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Scale-dependence is recognized as a ubiquitous feature of ecological systems. Ecologists have traditionally hypothesized a hierarchy of factors affecting the composition of ecological communities, with biotic interactions exerting a dominant influence at fine spatial scales, and abiotic factors such as climate driving patterns at broad spatial scales. However, the role of biotic interactions at macroecological scales has been increasingly questioned, with many ecologists hypothesizing that biotic interactions may have discernable effects on species distributions. Here, I evaluate the relative effects of climate and species interactions on composition patterns of tree species in the US Rocky Mountains. At fine spatial scales, I model the radial growth of trees along montane ecotones and evaluate sensitivity to temperature, precipitation, and interspecific competition. Climate has an overwhelming influence on radial growth of all species, and interactions among co-occurring tree species appear to be weak. Scaling the effects of biotic interactions to macroecological scales presents a complex statistical challenge, and I demonstrate that commonly used community-level models are an inappropriate technique, as they average species responses and fail to accurately reproduce co-occurrence patterns. As an alternative to community-level models, I use a novel Joint Species Distribution Modeling approach to demonstrate that the co-occurrence patterns of Rocky Mountain trees are overwhelmingly explained by climate, with little influence of interactions among tree species. I review evidence for the factors shaping North American tree species distributions and argue that species interactions may fail to affect macroecological patterns among Rocky Mountain tree species due to a historical legacy that has promoted strong responses to climate. Current tree distributions predominantly reflect the influences of climate with a likely influence of human land use.

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