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KUCHLER TYPE FIRE ECOLOGY AND MANAGEMENT

KUCHLER TYPE: Palmetto prairie
FUELS, FLAMMABILITY, AND FIRE OCCURRENCE : Fuels: In cut-over south Florida flatwoods, pineland threeawn comprised about 20 percent of the total biomass, other species 10 percent, and mulch constituted at least 50 to 60 percent of the total biomass 2 years after prescribed winter fire. Biomass yields at 3 to 4 years following fire averaged between 5,500 and 7,000 pounds per acre (6,188-7,875 kg/ha) [42]. After 3 to 4 years, a steady state was reached where a relatively constant 70 percent of the total biomass was mulch [42]. Flammability: Fire travels unevenly through the mosaic of palmetto prairie, flatwoods (various phases), and scrubs, reflecting the varying flammability of the different communities [2]. Pine flatwoods and palmetto prairie typically burn vigorously and completely (excepting overstory pines), leaving few unburned patches [3]. Swale and flatwoods transects ranged from 96 to 100 percent burned after a series of prescribed fires and one wildfire at the Archbold Biological Station [2]. The two most dominant species of palmetto prairie, pineland threeawn and saw-palmetto, are highly flammable, as is gallberry, even though saw-palmetto and gallberry remain green all year [3,48]. Abrahamson and others [4] rated pine flatwoods second in flammability among the seven community types occurring at Archbold Biological Station. Flammability appears to be related to the amount of grasses, saw-palmetto, pine duff, and litter [4]. Fire Occurrence: Southern Florida has the highest number of thunderstorm days per year (70-90) [45] and the highest frequency of lightning strikes of any region of the United States [3]. The National Oceanographic and Atmospheric Administration estimated that there are 10 to 12 cloud to ground lightning strikes per square kilometer per year in Highlands County, Florida. Only a small percentage of those strikes actually ignite fires: In one area, 30 fires resulted from an estimated 2,100 to 2,600 strikes in 14 years. At the Archbold Biological Station, a majority of the 56 lightning strikes recorded between 1967 and 1980 occurred between May and September, with peak lightning activity occurring in July. Almost all (97%) lightning fires occurred between May and September, most frequently at the beginning of the rainy season, since vegetation moisture content of many dominant species is lowest in spring [4,45]. In the presettlement period, fires were not restricted by roads, canals, and other structures, and many spring and summer wildfires probably burned over very large areas [3]. Fire Frequency: Christensen [9] estimated a natural fire frequency for Coastal Plain savannas (which probably includes palmetto prairie) as approximately every 2 to 8 years, based on the life history and fire adaptations of the dominant species, including pineland threeawn and saw-palmetto. Menges and others [37] estimated that flatwoods and southern ridge sandhills in central peninsular Florida burned an average of once in 3 to 10 years. Within a climatic region, fire frequency is often correlated with site fertility: Fires are more frequent on nutrient-limited or oligotrophic sites than on eutrophic sites [10]. The nutrient-poor soils of Coastal Plain savannas result in production of ligneous herbaceous fuels with a low nutrient content. Decomposition is slow, litter accumulates rapidly, and therefore the probability of another fire increases rapidly. Fire History: Fires have occurred in southern Florida for at least 50,000 years, as evidenced by the charcoal present in lakebeds [45]. Watts and Hansen [61] speculated that fires were relatively frequent between 50,000 and 20,000 years ago, then decreased in frequency, and increased again approximately 12,000 years ago. The earliest aboriginal occupation of Florida has also been estimated at about 12,000 years BP [45]. However, the flat terrace of extreme southern Florida has only been above sea level for a few thousand years [64]. Lightning fires were probably the main selective force driving the fire adaptations of palmetto prairie vegetation; however, extensive consistent burning by Native Americans, in a pattern different from that of lightning fires (increased frequency and in different seasons or habitats) constituted a strong selective force. There are, for example, instances where local genetic adaptations have arisen in time spans of only a few hundred years [45]. It is therefore quite conceivable that either Native American burning practices, or burning and other management practices implemented since European settlement, have combined to enhance fire adaptations in palmetto prairie species [45]. FIRE EFFECTS ON SITE : The nutrient budget of palmetto prairies and pine flatwoods is closely tied to fire regime. Over long periods of time, the average annual loss of carbon in many frequently burned communities may exceed 0.18 pounds per square yard (100 g/sq m). In pine flatwoods, 10 to 30 percent of the biomass produced each year may be recycled by fire. In addition, soil