Fire Effects in the Everglades

Fire Effects on Soils

Fires have variable effects on soil properties in the Everglades depending on the severity of fire. Surface fires have little effect on soil properties including physical and chemical make-up in comparison to peat or muck fires. Both carbon and nitrogen are lost in peat fires due to volatilization. Peat fires increase phosphorus concentrations in the upper soil layers and convert the chemical form of phosphorus from the organic to the inorganic form, making it more available for plant uptake. Phosphorus in the historic Everglades was mostly organic phosphorus. Calcium is also concentrated in the upper soil layers following severe fires indicating that soil physical properties change (bulk density increased in upper soil layers) (Smith et al. 2001).

Oxidation of soil in the Everglades including severe burns after severe drying has been shown to release large amounts of methyl mercury into the environment (Fink 2003). Methyl mercury bioaccumulates and is considered an environmental toxin. The frequency of soil dry down has been increased by human modification of hydrology. It is not clear if burning in addition to drying causes more methyl mercury release than severe drying alone.

Fire Effects on Vegetation

Depending on the severity of the fire, vegetation of the Everglades is often able to regrow following fires. Sawgrass, the dominant plant of the Everglades, is well adapted to fires. Its leaves are extremely flammable and will carry fire even over standing water. Its meristem is usually well protected from the heat of fires either by its spongy leaf bases that can absorb water or by standing water (Wade et al. 1980). Regrowth of sawgrass after fire is rapid, often as much as .75 to 2 cm leaf growth per day (Herndon et al. 1991), but regrowth rate varies with the severity and intensity of burn (Wade et al. 1980). Sawgrass stands often recover pre-burn biomass within 2 years of a fire (Wade et al. 1980) and sawgrass will withstand repeated burning (2 times in 2 years) without loss of vigor (Forthman 1973 cited in Wade et al. 1980). Recovery rates have also been correlated with rate of fire spread with slower regrowth after slow moving fires (Wade et al 1980). Longer residence times of slow moving fires have been postulated to cause more damage to meristems thus causing plants recover more slowly.

Sawgrass can be killed by fire in several ways:

Herndon et al. 1991). Wade et al. 1980). Burning stands with tussocks under low water levels can cause extensive mortality due to vulnerability of elevated meristems.

Sawgrass is often seen flowering 2 years following fire (Hofstetter 1974). It is thought that fires usually kill buds associated with flower stalks. If water is high at the time of burning flower buds may be protected and sawgrass has been observed to bloom the season following fire (Klukas 1973).

Severe ground fire may be an important stimulus for recent observed invasion of sawgrass marsh by cattail (Urban et al.1993, Smith and Newman 2001). Cattail has become more and more dominant over larger areas in the northern Everglades, especially in some of the water conservation areas (Davis 1994). Changes in hydrology and nutrient availability have been cited as reasons for increases in cattail (references cited in Smith and Newman 2001). Both nutrient enrichment and changes in hydroperiod have been direct results of human modification of the Everglades environment through water management and through nutrient inputs in runoff from agriculture sources (Davis 1994). However, these factors have not always explained increases in cattail abundance in the absence of human perturbations. Severe ground fire not only reduces soil elevations and thus increases water depth, but severe peat fires create pools of inorganic phosphorus (Smith et al. 2001) and thus may explain increases in cattail. Thus changes in the frequency and extent of severe ground fires may have affected increases of sawgrass in some areas of the Everglades

A recent study conducted in sawgrass marshes north of the Everglades showed that in marshes with a somewhat enriched nutrient status, surface fire might play an important role in stimulating cattail (Typha domingensis) expansion. Temporary, but dramatic increases in cattail within sawgrass marsh were observed after dormant and growing season surface fire (Ponzio et al. In press). Increases in cattail densities lasted two years returning to pre-burn levels by the third year or sooner. Most other studies have observed either no change in cattail (Typha spp.) following fires or cattail death following fire and subsequent inundation (reviewed in Ponzio et al. In press). Many questions still remain regarding the interaction of hydrology, nutrient status and disturbance such as fire in the population dynamics of cattail.

