Still standing: Snagfall the first decade after severe bark beetle infestation
Figure 6— An adult mountain pine beetle treading pitch while excavating a larval gallery in Pinus ponderosa var. ponderosa.
Mountain pine beetle (Dendroctonus ponderosae) attacks began in 2003, peaked by 2006, and killed 78 percent of overstory lodgepole pine in 133 plots distributed across a range of stand and site conditions on the Fraser Experimental Forest. The attacks have caused one of the most dramatic changes in forest condition in western North American forests in more than a century. The severity and extent of mortality prompted efforts to salvage dead timber, reduce canopy fuels, and regenerate new forests.
USDA Forest Service scientists studied watersheds on the Fraser Experimental Forest in north central Colorado in order to understand the dynamics of snagfall across a range of forest conditions typical of high-elevation forests affected by severe bark beetle outbreaks. Standing live and dead lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelmannii) were tagged following the peak of a recent mountain pine bark beetle outbreak, and then snagfall was sampled 10 and 12 years later. Less than 20% of the mountain pine beetle-killed pines tagged at the beginning of the outbreak have actually fallen in the first decade since the outbreak.
This study provides new insights regarding lodgepole pine snag dynamics and refines expectations regarding post-outbreak ecological conditions and commercial opportunities. Based on observations at Fraser Experimental Forest and other high-elevation lodgepole ecosystems, it was estimated that one-half of the beetle-killed trees will fall within about 15–20 years after the peak of the outbreak. The findings will help develop projections of post-mountain pine beetle surface and standing fuels, snow and water yield, harvest possibilities, cavity and coarse wood wildlife habitat, stand regeneration, and carbon storage.