Forest Management Options
This section provides information on managing forest stands where the mountain pine beetle is present or the stand is at risk of attack. Taking actions through thinning to prevent attack prior to mountain pine beetle arrival provides many more options in selecting the trees you want to keep. Forest stands are considered to be groups of trees over a larger area where trees are numerous enough that you are not considering the protection of individual trees, but rather managing the entire collection of trees. This section on Forest Management Options is not intended to focus on managing individual trees or trees in an urban setting. For information on urban or individual tree management see the drop down titled “Preventing MPB Attacks”.
Silvicultural manipulations (tree thinning):
Pine trees respond to mountain pine beetle by “pitching out” with pressurized resin. The ability of an individual tree to pitch out is directly related to that tree's general health and vigor. Therefore, thinning stands to reduce competition for light, nutrients, and water will enhance the vigor of residual trees and consequently promote resilience to beetle activity. Diversifying age classes and including tree species that do not host mountain pine beetle (such as western larch) will also minimize stand-level tree mortality.
Thinning must significantly open the stand to be effective. Thinning stands to about 80 square feet of basal area has been shown to be effective in many cases. Eighty square feet equates to about 19 feet between trees, when the average tree diameter is 11 inches. A consulting forester, DNRC Service Forester or Extension Agent can help you with this for your individual stand.
Research done in the late '80s [ Bartos, Dale L.; Amman, Gene D. 1989. Microclimate: an alternative to tree vigor as a basis for mountain pine beetle infestations. Res. Paper INT-400. Ogden, UT: USDA Forest Service, Intermountain Research Station.] showed an "immediate" benefit of thinning by creating conditions in stands (more light, heat, and wind movement) that beetles don't like very much. A year or so after thinning, an increase in tree vigor will generally make trees less susceptible to beetles. Thinning in the face of an on-going outbreak has also resulted in benefits to leave trees because beetles tend to avoid stands that are less densely stocked a benefit that is virtually immediate.
It should not be concluded that thinning will eliminate tree mortality in all cases. Cases have been noted where beetle populations were so extreme that mortality was high even in aggressively thinned stands. In the vast majority of situations thinning will reduce mortality, making it a highly recommended management option.
Blowdown is also a risk that should be considered in lodgepole pine thinning. Lodgepole pine does not have a deep root system. A tree’s root system anchors the tree to the ground and thus shallow rooted trees tend to be more easily blown down in heavy wind events. Trees grown in very tight spacing, which many lodgepole pine stands are, are also susceptible to blowdown when the stands are thinned. A consulting forester or DNRC Service Forester can provide assistance in assessing this potential threat.
Identifying and removing trees currently infested with mountain pine beetle offspring can directly reduce beetle populations in the stand. Accurate identification is critical for this tactic to be successful. Infested trees will commonly have an apparently healthy, green crown. Closer inspection might reveal pitch tubes on the bole, although these diagnostic structures are not always evident, especially in dry conditions. The presence of dry, granular, reddish-brown boring dust called frass between the bark flakes and surrounding the base of the tree is another good indicator of successful beetle attack. Peeling back the bark will expose galleries. Look for larvae, pupae, or adults under the bark to determine current infestation. Brood trees must be removed from the stand or treated as immature beetles can successfully continue development in a cut tree. Infested logs that must be left on site can be treated to kill the developing brood if they are still in the larval stage. Treatment includes any effort to speed up drying of the log. Such treatments usually include bucking logs in short lengths and splitting them. These split pieces need to be laid out in direct sunlight to allow for proper drying. Tarping or covering the logs with plastic has not been found to be effective in Montana. Drying logs will not work for trees the season after they were attacked since the beetles have almost completed development and drying won’t occur quickly enough to be effective. These trees need to be removed from the area. For more details see "Removal and Disposal of Infested Trees".
Mountain pine beetle will continue to develop in trees even after they are cut. Therefore, it is critical to remove infested logs from the site well before the next beetle flight period (June through August) in order to inhibit dispersal or take some action to treat the material (see above).