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Thinning Practices in Southern Pines - With Pest Management Recommendations

T. Evan Nebeker – Respectively, professor, Department of Entomology,
John D. Hodges – Professor, Department of Forestry, Mississippi State University, Mississippi State, MS,
Bob K. Karr – Assistant professor, Department of Forestry, Mississippi State University, Mississippi State, MS, and
David M. Moehring – Professor (deceased), Department of Forestry, Mississippi State University, Mississippi State, MS.

United States Department of Agriculture, Forest Service, Technical Bulletin 1703, December 1985.

Adverse Effects of Thinning

Thinning can produce positive and/or negative effects, depending on how, where, when, and why it is conducted. The presence of more than one kind of hazard at any given time and place poses some problems in designing an optimum thinning strategy. Further complicating the situation are the species, stage of stand development, anticipated direct damages to residual stems, site quality, growth rate, and susceptibility to such damaging agents as insects, disease, and windthrow.

As a prerequisite to making the best thinning prescription, the forester must have a perspective of thinning impacts gathered from published information and from experience. The primary focus of this section is to identify the negative effects of thinning involving such factors as logging damage, insects, and diseases.

Felling-Related Damage to Residual Trees

The degree of felling-related damage is influenced by several factors: the method of felling, the equipment and its configuration, tree species, spacing (density) and size class (age), and ground conditions. The type of damage encountered is usually in the form of limb breakage, bole wounding (upper and lower bole), and root breakage. Additional damage may involve bending and breakage of whole trees.

Spacing (density) and size class (age) influence the subsequent extent of injury to the residual stems. All types of felling injury (bark abrasion, stem bending or breakage, broken limbs) are minimal in trees 12 inches d.b.h. and over (King 1963).

Timing, as it relates to season and weather conditions, can cause differences in levels of stand damage (Moehring and Rawls 1970). Thinning during the period of most rapid growth (spring or early summer) can result in greater injuries to residual trees.

Skidding-Related Damage to Site and Residual Stems

In general, as the size of equipment increases, damage to the residual stand increases, and stem injuries are greater where arches are used than where logs are ground skidded (Benzie 1959). The same study also found that tree-length arch-skidding knocked over residual trees more often than log-length ground skidding but that ground skidding severed and bruised a higher percentage of the roots. In general, using rubber-tired skidders and skidding tree-length were the most damaging practices.

Volume loss in skid trails is significantly related to soil moisture and terrain (greater damage in steep terrain) (Peters 1977) and varies with rut depth and

Skidding damage.
distance from the residual tree (Nebeker et al. 1983). Soil characteristics and terrain also influence the extent of skidding-related damages because imperfectly drained soils are conducive to compaction (Moehring and Rawls 1970). Seedling survival is poorer on heavily compacted, light-textured soils. Likewise, seedling growth is significantly retarded in skid roads or compacted soils (Dickerson 1976; Foil and Ralston 1967; Hatchell 1981; Hatchell et al. 1970; Moehring and Rawls 1970; Perry 1964; Pomeroy 1949), with volume growth affected more than height.

Some tree species may be more sensitive to logging damage under certain seasonal and soil moisture conditions. For example, the diameter growth of loblolly pine can be reduced following wet-weather logging (Hatchell et al. 1970; Moehring and Rawls 1970).

Decreases in site index in pine plantations have been observed for trees growing on old woods roads. Significant losses in productivity also occur in disturbed areas following harvest (Peters 1977).

With regard to tree size classes, damage due to skidding is greatest on saplings followed by poles and sawlogs (Benzie 1959; King 1963).

As log length increases, damage to the residual stand increases. Doubling the length of a log quadruples the turning area, thus increasing the potential for damage (King 1963). Further, increasing traffic intensity correspondingly increases loss in basal area growth of loblolly pine during wet-weather logging (Moehring and Rawls 1970).

Indirect Thinning Damage

Thinning in southern pine plantations may increase the likelihood of indirect damage due to environmental factors or damaging organisms.

Wind. The severest wind damage appears to occur in larger diameter trees regardless of thinning intensity, with these trees tending to be more prone to windthrow and breakage (Nelson and Stanley 1959). Smaller diameter trees tend to sustain more severe lean without being windthrown. The presence of pathogens predisposes trees to windfall, with root rot the most important, followed by butt rots and trunk rots (Boyce 1948; Nelson and Stanley 1959; Powers and Verrall 1962). A greater chance if windthrow and wind damage is also related to geographic location, with particular reference to the Atlantic and Gulf Coastal Plains and hurricane frequencies.

Ice. In addition to wind-related problems, ice-related damage is also of concern following any silvicultural treatment. Abel (1949), Brender and Romancier (1965), McKeller (1942), Muntz (1947), and Williston (1974) suggest that susceptibility to glaze damage in the southeastern United States is related to tree species, with slash pine being most affected followed (in order) by longleaf, loblolly, and shortleaf pines. Larger trees suffer more damage than smaller ones in dense stands (Shepard 1975), and trees with a low diameter/height ratio

Ice damageto Loblolly Plantation in Virginia.
are more vulnerable to glaze.

Other factors contributing to the severity of ice damage include stand density, crown class, presence of pathogens, and geographic location (Brender and Romancier 1965; McKellar 1942; Nelson 1951; Shepard 1975; Williston 1974).

