Silvicultural Systems in the Management of Longleaf Pine

Longleaf pine is a low-risk species to manage. In addition to its BROKEN-LINK 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.)

Silvicultural systems

Clearcutting and seed-tree systems

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.

Shelterwood systems

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.