Distribution and biomass dynamics of the alpine treeline ecotone across the western United States

The alpine treeline ecotone is primarily caused by climate at continental and regional scales, but at finer scales exhibits increasing complexity and linkages with abiotic factors. In order to better understand the dynamics and geographic properties of treeline, a three-pronged study was conducted. The study’s first objective investigated what factors account for local treeline variation by comparing a climate-based predictive surface with actual treeline elevation. After finding that a mean growing season temperature of 10.27°C was the strongest predictor of treeline at continental scales, statistical analyses attempted to model the differences between predicted and actual treeline elevation based on local climatic and geomorphological controls. Site elevation and latitude were found to correlate most strongly. Roughness also exerted positive influences, as well as precipitation when coupled with certain terrain factors. The second objective explored the distribution of treeline carbon at site scales by analyzing field data gathered in August 2009 in Glacier National Park, Montana. This confirmed the expected relationship of upright trees containing the highest biomass density, tundra the lowest, and krummholz in the middle. A high degree of dead organic matter was discovered, which may have important implications for treeline as a carbon source or sink when considering treeline’s slow decomposition rates, and how these will change under climate warming. An additional analysis investigated the potential for relative biomass gain with future treeline advance, based on site and regional differences in slope and environmental lapse rates.