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publications > report > a survey of the effects of fire in Everglades National Park
U.S. Department of the Interior
A Survey of The Effects of Fire in Everglades National Park
William B. Robertson, Jr.
Submitted: February 15, 1953
The present report provides the results of recent field investigations of the effects of wildfire upon the vegetation of Everglades National Park. This project was carried on in the winter and spring of 1951-52 at which time the author held a temporary position as Fire Control Aid in Everglades National Park. Earlier field investigations in south Florida by the author supplied much of the original data presented here, and provided a background of knowledge of the area without which the present work would have been impossible.
The information presented is divided into four sections as follows: Fire history of the Everglades National Park area; Description of the burnable vegetation types; Effects of fire on the burnable vegetation types; and, Conclusions and recommendations. The following paragraphs summarize these sections.
I. Fire History - An attempt is made to reconstruct the history of fire occurrence throughout the geological existence of south Florida in its present relationship to sea level. Evidence is presented which strongly suggests that natural fire has been a constant factor affecting the local distribution of vegetation types through the ages, and that the arrangement of plant cover types has probably always been similar to that seen today. Fire frequency is believed to have increased as aboriginal peoples occupied the area. With white settlement came another marked increase in fire frequency and also an increase in the severity of fire damage as drainage lowered water levels in the Everglades. Records indicate severe and widespread fire in south Florida for more than 1/3 of the years between 1900 and 1952. A half-century fire chronology compiled from the scientific literature, from newspaper accounts and from interviews with local residents is given. A summary of rainfall records since 1900 is presented. Examination of these data indicate that the Lake Okeechobee-Everglades system is no longer an effective drainage unit, and that water levels and fire danger in Everglades National Park now depend entirely on rainfall south of the Tamiami Trail. This section is concluded with an account of fire occurrence since the establishment of Everglades National Park including a map of fire occurrence by years, and a graph of fire occurrence by months in the two chief fire types.
II. Description of Vegetation Types - This section presents accounts of the following burnable vegetations: Rockland pine forests; tropical hammock forests; bayhead forests; and Everglades marshes. For each the description includes as detailed a survey as is possible from data at hand of: The plant species and general aspect of the vegetation type; the major variations noted from stand to stand through south Florida; the factors which appear to govern local occurrence of the vegetation type; and, the major gaps existing in our present ecological understanding of the vegetation type. It is emphasized that present knowledge of the vegetation of Everglades National Park is incomplete and that these gaps hinder understanding of the effects of fire upon the plant cover.
III. Fire Effects - For each of the above cover types a discussion is given of fire effects upon the soil and upon the plant cover. Recovery of the plant cover after fire is discussed, and the influence of fire upon the successional relations of the plant communities is analyzed.
Pineland fires remove the ground cover vegetation and prune back the shrubs of the hardwood understory leaving bare limestone. The fires are ground fires which do not ordinarily kill the overstory pines. Recovery after fire is marked by an outburst of bloom of the small pine woods herbaceous plants, and by stands of tall broom grass on one-year old burns. A single fire kills few hardwood shrubs. The roots of these shrubs are deeply driven into the limestone, and are protected by it. They soon send up crown-sprouts and most individuals show a typical many-stemmed growth-form brought about by frequent fire-pruning. There is some evidence that hardwoods tend to be eliminated from the pineland by frequently recurring fires, and to be replaced by an understory of low palms, especially saw palmetto.
Two kinds of fire effects are noted in the case of hardwood hammocks; 1.) pruning back of the hammock edges; and, 2.) complete hammock destruction occurring when fires ignite the organic soil deposit of the hammock. In the latter case the trees of the forest canopy are commonly killed by fires burning around their roots, or later windthrown due to loss of supporting soil. Recovery is long-delayed in the case of complete burnouts, and some of the more sensitive epiphytic orchids and ferns may be lost entirely. In early stages of recovery, hammock interiors become clogged with a rank growth of fire weed shrubs and vines.
Fire prevents succession of hardwoods into pine forest by fire-pruning hardwood shrubs and cutting back hammock edges. In the Long Pine Key area of Everglades National Park this succession is rapid in the absence of fire. Here a fire-free period of 15 to 25 years is considered sufficient to establish a continuous young hardwood forest on most pineland sites.
Fire effects upon the bayheads of the Everglades are similar to those on upland hammocks, but more severe. Those tree islands occupy deep deposits of combustible peat and their occurrence requires the elevation above the surrounding marsh which the peat mass provides. Fires remove the peat entirely commonly leaving burn-out ponds, and a long period of plant succession must occur before bayhead forest can again occupy the site. Where these peat burn-outs result in establishment of ponds, they have the beneficial effect of furnishing a dry-season refuge for many glades water animals.
In sawgrass glades fire damage is severe only in the muckland area little of which now remains south of the Tamiami Trail. It is probable that over a period of years sawgrass fires have decreased the water storage capacity of the Everglades by destruction of the peat and marl seal over the highly permeable underlying limestone. Over most of the marl soil glades of the park no definite fire effect can be indicated. Much more information on the ecology of the many species of sedges and grasses which comprise the Everglades vegetation is needed before fire effects on stand composition can be satisfactorily studied.
With drainage much of the Everglades area has become suitable for invasion by woody plants, especially willow and the woody species of the bayheads. Fire acts to restrict this forest extension into the marsh. In spite of the severe fires of the last twenty years plant succession has entirely changed the aspect of considerable areas, from open herbaceous marsh to scrubby thickets.
IV. Conclusions - Fire is a natural environmental factor in Everglades National Park. Elimination of fire would result in eventual disappearance of the fire-maintained cover types, the pine forest and Everglades marsh prairies.
The severe and frequent fires occurring under present altered conditions are rapidly eliminating the hardwood forest types, and seem capable, also, of causing degenerative changes in the fire types. It thus seems imperative that an attempt be made to control all fires in the area with special efforts to protect the tropical hammock and bayhead vegetation.
Restoration of former water levels on the glades would change the necessities of fire control, and should bring about a situation in which only areas of special use or interest need be guarded from fire.
Careful long-term attention should be given to the study of fire effects on vegetation of Everglades National Park with particular concentration upon the problem of fire effects upon the stand density and composition of the sub-climax fire types. A program of investigation designed to meet this need is outlined.
The fire problem promises to remain one of key importance in Everglades National Park. Enlightened administrative procedures will require a background of full information on all aspects of fire effects in the area.
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U.S. Department of the Interior, U.S. Geological Survey
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