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Prescribed fire is used to various degrees in the management of southern pines. The following sections summarize information from Silvicultural Systems for the Major Forest Types of the United States (Burns 1983) and provide links to on-line information sources such as Silvics of North America (Burns and Honkala 1990) and the Fire Effects Information System.
Encyclopedia ID: p585
The natural range of longleaf pine (Pinus palustris Mill.) includes most of the Atlantic and Gulf Coastal Plains from southeastern Virginia, south to central Florida and west to eastern Texas, with extensions into the Piedmont, Ridge Valley, and Mountain Provinces of Alabama and northwest Georgia.
The longleaf pine forest is considered a fire subclimax type that has developed and maintained itself in close association with periodic fires. The species is the most fire-tolerant of the southern pines. It is also very sensitive to competition, especially as seedlings. This has restricted longleaf largely to sites that have been periodically burned and to poor sites supporting only a sparse cover of competing vegetation. Longleaf pine is a pioneer species that, given an adequate seed source, can invade abandoned fields or areas cleared by a catastrophic event such as blowdown or severe wildfires.
Once established, longleaf pine tends to perpetuate itself in areas where fires occur frequently. Needle litter from overstory pines helps to support hot surface fires. These fires slow or prevent the encroachment of hardwoods and other pines; they also provide a favorable seedbed by removing accumulated litter and exposing mineral soil. Grass-stage longleaf seedlings are highly resistant to fire and can even tolerate growing season fires in the open or under a light pine overstory. But under a medium to heavy pine overstory, most seedlings cannot survive the combination of slow growth resulting from overstory competition plus hot fires fueled by abundant needle litter. Therefore, longleaf pine usually originates in openings or under a light overstory where less intense fires still suppress hardwoods but do little harm to vigorous longleaf seedlings.
Longleaf pine can tolerate prescribed fire at all ages, except for young seedlings less than about 0.3 inch in root-collar diameter. Therefore, this species is better adapted to uneven-aged management than any other southern pine, as regular burning can be used to control hardwood competition. Although this type of management may best suit the goals of some landowners, especially those with a limited acreage, even-age management is, and will continue to be, a predominant form of management for longleaf pine.
Since fire is such an integral part of longleaf pine management, the management unit, where an even-age stand is established and maintained, should also be a convenient burning unit, bounded by roads and streams, to minimize the length of maintained firebreaks.
For additional information on the use of prescribed fire in the silviculture of longleaf pine, see:
Encyclopedia ID: p591
Longleaf pine is a low-risk species to manage. In addition to its fire tolerance it is also rarely bothered by the serious diseases and insects that afflict the other major southern pines. Site requirements are not demanding, and it can grow well on droughty, infertile soils. Once out of the seedling stage, mortality is low. Suppressed trees may eventually die, but the greatest single cause of loss is lightning, frequently followed by bark beetle attack (Ips spp.). Mortality from all causes among dominant members of maturing longleaf stands has averaged about one tree per 2.5 acres (1 ha) annually, and for half of the observed stands averaged one tree or less per 5 acres (2 ha) annually (Boyer 1979).
The major management problems for longleaf pine, in common with other southern pines, are associated with stand regeneration, especially natural regeneration. Problems with artificial regeneration are less imposing, so this approach is preferred despite its high cost. Natural regeneration requires effective competition control and seedbed preparation, which can be achieved only through broadcast cultural treatments using mechanical equipment, chemicals, or fire. With the exception of fire in longleaf stands, such treatments are nearly impossible to apply to a management unit comprised of pines of all ages and sizes. Longleaf pine can tolerate prescribed fire at all ages, except for young seedlings less than about 0.3 inch (0.8cm) in root-collar diameter. Therefore, this species is better adapted to uneven-aged management than any other southern pine, as regular burning can be used to control hardwood competition. Although this type of management may best suit the goals of some landowners, especially those with a limited acreage, even-age management is, and will continue to be, a predominant form of management for longleaf pine.
Most natural second-growth stands are even-aged, dating back to the time the old-growth overstory was removed. The association of regeneration with a catastrophic event (land clearing, blowdown, logging, hot fire, etc.) led to the predominance of even-age stands, often of considerable extent. Since fire is such an integral part of longleaf pine management, the management unit, where an even-age stand is established and maintained, should also be a convenient burning unit, bounded by roads and streams, to minimize the length of maintained firebreaks.
Rotations selected for longleaf pine depend on management objectives, site quality, cultural treatments, and thinning schedules, but usually range from 60 to 80 years for sawtimber. Thinning is important if management objectives are sawlog-sized products. Some results indicate that on a medium site the periodic cubic-foot growth of 35- to 40-year old longleaf pine does not increase much with increasing stand density above 60 square feet of basal area per acre (13.8 in2/ha). Periodic thinning from below, to leave about 10 square feet per acre (16.1 in /ha) of best dominants, costs little in potential volume growth, concentrates growth on quality crop trees, and minimizes the investment in growing stock (Farrar 1968).
(For background information, see Silviculture.)
Clearcutting, seed-tree, and shelterwood systems have all been applied to longleaf, but serious drawbacks have eliminated clearcutting and seed-tree systems as practical alternatives for natural regeneration (Croker 1975). Clearcutting a mature stand can destroy much of the advanced reproduction, if any is present. Because of past difficulties in successfully planting longleaf pine, loblolly, slash, or sand pine frequently were substituted following clearcutting. Increasing success with containerized and bare root seedlings has overcome some of these planting problems.
The limited seed dispersal range requires that most of the cleared area be within 100 feet (30.5 in) of a seed source. If there is an extended wait for a seed crop, the growing space will be occupied by low quality hardwoods and brush that must be eliminated, at some cost, to prepare for a seed crop that may or may not be adequate to regenerate the area. With 8 to10 scattered seed trees per acre (20 to 25/ha), the land is essentially out of production during the wait for a good seed crop. Even with periodic burning, the lower fire intensity resulting from a lack of heavy needle-litter fuels permits some encroaching hardwoods to regularly escape into a relatively fire-resistant size. When too much of this occurs, mechanical or chemical site preparation is required.
