Thinning Practices in Southern Pines - With Pest Management Recommendations
United States Department of Agriculture, Forest Service, Technical Bulletin 1703, December 1985.
Thinning Practices in the Southern Pines
Insect problems intensify as stands become crowded and vigor declines. Southern pine beetle infestations, for example, have long been associated with high stand density. Studies have shown that silvicultural techniques such as thinning offer the most promising and long-lasting means of preventing this situation. But by the same token, we know that the above- and below-ground injuries caused by harvesting and thinning operations serve as infection courts for disease organisms causing decay and deterioration. In fact, thinning can increase the incidence of annosus root rot. Wounded trees tend to be more susceptible to insect infestation as well. These conditions (which will be addressed in more detail later), together with a thorough understanding of the technology and effects of thinning operations, must be taken into account in the development of appropriate management recommendations.
In the South, the decision to thin or not is based primarily on product objectives. If pulpwood is the sole objective, the value of thinning(s) is questionable (Bennett 1963; Goebel et al. 1974; Schultz 1975), especially if there are no size restrictions on the product (e.g., when whole-tree chipping is used). Most studies indicate that, for pulpwood rotations, thinnings of normal intensity will have no influence on cubic volume yield or, more commonly, will reduce the total yield (Crow 1963; Mann 1952; Nelson and Arnold 1976; Wakeley 1969; Wheeler et al. 1982; Williston 1979). An exception would be extremely dense young stands where precommercial thinning may be necessary to prevent near stagnation of the stand and much reduced volume growth (Balmer et al. 1978; Balmer and Williston 1973; Bower 1965; Brender and McNab 1978; Cooperi 1955; Debrunner and Watson 1971; Grano 1969; Gruschow 1949; Guttenberg 1970; Lohrey 1972, 1973, 1977; Mann and Lohrey 1974).
When sawlogs or multiple products are desired, thinnings should be an integral part of southern pine stand management (Bennett 1963). Under such circumstances, the issues that must be addressed include: 1) The relationship between initial spacing and the need for thinnings, 2) the time to thin (age), 3) the intensity and frequency of thinnings, and 4) the most appropriate method of thinning.
The choice of initial spacing is critical in plantation management, especially for large product rotations. Numerous studies have shown that, for a wide range of initial stocking (e.g., 300 to 1,000 + seedlings/acre), and for rotations of 20 years or longer on given sites, tree heights and total volume production are essentially independent of initial stocking. These observations simply reflect the relationship between stocking (table 1) and the carrying capacity of the site.
At close spacings, the basal area carrying capacity is reached earlier, but once reached, the rate of volume production will be the same regardless of initial stocking. Although volume production may be independent of initial spacing, stocking will have a marked effect on the diameter growth of individual trees and thus on value of the final product, as well as the length of time necessary to produce a product of a desired size.
Choice of initial spacing for a given site will probably be determined primarily by the product(s) to be grown in the area. When sawlogs or multiple timber products are the objective, the owner has three alternatives: plant at wide spacing and do no thinning, plant at closer spacing and accept a somewhat longer rotation to get a product of the desired size, or plant at a closer spacing and thin to keep trees growing at an acceptable rate.
Wide spacings (10 by 10 feet or more) will produce more board foot volume on relatively short rotations (25 to 35 years) than will the more usual, closer spacings (Arnold 1978; Bennett 1963, 1969, 1971; Burton 1982; Shelton and Switzer 1980; Shepard 1973), but total volume will likely be less and the quality of the logs lower unless trees are artificially pruned (Bennett 1969; Box et al. 1964; Brender 1965; Feduccia and Moiser 1977; Ware and Stahelin 1948). Intermediate spacings (600 to 800 trees/acre, depending on site) coupled with thinnings will probably offer the best compromise where multiple products are the objectives. For rotations of 25 to 35 years, such spacings will yield about as much cubic foot volume as closer spacings and as much board foot volume as wider spacings, although the average tree diameter will be smaller. Stem quality, expressed in terms of branch size and number, sweep and fusiform cankers, will also be better with intermediate spacings in the absence of thinnings. Intermediate spacings with thinnings are also best for production of sawlogs plus cordwood (Feduccia and Mosier 1977; Ware and Stahelin 1948; Williston 1979). Frequent light thinnings (e.g., every 5 years) may yield a higher quality end product and perhaps more board foot volume than heavier thinnings (Farrar 1968; Feduccia and Mosier 1977; Fender 1968). However, economics may dictate heavier, less frequent thinnings. Some work has shown that a single thinning might be acceptable for rotations designed to produce sawlogs (Fender 1968; Hardie 1977; Parker 1979). Timing and frequency of thinnings should be determined to a large extent by site quality and length of rotation.
