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Prescribed fires are generally one of three types: head fires, backing fires, or flanking fires. Head fires burn with the wind or upslope. They are of relatively high intensity and move through fuels at a relatively high rate of speed. Head fires are often ignited in strips (called strip head fires) to speed the burning process and to provide the desired intensity. Fire intensity increases as the rear of a previously ignited strip merges with the advancing front of a subsequent strip (Brown and Davis 1973). Backing fires back into the wind or burn downslope. They burn with lower flame heights and lower intensity, and move through the stand at slower speed than head fires. Because of their lower intensity and slower speeds, backing fires are more easily controlled. Flanking fires are set moving parallel to and into the wind. They are generally used to supplement other burning techniques. For example, flanking fires can be used to speed the process of burning with backing fires. Flanking fires are set perpendicular to backfires. Where flanking fires merge, fire intensity increases, but not as much as it does with strip head fires.
The choice of which fire to use depends upon objectives, fuel and moisture conditions, and need to manage smoke. To understand fire behavior and fire effects, the difference between fire intensity and severity should be appreciated. Fire severity describes the condition of the ground surface after burning (Wells and others 1979), whereas fire intensity is the rate at which an ongoing fire produces thermal energy. Although an intense fire usually has severe effects, such congruence is not always the case. For example, any fire that consumes the entire organic layer and alters mineral soil structure and color would be classified as a "severe burn." A high-intensity fire in heavy fuels when the soil and forest floor are moist, however, would leave a large amount of residual forest floor and would not alter soil structure and color. Thus, the severity of such a high-intensity fire would be classified as "light".
Fire effects are related to intensity and duration of exposure. Fireline intensity is the heat output of a unit length of fire front per unit of time (Deeming and others 1977). Fireline intensity is directly related to flame height, which influences fire effects. As trees grow taller and their bark thickens, resistance to fire increases because crowns are higher above the heat of the flames and thicker bark insulates their cambium. The duration of exposure (residence time) also is an important consideration in prescribed fire. Living tissue can be instantly killed at a temperature of 147 oF; it also can be killed by prolonged exposures to lower temperatures (Hare 1965, Nelson 1952). Backing fires of low intensity can be lethal to small stems because the slow speed of the burning front enables lethal cambium temperatures to be reached just above ground.
Leaf litter is the primary fuel that sustains fire. Loading and thickness of the litter layer vary depending on site, stand age, and season (Kucera 1952, Metz 1954, Blow 1955, Crosby and Loomis 1974, Loomis 1975, Albrecht and Mattson 1977). Fuel weights in like stands on comparable sites vary little longitudinally, but increase northward because decreasing mean temperatures slow decomposition. Most hardwood stands have a litter loading from 1 to 4 tons per acre and a depth of 1 to 5 inches, depending on season. Litter loading and depth are greatest immediately after leaf fall in the autumn and decline until the following autumn. Hardwood leaves in general tend to cup and hold water after a rain, but the leaves of some species of oak tend to curl and dry quickly in comparison to other hardwoods, allowing fire to run through oak litter when other hardwood fuel types are too wet to burn.
Under mature southern pine stands on the Atlantic Coastal Plain, forest floor fuel loads range from about 1.5 tons per acre under an annual dormant-season fire regime to 13 tons per acre after 40 years without a fire. Live groundcover and understory fuels with a ground line diameter less than 1 inch range from about 0.75 tons per acre with annual burns, to over 11 tons per acre after 25 years. On Piedmont sites, roughly the same trends hold. Fuel weights are highest in loblolly and longleaf pine stands, and appreciably lighter under shortleaf and Virginia pine stands. Shortleaf and mixed shortleaf pine-hardwood stands in the Mountains may have substantially heavier fuel loads than similar stands in the Piedmont, at least in part because of a heavier understory component (Albrecht and Mattson 1977). Small woody fuels can be abundant in young stands originating after a major disturbance. Woody fuels are less abundant in mid-successional and mature stands, but increase in old-growth stands due to accumulation of large downed or standing dead woody material. When present, ericaceous shrubs such as mountain laurel and rhododendron can burn with extreme fire behavior resulting in a mixed-severity or stand-replacement fire (Waldrop and Brose 1999). Many of the firefighter fatalities in hardwood forests have occurred because of the explosive nature of these fuels.
