Promote Stand Resistance

Regulate Stocking. - High-hazard stands are characterized by dense stocking and slow radial growth (fig. 5). Thinning will stimulate growth and vigor in young stands and reduce the likelihood of future losses from the SPB (fig. 6). Low thinning or "thinning from below" is recommended to reduce competition and remove slow-growing trees, which are most subject to SPB attack (fig. 7). The poorer crown classes suppressed and intermediate trees are cut first. Dominant and codominant trees with large five crown ratios and desirable phenotypic traits should be favored as crop trees. They are most likely to respond to thinning and to provide the greatest number of silvicultural options in the management of residual stands.

Figure 5. - Slow radial growth is a common measure of high-risk stands.

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Thinning schedules depend on the close relationships between site quality, stand age, stocking, live crown ratio of individual trees, and growth rate. Root and crown competition among individual trees develop at ages 10 to 15 years on most sites. Crowding can occur earlier than age 10 on better-quality sites that contain a large number of trees. Initial thinning should be scheduled before live crown ratios drop to 40 percent. Delay will reduce stand vigor to levels unfavorable for growth but attractive to the SPB.

Figure 6. - Thinning promotes the rapid growth and vigor of your stands. (Photo courtesy of the Texas Forest Service.)

Figure 7. - Low thinning will remove small, slow-growing trees that are susceptible to SPB attack (top). Healthy dominant and codominant trees should be favored as crop trees (bottom).

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Degree of thinning is determined by the intensity of management, the kind of product desired, available markets, and natural conditions particular to each location. Depending on site quality, basal areas of 80 to 100 ft2/ acre are recommended to reduce the potential for SPB attacks. The risk of beetle attack in most instances will increase considerably when stocking levels exceed 100 ft2/acre. Wide spacings distribute growth on fewer stems, resulting in the production of small sawlogs by age 35 to 40 years on good sites. Close spacings are recommended for high yields of wood fiber. Thinnings should be scheduled as needed to maintain desired stocking levels and stand vigor.

Mix Pine and Hardwood.-The SPB prefers pure pine stands. A mixture of pine and hardwood species reduces the potential for spot incidence and growth. Stands composed of pines and hardwoods may be suited to owners of nonindustrial woodlands managing for products other than or in addition to timber; such stands often support diverse wildlife populations, are esthetically pleasing, and contribute to soil improvement. However, intensive

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Damage from recent logging (within the past year) favors SPB infestations. Careless cutting, skidding, and hauling often cause severe mechanical injury to above- and below-ground portions of residual trees (fig. 8). Moderately to severely damaged trees are high-hazard trees and should be removed from the stand as soon as possible.

Figure 8. - Trees severely damaged by logging equipment should be removed from the stand.

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Use of small harvesting equipment and removal of short roundwood are recommended for intermediate cuttings. Heavy equipment and tree-length logging generally increase the amount of damage to residual trees. Equipment operators and ground crews should be trained to minimize damage to residual trees and thus reduce the likelihood of bark beetle attacks.

Figure 9-Stands should be regenerated with pine most resistant to SPB attack. (Photo courtesy of the Mississippi Forestry Association.)

Figure 9. - Stand should be regenerated with pine most resistant to SPB attack. (Photo courtesy of the Mississippi Forestry Association.)

Regenerate Overmature Stands.

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Susceptibility of trees to SPB attack increases with age. Mature and overmature trees usually have slower radial growth, flat-topped crowns, and sparse foliage. These trees seldom respond to intermediate cuttings and should be replaced with the most resistant host species or a species mix suited to the area (fig. 9).

Many overmature pine stands throughout the South are being intentionally preserved for ecological, wildlife, or esthetic reasons. These stands are extremely susceptible to attack and should be monitored regularly to detect the buildup of SPB populations early, when remedial action can save the pine component.