Adapted from Feduccia, D.P.; Mann, W.F. Jr. Growth following initial thinning of loblolly pine planted on a cutover site at five spacings. Res. Pap. SO-120. New Orleans: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station; 1976. 8 p.
Configuration of the initial planting is also an important consideration. Growth of individual trees appears to be a function of available growing space (stocking density). Within reasonable limits, configuration (square or rectangular spacing) will have little or no effect on tree growth (Bennett 1960; Harms and Collins 1965) but may influence such future operations as the thinning method used in the stand. If very wide spacings (greater than 10 feet) are used, the manager may be limited in the type of mechanical thinning that can be employed to avoid reducing stocking to below the desired level (Bennett 1965).
Precommercial thinning. Most foresters believe that precommerical thinning is unnecessary in plantations established at spacings now commonly used in the South. However, there may be a need for such thinnings in dense, natural stands and in plantations established by direct seeding or supplemented with natural regeneration from surrounding stands. Precommercial thinning is probably justified if there are 1,500 or more well-spaced seedlings per acre (Balmer and Williston 1973). In such cases, tree growth can be accelerated by this practice (Lohrey 1972, 1973, 1977; Mann and Lohrey 1974), especially in slash pine, which has a greater tendency to stagnate at an early age than other southern pines. Thinning is best performed as soon as the seedlings are well established, usually between ages 2 and 5 (Balmer and Williston 1973; Grano 1969; Guttenberg 1970; Jones 1977; Mann and Lohrey 1974), before they have experienced severe intraspecific competition and while they still are small enough to permit thinning with relatively light equipment such as a rotary mower or light chopper. If stocking is fairly uniform, seedlings can be removed in strips. Where stocking is extremely high, cross stripping can be used to further reduce their numbers. The best response appears to be obtained with a residual stocking of 500 to 750 trees/acre (Grano 1969; Gruschow 1949; Jones 1977; Lohrey 1973; McMinn 1965).
First commercial thinning. The timing of the first commercial thinning should consider management objectives, operability, site quality, stand density, probability of subsequent thinnings, and rotation length.
Once management objectives are determined, other thinning variables are readily defined. If sawlogs or multiple products are the objective, early intervention may be required to increase the proportion of large, quality, merchantable stems in the final harvest (Burton 1982; Wahlenberg 1960), especially with shorter rotations in dense stands (600+ trees/acre) and on good sites where residual growth potential is high.
Wahlenberg (1946) suggested that intervention usually should be deferred in stands intended for sawlog production until the stems are clear of live branches or can be pruned to a height of 25 feet or more. With a desirable live crown ratio of 35 to 40 percent, this means that first thinnings should be delayed until total height is 38 to 42 feet. Corresponding tree ages would be 12 to 14 years on land with a site index of 95 and 20 to 22 on land with an index of 65. These guidelines, designed to insure desirable crown characteristics and provide rapid diameter increment, apply when tree density exceeds 90 + trees/acre. Mean crown ratios normally exceed 35 percent through age 25 at lower stand densities, even on land with a site index of 95 (Feduccia et al. 1979).
