Technical Commentary: Management of Lodgepole Pine Stand Density to Reduce Susceptibility to Mountain Pine Beetle Attack

Silvicultural strategies to reduce tree losses from mountain pine beetle attacks typically seek to reduce relative densities in order to increase tree resistance and thus lower potential beetle attacks. For lodgepole pine stan&, however, the tree mortality/relative density relationship is nonlinear. We describe three relative density zones corresponding to different levels of resistance to beetle attack. In the density management of young lodgepole pine stands, we propose two alternative strategies to reduce future losses from mountain pine beetle attacks. The first density management regime involves a low density (i.e., SDI 245). West. J. Appl. For. 11(2):50-53. Bark beetles in the genus Dendroctonus (Coleoptera: Scolytidae) are among the most damaging insects in western North American coniferous forests. The mountain pine beetle (MPB), D. ponderosae, is one of several aggressive species in the genus capable of sustaining outbreaks over large acreages, especially in lodgepole pine (Pinus contorta) forests. Silvicultural strategies designed to lower lodgepole pine stand susceptibility to beetles are effective prior to or in the early stages of outbreaks. Little can be done to reduce mortality once populations reach outbreak levels. The mountain pine beetle plays an important ecological role in lodgepole pine forests. Endemic populations of beetles act as thinning agents, attacking large older trees, creating openings for regeneration, and contributing fuel for eventual stand-replacing fires. Such trees generally have thin phloem, thus limiting brood production and maintaining beetle populations at endemic levels (Amman 1972). It is not known how populations witch from endemic to epidemic levels. During short-term disturbances, otherwise healthy, thick phloemed trees may become susceptible to beetles, thereby allowing the population to increase (Berryman 1982). In an epidemic, most trees greater than about 6 in. in diameter may be attacked (Cole and Amman 1980). NOTE: This research was supported by the Utah Agricultural Experiment Station, Utah State University, Logan, Utah 84322-4810. Approved as journal paper no. 4775. Amman et al. (1977) developed asystem of rating susceptibility of lodgepole pine stands to beetle attack using stand age and diameter as the principal indicators along w•th elevation and latitude. Commonly accepted silvicultural strategies to lower stand susceptibility to mountain pine beetle were greatly influenced by the work of Cole and Cahdl (1976) and Amman et al. (1977). Silvicultural strategies that alter tree species composition, age class distributions, and average stand diameter were summarized by Cole (1978) However, much of the basic research is based on mature unmanaged stands with high beetle populations and may not be germane to younger managed stands. A study by Anhold and Jenkins (1987) found that the tree mortality/relative density relationship for lodgepole pine stands is nonlinear Specifically, stands with either low or fairly high relative densities are resistant o beetle attacks. Conversely, stands with intermediate relative densities are susceptible to beetle attack. These results suggest alternative silvicultural strategies for "beetle-proofing" lodgepole pine stands. This paper describes two alternative density management regimes •ntended to reduce future losses from mountain pine beetle attacks in lodgepole pine stands. Silviculturists use indexes of relative density to characterize stand structure. The most effective of these indexes combine some expression of mean size (e.g., mean volume, height, or diameter) and absolute density (i.e., trees per umt area). A commonly used index of relative density is SDI,