Landscape Level Dynamics in an Endangered Mountain Ecosystem, Northern Rocky Mountains, U.S.A.

The purpose of this dissertation research was to investigate the extent of natural and anthropogenic impacts on declining whitebark pine communities. My research used dendroecology to study multicentury changes in these threatened communities to assess current and past forest dynamics and the overlapping effects of white pine blister rust, mountain pine beetle, and climate change in the northern Rocky Mountains.

I created whitebark pine (host) and subalpine fir (nonhost) chronologies and collected species composition, stand structure, and forest health data in sites along a latitudinal transect through the Rocky Mountains in western Montana. The standstructure data indicate over 70% of whitebark pines at all sites are declining or dead. The high mortality of whitebark pines was caused by the overlapping effects of past (1880s, 1920s, and 1970s) and current mountain pine beetle outbreaks, and more recently by white pine blister rust infection. Whitebark pine populations, in the majority of our sites, are being successionally replaced by subalpine fir, grand fir, and Engelmann spruce in all levels of the forest strata (trees, saplings, and seedlings). Shade-tolerant trees began establishing between 150 and 300 years ago at all sites, a finding that suggests 20th century fire suppression is not responsible for the successional replacement of whitebark pine forests.

Whitebark pine and subalpine fir growth respond strongly to drought and precipitation. This indicates wet conditions from precipitation and snowpack melt in the previous summer enhance tree growth during the following growing season. My climate reconstruction results support other climate reconstructions, east of the Continental Divide, that found precipitation and Palmer Drought Severity Index (PDSI) have more of an influence on subalpine forest growth than temperature. The reconstruction data contribute an important high-elevation component to existing drought reconstructions from lower elevations in the northern Rocky Mountains. Whitebark pine communities should continue to be monitored as continued periods of drought will likely make whitebark pines more susceptible to mountain pine beetle attack and weaken their resistance to white pine blister rust infection.