Rating the Susceptibility of Stands to Southern Pine Beetle Attack

G.N. Mason - Research Coordinator, IPM RD&A Program for Bark Beetles of Southern Pines,
P.L. Lorio, Jr. - Project Leader and Supervisory Soil Scientist, Forest Insect Research, USDA Forest Service, Southern Forest Experiment Station, Pinesville, LA,
R.P. Belanger - Principal Silviculturist, USDA Forest Service, Southeastern Forest Experiment Station, Athens, GA, and
W.A. Nettleton - Entomologist, Forest Pest Management, USDA Forest Service, Southern Region, State and Private Forestry, Pineville, LA.

Integrated Pest Management Handbook, USDA, Forest Service, Agriculture Handbook No. 645, April 1985.

Application and Evaluation

Methods of Application

Following are several approaches that may be used to implement hazard-rating systems.

Existing Stand Data

Most owners of managed forests maintain tract survey, management unit, or stand type maps from which timber inventory information is available. These records frequently include all data necessary to rate stands (or information from which these data can be derived). Computer storage and retrieval offer convenient acquisition of data and the capability of continuous updating, providing the ideal data base for SPB stand hazard rating. Stands may be rated manually from computer printouts or automatically from an information base after entering the appropriate rating routine. This approach has been applied to National Forest lands (Lorio and Sommers 1981) by use of the Southern Region's Continuous Inventory of Stand Conditions (CISC) data system, and to industrial holdings by use of other rating systems and similar standardized inventory records.

Aerial Photographs

In the absence of existing stand inventory data, large land bases can be rated from aerial photographs (Mason et al. 1981, Sader and Miller 1976) if input requirements for the rating system can be interpreted from the photographs. Gross stand stratification suitable for stand rating can be accomplished easily from small-scale (1:60,000 to 1:120,000) color infrared photography. Detailed site and tree conditions within stand types may be added by sampling representative stands using lager scale (1:5,000 to 1:12,000) aerial photographs or ground observations.

Ground Cruise

For individual tracts of particular concern or for small ownerships, onsite determinations of SPB hazard may be most practical. As with other application approaches, the data to be gathered depend upon the system selected. Stands vary greatly in tree size and distribution, stocking level, and species composition. The same standards used to account for such variability in timber inventory cruised are applicable to gathering data for hazard rating.

Combinations of Techniques

For a variety of reasons--data requirements of some systems, availability of funds, training requirements, size of the land area to be rated, and the like--field application through ground cruise may be most practical. However, fieldwork can be accelerated and simplified by first stratifying stand types on aerial photographs, or by taking advantage of resource inventory data or existing stand maps.

In situations where existing data and computerized systems are available, it is advantageous to rate large areas quickly with the computerized approach. Refinement, updating, or further improvement, possibly using a different rating system, may follow at a later date as additional data become available or as a part of regularly scheduled field activities.

Evaluation of Stand Rating Results

Large-scale tests of hazard rating systems have demonstrated that a relatively small proportion of the acreage is classified as either high or low hazard (Fig. 5A), but on a per-acre basis most SPB spots occur in high-hazard stands (Fig. 5B). Further, a larger proportion of trees is killed in these areas than in lower hazard stands (Lorio and Sommers 1981, Mason et al. 1981, Hicks and Mason 1982). The major objective in developing SPB stand rating systems has been to identify site and stand conditions most often associated with SPB infestation occurrence and subsequent volume loss. To be an effective management tool, a rating system should permit the manager to identify the smallest land unit acreage on which the greatest loss due to SPB might be expected. A classification system that too broadly classifies all stands where SPB spots occur as high hazard (and does not consider possible exceptions that may be influenced by stand damage, beetle population density, and similar factors) will result in an unrealistic number of stands targeted as high risk.

Figure 5A and 5B. - Classification of forest lands in east Texas according to southern pine beetle hazard class (5A). On a per-acre basis, almost twice as many spots occured in high- and very high-hazard stands compared with all other classes combined (5B). (Based on approximately 500 SPB spots in a 182,000-acre sample taken in 1973-78.).

Judgement of a rating system's adequacy should also be based on consideration of the overall timber resource and beetle population conditions over a large area, rather than on classification results for a few stands associated with a limited number of SPB spots. A thorough evaluation requires ranking a relatively large number of stands, then considering the total number of acres in each hazard class, size

and distribution of individual stands in the class, spot distribution, and spot size. Stand ratings should be based on a comparison of spot occurrence, total tree mortality, and resource value. If a system is judged on a spot-by-spot basis, one should not be alarmed to consistently find infestations in low- to moderate-hazard stands. Infestations can often result from factors that are known to override site/stand conditions (lightning strikes, ice damage, logging or storm damage, areawide drought or flooding, and the like). One must also be aware that high-hazard pockets may occur in low-hazard stands. Beetle population density within an area will also affect the proportion of spots that occur in high-, moderate-, and low-hazard areas (Mason et al. 1981). When beetle activity is low, a larger proportion of spots will occur in high-hazard areas. As beetle activity and population pressures on stands increase, a proportionate increase in spot numbers will occur in low- and moderate-hazard stands (Fig. 6). Stand condition is not a determining factor during epidemics. The distribution of spots among hazard types during periods of high populations will be proportional to the land area in each class (that is, if the greatest land area is in the moderate-hazard class, more spots will occur in that class). During these periods of populations increase and decline, other forms of tree stress--disease, attacks by other beetle species (Ips spp., etc.), logging, drought, lightning (Fig. 7), and other such events--play an important role in spot initiation and growth.
	Figure 6. - Population levels strongly influence SPB activity within various stand hazard classes. (Based on approximately 500 SPB spots in a 182,000-acre east Texas sample taken in 1973-78.).
	Figure 7. - Other forms of tree stress, such as lightning strikes (left), disease, associated insects, and other natural or human-caused events play an important role in SPB spot initiation and growth. These factors may override site and stand influences and must be considered when evaluating stand hazard rating results. (F-705624).

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