Western North American landscapes offer few opportunities for high elevation tree species migration

Background/Question/Methods

Rapid climate change represents a major threat to future forest composition and function in mountainous regions of western North America. For high elevation tree species, future availability of climatically suitable areas may become limited in mountainous regions where land area decreases with elevation. Alternatively, the climatic complexity of mountain ranges may reduce the distance species must migrate to keep up with climate change. Species distribution modeling (SDM) offers a method for assessing potential future changes in suitable areas, but the utility of SDM has been questioned. By integrating forest structural information relating to tree dominance and demography with distributional data from the USDA Forest Inventory and Analysis (FIA) program, we increased our confidence in the SDM approach and predict future availability of climatically suitable areas for four broadly distributed tree species in the western U.S. We assessed contemporary patterns of tree demography (i.e., seedling density and proportion of species basal area that was dead) and dominance (i.e., basal area and importance value) in relation to species probabilities of occurrence (i.e., climatic suitability) to examine the potential for climate-induced range shift in tree species of the dry domain of the western United States.

Results/Conclusions

Predicted climatic suitability from SDM analyses were positively related to species basal area, importance value, and seedling density, supporting the use of SDM to describe habitat suitability of the study species. Climatic suitability of both inhabited and unoccupied landscapes surrounding established populations will likely decline dramatically for high elevation species, indicating that both species persistence and migration may be threatened. Therefore, climate change over the next century may severely reduce the extent of climatically suitable areas for high elevation tree species, while lower elevation species are well-positioned to shift upslope across the region. Reductions in suitable area for high elevation tree species are so extreme that even unlimited migration would be insufficient to offset predicted habitat loss in the United States, underscoring their vulnerability to climatic change. Thus, FIA plots offer an opportunity to identify potential range change and changes to forest biodiversity, carbon storage, and hydrology at regional and continental scales.