Thinning Practices in Southern Pines - With Pest Management Recommendations
United States Department of Agriculture, Forest Service, Technical Bulletin 1703, December 1985.
Beneficial Effects of Thinning
The purpose of thinning is to increase economic gain. The gain may be achieved by offsetting the expense of carrying establishment costs to rotation age, increasing the value of the product, and/or increasing stand utilization. Large trees are more valuable than small ones because they are cheaper to transport and the resulting products have a greater value than those from small trees, particularly ones below sawlog size. Across the South, the value of sawlogs is two to seven times that of an equal pulpwood volume (Holley 1979). The landowner, whose only return on the raw material is the stumpage value, should be particularly aware of this differential when setting long-range management goals or determining end products from the land.
Although thinning is primarily aimed at improving the value of the residual stems, other benefits now being recognized are risk reduction for insect infestations, disease epidemics, and damage from abiotic agents. The mechanics by which thinnning reduces these risks is not fully understood.
Thinning increases the size of individual trees through redistribution and concentration of a site's growth potential on fewer stems (Shepard 1973). Some studies, however, have shown that thinning or wide spacings may not increase total volume and, for short rotations (20 years or less), may decrease total cubic foot volume (Mann 1952; Smith 1967; Wakeley 1969). For rotations of 35 years or more on good sites, the volume differential may be greater in thinned stands (Williston 1979). Again, the value of the wood is 200 to 700 percent greater, depending on local demands.
As a silvicultural practice, thinning concentrates the growth potential of a stand on crop trees and removes suppressed and dying trees. Although stem quality and total utilizable yield may be increased, the effect of thinning may provide marginal economic returns and only limited growth response in the stand over the rotation. Response to thinning is tempered by most of the factors that influence tree and stand growth – species, age, site index, and the number of trees/acre.
Precommercial thinnings/natural stands. Studies have shown large increases in rate and amount of diameter growth of precommercially thinned natural stands with densities of 1,500 or more stems/acre (Balmer and Williston 1973; Mann and Lohrey 1974). Many trees in such stands are unmerchantable, and the stands continue to grow at a reduced rate until competition takes its toll or until they are thinned. After thinning, diameter and basal area increments may triple within 3 years (Bower 1965). Maximum growth of residual trees occurs when thinning takes place at the onset of competition. Increases in height increment following thinning may also be significant in these very dense stands.
Perhaps the most significant thing noted in the precommerical thinning of natural stands is the importance of stand age at the time of thinning. Responses have been noted in first thinnings from 2 to 15 or more years of age. However, stands thinned at age 2 or 3 years have a greater time for response than do older stands. Delaying precommercial thinning can sharply reduce growth and increase costs.
Precommercial thinnings/plantations. Density control is one of the advantages of planting over direct seeding or natural regeneration. In the South, plantings seldom exceed 890 trees/acre. Consequently, with current spacings and where multiple products are desired, precommercial thinning is usually unnecessary. If thinning is undertaken in precommercial stands, it should be done early, certainly before age 10, to reduce operational costs and obtain the greatest growth response from the residual trees.
Increased utilization through the removal of suppressed trees increases economic gain from managed forests. Intermediate and suppressed pine trees can be utilized for pulp. Usually, they are taken with the larger codominant or dominant stems in a "free" thin, common in southern forest management. A "low" thin that takes only intermediate or suppressed trees would be of minimal commercial value. Their removal would do little to reduce competition between dominant and codominant trees. Consequently, economic gain is probably minimal with this type of low thin.
Maintaining proper stand density has been long recognized in the South as a means of reducing insect damage. Bennett (1968) presented general guidelines for identifying areas of high risk to southern pine beetle (SPB) attack and for reducing potential losses. Good forest management was his message, and maintaining proper stand density was the most important of several silvicultural recommendations. Dense stands with slow radial growth and reduced tree vigor were subsequently shown to be highly susceptible to SPB attack (Bennett 1971; Hicks et al. 1978). Most hazard-rating systems assign heavy weight to the pine basal area component (Ku et al. 1980; Lorio 1978).
The message from hazard-rating systems is clear. Healthy trees are less susceptible to attack by SPB than unhealthy ones. Uninfested trees are generally larger, have thicker bark, greater crown/bole ratios, larger crowns, and faster growth rates, and occur in less dense stands. Ku et al. (1980) noticed this difference between noninfested trees and trees infested with SPB in Arkansas. The infested trees were usually located in heavily stocked stands that were under stress. Lightning strikes and logging damage contributed significantly to the stressed state and susceptibility to attack, but high stand density was the most important factor predisposing stands to SPB attack. Therefore, he recommended that basal areas be maintained at not more than 100 square feet/acre on good quality sites. Others have given similar recommendations with the allowable basal area decreasing to 80 square feet/acre on lower quality sites (Belanger 1980; Hicks et al. 1979). Lorio (1980), using stocking levels rather than stand density, showed that loblolly pine stands infested with SPB were overstocked (i.e., over-crowded).
From these early recommendations up to the present, good forest management with scheduled thinning has continued to be recognized as a means of maintaining healthy stands and promoting resistance to SPB attack.
Some genetic improvement may also be achieved through thinning. Trees removed in thinnings from below are usually less vigorous (not growing as well), diseased, or have undesirable form, sometimes due to genetic factors. By removing such trees prior to regeneration of the stand, the forester can minimize undesirable traits.
Thinning changes the environment of the forest. The penetration of light, the temperature of mineral soil, and the availability of moisture and nutrients are all increased (Blair 1969). Understory vegetation quickly responds to these changes, producing a more favorable habitat for wildlife and cattle (Blair 1967; Halls and Schuster 1965). A relationship between forage increase and reduced basal area has been demonstrated. The diminished canopy that results from thinning allows greater amounts of rain to reach the forest floor, which increases the quantity of water from the watershed (Rogerson 1968; Ursic 1974).
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