Rating the Susceptibility of Stands to Southern Pine Beetle Attack
Integrated Pest Management Handbook, USDA, Forest Service, Agriculture Handbook No. 645, April 1985.
Why Hazard Rate Forest Stands?
Just as an abundance of suitable fuels increases the risk of forest fires, stands of dense, slow-growing natural or planted sawtimber have an increased potential for loss to SPB. Stands having these "high hazard" characteristics are basically more likely to suffer heavier losses over time than are those classified as low hazard. An understanding of SPB/site/stand relationships can be useful in planning and scheduling routine forest management operations to gain the added benefits of reducing beetle-caused losses. SPB stand hazard rating can complement forest management practices and reduce or prevent SPB losses in several ways.
Improved Scheduling of Management Operation Forest managers may find hazard ratings useful in making decisions and developing justifications regarding the need, scheduling, and timing of management operations. Management planning to prevent SPB losses applies throughout the life of the stand. Conditions associated with SPB hazard ratings may help identify the need and justify practices for intermediate treatments, harvest cuttings, or stand regeneration (Belanger and Malac 1980) (Fig. 2). This may be particularly true for unthinned stands carried beyond rotation age. |
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| Figure 2. - Stand hazard rating identifies areas with greatest potential loss to the SPB and helps justify thinning (shown here) or other intermediate silvicultural treatments. (F-705622). |
Combined Forest Inventory and Pest Assessment
Existing stand records, usually obtained for timber inventory purposes, may provide the basis to judge risk of losses to SPB. Stand hazard rating offers the landowner an opportunity to examine overall resource conditions, make judgments on needs and priorities for stand management actions, and weigh these alternatives against the likelihood of outbreaks and the costs of direct control.
A relatively small proportion of forested land is classified as high hazard to SPB attack. Ownerships with a predominance of low-hazard stands will have less potential for severe beetle-caused losses than those having a large proportion of high-hazard stand conditions. During the 1974-77 SPB outbreak in east Texas, for example, only 0.5 percent of a 240,000-acre study area was rated as very high hazard and 9.5 percent was rated high; 27 percent was moderate; 26 percent low; and 6 percent very low. The remaining 31 percent included nonhost hardwood, clearcuts, and nonforested areas (Mason et al. 1981).
Relatively low percentages of high- and moderate-risk stands were also reported on the Kisatchie National Forest in Louisiana, where only 23 percent of the forest was represented by these classes (Lorio et al. 1982) (Fig. 3). Similar distributions would be expected under mixed ownership in other States across the South. Individual land holdings that receive more intensive management treatments will have a greater proportion of low-hazard stands, whereas those with little or no management (or those having management objectives other than timber production) will have a larger proportion of stands in the higher hazard classes.
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| Figure 3. - Map illustrating hazard conditions on approximately 200,000 acres of the Kisatchie National Forest in Louisiana. Note the relatively small quantity of medium- and high-risk stands. (F-705623). |
Reduced Outbreak Potential
Severely damaged or weakened trees and stands are believed to serve as low-level SPB reservoirs from which future outbreaks develop when environmental conditions favor beetle development. Timely removal of these trees and stands during periods of low beetle activity may serve to prevent or slow the development of future outbreaks by removing sources of beetles that could otherwise contribute to future population buildups.
Low-level SPB populations are strongly dependent upon the availability of easily accessible and suitable host material for their survival. When SPB activity is low, most spots initiate in very high-hazard (optimal habitat) stands or in small high-hazard pockets in larger low- or moderate-hazard areas. As host and environmental conditions become more favorable for the beetles, spots increase in number and size, and the outbreak expands into less suitable (low- to moderate-hazard) stand types. Removal of high-hazard stands that harbor beetles over a broad geographic area will aid in preventing development of future outbreaks.
Improved Scheduling of Control Operations
As forest managers have become more familiar with pine bark beetles and their relationship to forest conditions, management practices, and production economics, they have also come to recognize that some spots should receive high priority for immediate attention, whereas others need not be controlled at all. Low-priority spots can be monitored over time, and appropriate actions taken if future development warrants. Techniques have been developed for setting control priorities based on spot size, the presence of newly attacked trees, and local stand characteristics (Billings and Pase 1979). Stand hazard rating, as it describes the nature and condition of the surrounding forest, can greatly improve the manager's ability to make SPB control decisions that are biologically and economically sound.
Improved Bark Beetle Survey Efficiency
During periods of low population, aerial surveys are conducted periodically to detect changes in SPB activity or to monitor spot numbers, sizes, and locations. Such flights over entire State or multicounty areas may reveal little or no apparent activity, but at considerable expense. The effectiveness of these
Figure 4A. - Large-scale hazard rating of
18,000-acre blocks in Texas (A) and counties in North Carolina (B) offers a broad view of forest conditions, some of which can be monitored. The ratings also provide a means for improving SPB aerial survey efficiency. |
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| Figure 4B. - Large-scale hazard rating of 18,000-acre blocks in Texas (A) and counties in North Carolina (B) offers a broad view of forest conditions, some of which can be monitored. The ratings also provide a means for improving SPB aerial survey efficiency. |
surveys during periods of low population can be enhanced by restricting flights to high-hazard areas where the likelihood of beetle infestations is greatest (Fig. 4). As beetle activity increases in these indicator areas, the survey can be expanded to include other, less susceptible zones.
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