Accurate prediction of climatically-driven range shifts requires knowledge of the dominant forces constraining species ranges, because climatically controlled range boundaries will likely behave differently from biotically controlled range boundaries in a changing climate. Yet the roles of climatic constraints (due to species physiological tolerance) versus biotic constraints (caused by species interactions) on geographic ranges are largely unknown, infusing large uncertainty into projections of future range shifts. Plant species ranges across strong climatic gradients such as elevation gradients are often assumed to represent a trade-off between climatic constraints on the harsh side of the range and biotic constraints (often competitive constraints) on the climatically benign side. To test this assumption, we collected tree cores from across the elevational range of the three dominant tree species inhabiting each of three climatically disparate mountain slopes and assessed climatic versus competitive constraints on growth at each species’ range margins.
We found that mean growth typically increased or decreased monotonically across tree elevation ranges, and that some species did show an apparent trade-off between a climatic constraint at one range margin and a competitive constraint at the other. However, even these simple elevation gradients resulted in complex interactions between temperature, moisture, and competitive constraints that sometimes precluded a simple interpretation. Our results suggest that tree species can be constrained by a simple trade-off between climate and competition, or even by dual climatic constraints due to contrasting drought stress and cold stress gradients. However, the intricacies of real world climate gradients and complexities of tree life histories complicate the application of this theory even in apparently harsh environments, such as near high elevation tree line.