Early observations of longleaf regeneration in nature indicated that some form of shelterwood system for natural regeneration might be best suited to this species (Croker 1956). Several advantages are immediately apparent. Final harvest of mature crop trees is delayed until adequate advanced reproduction is present on the site. This keeps the site in production with growth occurring to high-quality dominants while waiting for a seed crop. Shelterwood stands produce enough needle litter to fuel the hot fires needed to restrict hardwood and brush encroachment, and maintain an understory comprised largely of grasses and forbs. The presence of a shelterwood overstory also inhibits development of the brown-spot needle blight on established seedlings (Boyer 1975). An overstory of 30 to 40 square feet of basal area per acre (6.9 to 9.2 m2/ha) maximizes seed production and in a good seed year produces three times as many seeds as a seed tree stand.
The shelterwood system can be applied only in existing stands with sufficient dominant-codominant trees of seed-bearing size. Guidelines for use of the system have been published (Boyer 1979, Croker 1975). Briefly, either a two-cut or three-cut shelterwood system may be applied, the latter only in stands needing a thinning or improvement cut. The first cut in a three-cut system would be a preparatory cut. This should leave 60 to 70 square feet of basal area per acre (13.8 to 16.1 m2/ha) in dominant and codominant trees. Removal of all other trees permits crown development on the residuals. A well-managed longleaf stand periodically thinned will not need a preparatory cut. The first cut in the two-cut system (second in the three-cut system) is the seed cut. This cut is made 5 or more years ahead of the planned harvest date and reduces stand density to about 30 square feet of basal area per acre (6.9 m2/ha), leaving the best dominant trees. Residual large hardwoods are also removed. Even though stand density may be cut in half, per acre volume production of 50 to 70-year old trees would be reduced by only about one-third (Farrar 1979). Regular prescribed burning keeps down hardwoods. Growing season burns may be needed where brush is heavy. Usually a seedbed burn the year before a good seed crop is the only site preparation that is needed.
Seed crop prospects are monitored through annual springtime counts of flowers and conelets on selected sample trees in the regeneration area. Normally 750 to 1,000 cones per acre (1855 to 2470 cones/ha) are needed for regeneration, although two or three successive lighter cone crops combined may do the job. Stocking surveys of established seedlings should be made annually, beginning after the seed cut. Sometimes an adequate stand of seedlings is already present on the site, so further measures to obtain regeneration are unnecessary.
The regeneration goal should be establishment of about 6,000 seedlings per acre (14 830 seedlings/ha) under the shelterwood overstory. Distribution should be such that 75 percent or more of milacre (0.001 acre or 0.0004 ha) sample plots are stocked with one or more seedlings. This number will allow for logging losses and still provide enough seedlings so that the superior 10 to 20 percent can supply all the crop trees. Regeneration success must be based on seedlings at least one year old, due to the high risk of mortality during the first year after establishment.
Once an adequate seedling stand is established, the parent overstory can be removed. Prompt seedling release is not required for survival, so harvest of the overstory can be scheduled to meet the needs of management. Given a choice, overstory removal at seedling age 1 or 2 will have the least impact on the new stand (Boyer 1975).
The regeneration area should not be burned during the first two years after overstory removal, as accumulated logging slash and undecomposed litter can result in a fire too hot for newly released seedlings. After 2 years, seedling growth plus decomposition of organic debris reduces fire risk considerably. Prescribed burning can be resumed and applied as needed for control of brush and the brown-spot needle blight. A brown-spot survey sampling only crop seedlings will indicate if a burn is needed (Croker 1967). If these seedlings have an average of 20 percent or more of their foliage destroyed by the disease, a winter brown-spot burn should be prescribed (Croker and Boyer 1975). Spring burns may be most beneficial to the seedling stand during the early years after release. These burns not only kill hardwoods more effectively than fires in any other season but also may stimulate longleaf seedling height growth more than burns at other seasons or no burns at all (Grelen 1978, Maple 1979).
Precommercial thinning usually is not needed in longleaf stands, so the first commercial thinning brings the stand toward the desired density for optimum future growth. Dominant crop trees can be easily identified and leave trees marked for this and all subsequent thinnings. All other trees are removed during thinning if they have reached commercial size. Otherwise they are left until the next thinning.
All available information indicates that in the past, the longleaf pine forest type was maintained primarily as a result of wildfires that periodically burned through established longleaf forests. Exclusion of fire leads to serious regeneration problems, as past experience amply demonstrates. Management of longleaf forests should include prescribed fire at 2- to 5-year intervals through the rotation, as needed to prevent hardwood encroachment and excessive risk of wildfire damage through build-up of fuel on the forest floor.
Encyclopedia ID: p601
The longleaf pine forest is considered a fire subclimax type that has developed and maintained itself in close association with periodic fires. The species is resistant to fire. It is also very sensitive to competition, especially as seedlings. This has restricted longleaf largely to sites that have been periodically burned and to poor sites supporting only a sparse cover of competing vegetation. Longleaf pine is a pioneer species that, given an adequate seed source, can invade abandoned fields or areas cleared by a catastrophic event such as blowdown or severe wildfires (Boyer 1979, Eyre 1980).
Once established, longleaf pine tends to perpetuate itself in areas where fires occur frequently. Needle litter from overstory pines support hot surface fires. These fires slow or prevent the encroachment of hardwoods and other pines; they also provide a favorable seedbed by removing accumulated litter and exposing mineral soil.
Grass-stage longleaf seedlings are highly resistant to fire and can even tolerate growing season fires in the open or under light pine overstories. But under medium to heavy pine overstories, most seedlings cannot survive the combination of slow growth resulting from overstory competition plus hot fires fueled by abundant needle litter. Therefore, longleaf pine usually originates in openings or under light parent overstories where less intense fires still suppress hardwoods but do little harm to vigorous longleaf seedlings.
Reduction in the frequency or exclusion of fire leads to substantial changes in the longleaf pine ecosystem. The open, park-like longleaf forests, with an understory comprised mainly of grasses and forbs, is invaded and eventually superceded by hardwoods as succession on these upland sites moves through pine-hardwood types toward eventual dominance by climax hardwoods. Dominance by other pines may precede hardwoods where presence of a slash pine (Pinus elliottii Engelm.) or loblolly pine (P. taeda L.) seed source permits these species to gain a foothold on longleaf uplands. In the absence of fire, the grass-stage of longleaf pine seedlings gives them a great competitive disadvantage in the presence of the fast-growing seedlings of other pines and hardwoods.
Encyclopedia ID: p602
Slash pine occurs naturally within 150 miles of the Atlantic and Gulf Coasts from South Carolina, to Louisiana, and throughout most of Florida. It is represented by two varieties: from central Florida northward by the common variety, typical slash pine (Pinus elliottii Engelm. var. elliottii), and in southern Florida by the South Florida strain (Pinus elliottii var. densa Little and Dorman).
