METAL MOBILITY IN BARK BEETLE-INFESTED FORESTS

Recent mountain pine beetle (MPB) infestation has resulted in devastating tree death in the Rocky Mountains of the western United States and Canada. The resulting and extensive land cover change not only impacts the aesthetic and commercial value of affected forests, but also water and nutrient cycles in these important mountain watersheds. In addition to hydrologic changes, MPB infestation has the potential to alter terrestrial and atmospheric biogeochemical cycling associated with killed stands. Previous research has focused on changes in nutrient cycles, and suggests the potential for increased dissolved organic carbon (DOC), nitrogen, and base cation fluxes as trees are killed and decompose; however existing literature has not addressed heavy metal mobility as a result of these biogeochemical changes. Here, we investigate potential changes in metal mobility under different phases of attacked trees through a combination of field analyses, batch digestions and flow-through soil column experiments. Sequential extractions suggest the potential for leaching of heavy metals from soils during the early stage of tree death, but also for accumulation of some metals, such as zinc, from needle litter input. Column experiments found that large pulses of needle leachate can increase soil water metal concentrations, such as zinc, copper, and aluminum. These trends in metal mobilization from the column experiments were also consistent with soil water observations in the field. Geochemical modeling suggests that this observation can be attributed to complexation with DOC in pine needle leachate. Ultimately, the combined field, lab, and modeling results of this work provide insight into a possible water quality concern for important headwaters in the Rocky Mountain West.