and lower branches burned are also common. In areas where scrub oaks and other shrubs have grown tall, or where there is a subcanopy of turkey oak or scrub hickory, those taller trees may be consumed incompletely. Moreover, fire intensity is usually greater in the interior than on the edge of burn units (Outcalt and Greenberg 1998).

Effects of varying fire intensities on vegetation and wildlife responses are not well studied, but can be important in predicting fire effects (see Fire Effects in Florida Scrub). For example, higher char height is associated with bark beetle damage and subsequent mortality of south Florida slash pine (Menges and Deyrup 2001). Patterns of consumption can also affect life history responses to fire.

There have been some attempts to measure and manipulate fire intensity during prescribed burns. Measurements of fire intensity have been made using thermocouples, pyrometers, and calorimeters (Wally et al. submitted). Pyrometers may be the most practical means of estimating relative fire intensity in planned fires and are correlated with temperatures obtained by thermocouples. Post-hoc, fire intensities can be gauged by char height, residual twig diameter, and other means. Mechanical pretreatments have treatment-specific effects, with fire intensity depending in part on the lag between the pretreatment and fire (Wally et al. submitted).

Organic soil in many Florida ecosystems can burn, especially during droughts and with artificial drainage. This can create horrendous smoke management problems. Since Florida scrub does not usually have organic soils, this is generally not an issue. However, duff buildup in long unburned areas can create flammable top layers to soils. This duff layer can be exposed by initial restoration fires and is thereafter very vulnerable to ignition when dried and exposed to sparks from nearby fires (Menges, personal observation).