| Summary: | Shifts in northern vegetation with warming are well documented, particularly at tree-line where species often are at their physiological limits. I investigated ecological and evolutionary controls on understory species composition at tree-line in the Yukon Territories of Canada. I found that plant functional type abundance was more strongly influenced by aspect and elevation classes than by fine-scale soil parameters. South-facing, diffuse slopes, had greater shrub and lower bryophyte abundance than north-facing, abrupt slopes. I explored relationships between species diversity, co-occurrence, and overlap among aspect and elevational classes, and also examined the phylogenetic structuring of shrub species found on my gradients. I found that species occurrences, overlap, and phylogenetic structuring varied more strongly with aspect than elevation. Overall, my research emphasizes that both environmental and evolutionary controls influence plant communities at tree-line, and that differences between north- and south-facing slopes might make them respond uniquely to environmental change. Changing Cold Regions Network Northern Scientific Training Program
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