It is generally accepted that the first thinning should be delayed until revenue received from the trees removed will pay the cost of the operation (Wahlenberg 1960). An exception is dense stands (1,000 + trees/acre), in which stagnation is likely, especially if slash pine is the favored species (USDA Forest Service 1971). First thinnings should be made just prior to overcrowding, reduced diameter growth, and heavy mortality and before the live crown ratio is reduced to below 35 percent of total height (Mann 1952; Wakeley 1954). The beginning of suppression-caused mortality in 4- to 5-inch-diameter trees serves as a good signal for a first thinning in most stands (Mann 1952). Generally, this occurs between 18 and 20 years for average density and site conditions, but may be as early as 13 to 15 years at closer initial spacings and on highly productive sites (Wakeley 1954).
As indicated above, the need for a first thinning can be earlier on land with high site quality than on land with low site quality. The significance of site in bringing about the first intervention, however, is better understood when considered with stand density. Time of the first thinning, even on the best sites, can be delayed in stands with poor survival and low initial planting density. Site occupancy and stand differentiation are delayed under these conditions, and maintenance of high crown ratios sustains better diameter growth than can be attained in more dense stands. However, stem quality relative to natural pruning length will be generally poorer, except when surviving trees occur in dense patches (Wakeley 1954). Similarly, increased stand density (600 + trees/acre) would move the initial thinning date forward in time when sawlogs and multiple products are the management objectives.
Little information is available on the influence of rotation length and use of additional thinnings on the timing of the first thinning. However, yield simulation work2 has shed some light on the relationships. These simulations verified earlier observations relative to time of thinning and site and stand density relationships. The results indicate that, to maximize total and sawlog volumes, stands with 450 trees/acre on land with a site index of 80 and stands with 300 to 600 trees/acre on land with an index of 95 should be thinned at 16 to 20 years of age. Stands with higher densities on both sites should be thinned between 13 and 16 years of age.
Additional thinnings increase net total volume yield over the rotation by harvesting more of the trees that are likely to die early. Potential gains are greatest in dense stands (600 + trees/acre) on the better sites. With two thinnings, the first is done at a younger age to reduce mortality and to increase subsequent average diameter of residual stems. For lower densities on the same sites, single thinnings appear best in maximizing sawlog production. On lower density sites, benefits from second thinnings are limited by unworkable stand volumes until late in the rotation. In all instances, average tree size is greater with two thinnings than with one.
Increasing rotation length delays the time of the first thinning. Yield simulations indicate that the first thinning can be delayed for 2 to 4 years when rotation length is increased from 30 to 36 years. Delay time increases as density increases from 300 to 750 trees/acre. The first thinning should take place 7 years later on poor sites than on good to excellent sites. Site has no effect in altering volume differences between the two rotation options.
Biological considerations in defining thinning intensity concern the tradeoffs between net volume production and average tree size. Stand volume growth is directly proportional to the residual basal area left after thinning, whereas diameter growth is the reverse (Enghardt and Mann 1972). For loblolly pine, there is a broad range of residual densities over which maximum net volume growth occurs after thinning (Nelson 1961). Obviously, d.b.h. per unit volume is maximized near the low end of this range.
Heavy thinning promotes rapid diameter growth by favoring large live crown ratios and improved canopy exposure, but with resultant underuse of site resources, reduced net volume production, and increased risk of mortality and quality reduction from damaging agents. Light thinnings increase site utilization and volume increment but require greater frequency to achieve stated tree size goals (Wahlenberg 1946). Wakeley (1954) cautions that no attempt should be made to reduce stand density to final sawlog trees without additional thinning. Intensive thinning, like wide initial spacing, wastes growing space, lowers product quality, and allows undesirable vegetation to occupy the site.
Light frequent thinnings are recommended for maximum pulpwood plus sawlog production. The strategy is to keep the stand open enough to prevent mortality from suppression. Earlier and more frequent thinnings may be required in high-density stands on good sites than in those on poor sites with low growth potential. Increasing thinning intensity would delay subsequent interventions, such that on better, more densely stocked sites, two thinnings could be achieved in a 30-year-rotation.