Virgin stands of slash pine were largely confined to ponds or pond margins and to narrow strips along creeks, bays, and other minor drainages where ample soil moisture or standing water protected the young trees from wildfires, and to hummocks in the Everglades that provided sufficient elevation above the water level for pines to grow.
Fire is both an agent of destruction and a valuable tool in the management of slash pine. Young trees are quite susceptible to injury by fire until they are 10 to 15 feet tall and have bark thick enough to insulate the cambium from lethal temperatures. Even mature stands may be damaged by head fires in areas with large fuel accumulations. The principal uses of fire in slash pine management are for site preparation, hazard reduction, and control of understory vegetation. Fire also improves forage production for livestock and habitat for wildlife.
For additional information on the use of prescribed fire in the silviculture of slash pine, see:
Encyclopedia ID: p595
Slash pine may be regenerated naturally in uneven-aged stands by the group-selection method and in even-aged stands by either the seed-tree or shelterwood system. Regeneration can also be achieved in even-aged stands by clearcutting with seed- or seedlings-in-place (Jones 1969, Langdon and Bennett 1976, Langdon and Schultz 1973). Because the species is intolerant, or nearly so, even-aged systems are preferred. Another reason for favoring even-aged systems is that they permit the use of prescribed fire to control stand composition and understory development.
(For background information, see Silviculture.)
Both the seed-tree and shelterwood systems entail removal of the overstory in two stages. They differ from each other only in the number of trees retained to regenerate the area, 6 to 10 trees per acre (15 to 25 trees/ha) in the seed-tree method and 25 to 40 trees per acre (60 to 100 trees/ha) in the shelterwood method. These trees should be full-crowned, disease-free dominants that are proven cone producers. They should be well spaced over the area to insure adequate seed dispersal.
Areas being regenerated by these methods should be prescribe burned prior to the harvest cut to control understory vegetation and prepare the seedbed. This burn should be made no earlier than the winter immediately preceding dispersal of a seed crop adequate to restock the area. Mechanical site preparation may be substituted for prescribed burning to ready the site for natural regeneration. But care must be taken to avoid injury to roots of the seed trees. Mechanical injury to roots renders the trees susceptible to attack by insects and disease organisms. Seedbeds deteriorate rapidly and must be refreshed if an adequate stand of seedlings is not established within about 2 years. The seed or shelterwood trees should be retained for 1 year after an adequate seed crop as insurance against drought losses during the critical first summer following germination. They should be harvested as soon thereafter as practical, certainly within 3 to 5 years, to insure vigorous growth of the new crop and to minimize logging damage to seedlings (Feduccia 1978).
Occasionally an adequate stand of natural regeneration becomes established under lightly stocked mature stands where there has been no preparatory cut. Under these conditions the overstory should be harvested immediately to promote growth of the seedling stand. A variation of this clearcutting with seedling-in-place system is clearcutting with seed-in-place. In this method, the overstory is harvested during or immediately after seed dispersal. The system is less reliable than other even-aged systems, because the seed source is removed before the new stand is established. Widespread success in direct seeding of slash pine suggests that this disadvantage may not be serious if the seed crop exceeds approximately 5 pounds per acre (5.6 kg/ha) (Derr and Mann 1971). Another disadvantage of this system is that seed crops must be estimated from flower or conelet counts unless the site preparation bum is made in late summer, after cones are fully grown.
Successful use of the group-selection method of unevenaged management requires the harvesting of groups of mature trees to create openings about 1 acre (0.4 ha) in size. Larger openings are impractical because the seeds are not dispersed over large distances. Smaller openings are undesirable because the surrounding overstory trees retard seedling growth. Area-wide stand management treatments, such as prescribed burning, are not practical with uneven-aged management.
The preferred method of regenerating slash pine among large forest landowners is clearcutting followed by chemical or mechanical site preparation, then direct seeding or planting of nursery or container-grown stock. In 1981, plantations comprised 56 percent of the acreage in slash pine forest types in North Carolina, South Carolina, Georgia, and Florida.
The proportion of slash pine acreage established by artificial methods is probably higher in the Gulf region than in the Southeast because of widespread planting of the species north and west of its natural range. With increasing availability of genetically superior stock, planting should continue to be the preferred method of regeneration of slash pine.
Encyclopedia ID: p598
Loblolly pine (Pinus taeda L.) is the most important forest species in the southern United States. It is a dominant pioneer species in secondary succession on the uplands; however, in later stages of succession it is commonly found with and often replaced by a broad spectrum of hardwood species when the successional trend is unchecked. While oaks, gums, and hickories are the most common associates, species composition varies by soil, moisture regime, and topographic position.
Although young loblolly pine stands less than 12 to 15 feet tall are highly susceptible to wildfire, prescribed fire is an excellent management tool for stands over 15 feet tall. Prescribed fire is effective for manipulating understory vegetation, reducing excessive fuel (hazard reduction), disposing of logging slash, preparing planting sites and seedbeds, and improving wildlife habitat. Responses of the understory to prescribed fire vary with frequency and season of burning. Periodic winter burns will keep the hardwood understory in check, while a series of annual summer burns will usually reduce vigor and increase mortality of the hardwood rootstocks.
While prescribed fire can be used in loblolly stands, care needs to be taken not to stress the trees. The added stress from burning may induce or exacerbate the effects of southern pine beetles. This is especially critical when burning dense stands or when using growing season burns.
For additional information on the use of prescribed fire in the silviculture of loblolly pine, see:
Encyclopedia ID: p590
Either even-aged or uneven-aged management systems can be used with loblolly pine. Even-aged stands can be established by natural or artificial regeneration methods. Uneven-aged stands are usually maintained by selection cutting methods that promote natural regeneration. Even-aged management is generally more practical and economically efficient on such large holdings as industry lands and the National Forests. Even-aged methods allow cultural operations to be concentrated in time and space and permit management areas to be easily defined and treated. The most common regeneration method used in the last decade on most southern forest lands has been clearcutting and planting. Planting has proven to be a reliable regeneration method for loblolly and has the advantages of providing control of spacing and stocking, providing for rapid and timely restocking, allowing the use of genetically improved seedlings, and permitting shorter rotations. Also, planting is not dependent on an on-site seed source. However, clearcutting, followed by site preparation and planting, is the most costly regeneration method. It requires a large capital investment and there is a long waiting period before financial returns can be realized from a given stand. But for large forest tracts, regulation of yields can be set up to provide a steady sustained flow of multiple forest products and income over time.
