| Whitebark pine (Pinus albicaulis), a high elevation foundation species, is experiencing population declines throughout the northern part of its range. The introduced fungal pathogen, Cronartium ribicola (white pine blister rust), infects whitebark pine and kills cone-bearing branches and trees. Blister rust has spread nearly rangewide and damage and mortality are highest in the northwest US and southwest Canada. Mortality caused by mountain pine beetle (Dendroctonus ponderosae) population upsurges, and successional replacement and loss of regeneration opportunities from fire suppression, are also impacting some whitebark pine populations. Within this dissertation, I present three manuscripts that address the impact of whitebark pine's decline on species interactions and ecological processes within subalpine forests. Research was conducted in three ecosystems in the Rocky Mountains USA that are distinct in whitebark pine health conditions (rust infection and mortality) and abundance. In the first manuscript, I explore how the relationship between whitebark pine and Clark's Nutcracker (Nucifraga columbiana), its primary seed disperser, is being affected by whitebark's decline. Nutcrackers were less likely to use and disperse seeds from forests where cone production is below a threshold. In the second manuscript, I describe habitat use of whitebark pine forests by red squirrels (Tamiasciurus hudsonicus). Squirrel residency and impact of cone predation increased with decreasing whitebark pine abundance. The third manuscript focuses on the tree-level ecological process, predispersal cone survival, as a function of coarse scale whitebark pine abundance. Surviving trees in high mortality forests were found to have a lower rate and higher variability of cone survival, suggesting that the putative levels of rust-resistance in surviving trees of high mortality forests may not be passed on to future generations. At the ecosystem level, the Northern Divide had the highest levels of rust infection and tree mortality and lowest nutcracker interaction and regeneration levels; the Greater Yellowstone had the lowest infection and mortality levels and nutcrackers were present and dispersing seeds at all research sites in all years, while the Bitterroot Mountains were intermediate in these comparisons. These findings provide important components for developing a long-term strategy to conserve and restore whitebark pine ecosystems in the Rocky Mountains. |
