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Abstract:
A species’ climate niche summarizes the observed climatic conditions at its range limits. This information can be used to predict range shifts of species under climate change, but it does not explain why they occur under a given climate or are absent from another. Functional traits associated with the climate niche, however, allow for such an explanation. We show that key plant functional traits predict the climate ranges of North American trees and discuss the underlying filter mechanisms that define “no-go areas” for specific trait expressions. This approach replaces species idiosyncrasy by the generality of traits, puts biogeography on more functional grounds, and yields products that will serve the improvement of next generation global vegetation models. Using functional traits to explain species’ range limits is a promising approach in functional biogeography. It replaces the idiosyncrasy of species-specific climate ranges with a generic trait-based predictive framework. In addition, it has the potential to shed light on specific filter mechanisms creating large-scale vegetation patterns. However, its application to a continental flora, spanning large climate gradients, has been hampered by a lack of trait data. Here, we explore whether five key plant functional traits (seed mass, wood density, specific leaf area (SLA), maximum height, and longevity of a tree)—indicative of life history, mechanical, and physiological adaptations—explain the climate ranges of 250 North American tree species distributed from the boreal to the subtropics. Although the relationship between traits and the median climate across a species range is weak, quantile regressions revealed strong effects on range limits. Wood density and seed mass were strongly related to the lower but not upper temperature range limits of species. Maximum height affects the species range limits in both dry and humid climates, whereas SLA and longevity do not show clear relationships. These results allow the definition and delineation of climatic “no-go areas” for North American tree species based on key traits. As some of these key traits serve as important parameters in recent vegetation models, the implementation of trait-based climatic constraints has the potential to predict both range shifts and ecosystem consequences on a more functional basis. Moreover, for future trait-based vegetation models our results provide a benchmark for model evaluation.