Resource managers generally prescribe burning conditions that limit fuel consumption to 1 to 3 tons per acre during passage of the flame front. Residual smoldering combustion can more than double these values, especially under drought conditions, or 5 to 6 years after a major disturbance when large down woody fuels become partially decomposed. More detailed descriptions of fuels and fire behavior can be found elsewhere (Hough and Albini 1978, Johansen 1987, Wade and Lunsford 1989, Wade and others 1993, Wade 1995, Cheney and Gould 1997).
Prescribed fire is often used to reduce fuel lands from dangerous levels to protect forests from wildfire. Most wildfires are accidental; campfires, debris burning, or sparks from machinery are common ignition sources. Burning embers carried aloft by the convection column (rising hot air and gases) may ignite numerous spot fires far away from the main fire. Nevertheless, arson remains a serious problem in Southern forests. Fires set by arsonists are difficult to control because they often occur during times of extreme fire danger. Wildfires in recently harvested stands can be intense because of heavy fuel loadings (Sanders and Van Lear 1987).
Pine stands usually develop an understory of hardwoods, shrubs, and vines. When draped with pine needles, this understory becomes highly flammable. If the condition extends over a large area, the whole forest is at risk of destruction by wildfire. In hardwood stands, rhododendron and mountain laurel often form thickets of highly flammable fuels, which allow fire to climb into the canopy. Prescribed fire is an economical way to reduce dangerous fuel accumulations. Wildfires that burn into areas previously subjected to prescribed fires cause less damage and are controlled more easily. The appropriate interval between prescribed burns for fuel reduction varies with several factors, including the rate of fuel accumulation, which is high in pine stands in the Coastal Plain because of the rank understory. Past wildfire occurrence and the values at risk are other factors to consider. The interval between fires in pine stands can be annual, but a 3- to 4-year cycle between fires usually is adequate after an initial fuel reduction burn (Wade and Lunsford 1989).
Judicious use of fire reduces the large amount of highly flammable fine woody material present after clearcutting by more than 90 percent (Sanders and Van Lear 1987). Site-preparation burns in pine plantations are normally conducted in the summer and are of moderate to high intensity. They are used to reduce logging debris, control hardwood sprouts, and improve the plantability of the site. Because of their intensity, these burns must be conducted under the proper fuel- and soil-moisture conditions to prevent damage to the soil, especially in the steep terrain of the Southern Appalachians (Swift and others 1993, Van Lear and Waldrop 1989). Broadcast burning late in the summer following long periods without rain can completely remove organic layers from the soil. Such burns reduce logging debris, ensuring that the site will be plantable, but they can cause site damage from accelerated erosion and loss of nutrients and organic matter. In addition, severe burns may contribute to poor initial survival of planted seedlings because of the loss of mulching effects of a residual forest floor. Both onsite and offsite damage from broadcast burning can be minimized by burning earlier in the summer, soon after soaking rains.
Prescribed fire prior to harvest is used to prepare seedbeds for natural regeneration of pine. Low-intensity burns are used to protect the stand that is being regenerated when seed trees are retained for future cavity trees for red cockaded woodpeckers. One or more winter burns may be required to reduce fuel loadings. A final summer burn is used to prepare the seedbed and reduce the vigor of understory hardwoods. Dormant season logging further enhances seedbed preparation and allows seeds to germinate the following spring.
Mixed pine-hardwood stands can be regenerated after clearcutting in the Southern Appalachians with minimal adverse site effects using the fell-and-burn technique (Abercrombie and Sims 1986, Danielovich and others 1987, Phillips and Abercrombie 1987). As the name suggests, fell-and-burn requires two steps after clearcutting of hardwood or pine-hardwood stands. First, residual stems over 6 feet tall are felled with chainsaws during early spring after full leaf development when carbohydrate reserves in the roots are low. Allowing full leaf development is important for two reasons: (1) leaves on the felled trees speed the drying of small twigs and branches, which serve as fuel for the broadcast burn, and (2) leafing out reduces root reserves and therefore reduces the vigor of hardwood sprouts. The harvested stand is burned in mid-summer, within 24 to 48 hours after a soaking rain. The damp forest floor reduces fuel consumption, minimizing heat penetration into the soil and protecting against erosion. Pine seedlings, planted at a wide spacing the following winter, generally compete well with hardwood coppice.
