Whitebark pine encroachment into lower-elevation sagebrush grasslands in southwest Montana, USA

Background: Projections for the future health and abundance of whitebark pine (Pinus albicaulis Engelm.) populations in western North America are dire. Not only has the species been declining due to the combined effects of fire exclusion policies, mountain pine beetle (Dendroctonus ponderosae Hopkins 1902) outbreaks, and white pine blister rust (Cronartium ribicola J.C. Fisch), many are predicting that climate change will amplify these effects and whitebark pine habitat may be lost in many areas. One method to evaluate the validity of future predictions is to examine current migration patterns of whitebark pine into surrounding non-forested areas. In this paper, we documented the findings of a case study surveying the encroachment of whitebark pine into adjacent lower-elevation sagebrush grasslands and higher-elevation subalpine meadows in southwestern Montana, USA. Results: We found abundant evidence of whitebark pine trees (>40 trees ha(-1)) in sagebrush grasslands below the mature seed source, but there was no evidence of any whitebark pine regeneration in subalpine meadows above the seed-source stand. Conclusions: These results are in stark contrast to the findings of statistical modeling studies that predict that whitebark pine will expand into higher-elevation areas as a response to climate change. We believe that the reason for this low-elevation encroachment may be in response to fire exclusion rather than climate change. Although observations in this case study are for a limited geographic area, they do serve to illustrate the complexity of predicting ecological responses of tree species to climate change.