Damage is more extensive in row-thinned than in selectively thinned or unthinned stands of loblolly pine (Shepard 1975). In unthinned loblolly, denser stands suffer less damage, while in row-thinned loblolly, denser stands have more damage after thinning. Brender and Romancier (1965) suggested that increased thinning intensity affects the degree of glaze damage.

Insects and disease. The most damaging insects in thinned stands include the black turpentine beetle and the three southern Ips engraver beetles found in the southeastern United States. Anderson and Mistretta (1982) suggested that these species, plus the southern pine beetle and the southern pine coneworm, commonly attack trees infected with fusiform rust, annosus root rot, and/or littleleaf disease. The black turpentine beetle is attracted to the oleoresin produced on stumps of recently cut and injured trees (Feduccia and Mann 1975). Infestations of the black turpentine beetle can be reduced substantially by minimizing injuries to residual trees during logging and avoiding harvesting on waterlogged soils to prevent excessive root damage (Bennett and Ostmark 1959; Feduccia and Mann 1975).

The relationship between stand manipulation (i.e., thinning) and pest organisms has been noted in general, with few specific studies to evaluate this relationship. Mason (1969) investigated the behavior of Ips spp. populations after summer thinning in a loblolly pine plantation. He reported that thinning attracted large numbers of Ips avulsus (Eichh.) and I. grandicollis (Eichh.), which infested slash in the experimental area. However, the beetles did not attack residual trees and upon emergence dispersed to new sources of attraction. Mason conculded that in pulpwood stands in the mid-South, Ips spp. rarely pose a problem to residual stands following summer thinning (1969). Similar observations5 were made following thinning of a loblolly pine plantation on an experimental forest in Mississippi during the winter and spring of 1977-78. Large numbers of Ips spp. were attracted to the slash and freshly felled trees, and little residual stem mortality occurred. However, during 1981 and 1982, some mortality of residual stems occurred when thinning slash was left around the base of residual trees.6 Others observed mortality in precommercially thinned plantations but could not clearly associate it with the distribution of logging slash and an Ips spp. buildup. Ips spp. may also attack living trees after natural catastrophes such as ice storms or severe drought (Brender and Romancier 1965; Mason 1969).

The southern pine beetle is considered the most destructive insect of southern pines, but outbreaks are usually not associated with thinning in young stands unless there is severe damage to residual trees. However, thinning may be important in preventing losses to the SPB.7 Several studies (Hicks et al. 1980; Ku et al. 1980; Lorio 1978) have shown that infestations most often occur in denser stands. Trees in such stands are more apt to be under stress and to be less vigorous than trees in less dense stands. Thus, thinning may improve

Bark beetle damage.
the vigor of residual trees and make them more beetle resistant.

The impact of thinning on host susceptibility to bark beetles has been recently explored by Nebeker et al. (1983) and Nebeker and Hodges (1983). Preliminary findings indicate that, if properly done, thinning can result in increased growth rates and improved resistance to pest attack. However, if there is considerable disturbance, there can be initial severe damage to the site, reduced growth in residual trees, and increased susceptibility to pest attack. Additional studies have also focused on the influence of harvesting on the forest ecosystem and associated pest damage (Hedden 1983).

Recent studies have focused more closely on the changes in host condition resulting from silvicultural practices (Blanche et al. 1983) than does this discussion. In addition, a broader treatment is the host is presented in the proceedings of a recent symposium (Kellison and Gringrich 1982).

In thinned loblolly and slash pine stands, disease may be a more serious problem than insects on certain sites. Annosus root rot can enter previously healthy stands, primarily after thinning, where it colonizes freshly cut stumps and eventually spreads to adjacent trees through root contact (Campbell 1965; Hodges 1974). In the latter study, annosus root rot was present in 59 percent of all thinned loblolly pine plantations and 44 percent of slash pine plantations on high-hazard sites in the southeastern States. Losses averaged 2.8 percent

Aerial view of infection center
at Nacogdoches city plant.
of the loblolly trees examined and 2.2 percent of the slash. Losses in some plantations exceeded 30 percent. A southwide study (Powers and Verrall 1962) showed that mortality associated with annosus root rot increased with the number of thinnings and the years after thinning. Mortality, however, is only part of the problem, for growth (height and diameter) in some trees may be reduced by a third even though the crowns appear vigorous and there is no outward sign of infection (Bradford et al. 1978; Froelich et al. 1977).

The probability of serious annosus root rot infection occurring after thinning is highly dependent on site. Froelich et al. (1966) developed methods for assessing site hazard in the Gulf South. In general, soil texture or texture-related variables provide the best indication of disease hazard (Kuhlman et al. 1976). Low-hazard sites include Piedmont soils, lowland flatwood soils (poorly drained, silty or sandy surface soil, heavy clay subsoil), and shallow upland Coastal Plain soils. High-hazard sites include both deep sandy and silty soils. Deep, well-drained sandy soils are extremely hazardous in the Southeast. These soil types occur mainly in the Sand Hills and well-drained areas of the Coastal Plain. Thick loessial deposits in north Mississippi represent high-hazard silty soils.

5 Nebeker, T.E. Unpublished report. Mississippi State, MS: Mississippi State University.

6 Ibid.

7 Ibid.

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