The last half-century has witnessed substantial alterations in the disturbance regimes of western forest ecosystems resulting from both changes in climate and human activities. While much attention is being focused on disturbances like fire and floods, eruptive bark beetle species have had equal or greater impact on western forest communities. Like all insects, bark beetles are highly sensitive to slight shifts in temperature, and recent warming trends may have resulted in increased outbreak frequency and range expansion into environments that were previously unoccupied by the beetles, such as high-elevation and high-latitude forests. Mountain pine beetle outbreaks are more common in lower elevation lodgepole pine forests but are thought to be relatively rare events in whitebark pine ecosystems. Outbreaks of mountain pine beetle in whitebark pine may be somewhat novel, with only a few documented outbreaks over the twentieth century. In recent years, however, mountain pine beetle outbreaks appear to be occurring more frequently in whitebark pine forests. The recent widespread appearance of mountain pine beetle in whitebark pine forests may be the result of changes to the life-cycle characteristics of mountain pine beetle or host susceptibility brought about by a warming climate. The long-term dynamics of mountain pine beetles in forests and the potential role of climate variability remains little understood because outbreaks are infrequent and historical records rarely extend beyond the twentieth century. This limits the number of events that can be examined and hinders the search for potential mechanisms producing outbreaks. This collaborative research project will increase the record of mountain pine beetle outbreaks in two watersheds using paleoecological reconstruction methods. The investigators will calibrate the historical record of mountain pine beetle outbreaks in common (e.g. lodgepole pine) and potentially novel (e.g. whitebark pine) forest ecosystems to extend outbreak histories further back in time by integrating dendrochronological techniques with lake sediment analyses. The use of these two paleoecological reconstruction methodologies takes advantage of the main strengths of each -- the annual resolution afforded by tree rings and the extraordinary depth of time that can be gleaned from lake sediments, which often exceed several thousand years. The investigators will develop detailed records of forest successional patterns following the most recent mountain pine beetle outbreaks using tree rings for comparison with longer records of vegetation and disturbance history available from pollen and insect remains found in lake sediments. Annually resolved reconstructions of summer temperature will be developed using tree-ring records from temperature sensitive trees to assess the role of climatic patterns related to bark beetle outbreaks. Records of successional patterns, climate variability, and beetle activity developed from tree rings will be compared with lake sediment records where the proxies overlap temporally to calibrate the sediment record and improve interpretations further into the past.
The results of this project will be relevant to management efforts in the forests where the study is conducted, and they will provide a number of longer-term benefits. First, the project will investigate the potential use of proxy records to reconstruct past mountain pine beetle outbreaks. Second, the project will provide a new method of pollen analysis that includes comparisons between instrumental and dendrochronological reconstructions of climate. The calibration of relationships between pollen and reconstructed climate from tree rings for approximately the last 1,000 years will strengthen inferences of climate conditions for periods of time for which instrumental and dendrochronological data are not available. Third, the results will be used to analyze forest successional dynamics following mountain pine beetle outbreaks and provide information regarding how vegetation composition may shift as a result of mountain pine beetle infestations. In addition to contributing to the science of beetle outbreaks, forest succession and climate controls, there will be direct management applications as a result of the research. The study will identify the linkages between mountain pine beetle outbreaks and summer climate conditions that can be used to better inform management personnel charged with developing plans for forest sustainability in the face of a changing climate. Whitebark pine is a keystone species in these ecosystems and is currently facing a number of threats including potential increases in mountain pine beetle activity. Understanding how climate, vegetation, and mountain pine beetles interact at long time scales will provide managers with critical information to develop management plans that could mitigate the deleterious impacts of a changing climate. The investigators will make the results accessible by working closely with resource managers throughout the study so that the managers are aware of the research as it progresses and can use significant findings in a timely manner.