A variety of silvicultural systems are suitable for loblolly pine. Reproduction cutting methods such as seed-tree, shelterwood, and clearcutting establish even-aged stands while selection cutting develops or maintains an uneven-aged forest.
(For background information, see Silviculture.)
Success of the seed-tree method depends on proper manipulation of the seed supply and seedbed conditions. The reproduction cut should be made prior to seed fall in a good seed year leaving 4 to 12 evenly distributed, well-formed trees per acre (10 to 30 trees/ha). The number of seed trees left depends on tree size and site conditions. The seed trees should be at least 10 inches (25.4 cm) d.b.h., but preferably 12 to 16 inches (30.5 to 40.6 cm) (Williston and Balmer 1974). Crown release of the seed trees 3 years before the main harvest cut can increase seed production of loblolly pines that have been grown in closed stands. Disking prior to seed fall or prescribed burning in advance of the reproduction cut will prepare a seedbed and assist in controlling small hardwoods. Larger hardwoods are often controlled with herbicides. Well-stocked stands usually result if adequate seed fall occurs within a year after seedbed preparation and the reproduction cut. Delayed seed fall may require the receptive seedbed to be maintained-normally by prescribed fire. The seed trees should be removed within 3 to 5 years after adequate reproduction has become well established. This would be approximately 1,000 well-distributed seedlings per acre (2470 seedlings/ha).
The shelterwood system has been most successful in the central and eastern parts of the range, where greater summer rainfall enhances seedling establishment. A two-cut shelterwood system is normally used in loblolly pine. In the first cut, all but 20 to 30 of the best seed trees per acre (49 to 74 trees/ha) are removed. The leave trees should comprise 20 to 30 square feet of basal area per acre. (4.6 to 6.9 m2 /ha). Prescribed burning is the most practical method of site preparation and control of small hardwoods, and should be done before the first cut is made. The overwood, which helps retard the growth of competing hardwoods, is usually removed as the second cut within 5 years after establishment of reproduction. Shelterwood cutting can result in too much reproduction, particularly when a good seed crop occurs following intensive site preparation. If overly dense stands develop they should be precommercially thinned within 3 to 5 years after overstory removal.
The clearcutting method can be used to naturally regenerate small blocks, patches, or narrow strips, if there is an available seed source from adjacent stands. The longer axis of the clearcut areas should be made perpendicular to the direction of prevailing winds and in most cases the clearcut should not exceed 300 to 400 feet (approximately 90 to 120 m) in width to insure adequate seeding over the entire area. Larger clearcut areas can be regenerated naturally with either seed- or seedlings-in-place. Seed-in-place involves clearcutting the stand after peak seedfall but prior to germination. Thus, logging must be done during the late fall and winter following a good seed fall.
If the management objectives are to maintain an unevenaged stand, where seedlings, saplings, pulpwood, and small and large sawtimber are all represented, and to harvest at relatively frequent intervals, selection cutting is the best alternative. The selection method involves periodic cutting, at 5 to 10 year intervals, of selected trees from all merchantable diameter classes. In fully stocked stands, harvest-cut volumes generally approximate growth for the cutting period or cutting cycle. In stands that are not fully stocked, only a portion of the periodic growth is cut. Trees selected for harvest can be single isolated trees or groups of trees. If regeneration is not necessarily needed following a particular harvest cut, single-tree selection is suitable. However, if regeneration is badly needed, the group selection or a combination of group and single-tree selection may be required. In many cases, site and seedbed preparation is achieved by the logging operation and the use of chemicals to control larger competing hardwoods. In some cases more intensive control of competing vegetation is required. This can be accomplished mechanically with chemicals and sometimes with prescribed fire. However, blanket application of prescribed burning or nonselective herbicides should be avoided where new regeneration is present. Throughout much of the loblolly pine range, uneven-aged management is an effective means of rehabilitating understocked stands and is especially suitable for small forest properties. It requires a low capital investment, provides periodic income without interruption for stand regeneration, permits net income to be spread out over time, and affords a reserve of large timber to take advantage of favorable market conditions.
Silvicultural systems that depend primarily on natural regeneration have some inherent shortcomings that must be accepted. Natural regeneration results in less control over spacing and initial stocking than does planting and some forms of direct seeding. The irregular stands that develop can cause problems with mechanical harvesting and provide poor access for fire equipment. If overly dense natural stands become established, precommercial thinning is often required to maintain maximum growth of crop trees and promote early sawlog production. In addition, managing for natural regeneration does not permit the use of genetically improved stock; however, it does permit leaving seed trees of good form, fast growth, and disease resistance, all of which contribute to stand improvement. Despite the shortcomings of natural regeneration systems, they can be used effectively and cheaply in many situations. For example, on those nonindustrial holdings or on sensitive sites where the vigorous and expensive site preparation measures required for artificial regeneration are not feasible, natural regeneration systems may be the best alternative.
Encyclopedia ID: p603
Shortleaf pine (Pinus echinata Mill.) is the most widespread of any pine in the southeastern United States. It extends from southeastern New York and New Jersey into Pennsylvania, southern Ohio, Kentucky, southwestern Illinois, and southern Missouri, south to eastern Oklahoma and eastern Texas, and east to northern Florida and Georgia. The adaptability of shortleaf pine to a great variety of site and soil conditions partly accounts for its wide occurrence.
Shortleaf pine is generally fire resistant, although wildfires in young stands are very damaging, since they easily extend into the crowns. The crowns are usually killed, but shortleaf pine seedlings and small saplings can sprout from the base and form new stems. Larger trees may be killed by very hot fires, particularly if there is a large amount of fuel near the tree base. Fire-damaged trees are also more susceptible to attack by insects and diseases. After the trees grow to large sapling- or small pole-size, prescribed fire can be used to reduce both understory competition and wildfire hazards. Under proper conditions, fire can be used to improve wildlife habitat and is used extensively for site preparation for both natural and artificial regeneration.
Shortleaf pine is generally managed in even-aged stands, primarily because it is intolerant of shade. In the past, fire undoubtedly played an important role in establishing and maintaining the species.
