RANGE SHAPE AND RANGE ELONGATION OF NORTH AMERICAN TREES

Understanding the characteristics of geographic ranges is a basic premise of all biogeographical research. One such measure of geographic ranges is range shape. The shapes of ranges reveal processes that affect the habitable limits of their respective species. I investigated the relative range elongation, with respect to maximum north-south distance and maximum east-west distance, of 269 North American tree species. These data provide evidence that species with smaller ranges tend to be elongated north-south and species with larger ranges tend to be elongated east-west. These results are similar to previously published data on land mammals and land birds of North America, but differ from data published on North American ectotherms. This pattern is likely a result of species with small ranges being limited by local north-south-trending topography and species with large ranges being limited by major east-west-oriented climatic zones, and is not merely an artifact of Rapoport's Rule. Previously, range elongation has been limited to graphical representation with no ability to compare range geometry with other biogeographical characteristics. Now, an explicit measure of range shape exists to compare to other characteristics.