Year of Award

2012

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Resource Conservation

Department or School/College

College of Forestry and Conservation

Committee Chair

Cory C. Cleveland

Commitee Members

Anna Sala, Sasha Reed, Solomon Dobrowski

Keywords

biogeochemistry, carbon, Dendroctonus ponderosa, disturbance, mountain pine beetle, nitrogen, nutrient cycling, Pinus albicaulis, whitebark pine

Abstract

Ecological disturbances can significantly impact biogeochemical cycles in terrestrial ecosystems, but the effects of the current widespread mountain pine beetle outbreak on ecosystem processes like carbon (C) and nitrogen (N) cycling are poorly understood. This is especially true in high elevation whitebark pine (WbP) (Pinus albicaulis) ecosystems of western North America. WbP has been described as a keystone species, providing a critical food source and regulating hydrologic regimes. However, widespread WbP mortality caused by the mountain pine beetle drives structural and physiological changes in WbP forests, which could result in shifts in pools and fluxes of C and N within these ecosystems. To assess the biogeochemical consequences of the mountain pine beetle outbreak on whitebark pine ecosystems, I measured above and belowground nitrogen and carbon pools and fluxes around trees at three different times since beetle attack, including unattacked trees. Litterfall inputs under beetle-attacked WbP trees were more than ten times higher than those under unattacked trees. In response, soil NH4+ concentrations in the organic horizons increased from 15 µg N/g soil under unattacked trees to 33 µg N/g soil under attacked trees. However, there were not significant differences in ammonium (NH4+) concentrations in the mineral soil horizons. Overall, soil nitrate (NO3-) concentrations were low and highly variable, but generally increased following beetle attack. Additionally, there was no change in microbial biomass N in the soil between attacked and unattacked trees, implying that changes in N cycling in response to the initial stages of WbP attack were subtle. Soil CO2 efflux rates were generally higher under unattacked trees, but overall, the similarities were more apparent than the differences. My results indicate that while beetle attack drives a large pulse of C and N canopy to the forest floor after beetle attack, changes in litterfall quality and quantity do not have immediate and profound effects on soil biogeochemical cycling. However, continuous observation of these important ecosystems will be crucial to determining the long-term biogeochemical effects of the mountain pine beetle outbreak.

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© Copyright 2012 Megan Keville