Using fire to regenerate hardwoods generally has not been recommended because of the fear of damaging stem quality and because of the danger of erosion, particularly on steep slopes (Van Lear and Waldrop 1989). Nevertheless, many oak stands that currently occupy better sites in the Appalachians no doubt became established 60 to 100 years ago when burning was a common practice. Observations of conditions after wildfires are the basis for avoiding burning in hardwoods. Wildfires, however, burn with higher intensity and severity than prescribed fires (Abell 1932, Nelson and others 1933, but see Wendel and Smith 1986). A low intensity winter backing fire in mature hardwood stands probably has little adverse effect on crop trees (Sanders and others 1987).
Excluding fire or other disturbances like grazing from mature oak stands may have altered the ecology of mixed-oak, cove hardwood, and pine-hardwood cover types to the detriment of advanced oak regeneration (Little 1974, Van Lear and Johnson 1983). Fires every few years may be the key to enabling oaks to become dominant over their associates in the advance regeneration pool. Oak seedlings are less susceptible to root-kill by fire than other species, providing oaks a competitive advantage (Niering and others 1970, Swann 1970, Langdon 1981). The combination of season, frequency, and number of burns to foster oak regeneration in the Appalachians has not been determined but multiple prescribed burns are probably necessary to promote development of advance oak regeneration prior to harvest (Thor and Nichols 1974, Carvell and Tryon 1961, Keetch 1944). Oak seedlings initially may be more readily established on burned areas in part because the openings encourage activity by blue jays. Jays hoard and scatter acorns and they seek out areas of thin litter, low vegetation, and full sunlight to bury nuts (Healy 1988). Germination can be enhanced by fire as weevil and beetle species that prey on germinating acorns are reduced on burned seedbeds (Galford and others 1988). Once established, subsequent fires favor oak seedlings over other hardwoods and single prescribed fires have little effect on species composition in the understory (Johnson 1974, Wendel and Smith 1986, Teuke and Van Lear 1982, Augspurger and others 1987, Waldrop and others 1985).
The task of regenerating oaks, particularly northern red oak, is especially challenging on moist, fertile cove sites. Fire exclusion allows other understory species to compete vigorously with oak seedlings and usually overtop them (McGee 1979). In addition, control of the subcanopy and midstory is necessary to allow enough light to reach the forest floor and favor advance oak regeneration (Van Lear and Waldrop 1988). Fire has been successful for regenerating yellow-poplar on cove sites. This shade-intolerant species is well adapted to fire disturbance. Its light seeds are disseminated by wind and gravity, and they germinate rapidly on fire-prepared seedbeds. Yellow-poplar seeds also remain viable in the forest floor for 8 to 10 years (Little 1967) and will germinate after a fire creates the needed site conditions (McCarthy 1933, Sims 1932, Shearin and others 1972).
Prescribed burning is used in pine stands to control competing hardwoods that develop from root or stump sprouts after harvesting, or that encroach from adjacent areas such as depressional wetlands. Control is desirable to decrease competition for water, nutrients, and growing space; to reduce risk of wildfire damage in stands with palmetto, gallberry or wax myrtle understories; and to aid in stand management and regeneration. In most situations, total eradication of the understory and midstory is neither practical nor desirable. Burning can reduce the size, but not the number of hardwood stems in understories. Effects depend on the frequency and timing of prescribed fires (Thor and Nichols 1974, Waldrop and others 1987). Low-intensity fires generally kill most hardwood stems up to 3 inches in diameter. Summer fires are more effective than winter fires in killing hardwood rootstocks, but numerous summer fires in successive years are necessary (Waldrop and others 1987).
Periodic prescribed burns can control the size of hardwoods, reduce wildfire hazard, and facilitate stand regeneration. Burning at about 5-year intervals controls the size of sprouts developing from top-killed rootstocks. By controlling the size of understory hardwoods, pines can be maintained more easily on sites where they are the species of choice. If not controlled, hardwoods will form a midstory and capture the site once the pine is harvested (Wade and Lunsford 1989). If a large pine component is desired in the next rotation, these unmerchantable stems must be removed during suite preparation at additional expense and risk of compaction.
