Erin McCann's 2020 ESA talk.

Tree species are largely predicted to shift their ranges to higher latitudes and elevations to track shifting climate envelopes as global temperatures warm. However, many idiosyncratic responses have also been documented with some species migrating in unanticipated directions while others expand or contract their ranges. Understanding how and if species distributions will shift in response to climate change is critical for understanding future patterns of biodiversity and ecosystem function. Functional traits may be useful in explaining the variation in species’ responses and may make it possible to develop a trait-based predictive framework for range shift dynamics. In sessile, long-lived organisms, such as trees, life stages can be used as a proxy for time, with the seedling distributions substituting for the species’ future distribution. Here, we used Forest Inventory Analysis data to develop species distribution models of adults and seedlings for 25 tree species in the Western United States. We then assessed disparities between conspecific adult and seedling distributions and used these as an indication of species’ future range changes. Lastly, we evaluated variation in species' responses relative to dispersal traits (seed dry mass and dispersal mode).
Overall, we found significant disparities between seedling and adult distributions suggesting that Western US tree species are shifting their ranges in response to climate change. Specifically, we found that 18 of 25 species had seedlings occupying higher latitudes than adults, and 10 of 25 species had seedling ranges occupying higher elevations than adults, suggesting some species may be moving to cooler climates. These disparities between seedling and adult ranges were in part predicted by dispersal traits, where animal dispersal was more commonly associated with northward migration than wind or gravity dispersal and seed dry mass was positively correlated with species expansion to higher elevations. Lastly, we found that for 12 species, adult distributions existed at higher elevation than their conspecific seedlings and only 3 of the 25 species we modeled showed no differences in latitudinal distributions between life stages. Overall, our results highlight the varied nature of tree migration patterns in the face of climate change and highlight that dispersal mode may be a critical determinant of species ability to effectively colonize areas outside of their current range.

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