Mountain pine beetles (Dendroctonus ponderosae; MPB) are causing extensive mortality of whitebark pine (Pinus albicaulis) throughout the species’ range. In the highest mountains where these trees grow, they reach alpine treeline–the climatic boundary where growth forms transition from trees to shrub-like krummholz. Although treeline whitebark pine exist within a landscape of widespread MPB-caused mortality, these populations appear to have survived the most recent outbreaks. This observation motivated the hypothesis that stunted treeline growth forms escape MPB attack because their stems are too small to attract beetles, and that these habitats serve as refuges for whitebark pine. The purpose of this research was to determine if treeline ecotone populations could serve as refuges for whitebark pine from the most recent MPB outbreaks in the US Northern Rocky Mountains, and whether these populations represent the greatest potential for future whitebark pine population recovery. I surveyed mortality along 500 m transects at alpine treeline and other forest edges at 10 randomly selected sites in the US Northern Rocky Mountains. I found consistent mortality gradients at alpine treelines, but not at other edges. This supports the hypothesis that treeline habitats are refuges, and that treeline whitebark pine may avoid MPBs because of transitions in growth form. I observed limited reproductive output at alpine treelines, but these habitats may still serve as population refuges over long time periods. However, smaller surviving whitebark pine exist at greater density than dead ones (by ~200 per ha) throughout my study region, indicating a strong potential for these forests to recover from the recent MPB outbreaks in the short-term without a treeline refuge.

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Nov 15, 2018
Colin T. Maher
Fire Effects, Ecological - Second Order, Vegetation, Wildlife, Fire & Wildlife, Invertebrates, Mountain pine beetles