Thinning guidelines for southern pines frequently suggest removing 30 to 45 percent of the stand basal area (Farrar 1968; Morris 1958). The percentage of the basal area to be left tends to increase with increasing site quality because of greater productive capacity and as the age of intervention is increased. Residual basal areas range from 60 to 90 square feet/acre (Bull 1950; Nelson 1961) and tend to be lower on poor than on good sites. For the same site, residual basal areas are somewhat lower for early interventions than for late.
As thinning intensity increases, the method used becomes more important because of its effect upon the growing stock left (Brender 1965). This relationship increases in importance as densities drop below 450 trees/acre.
Thinning frequency is influenced by management objectives, stand density, and site quality. The number of thinnings is determined by stand density at the time of the first thinning (Andrulot et al. 1972). The interval of cutting is influenced by economic factors associated with operability, but the biological interval is frequently defined by the length of time required for trees to grow 10 feet in height (Brender 1965). From the biological viewpoint, the interval between thinnings will increase as site quality decreases and stand age at the first thinning increases.
To maximize multiple product yields in short rotations, early and frequent thinnings are needed in stands of 600 + trees/acre (Fender 1968). Such thinnings salvage trees that are likely to die early and maintain good diameter growth on crop trees. At extreme stand densities (often true of direct seeded stands), a precommercial thinning may be necessary to achieve multiple product goals over short rotations. For dense stands, the first thinning should be earlier on better quality sites to capture the full growth potential of the site (Jackson 1970). Regardless of intensity, the longer initial thinnings are deferred, the slower will be the response in diameter growth (Mann and Enghardt 1972).
Two considerations are paramount in choosing a thinning method for southern pine plantations: 1) The growth response and quality of residual trees, and 2) the costs involved in marking and harvesting. Unfortunately, the method that results in the greatest growth response and best quality trees may also be the most expensive. The best choice will often represent a compromise between cost and quality.
Thinning methods for the southern pines include:
Selective methods. Trees to be removed are marked individually based primarily on their position in the crown canopy, although other considerations (e.g., damage from disease, insects, and wind) may take precedence. These are the classic European methods of low, crown, and selection thinning, or some combination of the three ("free thinning").
Mechanical methods. Trees are removed strictly on the basis of spacing with little or no regard to crown position. Row or corridor thinnings are the primary examples of this type of thinning. Leave-tree or D + x thinnings3 are also mechanical-type thinnings, but some emphasis is placed on selecting better trees for the residual stand.
Mechanical plus selective method. In this technique, the stand is first thinned mechanically, usually by rows, and then selectively within the leave rows.
McMillan 1955; Collicott and Strickland 1967; Enghardt 1968; Gilmore and Boggess 1969; Grano 1974; Whipple 1962). Research also indicates that mechanical methods generally leave the stand more susceptible to damaging agents such as wind and ice (Belanger and Brender 1968; Enghardt 1968; USDA Forest Service 1971). Furthermore, in stands with a high incidence of diseased or damaged trees, mechanical methods may be ineffective (i.e., leave too many defective trees at the expense of better ones).
Thus, from a biological standpoint, selective thinnings appear more desirable than mechanical ones, and for some landowners (e.g., the private nonindustrial owners who mark and harvest their own timber), selective thinning is no doubt the best approach (USDA Forest Service 1971). For owners who have the option of using mechanical harvesting equipment and must harvest large areas over short periods, row thinnings may be more economical (Enghardt 1968).
If strict row thinning is employed (no thinning in residual rows), it appears that, for 8- by 8-foot or closer spacings, removal of every third row will give the best results (Belanger and Brender 1968; Collicott and Strickland 1967; Little and Mohr 1963). Third row thinning will give some release to all residual trees and, unlike alternate row thinning, will usually not reduce stocking below acceptable levels, nor will the residual stand be as susceptible to wind and ice damage (Belanger and Brender 1968).