In other freshwater marsh systems of the Everglades vegetation responds much like it does in other southeastern systems (See Freshwater Marshes). Woody vegetation is pruned back by fires and kept out of marl prairies or from the edges of tree islands.

Many tree islands that burn can recover from fire if the fire is not severe. Even low intensity fires with flames only 20cm long can topkill trees by girdling stems and roots (Robertson and Platt 2001). Many of the hardwoods resprout giving recently burned hammocks and bayheads a scrubby appearance (Gunderson 1994). Recovery can also be from seed. Australian pine and Brazilian pepper often invade severely burned tree islands (Wade et al. 1980).

Epiphytes, common inhabitants of tree islands especially those that have tropical hammock vegetation, are not typically severely affected by fire, but changes in fire regime, and disturbances in addition to fire affect epiphyte populations. Fires in hammocks usually only directly kill epiphytes that have fallen to the ground or those less than 1 m above the ground (Robertson and Platt 1992). The interaction of fire and hurricanes can affect epiphytes more than fire alone. In high winds, trees top-killed by fires loose branches on which epiphytes grow, lowering the vertical distribution of epiphytes (Robertson and Platt 2001). An exotic plant, old world climbing fern (Lygodium microphyllum), can grow into the canopy of tree islands where it acts as a ladder fuel, conveying fire into the canopies of trees. This can cause tree and epiphyte mortality not seen under the natural fire regime (Ferriter 2001).

Fire Effects on Animals

Direct animal mortality has been observed as a result of fires in the Everglades. Cotton rats, marsh rabbits, box turtles, snakes, and small alligators have all been observed to perish as the result of fires (Wade et al. 1980). Less mobile animals have been perceived as most vulnerable to fires. Direct bird mortality is generally thought to be restricted to nestlings on or near the ground. Recent observations of wading bird mortality due to fires showed that a breeding colony located in a willow /button bush thicket was unharmed by two fires, however, a number of mature white ibis were killed by fire while foraging away from the breeding colony (Epanchin et al. 2002). The authors speculate that the adult birds were trapped by flames or were disabled due to smoke inhalation. The breeding colony was unharmed because fires did not burn into the willow thicket. In fact the majority of wading bird breeding colonies in the central Everglades are located in willow thickets. These wet sites provide protection against predation but in addition, this vegetation type provides some protection from fire (Epanchin et al. 2002).

Fires create direct opportunities for animal foraging, and vegetative response to fires can create food sources. Birds have been observed preying on insects fleeing fires. Snakes have also been observed along flanks of fires feeding on escaping small mammals (Wade et al.1980). Post-fire regrowth attracts deer (Klukas 1973) and fires remove excess biomass in sawgrass, allowing waterfowl access to sawgrass seed (Yates 1974 cited in Wade et al. 1980).

Fires also affect animals through habitat alteration. Fire was found to have little effect on round-tailed muskrat populations, except forcing moves to unburned vegetation (Tilmant 1975 in Wade et al.). Fires that burn edges of tree islands can restrict the ranges of edge specialist birds, and conversely complete burn out of tree islands can encourage edge specialists when scrubby vegetation reclaims the site (Gunderson and Loftus 1993). Peat fires create deeper water habitat that is utilized by aquatic animals (Gunderson and Loftus 1993) and wading birds (Hoffman et al. 1994). Peat fires are a source of nutrients (particularly phosphorus, the most limiting nutrient in the Everglades) and may increase productivity of aquatic organisms (Robertson and Fredrick 1994). Though the effects of fire on the Cape Sabal seaside sparrow are unclear, fire plays a role in maintaining habitat (For more information on the Cape Sable seaside sparrow see Coastal marsh Birds).