conditions favor vegetation characteristics that increase the probability of low intensity, frequent fire. Nitrogen and phosphorus are the nutrients most frequently limiting to plant growth in Coastal Plain communities [10]. Fire causes an overall loss of nutrients, but increases short-term nutrient availability as a consequence of mineralization, addition of nutrients as ash, changes in microbial activity, or alteration of soil ion exchange. Most nutrients are not present in available form immediately after fire; there is a delay during which nutrients are converted to available forms. Direct addition of mineral nutrients in the ash is not as important as changes in mineralization rates to postfire soil fertility. Nutrient loss to surface runoff is negligible due to the lack of topographic relief. Most nutrient loss is due to volatilization and leaching into groundwater. Annual prescribed fire could result in overall loss of nutrients and effectively lower productivity [10]. However, Wells [58] reported that annual burning of pine flatwoods in South Carolina did not reduce nitrogen. Following prescirbed fire for fuel reduction in a slash pine/saw-palmetto-gallberry type, nutrients remaining in unburned and partially burned plant materials were measured. Carbon was found to have been largely volatilized. Nitrogen, phosphorus, magnesium, and sulfur loss had a significant positive correlation with fuel weight loss (i.e., much of those nutrients were volatilized); potassium, calcium, and manganese were not as strongly correlated. Copper, aluminum, and molybdenum increased following fire; iron, strontium, boron, barium, and zinc loss was medium to large [25]. Temperatures at the soil surface, measured 5 days after prescribed fire in palmetto prairie near Archbold Biological Station, were up to 20 degrees Fahrenheit (11 deg C) higher than on unburned sites [2]. FIRE EFFECTS ON VEGETATION : The vegetation of palmetto prairie is highly adapted to fire; most of the species are usually found only in frequently burned (less than 50 years without fire, most less than 20 years) ecosystems [3]. Many species sprout from underground stems or roots [3]. Gallberry, for example, sprouts from roots and saw-palmetto sprouts from stems that are partly to completely underground. On southern Florida range, gallberry burned in January was visibly sprouting by late February, exhibited the most rapid growth from mid-April to mid-May, and was 2 feet tall (0.6 m) by November. Saw-palmetto burned in November produced one fully expanded leaf by January, seven to eight leaves by the end of 1 year, and three or four more leaves by the end of the second year. Unburned saw-palmetto produce an average of five to six leaves per year. Saw-palmetto burned in late fall is stimulated to grow all winter whereas unburned saw-palmetto is relatively dormant in winter. Postfire sprout numbers of both saw-palmetto and gallberry usually exceed prefire stem numbers, though a number of sprouts die the second year. Foliage cover per stem increases gradually with protection from fire [20]. Blades of pineland threeawn appear within 3 days of a winter fire; this species comprised at least half of the herbaceous vegetation through a 2-year period following fire on south Florida pinelands [20]. The date of burning affects the number and density of pineland threeawn reproductive tillers; pineland threeawn burned in late spring (May) produces substantially more seed than that burned in early spring (March or April) [42]. Fire occurrence in a particular season also initiates or enhances the flowering of other herbs inhabiting palmetto prairie. Cutthroat grass (Panicum abcissum) and beardgrasses (Andropogon spp.) do not flower unless burned in the spring or summer. Catesby's lily (Lilium catesbaei), yellow bachelor's button (Polygala rugelii), deer tongue (Carphephorus paniculatus), and white-topped aster (Aster tortifolius) flower more conspicuously following fire [1,4,42]. Robinson [46] compiled a list of the fire effects on 30 proposed, threatened, and endangered plant species occurring in pine flatwoods communities. Fire is rated beneficial for all of the listed species [46]. Gallberry flatwoods recovered to near prefire vegetative cover in less than 6 months following prescribed winter fire. Pineland threeawn flatwoods whose prefire cover averaged 82 percent had recovered to 56 percent cover at approximately postfire year 1, 60 percent cover by year 2, and 79 percent cover by year 3. Burning produced no appreciable change in vascular plant community composition, although there was some shift in the relative abundance of woody species. In gallberry flatwoods, there was a decrease in the importance value of fetterbush, gallberry, and saw-palmetto; and an increase in the importance value of dwarf huckleberry (Gaylussacia dumosa) and ground blueberry [2]. A discussion of species specific responses is available [1]. Effects of Fire Exclusion: Some species decline with fire exclusion. Ground blueberry increases in shoot density and dominance following fire, but rapidly declines in density as it is overtopped by taller