Some of the biological disadvantages of strict row thinnings can be overcome by a combination of row thinning and selective thinning within the leave rows (Bennett 1965; Brender 1965; Collicott and Strickland 1967; Enghardt 1968; Grano 1974) and at little additional harvesting cost. With this method, complete rows of trees are removed at selected intervals (e.g., every third, fourth, of fifth row), and a selective thinning is performed within the leave rows. The distance between cut rows will be determined primarily by equipment limitations. The wider the distance, the closer the cut will be to a selective type thinning, but harvesting costs will also increase., . The most common application of this method involves harvesting of every third row and selective thinning within the leave rows (Collicott and Strickland 1967). Trees removed in the selective thinning are primarily those in the lower crown classes and poorly formed or diseased trees. For that reason, post-thinning mortality should be less than that for row thinning alone and comparable to that for selective thinning alone (Collicott and Strickland 1967).
Initial spacing, tree condition (in terms of disease incidence and severity and deformities), and stand age have a strong bearing on the choice of a thinning method for plantations. Wide spacings (15 feet and wider) will usually dictate the use of selective thinning (or possibly no thinning) because removal of entire rows will create a situation where the remaining trees are unable to utilize all the growing space (Bennett 1965; Enghardt 1968; USDA Forest Service 1971). When the incidence of diseased or malformed trees is high, row thinnings alone would be inappropriate, but row plus selective thinning might be satisfactory (Collicott and Strickland 1967; Enghardt 1968).
There is little information on the relationship between age and thinning method, but if either row or row plus selective thinning is used, it should be done when the trees are fairly young, before much crown class differentiation has occurred and before competition has resulted in serious loss of crowns on most trees. For loblolly pine on average or better sites and plantings of 700 or more trees/acre, this will usually mean that the thinning should be done before age 17, usually between ages 12 and 17.
Many types of thinning systems are used in southern pine plantations but they can be divided into two major categories: Labor-intensive systems, and mechanized systems.
of this system include: 1) The capital investment required is low. 2) High maneuverability is possible, allowing selective thins with little residual stand damage. 3) Soil disturbance is minimal. 4) It can be used to harvest small, remote tracts not economically harvested by other systems. The disadvantages are: 1) It is labor intensive with a low productivity rate. 2) It is sensitive to adverse weather or ground conditions, with the result that production potential is lost. 3) Harvesting costs are comparatively high as a result of low production.
A second labor-intensive thinning system introduced into the South is the Nordfor system,4 which employs a five-person crew (three cutters and two winch operators), and the Nordfor Tiltwinch. The system is designed to perform a first thinning of natural stands or plantations on areas where terrain conditions are unsatisfactory for conventional systems. The system's advantages are: 1) Residual stand damage and soil disturbance are minimal. 2) It can be used where terrain or soil conditions prevent a more conventional system. 3) It requires relatively low total capital investment. The disadvantages are: 1) It is highly labor intensive, requiring considerable manual labor and resulting high worker turnover. 2) It requires a continuous, expensive training program due to high worker turnover. 3) It achieves a low production rate resulting in a comparatively high harvesting cost. 4) It is sensitive to stand factors, such as tree spacing or density, average d.b.h., and planting quality.
Mechanized systems. In the last two decades, the forestry industry has moved toward mechanization of harvesting because of labor shortages, high wood product volume requirements, advances in technology to meet rising costs, and potential gains in efficiency through use of large-scale systems.
As this move toward mechanization has advanced, it has become evident that few machines, either existing or under development, are really suitable for thinning. With few exceptions, existing machines have been designed for use in a final harvest and, when used for thinning, cause extensive site and residual stand damage. Consequently, equipment manufactures have begun to develop and market some machines designed especially for thinning southern pine plantations.
A typical mechanized system consists of felling, primary transport, loading, and secondary transport units. Most of these mechanized systems produce tree-length material.
Primary transport, from the stump to the loading area, is usually handled by either skidding or forwarding equipment. In skidding, the log is dragged on the ground with only the butt end elevated, while forwarding involves loading and carrying the entire stem or bucked-up section. Although skidding is the most popular form of primary transport, forwarders are used in many thinning operations in the South.