For additional information on the use of prescribed fire in the silviculture of shortleaf pine, see:
Encyclopedia ID: p594
Shortleaf pine is generally managed in even-aged stands, primarily because it is intolerant of shade. In the past, fire undoubtedly played an important role in establishing and maintaining the species. Both natural and artificial regeneration methods are used to establish even-aged stands. In recent years management has shifted from dependence on natural regeneration to planting and direct seeding. Artificial regeneration methods reduce the time required for establishment, provide better control of spacing, and allow establishment of genetically improved trees. Since fast-growing, better formed, pest-resistant strains of shortleaf pine are now generally available, artificial regeneration will likely predominate.
(For background information, see Silviculture.)
Even-aged stands of shortleaf pine are established artificially by planting or direct seeding. Both of these options require removal of all merchantable trees, control of competing vegetation, and, for direct seeding, removal of litter. The most common site preparation procedure following clearcutting is chopping or crushing of residual trees followed by a hot fire to control hardwoods, reduce ground cover, and expose mineral soil. Residual trees also may be sheared and burned or sheared and windrowed. The prepared sites are then machine- or hand-planted, or seeded.
Trees planted in cutover areas are susceptible to damage and loss by pales weevils. To avoid the problem, seedlings should be dipped in insecticide prior to planting, treated with insecticide at planting time or in late winter, or planting should be delayed 1 year. This delay, however, results in the loss of a years growth and allows competing vegetation to develop 1 year ahead of regeneration.
Establishment of even-aged stands with natural regeneration may be accomplished by clearcutting strips or patches no greater than 200 feet (61 in) wide to allow seeding from nearby trees, felling the entire stand after seed fall or cone ripening, leaving seed trees, or shelterwood cutting (discussed below; Mann 1973). Of these natural regeneration methods, clearcutting strips or patches is most compatible with the use of fire and heavy equipment to control competing hardwoods and prepare a suitable seedbed. A major disadvantage of depending on natural regeneration is the lack of adequate seed production every year, especially in the inland and westerly areas. Sufficient seeds must be available soon after site preparation or development of competing vegetation will necessitate retreatment of the area (Mann 1973).
The second method, felling the entire stand in conjunction with cone and seed maturity, has been successful in some areas of the Southeast. Seedbed preparation is completed in advance of cutting, and seeds from the existing trees regenerate the area. One disadvantage of this method is that harvesting must be done after cone ripening and must be completed before seed germination in late winter or early spring. Seeds will be better distributed over the area if most are allowed to fall before harvesting. Regeneration can be obtained by cutting the trees between cone ripening and seed fall, since the cones usually remain in the slash instead of being removed during logging (Mann 1973). A second major disadvantage is that the method can only be used when an adequate seed crop is present.
The seed-tree method involves leaving 10 to 16 of the best seed trees per acre (25 to 40/ha) to regenerate the area after harvesting. Where stands are dense, crown release of the seed trees about 3 years before the main harvest will improve seed production. Well-stocked stands should result in 1 to 3 years if seed fall is adequate. Mechanical site preparation can be used, but care must be taken to avoid damaging the seed trees, which could induce disease or insect attack. Prescribed burning can be done before or after the main stand is cut, but requires great care. Disturbance during logging may provide adequate seedbed preparation (Yocom and Lawson 1977). Seed trees are also more vulnerable to loss or damage from lightning, wind, and insects than those in fully stocked stands. The loss may be substantial since these are high value trees and are the only source of natural regeneration.
The shelterwood system has been successful, particularly where summer rainfall is sufficient for good first-year survival and where half of the stand or approximately 50 to 60 square feet of basal area per acre (11.5 to 13.8 m2/ha) remains. Prescribed burning is the most practical method of site preparation because the residual stand is too dense for easy operation of equipment. In many areas, litter decomposes rapidly, thus eliminating the need for burning. Chemical hardwood control, if needed, is usually easier to accomplish before the initial harvest, but may be done afterward.
Shortleaf pine can be managed in uneven-aged stands with the selection silvicultural system, an alternative that may be especially attractive to managers of small tracts (Reynolds 1969, Williston 1978). The selection harvesting method utilizes natural regeneration and involves removal of trees singly or in small groups with the objective of achieving a balanced uneven-aged structure. This structure is determined by the selected basal area, diameter, and number of trees in each diameter class--all of which depend on the land managers objectives, site quality, and other factors. Selection harvesting has been primarily used in understocked stands where cutting of trees and controlling hardwoods has created openings large enough for reproduction to become established (Reynolds 1969). Usually, the primary product target is sawlogs, but smaller trees are removed to achieve the desired number of trees in each diameter class. The selection harvesting method could be used to grow smaller trees as a primary objective if enough trees (10 to 15/acre or 25 to 37/ha) of seed-bearing size are produced, although it would probably not be very efficient for simple fiber production. In general, the method is more difficult to use, requires more care, and may be economically less efficient than alternative methods.
Fire generally cannot be used with the selection harvesting method for seedbed preparation since seedlings in nearby openings would be killed. Openings may also be too small for efficient use of machines, therefore seedbed preparation is usually limited to disturbance during logging. Regeneration must be obtained promptly or expensive hand methods may be needed to maintain control of encroaching hardwoods.
Regeneration systems depending primarily on natural regeneration have several additional disadvantages. Loss of growth during the extended establishment time can he substantial, since seed crops may not occur for 3 to 4 years. Also, they are not amenable to establishment of genetically improved trees, and there may be problems with either too few or too many trees per unit area. Too few trees may require additional site preparation and artificial regeneration, while too many will likely require precommercial thinning.
Despite the shortcomings of natural regeneration systems, they can be used effectively and cheaply in some situations, and may be the best choice for small ownerships. Also, the greater expense and soil disturbance usually associated with artificial regeneration may not be justified on sensitive sites or those that have low productive potential.
Encyclopedia ID: p599
Sand pine is native to the droughty, acid, infertile, marine deposited sandhills of Florida and Baldwin County, Alabama. It is represented by two varieties, the Ocala variety (Pinus clausa var. clausa D. B. Ward) found in central Florida, and the Choctawhatchee variety (P. clausa var. immuginata D. B. Ward) which is found along the Gulf Coast of northwest Florida from the Apalachicola River westward into Alabama.