species during intervals without fire. In intervals without fire, ground blueberry can survive with little aboveground biomass. When fire releases nutrients and decreases shade and competition for moisture, ground blueberry responds with renewed vigorous growth [3]. Herbs are also less common in pineland threeawn flatwoods that have not experienced recent fire than in recently burned stands, largely due to shading by tall shrubs [4]. Little bluestem (Schizachyrium scoparium), and slender bluestem (S. tenerum) are suppressed by the accumulation of litter in unburned stands; they remain vigorous on burned stands. However, little bluestem decreases with burning on grazed lands [16]. The relative abundance of species may be altered by fire exclusion as well. The understory of a South Florida slash pine stand that was last burned in 1941 had greatly reduced pineland threeawn cover by 1963 [20]. Pineland threeawn may survive 2 to 4 decades of fire suppression but with a decline in vigor and cover. A longleaf pine flatwood that had not experienced fire for 25 years was overgrown with saw-palmetto and gallberry. Even though pineland threeawn had not been observed on the site, it sprouted from dormant tussocks within a few weeks of a winter precribed fire. It was, however, eliminated from a longleaf pine site over 34 years of fire exclusion [11]. In infrequently burned pine flatwoods, saw-palmetto, gallberry, and fetterbush increase in dominance (largely due to height growth) [11,20]. Other species increasing with fire exclusion include sand live oak (Quercus geminata), southern bayberry, and dahoon (Ilex cassine), all of which grow rapidly to small tree size if not top-killed by fire [53]. At Archbold Biological Station, changes in vegetative composition with fire exclusion have been recorded. There have been no fires on the study sites since 1926 or 1927; prior to that fire frequency in the southern ridge sandhill area was about once every 9 years. It was reported that flatwoods/bayheads sites (lumped together for the purposes of the study) showed some degree of convergence towards bayheads; sites formerly dominated by flatwoods shrubs have bayhead trees such as redbay (Persea borbonia) emerging as dominant species [37]. Monk [38] also suggested that bayheads may develop from low pine flatwoods in the absence of fire. FIRE EFFECTS ON RESOURCE MANAGEMENT : Palmetto prairie forage production was 65 pounds per acre (73 kg/ha) 3 weeks following burning, 1 ton per acre (2,250 kg/ha) 1 year following burning, and 1.75 tons per acre (3,938 kg/ha) after 2 years. Species included pineland threeawn, panicums, and bluestems [21]. Forage quality is highest immediately after a fire, but declines rapidly to borderline for cattle nutrition by 3 months after a fire [20]. Cattle will graze pineland threeawn for 2 to 3 months after burning, but then seek other forage plants [21]. Pineland threeawn remained vigorous under heavy grazing when burned at intervals longer than 1 year, but decreased slightly in cover on ranges that were burned annually and heavily grazed [18]. In southern Florida, pineland threeawn cover was unchanged when burned biennially and grazed heavily [20]. Month of burning affects early forage production. Ranges burned in March or May averaged two and four times as much herbage, respectively, in the 60 days after fire as ranges burned in October or November [34]. Cattle grazed newly emerged saw-palmetto leaves following fire [22]. Treeless ranges produced three times as much early growth herbage when burned in May than on ranges burned in November. Yields by 2 years after fire were not affected by season of prescribed fire. The difference in season of burning was much less pronounced on ranges with trees present, since ranges with trees produce less forage than open sites [28]. Gallberry top-killed by fire does not produce flowers or fruit until the second year, but then bears profusely. Burned saw-palmetto blooms and produces a few fruits the first year, but at a lower than normal rate. Full fruit productivity for gallberry is regained in 8 to 10 fire-free years after biennial burning [20], although the fruit production of most other shrubs in open pine flatwood peaks at 3 to 5 years after burning [29]. Direct Fire Effects on Wildlife: In longleaf pine flatwoods on the St. Marks National Wildlife Refuge no dead herpetiles were found in an area where severe fire consumed vegetation down to bare sand, although a later search resulted in the discovery of one dead box turtle. Several living box turtles had fire scars on the carapace. Twenty-six species of herpetiles were active in the burned area 3 weeks after the fire. In adjacent slash pine/saw-palmetto flatwoods, the flatwoods salamander occurred at similar activity levels in the winter and spring of 1979 on sites that were either unburned since 1977 or burned in January 1979 (at the start of the sampling program). The fact that the salamanders survived a winter fire during the active breeding season makes it extremely likely that they can survive summer fire when they are dormant and protected in soil burrows [36]. Herpetiles that appear to be most at-risk from fire are reptiles in mid-ecdysis (i.e. are actively shedding skin) [36]. Wood [59] reported no fire-caused mortality of tree squirrels, furbearers, or black bears (Ursus americanus). Taylor [54] however, reported that a fast moving grass fire killed five marsh rabbits (Silvilagus palustris), although eastern cottontails (S. floridanus) easily escaped. Indirect Fire Effects on Wildlife: Prescribed fire is widely used by wildlife managers in the Southeast. White-tailed deer have been observed to lick the nutrient-rich ash on fresh burns. In winter, white-tailed does use freshly burned areas; the extra nutrients may be beneficial during early gestation. April through July lightning fires are thought to provide nutrient-rich ash that is beneficial during late gestation [15]. Mourning doves (Zenaida macroura) have been observed foraging on fresh burns [32]. At Myakka River State Park, palmetto prairie that had been overgrown by shrubs was prescribed burned in winter and summer to compare season of fire on bird densities. Bird abundance on the winter burn was not different from the unburned control, but was lower on the summer burn [13]. Managers of hunting plantations burn ranges annually to improve northern bobwhite habitat. Time of burning has an important effect on food availability to northern bobwhite. Normally, late winter and early spring are critical periods when food is relatively scarce. Spring fires are detrimental to food production for game birds. Production of northern bobwhite food is delayed 1 month on ranges burned in January, 2 months on March burns, and 3 months on May burns [28]. Hughes [28] recommends that managers burn separate parts of the range area in different seasons to keep forage green and palatable all year. Fall burning is best where wildlife habitat is desirable because burning in fall favors spring production of food plants. However, fall prescribed fire involves increased risks, and there is potential for loss of nest sites for birds. Hughes [28] recommends that for wildlife use, cattle should be kept off of ranges for approximately 6 weeks following fall prescribed fire, and 4 weeks after spring fires to maintain wildlife forage availability. Prey availability for raptors is altered by fire; most hawks and owls in palmetto prairie depend on hispid cotton rat (Sigmodon hispidus) and eastern cottontail. Frequent fire maintains habitats suitable for many common mammalian prey species, but temporarily exposes them when cover is consumed. The fulvous harvest mouse (Reithrodomys fulvescens) and the cotton mouse (Peromyscus gossypinus) increase after fire; the hispid cotton rat, eastern harvest mouse, and round-tailed muskrat (Neofiber alleni) decrease after fire; oldfield mouse (P. polionotus) and Florida mouse (P. floridanus) populations remain unchanged [32,54]. Cooper's hawk (Accipiter cooperii) and sharp-shinned hawk (A. striatus) are the primary predators of northen bobwhite and the larger passerines that are abundant in fire-maintained game lands including palmetto prairie [32]. FIRE USE CONSIDERATIONS : Temperatures as high as 977 degrees Fahrenheit (525 deg C) have been recorded at Archbold Biological Station prescribed surface fires in flatwoods [4]. In south Florida cutover pine flatwoods with low coverage of saw-palmetto, aboveground temperatures measured in both headfires and backfires averaged 415 degrees Fahrenheit (212 deg C), with no significant differences between the headfires and backfires. Only two of five sites showed temperatures as high as 120 degrees Fahrenheit (49 deg C) at 0.125 inch (0.3 cm) below the surface; all other sites were cooler at that level and deeper [42]. These data indicate that fire in palmetto prairie, as in other grassy ecosystems usually passes over the surface too rapidly to heat the soil to any great degree. Nelson and Adkins [41] report on a laboratory study of flame height, length, and tilt angle in relation to wind speed for slash pine/saw-palmetto-gallberry fuels. They also provide comparable field data on winter fire in a longleaf pine-slash pine/saw-palmetto-gallberry type. Hough and Albini [26] characterized the fuel complex of a palmetto-gallberry type, adjusting parameters so that use of the Rothermel model of fire spread and intensity is practical for this type. This type is a complex association of saw-palmetto and gallberry beneath slash pine or slash pine and longleaf pine; it appears to be very similar to palmetto prairie in most respects. In this complex, live and dead fuels accumulate so rapidly that in 5 years wildfire can seriously damage the pines [26]. McNab and others [35] estimated fuel weights in slash pine/saw-palmetto stands, and developed equations to be used to estimate the dry weight of total fuels and fuels by size class. They concluded that most of the components of this fuel complex can be estimated by the age of the rough, understory height, and saw-palmetto cover [35]. Hough [24] reported a proposed method for estimating the weight of fuel burned by prescribed fire, and also measured the caloric value of some