The major advantages of forwarders are that they reduce residual stand damage and soil disturbance because the wood is in short lengths and is carried off the ground; they are more flexible in laying out the logging plan, because travel corridors need not be straight lines; and they have an integral loading capability. The major disadvantage is that wood has to be bucked and bunched to achieve good production rates, resulting in higher labor requirements and harvesting costs.
Though used extensively, few skidders are really suited to thinning operations because of their large size, which restricts maneuverability in densely stocked stands. Some equipment companies have produced and marketed small skidders better suited to thinning activities.
Although other machines, such as farm tractors, crawler tractors, and track-type skidders, are often used for skidding, the articulated rubber-tired skidder is the primary equipment used in the South. Two main types of rubber-tired skidders are the cable skidder and the grapple skidder.
The cable skidder is able to recover all stems cut and, with its winching capability, to keep away from the immediate vicinity of residual trees. With proper use of the skidder, residual damage to stems and roots is minimal. However, extensive residual stand damage and soil disturbance can result when many trips are made over the same skid trail.
The grapple skidder (named for the armlike assembly mounted on the rear) is used to pick up and drag logs to the landing site. Its major advantage is high production rates (when used with bunched stems), resulting in a lower harvesting cost and fewer trips through the stand, thus reducing soil and residual stand damage. Again, the major disadvantage is the considerable soil disturbance and residual stand damage that occur when many trips are made.
Most mechanized operations use some type of feller-buncher, which consists of two pieces of equipment, the cutting head and some type of carrier. The directional shear and the chainsaw are still commonly used, chainsaws most often for selective thinnings.
Another type of carrier that has been used with some success in thinning operations is a small, highly maneuverable skid-steer-type machine, usually rubber-tired, with either a braking or hydraulic drive system for steering.
Secondary transport equipment used in harvesting operations is selected on the basis of the planned end product. However, diesel tractors predominate because of their reliability and good fuel economy in comparison with gasoline tractors. The trailer depends on the end product, e.g., pole trailers for tree-length (longwood) stems, chip vans for chips, etc.
Loading equipment includes front-end loaders with a log grapple, side loader, pallet rigs, and knuckle-booms. Although not technically a loader, the in-woods chipper chips and loads felled material into vans for hauling.
Overall, mechanized harvesting systems have a number of advantages: 1) They result in higher production rates and worker productivity, so that harvesting costs per unit of production are lower. 2) They are non-labor intensive and, with proper maintenance, mechanically reliable. 3) They are intensive to adverse weather and ground conditions. However, they have the disadvantages of being capital intensive because of high equipment costs, requiring relatively large timber volume and tract size to be profitable, requiring extensive operator training, and causing extensive residual stand damage and soil disturbance if improperly used.
A last piece of equipment that may be used in thinning operations is the multifunction processor, which is designed to completely or partially process the stem, in some combination of felling, limbing, bunching, and forwarding. Examples of this type of machine are the Timberjack RW 30, which fells, delimbs, and can either forward and/or accumulate bunches, and the Koehring Shortwood Harvester, which fells, limbs, bucks, and forwards the stems to the loading area. Multifunction systems presently consist of technically complex equipment with relatively low output and high cost.
Future thinning operations will most likely require small equipment that is better suited to perform a selective type thinning than present-day equipment. In addition, silvicultural techniques like variable row spacing or wider spacing will be considered. At least one company is now planting rows at 11-foot intervals to allow for machine access and maneuverability and also spacing trees wider within rows to eliminate the first thinning, which is usually of limited commercial value.
The use of various machines, systems, and techniques for thinning is being continuously evaluated. The most important factor in cost-effective thinning to be emphasized here is planning. The planning process should be continuous and flexible, begin before the establishment of a new stand, and recognize that all operations are interconnected and that each could affect all others through the rotation.
3 In the D + x method, the average spacing between residual trees in feet is equivalent to the average tree dimension in
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