Status and dynamics of whitebark pine (Pinus albicaulis Engelm.) forests in southwest Montana, central Idaho, and Oregon, U.S.A

Whitebark pine (Pinus albicaulis) is a vital component of high-elevation forest communities across western North America, but declines in its health and dominance have raised concerns about the potential loss of this foundation species from many of the places it is currently found. The factors implicated as driving mechanisms of these declines include the exotic fungal disease white pine blister rust (Cronartium ribicola), outbreaks of the native mountain pine beetle ( Dendroctonus ponderosae), climate change, and fire suppression, but much of the research that links these mechanisms with whitebark pine declines is geographically restricted to the Northern Rockies, an important but relatively small part of the range of whitebark pine. My dissertation research developed baseline data on the status of whitebark pine communities and critically assessed the effects of blister rust, mountain pine beetle, and fire suppression on whitebark pine communities across the central distribution of the species. Specifically, I assessed (1) blister rust infection levels and the causes and rates of whitebark pine mortality, (2) patterns in the abundance and distribution of whitebark pine regeneration, and (3) patterns in disturbance, succession, and the effects of fire suppression on the structure and composition of the whitebark pine communities at my sites. Blister rust rates were generally lowest in western Oregon and highest in central Idaho. Mortality rates varied widely but mountain pine beetle activity was the primary cause of whitebark pine death at most sites. Whitebark pine regeneration was nearly ubiquitous and more abundant on cooler, drier sites with lower subalpine fir abundances and higher rates of mountain pine beetle-related mortality. Many of the stands I examined contained post-fire cohorts, but several stands also contained cohorts that established following episodes of mountain pine beetle-related mortality, illustrating the need for multiple lines of evidence when reconstructing fire history in whitebark pine forests. Patterns in succession and forest composition were strongly influenced by site-specific climate conditions and I found limited evidence of advancing succession due to fire suppression at my sites. The diverse and complex dynamics of whitebark pine communities require a nuanced discussion of its current and future status.