Pine Bark Beetle and Dwarf Mistletoe Infestation in a Remnant Old-Growth Stand

We examined the incidence of bark beetle (Coleoptera: Curculionidae: Scolytinae) colonization and subsequent mortality, and the presence of dwarf mistletoe (Arceuthobium vaginatum subsp. crytopodum) within a rare remnant old-growth stand of ponderosa pine. In 2011, 96% of the old growth trees were remaining with additional dense ingrowth of ponderosa pine (Pinus ponderosa Douglas ex. Lawson) within the understory of the oldest trees. We compared the old-growth stand to abutting stands of ponderosa pine: a low-density seed-tree cut and a high-density post-1900 second growth. The old-growth stand had levels of bark beetle colonization similar to the seed-tree stand, and both of these stands had significantly more bark beetles than the high-density stand. Across all sites, bark beetles were found in higher numbers in the 51-60 cm diameter class. We found a trend between the proportion of trees infected per site and the percentage of trees with bark beetle activity, although a direct relationship between a tree's dwarf mistletoe infection and beetle attack was not found. This work indicates that large, older trees are not immune to endemic bark beetle attack and subsequent mortality. In light of current efforts to implement forest restoration practices that favor older and larger trees, managers of ponderosa pine forests should be aware that risk from bark beetle attack may increase as basal area increases within the stand.