Considerable differences exist between the two varieties and how prescribed fire can be used with each. Because of its thin bark, sand pine is relatively susceptible to fire-caused damage and mortality. The Choctawhatchee variety, however, with its typical sparse understory can be prescribed burned under proper conditions. The evergreen shrub understory of Ocala stands is usually dense enough to shade out grass and low herbaceous cover. Because of this lack of low base fuels, headfires are necessary to burn the understory, but these are too explosive to use without high risk of losing the entire stand. Thus, prescribed burning is impractical in the Ocala type.
For additional information on the use of prescribed fire in the silviculture of sand pine, see:
Encyclopedia ID: p593
Sand pine is best suited to even-aged management. Both form and branch pruning are considerably improved when trees are grown in dense, even-aged stands. Under natural conditions, stands are typically dense, pure, and single-storied, although uneven-aged stands do develop during the initial invasion stage of scrub-oak sites (Britt 1973).
(For background information, see Silviculture.)
Choctawhatchee sand pine can be successfully regenerated by either seed-tree or shelterwood systems (Britt 1973). In both methods of regeneration an initial cut is made to stimulate seed production, followed by a final harvest after adequate regeneration is obtained, normally 5 to 10 years. Disadvantages are the possible loss of trees from Ips beetle attacks after the initial cutting and damage to regeneration during the final cut. Density control can also be a problem with natural regeneration systems.
Because of its serotinous cones, the seed-tree and shelterwood systems are not suitable for regeneration of Ocala sand pine. Attempts have been made to get natural regeneration by using the heat from the sun to open cones in logging slash, but stocking has been below acceptable levels (Price 1973). Burning logging slash to release seeds has also been tried but it gave poor results because available cones (and logging slash) were unevenly distributed and fire destroyed the seeds (Cooper et al 1959).
The most successful system for regenerating Ocala sand pine is clearcutting, site preparation, and direct seeding (Price 1973). Double chopping with a heavy, duplex brush chopper is the preferred method of site preparation because it gives good control of competition and adequate exposure of mineral soil. A prescribed burn may be applied between chops if slash is especially heavy or as the only site preparation on sites with limited competition. Broadcast seeding at a rate of 0.5 to 1.0 pound per acre (0.6 to 1.1 kg/ha) should be done from October through November when soil temperatures are the most favorable for seedling establishment. Some method of covering the seed with a layer of soil 0.25 to 0.75 inches (0.6 to 1.9 cm) thick should be used to reduce seed predation and increase germination. Seeding can also be done with a scarifier-seeder, which uses less seed and gives stocking and spacing control by seeding on patches (Outcalt 1990). Adequate natural seeding occurs on some sites following mechanized harvesting. This is most successful when harvested trees are placed in windrows and delimbed with a flail chain drum on the front of a skidder. The rotating chains distribute the cones across the site, where the heat of the sun reflecting from the sand opens the serotinous cones and releases the seed. Ocala sand pine can also be planted, but due to its lack of dormancy, survival is generally poor (about 60 percent) and variable (Bums and Hebb 1972, Hebb and Burns 1973).
Unlike Ocala, the Choctawhatchee variety is easily planted with high and consistent survival rates even on sites outside its natural range (Bums 1973). Growth and survival on sandhill sites in Georgia and South Carolina indicate that sand pine will outperform other pine species normally planted on these deep sands (McNab and Carter 1981, Preston and Price 1979). Deep planting is recommended, with seedlings set to a depth that results in the lower branches remaining covered after the soil settles (Bums 1973). Recommended planting densities for 25- to 35-year rotations are 500 to 550 seedlings per acre (1235 to 1360/ha) if no thinning is planned, and 725 to 775 per acre (1790 to 1915/ha) if an intermediate thinning at about age 20 is anticipated.
Many of the sites where sand pine currently is being established are scrub oak-wiregrass areas with no existing seed source. These sites can be converted by underplanting sand pine among the existing vegetation. Growth of seedlings can be substantially increased by release soon after establishment (Bums 1973). This is an economically attractive strategy for landowners with small holdings who may not be able or inclined to make a large investment in stand conversion. On larger areas, double chopping followed by direct seeding or planting is more practical. Because spacing can be controlled, plantation establishment by planting the Choctawhatchee variety is the preferred procedure.
Overstocked stands of sand pine can result from regeneration by seed-tree, shelterwood, or direct seeding. Such stands should receive a precommercial thinning to prevent stagnation and growth loss. Mechanically thinning seedling-or small sapling-sized stands in strips using drum choppers or rotary mowers is the most practical method of reducing their density. Trees in older stands and plantations will respond to thinning (Bums 1978). Thinning in the traditional manner can be used in older stands for regulation of product size. This is more applicable to the Choctawhatchee variety which has better form, smaller branches, and higher wood density, and thus is better suited to sawlog production than is the Ocala variety. Thinning should be done during the dormant season to lessen the risk of bark beetle attacks.
Site preparation for conversion of scrub oak-wiregrass sites reduces available wildlife foods. Some useful seed-producing species will invade and grow on these sites for a few year after chopping, but they soon give way to grasses (Hebb and Burns 1973) Undisturbed strips of scrub vegetation can be left in larger plantations to increase wildlife use (Bums and Hebb 1972). In some cases these can be provided along natural drainages. Even dense stands of Ocala sand pine contain many understory shrubs that provide mast and forage for wildlife. Production can be increased by clearcutting or thinning (Harlow et al 1980). Prescribed burning every 3 to 4 years will improve the quantity and quality of forage under Choctawhatchee stands (Lewis 1973).
Encyclopedia ID: p600
Virginia pine (Pinus virginiana Mill.) is widely distributed in the northern Piedmont and the foothills and lower elevations of the Appalachian and Allegheny Mountains. It is also found on the western edge of the Coastal Plain in New Jersey and New York.
Extreme care needs to be taken when using prescribed fire in Virginia pine stands. It has a thin bark and is easily damaged or killed by fire. Fire should not be used for seedbed preparation or hardwood control. The only time the use of fire can be considered would be after a harvest cut under even-aged management. Clearcutting is best adapted to this species silvical characteristics. Clearcutting allows full sunlight to reach the new trees, produces a stand of the same age and height that protects the trees from windthrow, and allows the use of fire in site preparation. Fire may also be used with strip cutting a stand if the strips are 100- to 400-feet wide. Slash on these cut areas can be burned and hardwood control measures applied. Seedbed preparation using logging equipment or fire is applied to the harvested strips. Seed is supplied by trees in the adjacent uncut strips. After 3 to 10 years the uncut areas are harvested. Regeneration of the uncut strips is more difficult. A light fire can be used to prepare a seedbed in the uncut strip but damage to residual trees may negate the benefit of the fire if mortality results.
