Silvicultural Systems in the Management of Loblolly Pine

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.)

Seed-tree method

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).

Shelterwood system

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.

Clearcutting method

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.

Selection cutting

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.