Impacts of mountain pine beetle outbreaks on lodgepole pine forests in the Intermountain West, U.S., 2004–2019

Mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae), is the most important forest insect in western North America. We determined causes and rates of tree mortality and changes in forest structure and composition associated with D. ponderosae outbreaks in the Intermountain West, U.S. during 2004–2019 based on a network of 125 0.081-ha circular plots installed in Colorado, Idaho, Montana, Utah and Wyoming. Incipient populations of D. ponderosae began in 2004; peaked in 2007; and returned to endemic levels in 2011 in Idaho, Montana, Utah and Wyoming. In Colorado, incipient populations began in 2004; peaked in 2009; and returned to endemic levels in 2012. A total of 5107 trees died, 98.6% were lodgepole pine, Pinus contorta Dougl. ex Loud. Fifteen contributing factors were identified, including (in order of importance, highest to lowest) D. ponderosae, unknown causes, pine engraver, Ips pini (Say), wind, breakage and/or adjacent tree fall, Pityogenes knechteli Swaine/Pityopthorus confertus Swaine, suppression, spruce beetle, Dendroctonus rufipennis (Kirby), root disease, western balsam bark beetle, Dryocoetes confusus (Swain), lodgepole pine dwarf mistletoe, Arceuthobium americanum Nutt. ex. Engelm., stem diseases, woodborers, North American porcupine, Erethizon dorsatum (L.), mule deer, Odocoileus hemionus (Rafinesque), and lodgepole pine beetle, Dendroctonus murrayanae Hopkins. Most tree mortality (68.8%) was attributed solely to D. ponderosae, although D. ponderosae also occurred in association with other contributing factors. Overall, significant reductions in mean dbh (by 5.3%), mean quadratic mean diameter (by 8.6%), mean tree height (by 15.9%), mean number of trees (by 40.8%), mean basal area (by 52.9%), and mean stand density index (SDI) (by 51.8%) were observed. Significant reductions in tree density were observed in all diameter classes, except the smallest (midpoint = 10 cm, 5-cm classes). Significant increases in the mean number of snags (by 1324.7%) were observed, and most snags remain standing (71.3%). Pinus contorta remains the dominant tree species, and while significant increases in the number of subalpine fir, Abies lasiocarpa (Hooker) Nuttall, seedlings and saplings were observed, a long-term shift in tree composition is unlikely. Tree mortality (number of trees killed) was positively correlated with the initial number of live trees, basal area of live trees, SDI, and aspect, but not slope or elevation. The implications of these and other results to recovery and management of P. contorta forests in the Intermountain West are discussed.