For additional information on the use of prescribed fire in the silviculture of Virginia pine, see:
Encyclopedia ID: p596
Because of its silvical characteristics, Virginia pine should be grown in even-aged stands (Sowers 1956). Also, Virginia pine has thin bark and is easily damaged or killed by fire (Carvell 1979, Fenton 1960, Slocum and Miller 1953); therefore, fire normally cannot be used for seedbed preparation or hardwood control, except at the harvest cut under even-aged management.
(For background information, see Silviculture.)
For regenerating Virginia pine stands, clearcutting is best adapted to this species silvical characteristics. Clearcutting allows full sunlight to reach the new trees, produces a stand of the same age and height that protects the trees from windthrow, and allows the use of fire in site preparation. The clearcut area can be planted, seeded, or reproduced by natural regeneration.
Like the seedlings of other southern pines, Virginia pine seedlings must be properly handled and planted and all necessary hardwood control completed (Kundt 1979). Direct seeding may also be used (Sowers 1964). For natural regeneration, Virginia pines prolific seed crop is usually more than adequate and site preparation and hardwood control will increase regeneration success.
Clearcutting in strips or patches also can be used to regenerate Virginia pine. (Patch cutting can also be classified as group selection under uneven-aged management if the patches are small and cut out of the stand over a period of many years. Unless these conditions are met, this cutting method is considered even-aged management.)
In strip cutting, the stand is cut in 100- to 400-foot (30.5 to 121.9 in) wide strips. Slash on these cut areas can be burned and hardwood control measures applied. Seedbed preparation using logging equipment or fire is applied to the harvested strips. Seed is supplied by trees in the adjacent uncut strips. After 3 to 10 years the uncut areas are harvested. Regeneration of the uncut strips is more difficult. A light fire can be used to prepare a seedbed in the uncut strip but damage to residual trees may negate the benefit of the fire if mortality results (Sucoff 1961).
Thinning uncut stands would stimulate production of seeds and establishment of advanced regeneration but risks of windthrow and ice damage would be increased. Perhaps the best procedure for regeneration of the uncut areas is to time the harvest cut in the fall or winter following a relatively good seed crop of the residual stand. Seed shed before harvest or from cones on logging slash provide seed for regeneration. Logging equipment provides some seedbed scarification and exposure of mineral soil to seed in the surface duff. Hardwood control in the uncut strips may be difficult, however, and requires weeding to reduce hardwood competition. Planting or direct seeding could also be used to ensure regeneration of the residual strips or patches.
Shelterwood or seed-tree cuts are generally not recommended for regenerating Virginia pine. The major reason is the susceptibility of Virginia pine to ice or windthrow when released after having been grown under relatively dense stocking levels.
Single-tree selection is not recommended for Virginia pine. As the species is shade intolerant, pure pine regeneration in small openings would not be expected but instead a single-tree cutting would undoubtedly favor hardwood invasion and gradual reduction of the pine-growing stock.
Group selection in the classical sense is also not recommended because the even-age structure of the stand would deteriorate into a gradual transition to hardwood species. Site and seedbed preparation, hardwood control, and thinning would also be difficult to manage.
Hardwood encroachment is a serious problem in the regeneration of Virginia pine. Virginia pine, as a pioneer species, achieves its best growth and development in old abandoned fields. The exposed mineral soil is an ideal seedbed and the competition from native grasses does not seriously restrict establishment. Initial stocking is frequently 1,000 to 3,000 stems per acre (2415 to 7415/ha) at age 3 with gradual reduction to 500 to 400 stems per acre (1235 to 990/ha) at age 40 to 50. Following establishment of Virginia pine on an old-field site, invasion of hardwood species begin as the litter depth increases. These associated hardwood species are relatively tolerant and, although they grow slowly as understory species, they respond to any openings in forest canopy. It is therefore important to exert hardwood control at harvest time. If first-cycle Virginia pine stands are harvested without hardwood control, the second-rotation stand will be mixed pine-hardwood and further cutting of the pine will result in an almost pure hardwood stand.
Hardwood competition may not be a serious problem when regenerating ridge-top stands or stands that are growing on droughty, poor sites. These sites can be maintained by cutting all large hardwoods at the time of the harvest cut and allowing Virginia pine seedlings to compete with smaller hardwood sprouts and seedlings. On the very poorest sites, such as the shale barrens or rocky outcrop, Virginia pine may be one of the few tree species that survive the harsh environment. On these sites, Virginia pine serves primarily to protect the soil, and growth is so poor that the stands are rarely or never harvested.
Land use patterns throughout most of the Virginia pine range have recently changed so that agricultural land is rarely abandoned and allowed to revert to forest growth, and will probably remain for an extended period in crop or pasture production. In the Virginia Piedmont, Virginia pine growth normally exceeds that of shortleaf pine when the two species are found in mixed natural stands. On the poorer sites and more mountainous areas, Virginia pine remains the best suited species. Careful management, hardwood control, and regeneration strategy will be required to continue production from Virginia pine as a source of forest and related products.
Vigorous establishment of Virginia pine has severe limitations on future stand development. Even-age stands of several thousand trees per acre do not express dominance as clearly as other species and the major crop trees may appear as codominants at age 20 to 30 years. Thinning can be used to regulate stand density and increase the average diameter of the stand (Carvell 1966). If thinning is done in Virginia pine stands, it should be completed before age 12 to 15 years. This may be a precommercial thinning. Later thinnings apparently do not result in substantial growth increments on the residual trees. Thus, growth is primarily a function of basal area stocking and site index. On the better sites, relatively high basal areas up to 180 square feet per acre (41.3 m2/ha) have been observed. Thinning these stands reduces the growing stock and net reductions in total volume can be expected. Residual trees do show a diameter response but total volume will not equal the unthinned stand (Belanger and Bramlett 1979, Slocum and Miller 1953). Unless excessive sterns per acre are present, no thinning is recommended for Virginia pine stands that are grown on a 40- to 50-year rotation for small sawlogs and pulpwood on the better Piedmont sites.
Similarly, improvement cuts after stands reach 30 to 40 years would be unlikely to show an economic gain. Stand appearance would be improved by partial cutting but the disadvantages of increased hardwood encroachment and susceptibility to damage by ice or windthrow probably more than offset any economic return. Sanitation cuts could include removal of trees damaged from bark beetles, ice, etc., but again, most forest managers would normally recommend clearcutting the stand if the damage was severe and otherwise avoid partial cuts before harvesting fully stocked stands.
