Changes in litter inputs and decomposition in headwater streams during a mountain pine beetle infestation of whitebark pine

Headwater streams in forested landscapes are generally lacking in primary productivity and rely on allochthonous inputs to fuel secondary production. This close association with the forested landscape makes these streams sensitive to terrestrial disturbances. In the Greater Yellowstone Ecosystem (GYE) a climate change induced mountain pine beetle (Dendroctonus ponderosae, MPB) outbreak has been decimating whitebark pine (Picea albicaulis, WbP) forests over an extremely short time period. Among the possible implications of widespread WbP die-off, I predicted an increase in litter inputs to headwater streams and an increase in the quality (nitrogen content) of these litter inputs both of which would alter rates of organic matter processing and invertebrate communities in headwater streams. To test these predictions I quantified the mode of transport and rate of litter inputs to headwater streams in the GYE within ten streams distributed between two basins of differing levels WbP mortality and surveyed benthic invertebrate populations within those streams to determine whether MPB-mediated WbP mortality had an effect on detritivore communities. In addition, I carried out an experiment to compare differences in decomposition between needles from healthy WbP and those killed by MPB in a single stream within the high-mortality basin. Inputs of WbP needles were higher to streams in the basin with greater WbP mortality and these inputs were primarily entering streams via vertical transport. However, despite the increased quantity of inputs into these streams I found that invertebrate densities declined as WbP mortality increased. Furthermore, I discovered that naturally senescent needles from healthy trees decomposed faster than needles from MPB-killed trees despite higher nutrient levels in the MPB-altered needles. Although MPB attack does result in WbP litter with higher C:N than that which is associated with healthy trees, these needles may also contain elevated levels of defense compounds with insecticidal and anti-fungal properties. Further research is needed to determine whether possible MPB-induced increases in monoterpene concentrations affect needle decomposition. However, my results indicate that MPB infestations slow rates of nutrient processing in headwater streams despite an increase in litter quantity and quality.