South Florida Fire Environment

• South Florida experiences highly seasonal rainfall with > 75% of annual total of 135 cm falling during May to October rainy season. Progressive drying during the winter leads to driest conditions in April and May. Lightning begins in May with advent of summer thunderstorms, peaks in mid-summer, and ends in October.
• The greatest area burned by lightning-caused fires is early in the rainy season (Figure 1).
• Human-caused wildfires increase in number and size during the dry season and also burn the greatest area in May.
• Humans have inhabited South Florida for at least 5,000 years and probably supplemented lightning-ignited fires the entire time.
• Wildland fires can occur at any time of the year, but the least area burns in September and October when water levels are highest.

Study Objective

• A wide range of potential prescribed burning regimes is feasible in South Florida pinelands. The main objective of this project is to establish the baseline vegetation conditions and begin the experimental treatments for a long-term study of season and frequency of burning.
• The research will provide detailed data on responses such as changes in species composition and dominance relationships to different burning regimes.
• The results will be considered along with wildlife, public safety, and other management concerns in refining the prescribed burning program at Big Cypress National Preserve and other South Florida public lands.
• Here we report some early results on fire-induced mortality of pines.

Study Location

• Big Cypress National Preserve covers 295,000 ha of southwestern peninsular Florida (Figure 2). About 43,500 ha of the preserve are pine forest.
  • Big Cypress conducts the largest management-ignited prescribed fire program in the National Park Service, averaging over 15,000 ha per year.
  • Prescribed fire is used to reduce hazardous fuels which threaten property, human safety, and endangered species or other important natural resources. It is also used to meet other resource management objectives such as exotic species control.

• Raccoon Point is an unlogged area of eastern Big Cypress (Figure 2). The vegetation consists of a mosaic of pinelands and cypress domes.
  • The substrate is a thin layer of sand overlying limestone. The pine forests are often called wet pine flatwoods because there is standing water for significant periods during the rainy season.
  • The sole overstory tree of the pinelands is South Florida slash pine (Pinus elliottii var. densa), a species that shows ecological similarities to longleaf pine (P. palustris).
  • Cabbage palm (Sabal palmetto) and saw palmetto (Serenoa repens) can dominate the pineland shrub layer. Hardwoods are relatively sparse. Cypress trees (Taxodium ascendens) form dome-shaped stands in bowl-shaped depressions in the limestone and are also scattered in pinelands.
  • The herb layer in the pinelands is dominated by grasses and includes several species found only in South Florida.

Methods

• The long-term study involves six experimental treatments, combinations of 3 seasons and 2 frequencies of prescribed burning.
  • The three seasons are winter (January-February, dry season, plants most dormant), spring (May-June, early in wet season, conditions relatively dry), and summer (July-August, middle of wet season, conditions wet).
  • The short frequency is every three years, and the long frequency is every six years.

• The study area is divided into 18 experimental units, each containing at least 50 ha of pineland (Figure 3). There are three replicates of each treatment, one replicate burned each year for three consecutive years.
• Each unit has three 1.0 ha tree plots with all trees > 5 cm diameter and palms with stems > 1.4 m tall mapped and tagged.
• Experimental burns are carried out by the Big Cypress National Preserve fire management staff.
• Fire behavior measurements to characterize treatment burns include pre- and post-burn fuel sampling to calculate fuel consumption and measurement of fire temperatures with temperature-sensitive paints. We also measure height of bark char and percentage of the needles scorched of all the pines.
• Mortality of pine trees is assessed one year after the burns.

Results

• The prescribed burns began in spring 1996 and all 18 of the experimental units have received their initial treatment burns. Two of the spring burns were unavoidably delayed until this year (2000) so we are reporting on the first 16 burns (6 each of winter and summer and 4 spring).
• A total of 16,370 trees have been tagged and mapped.
  • The mean density of pines is 227 ha^-1 (range 72-438). The plots also contain an average of 52 cabbage palms and 24 cypress trees.
  • The largest pine tree in each plot ranged from 32 to 52 cm dbh. The mean pine basal area was 8.4 m²/ha (range 4.2 to 14.0).

• A total of 301 pine trees (2.8%) were found dead one year after burning, excluding those obviously killed by other means such as lightning or windthrow. Due to high variability from plot to plot, seasonal differences were non-significant (Table 1).
• There were significant differences among seasons in several measurements of fire behavior, however. Winter burns had the most severe values in all cases (Table 1). On the other hand, summer burns had the lowest values measuring fire severity, except for crown scorch. One of the problems with summer burning in South Florida is the lack of dependable winds.

• While mortality did not show a significant relationship to season of burning, mortality was significantly related to several measures of fire behavior at the plot level (Table 2). Crown scorch appears to be a better predictor of mortality than height of bark charring.
• At the individual tree level, dead trees averaged 90% scorch while those that survived averaged only 40% scorching. South Florida slash pine has a high tolerance for scorching--of the 2217 trees whose entire complement of needles was killed, 1981, or nearly 90% survived.

• As expected, smaller trees were more likely to die than larger trees (Figure 4). More than 10% of the trees in the smallest size-class died, while less than 1% in the 20-25 cm size-class died. The mean diameter of trees that died was 12.0 cm compared to 20.3 cm for those trees still alive.

• When the data from the final two burns are available, a more thorough analysis accounting for variability in size classes present in different units may show a significant effect of season. In the interim, there is little indication that impacts on mature trees should influence the season in which prescribed burning is conducted in South Florida pinelands.

Related information:

SOFIA Project: Experimental Study of Fire Regimes in South Florida Pinelands and Associated Cypress Wetlands

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U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 22 December, 2004 @ 02:52 PM (KP)