Mountain pine beetle impacts on tree resources and growth release in lodgepole pine dominated subalpine forest

Over the past decade and a half Western North America has experienced a mountain pine beetle (Dendroctonus ponderosae Hopkins) (MPB) outbreak on a scale not previously recorded. Millions of hectares of lodgepole pine (Pinus contorta) in high elevation forests have been infested. Although bark beetles are an important part of the natural disturbance regime in this region, the current unprecedented level of tree mortality will have a significant impact on resources and light availability for the remaining live trees in the forest. This thesis investigates the impacts of extensive MPB-induced tree mortality on essential tree resources such as light, soil moisture, and soil nutrients, and how ensuing changes in these resources affect annual growth in surviving canopy-level trees. Using a decade-long chronosequence of mountain pine beetle disturbance in a lodgepole pine stand, it was revealed that increased annual growth does occur in surviving canopy-level trees surrounded by high levels of neighboring tree mortality, but not in control plots representing a healthy intact forest. In infested plots where annual growth increase occurred in survivors, an increase in inorganic soil nutrients was observed, specifically, immediately after the transition from an intact canopy to one that has lost all needles and fine branches as a result of tree death. The spike in soil inorganic nitrogen that occurred during this period may have been taken up in part by surviving trees, as suggested by significantly lower leaf C:N. Other tree resources showed increase trends but lacked significance when compared to control plots. A confounding factor in the study site was a gradient of decreasing basal area (BA) with elevation, leading to differing initial BA in stands prior to infestation. Increased annual growth in surviving trees following bark beetle infestation is not simply a product of increased soil nutrients. Other resources likely limiting the post-infestation timing of growth increase may include available light, which can be restricted to low levels even after canopy deterioration due to initial stand density and BA encompassing the surviving tree. This study highlights the importance of understanding pre-infestation stand characteristics, and suggests promising areas for future research (light environment, fluxes in soil moisture, nutrients and stand carbon storage)