fuels as determined by oxygen-bomb calorimetry [23]. Sackett [47] published a schedule of fuel accumulation rates for palmetto prairie species. Specific burning guidelines for fuel reduction prescribed fires are available [47]. FIRE MANAGEMENT CONSIDERATIONS : The natural fire season for palmetto prairie in presettlement times was spring and summer. Humans have added a second fire season in winter, using prescribed fires which burn with lower severity. Currently, wildfires are much smaller than they were before European settlement, leaving many areas (particularly those not managed for cattle) unburned. The impact of winter prescribed fire on vegetative composition is different from that of the natural fire regime (spring and summer fire) and has resulted in an alteration of the relative abundance of many species [14]. Palmetto prairie and pine flatwoods are prescribed burned for many reasons including forage improvement for cattle, maintenance of open conditions, shrub control, fuel reduction, and wildlife habitat improvement [14,48]. Many cattle ranges in southern Florida, which are grazed all year long, are burned as often as there is sufficient vegetation to carry fire; fire is the primary tool for manipulating vegetation [3,28]. Areas are burned in rotation through the burning season to extend the period of quality herbage production and to reduce gallberry and saw-palmetto competition [3]. Fire will not eliminate saw-palmetto, however, and very frequent winter fire may increase it [31,49,57]. Fire effects on palmetto prairie include a stimulation of new growth, an increase in the nutrient content and palatability of grasses and forbs, and increased accessibility to forage plants [3]. However, frequent burning has led to the decline of lopsided indiangrass (Sorghastrum secundum), chalky bluestem (Andropogon capillipes), creeping bluestem, and goobergrass, which are the major preferred and palatable native forage grasses. These grasses are also sensitive to grazing [49]. Sievers [49] recommended that range managers burn pastures no more often than every third year, and then only between January and March when the preferred grasses are dormant. He also recommended that grazing on ranges in excellent condition be deferred for 60 to 90 days following fire. On ranges already dominated by pineland threeawn grazing can occur immediately after fire, but cattle should be removed in 6 to 8 weeks to allow creeping bluestem establishment [49]. In areas to be managed as nature preserves or natural areas, such as Myakka River State Park, fire has been used to preserve the original character of plant communities. Myakka River State Park was predominantly open (nonforested) in the 1940's. By the 1980's, oaks (Quercus spp.) expanded into palmetto prairie where fire was excluded. These oaks have a dense understory of saw-palmetto. With the introduction of winter prescribed fire in the 1960's, the expansion of oaks was halted, but the existing trees survive the winter fire regime. A summer prescribed fire was sufficiently intense to kill oak overstory trees, but left south Florida slash pine and the dense understory of saw-palmetto. It is therefore suggested that summer fire is needed for canopy reduction (where oaks have invaded); however, the danger posed by heavy fuel accumulations may require fuel reduction fires prior to summer prescribed fire [27]. Sackett [47] recommended a 3-year interval for prescribed fire as optimum for fuel reduction and reduction of wildfire damage risk. Fire consumes much aboveground biomass and litter, and appreciably increases the rate of nutrient turnover. Phosphorus and cations in phytomass are mineralized. Soil pH is raised, stimulating nitrogen-fixing bacteria. Some nitrogen is lost to volatilization, and the amount varies with fire intensity [3]. Frequent low-intensity fires release small pulses of nitrogen and volatilize only 20 to 40 percent of the forest floor nitrogen. This much is replaced in about 5 years from atmospheric precipitation alone, not counting what is added through nitrogen fixation [17]. Very frequent burning could result in sufficient nutrient loss to decrease plant production. Walker and Peet [65] reported that biomass production in annually burned mesic pine/wiregrass (Sporobolus teretifolius, Muhlenbergia expansa, and others) savannas was lower than in pine/wiregrass savannas that were burned infrequently. The long-term effects of prescribed fire may be small compared to the effects of grazing; grazing results in increased grass production and floristic changes (which vary with grazing intensity and precipitation, and are compounded by fire). Grazing animals also cause soil compaction and a concomitant reduction in water infiltration and percolation [63]. Soil compaction coupled with trampling reduces the growth and survival of pine seedlings [8]. REHABILITATION OF SITES FOLLOWING WILDFIRE : NO-ENTRY

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Information Courtesy: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. Fire Effects Information System

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