Silvicultural treatments to reduce losses in existing stands
Good stand management offers the cheapest, most practical, and longest lasting means of preventing SPB infestations, especially where the beetle epidemics occur frequently. Recommended practices are:
- To reduce stand susceptibility and damage from southern pine beetle, periodically reduce stand density to increase or maintain tree vigor.
- Southern pine beetle infestations are often associated with poor tree vigor. Tree vigor is related to site, tree, stand, and environmental conditions.
- Poor tree vigor is usually associated with densely stocked stands and declining or slow radial growth.
- Other factors that affect vigor include age, species composition, soil texture, and type, drainage patterns, and stand disturbances associated with cultural practices.
- Thinning, especially of weak, less vigorous trees in lower crown classes, that are susceptible to southern pine beetle attack, reduces competition and enhances the vigor of residual trees.
- Thinning to reduce southern pine beetle hazard is recommended when stand basal area approaches 120 square feet/acre or when live crown ratios of dominant and co-dominant trees drop to about 40 percent.
- Thinning stands back to 60 to 90 square feet/acre basal area reduces the risk of attacks and may also help to slow infestation expansion (spot growth) if an attack does occur.
- For greater effectiveness, thinning should be conducted in winter when the beetle is least active.
- Any thinning strategy to reduce the risk of southern pine beetle attack should be compatible with management goals and consider such things as site and stand factors, equipment, seasonality, market price for small diameter trees, and product objective. Management of other potential hazard (e.g., fusiform rust, annosus root rot, Ips spp., and black turpentine beetle) that might conflict with recommendations for southern pine beetle must also enter into the decision-making process.
Timing of the First Thinning
- Precommercial thinning is probably justified in dense, natural stands and in plantations established by direct seeding or supplemented with natural regeneration from surrounding stands if there are 1,500 or more well-spaced seedlings per acre
- The first thinning often is performed as soon as the seedlings are well established, usually between ages 2 and 5 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 seedling numbers. The best response appears to be obtained with a residual stocking of 500 to 750 trees/acre.
- First commercial 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.
- 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. Generally this occurs at 13 to 18 years.
- Thinning guidelines for southern pines frequently suggest removing 30 to 45 percent of the stand basal area.
- The percentage of the basal area to be left tends to increase with increasing site quality because of greater productive capacity. Residual basal areas range from 60 to 90 square feet/acre and tend to be lower on poor than on good sites.
Thinning younger stands is an important tool in long term prevention of southern pine beetle.
Thinning older stands is an important tool in long term prevention of southern pine beetle.
Setting Control Priorities for the Southern Pine Beetle
|All southern pine beetle spots (groups of infested trees) do not have the same control priority. The following guidelines should help you set priorities for controlling individual spots.|
A. Classify the infested trees according to the stage of attack shown below.
B. Collect spot expansion data:
- Walk completely around the spot and look for stage 1 trees, which indicate the area of most recent beetle activity. Areas with stage 1 pines are called "Active Heads." Check to see if the spot is expanding in more than one direction. Large spots can have more than one "active head."
- Determine the number of stage 1 and 2 trees. For large spots that have more than 50 trees, it is not necessary to examine each tree. Just walk the boundaries and estimate the number of these trees in the spot.
- From a location about 20 feet (6 m) in front of the active head(s), determine the pine basal area (a measure of stand density) in square feet per acre. A 10-factor prism is useful for this purpose.
- Note whether most trees in the spot are pulpwood (less than 9 inches in diameter) (23 cm) or sawtimber size (more than 9 inches in diameter).
- If only stage 3 trees are present, control is not necessary.
- Determine the control priority for the spot. See C, below.
C. Guide to southern pine beetle control priorities (May through October):
||Key to spot growth
||Your spot's classification
||Stage 1 trees
||Stage 1 (fresh attacks)
and Stage 2 trees
||1 to 10
11 to 10
21 to 50
more that 50
||Pine basal area
at active head
||less than 80
80 to 120
more than 120
||Stand class by
(9 inches or less)
(more than 9 inches)
Add up the risk rating points that apply to your spot:
|70 to 100
|40 to 60
|0 to 30