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Abstract

Determining the factors that govern a species range limit is essential for predicting their migration with climate change. Pseudotsuga menziesii var. glauca (Interior Douglas-fir) is an ecologically dominant tree species central to the timber industry in British Columbia, whose range extends to the northern edge of the Interior Plateau, where it abruptly stops. My hypothesis for this range stagnation, informed by population genetics models, is that steep environmental gradients limit adaptation of the edge populations due to genetic drift and a heightened cost for selection. This study investigates the extent to which steep climatic gradients limit the northward range expansion of P. menziesii var. glauca. I compared the spatial variation across the species range boundary for five climatic variables at the northern edge to the other edges. Using LOESS models, I analyzed climatic changes within rectangular transects perpendicular to the range boundaries. I find that the environment varies across the northern edge of the P. menziesii var. glauca more gently than it does across the other edges, suggesting that gradient steepness does not limit expansion at the northern range edge. Ultimately, these results imply that the P. menziesii var. glauca northern range limit is governed by factors other than climate abruptness and requires further investigation.