Encyclopedia ID: p597
Pitch pine (Pinus rigida Mill.) occupies a wide geographic range from central Maine to northern Georgia. Competition often restricts pitch pine to soils low in natural fertility. In the Mid-Atlantic and Southern States it generally occurs on ridges, steep slopes with a southwest aspect, and plateaus where soils tend to be shallow.
Pitch pine is outstanding among eastern conifers in its ability to survive all but the most severe fires. Trees will produce new needles even if all foliage is killed, and new sprouts will develop from dormant buds on trees with pronounced basal crook if the main stem is killed by fire. Although this ability is an asset in survival of the species, it can be a liability. Stems deformed by fire and forced to develop new crowns are poorly formed and slow-growing. Also, the ability to survive and recover from such damage may be greater in slow-growing strains that in fast-growing ones (Garrett and Fleming 1983).
Pitch pine is intolerant of shade and can be maintained best in stands by forms of even-aged management such as clearcutting where advance reproduction is present, or the retention of seed-trees where nonserotinous cones are present. Fires have been primarily responsible for the present distribution of this type, but fire is also responsible for the slow growth and characteristic poor form of the species on some sites. Pitch pine can be killed by intense heat and resulting stands, if from sprout origin on older trees, may be inferior to either seeded or planted stands. On the Pine Barrens of New Jersey, it is common practice to burn periodically to eliminate fuel buildup and thereby reduce the danger of uncontrolled wild fires. Prescribed burning is also used to prepare seedbeds before final harvest cuts.
If natural regeneration from seed is expected following harvest operations, it is essential that proper seedbeds be prepared and that openings be no smaller than 1 acre. Either controlled burning or some form of scarification before harvesting has been effective in exposing the necessary mineral soil. Direct seeding on prepared sites will also produce fully stocked stands if moisture conditions are adequate for several years following seeding. Small rodents, birds, and insects consume some seed on exposed sites, but the supply of seed is usually more than adequate for regeneration purposes (Garrett and Fleming 1983).
For additional information, see:
Silvics of Pitch Pine, from the on-line Silvics of North America (Burns and Honkala 1990), provides information on pitch pines habitat (range, climate, soils, topography, associations), life history (reproduction and early growth, sapling and pole stages to maturity); special uses; and genetics.
Pinus rigida, from the on-line Fire Effects Information System, provides a review of the fire ecology, fire effects and management considerations of pitch pine.
Encyclopedia ID: p592
The "fell and burn" technique was developed by Abercrombie and Sims (1986) as a low-cost, post-harvest site-preparation tool for pine-hardwood ecosystems in South Carolina. This technique uses prescribed fire in combination with properly timed felling of unmerchantable trees (Abercrombie and Sims 1986; Phillips and Abercrombie 1987). As the name suggests, "fell and burn" treatment consists of two steps. After clearcutting hardwood or pine-hardwood stands, residual stems over 6 feet tall are felled with chainsaws. Felling is conducted during early spring following full leaf development when carbohydrate reserves in the roots are low. The presence of leaves on the felled trees also serves to speed the drying of small twigs and branches. Burning is conducted in mid-summer within 24 to 48 hours after a soaking rain, to ensure that a residual forest floor and root mat will provide protection against erosion and that heat penetration into the soil will be minimal. Pine seedlings are planted the following winter. In the southern Appalachians, shortleaf pine or white pine is planted, often at a 10 x 10 foot spacing. In the piedmont, loblolly pine is often planted at a wider spacing.
The fell-and-burn technique allows planted pines to become established by controlling hardwood growth (Danielovich and others 1987). Studies have shown that this treatment also increases the density of other non-planted pine species typical of the southern Appalachian sites (pitch pine, Virginia pine) (Vose et al.1994, Clinton et al.1993). In addition to restoring the commercial viability of these stands, this treatment may also be an effective means of restoring pre-existing (i.e., before fire suppression) levels of species composition and productivity (Clinton et al. 1993, Clinton and Vose 2000).
Despite the usually numerous amount of hardwood sprouts following fell-and-burn treatments, pine survival is high (over 90% and 75% after 1 and 4 years, respectively) and pines are taller than competing oaks (Phillips and Abercrombie 1987). Sprouts that develop after chainsaw felling are forced to develop from below ground buds and are less vigorous because they are top-killed. Another advantage of this technique is that burning removes over 65% of the woody fuels less than 3 inches in diameter (Sanders and Van Lear 1987), making the site more accessible for planting. Often the primary objective of the fell-and-burn treatment is to reduce the competitive influence of mountain laurel sufficiently to allow planted white pine and hardwood sprouts to become established. However, Clinton and Vose (2000) point out that prescribed fire rarely completely eliminates mountain laurel, and therefore competition for light and other resources may intensify over time.
Studies have shown that the fell-and-burn technique has few adverse effects on soil. For example, although burns can remove much of the surface forest floor, most of the root mat remains (Danielovich 1986). Root mats act similar to mulch, increasing water holding capacity, reducing evaporation, and preventing erosion. Other studies have shown that the fell-and-burn technique does not cause significant erosion (Van Lear and Danielovich 1988, Swift et al. 1993). Lack of erosion following is attributed to large stems and stumps acting as debris dams, vigorous shrub and herbaceous regrowth, and burning under moist conditions so that the root mat remained intact. Swift et al. (1993) also reported other benefits of this treatment including increased soil moisture and increased soil temperatures. Clinton et al. (1996) found that losses of aboveground N pools were large following a fell-and-burn treatment in the Nantahala National Forest, losses which equaled or exceeded N losses expected from whole-tree harvest. Despite these above-ground losses, forest floor N was 90% of its pretreatment amount. Knoepp and Swank (1993) found that the same burn increased available soil N (NH4) but found little movement of dissolved inorganic N off site during the first year after burning.
The fell-and-burn technique was first developed as a less expensive alternative to pine plantation management that would attract private landowners to put their unmanaged stands into timber production. During 1988, the total cost of regenerating by the fell-and-bum technique was less than $100 per acre, including site preparation and planting costs (Phillips and Abercrombie 1987). More recent estimates of the fell-and-burn technique range as high as $250 per acre; stand replacement fires have been suggested as a lower-cost alternative (Vose and Swank, 1993).
Encyclopedia ID: p589