Bark beetles are native insects that exert dramatic effects on forest ecosystems. These include contributions to basic ecosystem processes such as nutrient cycling and gap formation, impacts on biodiversity, and, at times, widespread mortality to millions of trees. The frequency and extent of intermittent bark beetle outbreaks appear to be influenced by several human impacts, including climate change, forest fragmentation, and various management policies. This research will use ground sampling, tree physiological and biochemical analyses, and satellite imagery to evaluate the effects of a major form of natural and human disturbance, fire, on susceptibility of lodgepole pine to the mountain pine beetle, and on mountain pine beetle reproduction. It also will test how fire injury affects reproduction by other, non tree-killing bark beetles that compete with mountain pine beetle. This work will contribute to our basic understanding of how disturbance agents interact in conifer forests, and provide useful information for resource managers. It will contribute to science infrastructure through the training of undergraduate students, graduate students, and postdoctoral associates. This interdisciplinary study will be conducted in the Greater Yellowstone Ecosystem, where it will interface with other collaborators from various institutions.
We sampled lodgepole pines that had experienced varying degrees of wildfire injury for attack and development by the mountain pine beetle, in the Greater Yellowstone Ecosystem. Wildfire predisposed trees to attack, but moderately injured trees were most preferred. This relationship was influenced by stand-level beetle population size: both healthy and fire-injured trees of all classes were attacked where populations were high, but no healthy trees, and only low- and moderately injured trees were killed where populations were low. The number of adult brood produced per female was likewise curvilinear, being highest in moderately injured trees. There was high intraspecific competition arising from the large number of beetles needed to overcome defenses in healthy trees, and high interspecific competition and low substrate quality in more injured trees. We conducted chemical analyses of lodgepole pines of various fire injury classes, in terms of resistance parameters against mountain pine beetle and its associated fungus. Fire injury reduced the total monoterpene concentration of induced but not constitutive phloem tissue. It also affected the relative composition of various monoterpenes in both induction and constitutive phloem. We developed a remote sensing model to estimate incremental proportional mortality due to bark beetles from remotely sensed imagery for all conifer species in the Greater Yellowstone Ecosystem. We used 300 plots of field data from 2006-2012 to estimate mortality as a function of the difference in the Normalize Difference Infrared Index (NDII) and Normalized Difference Vegetation Index (NDVI) between disturbance year (i.e., year of field sampling) and a previous year exhibiting minimal disturbance. We were able to predict percent mortality with R^2=0.826 using NDVI and R^2=0.837 using NDII. Remote sensing models were validated using a split sample approach, with a validation RMSE < 10% mortality. We found no relationship between mapped locations of fire and increased beetle mortality in unburned nearby fires in the years following the fire. The proportion of the landscape impacted by fire is likely not extensive enough to exhibit an increased signal in beetle damage, despite the demonstrated preference of beetles for fire damaged trees. Our analyses of mountain pine beetle dynamics in mixed whitebark - lodgepole pine forests showed differences both between their defensive chemistries and in beetle preference behaviors.
