Abstract: Geographical range size is a fundamental ecological characteristic of a species, and the product of complex interactions of many factors in its history. Here, we investigate the causes of range size variation among 43 species of the woody plant genus Betula (birches), which each occupy areas of between one and 20 million square kilometers in the northern hemisphere. We find their distributions are more affected by temperature variables than by precipitation variables. The climatic niche breadth, median latitude, width of seed wings, degree of bark peeling, and ploidy of species all have significant impacts on range size variation, but number of leaf veins and life form do not. Many of these attributes, and range size itself, have a phylogenetic component and, once phylogeny is accounted for, ploidy no longer has a significant effect on range size, and climatic niche breadth is clearly the most important factor. Our results therefore support the niche-breadth hypothesis for range size variation and to a lesser extent also support the dispersal-ability hypothesis and Rapoport's rule that range size decreases toward the tropics. The climatic niche breadth of Betula species is likely to be a key attribute in their ability to avoid decline or extinction under climate change.

Key words: birch, climatic niche breadth, geographical range, phylogenetic signals, polyploidy.