Prescribed fire shows promise for the control of laurel and rhododendron in the Southern Appalachian Mountains. Fire suppression in this region has resulted in dense stands of these evergreen shrubs. These thickets compete with and substantially limit reproduction and growth of both woody and herbaceous vegetation (Van Lear and Johnson 1983, Swift and others 1993) and therefore are thought to have a major negative impact on hardwood species. Hence, the objective of many prescribed burns in the Southern Appalachians is the control of these evergreen species. Fire initially decreases mountain laurel density, but, in time, laurel regains dominance in the understory because it sprouts quickly after fire or fell-and-burn treatment (Elliott and others 1999). Mountain laurel sprouts grow slowly, however, allowing planted pine and other species to get established in the midstory and overstory before this shrub dominates the understory (Williams and Waldrop 1995).
Some threatened and endangered species require fire to become established and survive. Although the role of fire in the ecology of many threatened and endangered species is not well understood, fire has played an essential role in maintaining most ecosystems in the South (Spurr and Barnes 1980). The once extensive longleaf pine ecosystem was fire maintained (Stout and Marion 1993, Ware and others 1993). The forest mosaic of the Southern Appalachians was largely a product of fire disturbances interacting with complex topography. For example, the grassy balds on the summits of high Appalachian peaks may have been created and maintained by fire (Clements 1936), though other explanations are credible (Whittaker 1956).
Many plants have structural adaptations, specialized tissues, or reproductive features that favor them in a fire-dominated environment. Such traits suggest a close association with fire over a very long period of time. Many endemics are only found the first 1 to 2 years after a fire. Changes in the "natural" fire pattern as a result of attempted fire exclusion have led to dramatic decreases in many of these fire-tolerant or fire-dependent species. Many picturesque flowers, including several orchids, currently listed as threatened or endangered are benefited by fire.
Prescribed burning, however, does not automatically help perpetuate plant and animal species. It may be necessary to burn during the same season in which the site historically burned. The interval between prescribed fires as well as fire intensity may be important. The individual habitat requirements of a species must therefore be understood before fire can be prescribed to benefit that species.
Fires affect vegetation by altering or maintaining successional stages. When fire was more frequent in the South, fire dependent or fire-associated species dominated the overstory and understory of many forest stands. In the absence of fire and other similar disturbances, forests have gradually changed composition from predominantly longleaf pine and pine-hardwood to communities dominated by other pines in the Coastal Plain and hardwoods in the mountains and Piedmont. When fire is excluded or suppressed, fire-intolerant hardwoods compete with pine species. Many threatened, endangered, or sensitive plants are understory or midstory plants of fire-dominated communities. For example, mountain golden heather, turkeysbeard, sandmyrtle, and twisted-head spike-moss grow in the Appalachians on ledge habitats created and kept open by natural fires and severe weather (Morse 1988). Burning also enhances habitat preferences of several endangered animal species, including the Florida panther, gopher tortoise, indigo snake, and red-cockaded woodpecker (Wade and Lunsford 1989).
Prescribed fire improves habitats of certain wildlife species, but, it also degrades habitats for other species. Each of the hundreds of wildlife species in the South responds differently to fire, depending upon the frequency, intensity, severity, and season of burning. To effectively use prescribed fire to benefit wildlife requires an understanding of the habitat requirements of each species (Harlow and Van Lear 1981, 1987, Lyon and others 1978, Wood 1981). Some general guidelines for burning to enhance habitat for game species are show in Table 2.
Prescribed burns to improve wildlife habitat in existing pine stands historically have been conducted in the winter (Mobley and others 1978) to avoid the spring nesting season. Burns are at about 3- to 5-year intervals favor deer and turkey. On the lower Coastal Plain, bobwhite quail are favored by burning at 1- to 2-year intervals. Appropriate burning frequencies for other species are not well known. Low-intensity burns in hardwood or mixed pine-hardwood stands in improve wildlife habitat because increasing sprouting of advance regeneration and stimulation production of herbaceous forage. More intense site-preparation burns can also be beneficial where they increase the abundance of legumes and other herbaceous and perennial plants that are preferred by many wildlife species.
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content: John Stanturf |
created: 21-NOV-2001 |