How do climate and dispersal traits limit ranges of tree species along latitudinal and elevational gradients?

Aim: We compared the upper range limits of tree species along latitudinal and elevational gradients to assess the influence of specific climatic factors - winter temperature, summer temperature and growing season length - in determining species range limits. We analysed the degree and direction of climatic mismatches between latitudinal and elevational limits to determine whether mismatches could be explained by species dispersal traits. Location: Eastern North America and the Great Smoky Mountains (Tennessee and North Carolina), USA. Methods: We determined the climatic limits for each of 28 common tree species along a latitudinal gradient in eastern North America and across an elevational gradient in the Great Smoky Mountains. We calculated the degree of climatic mismatch between species limits along the two gradients and tested for relationships with species dispersal traits (seed mass, maximum height, seed and pollen dispersal mode). Results: We found strong positive relationships between species latitudinal and elevational climatic limits. Winter temperatures were much lower at species poleward limits compared with their upper elevational range limits, but there were close matches between limits for variables related to summer temperature and growing season length. The degree of climatic mismatch was influenced by species dispersal traits; species with unassisted seed dispersal and low maximum height tended to have lower latitudinal than elevational limits. Main conclusions: Our results suggest that low summer temperatures and a short growing season length limit species distributions along both latitudinal and elevational gradients, whereas winter temperature does not play a critical role. The failure of some species with poor dispersal ability to grow as far poleward as expected based on their elevational limits supports the hypothesis that historical dispersal constraints may limit species distributions along the latitudinal gradient.