METAL CYCLING IN MOUNTAIN PINE BEETLE IMPACTED WATERSHEDS

Rapid changes in tree mortality rates, such as those induced by bark beetle infestations, may have long lasting impacts on the hydrological and geochemical cycles within affected areas. In Rocky Mountain watersheds impacted by the mountain pine beetle, tree death occurs an average of three years after the initial infestation. In this short period of time the trees stop transpiring, defoliate, and die. The rapid deposition of pine needles to the forest floor, and subsequent decomposition of the needles, increases organic carbon (OC) availability and release metals that are stored in the impacted watersheds. Consequently, both OC and metal fluxes into and through the beetle-infested watersheds, may be larger than those in non-infested watersheds. The duration of the increased fluxes is unknown. They may end quickly or persist while the watershed strives to re-equilibrate. We used geochemical modeling coupled with field and lab observations to explore the relationships between this increased OC flux and the retention and/or loss of metals within the soils to estimate changes in water quality over time. Four watersheds along Keystone Gulch Rd., located in Keystone, CO, were chosen for soil and water sampling because of their similar bedrock, drainage area, tree density and type, aspect, and their varying degree of pine beetle infestation. Sequential extractions using simulated rainwater, MgCl₂, and pyrophosphate (representing soil pore water, exchangeable fraction, and organically bound metals) were performed on the Keystone Gulch soil samples to develop a better understanding of the distribution of metals in soils. Samples were classified by degree of beetle impact within and between the watersheds. The most obvious differences in the soil extractions between the four watersheds were observed for aluminum and iron and to a slightly lesser extent copper and zinc. In general, aluminum, iron, and zinc concentrations were higher while copper concentrations were lower in soils from less beetle-impacted watersheds. Metal concentrations in stream waters will be evaluated in the context of metal mobility through and out of the watershed.