The niche breadth–range size hypothesis states that the geographic range size of a species is positively correlated with its environmental niche breadth. We test this hypothesis and examine whether the correlation varies with climate change and among taxa through modeling (processing Maximum entropy (Maxent)) potential distributions in the present and future climate scenarios of four sympatric Ostrya species in China and with different geographic range sizes, including extremely rare O. rehderiana. Potential geographical distributions of narrow- versus wide-ranged Ostrya species were predicted based on their niche breadths. Niche equivalency and similarity tests were performed to examine niche overlap between species pairs. Potential distribution areas of wide niche breadth species (O. japonica and O. trichocarpa) were significantly wider than those of narrow niche breadth species (O. multinervis and O. rehderiana) although niche divergence was hardly observed among them. In the future scenarios of global climate change, wide-ranged O. japonica would have wider potential distribution than in the current scenario, even expanding their geographic range. Conversely, suitable habitats of narrow-ranged O. multinervis and O. rehderiana would be reduced strikingly in future scenarios compared to in the current scenario, and they might be subjected to a high risk of extinction. Potential distribution range sizes of the Ostrya species would positively correlate with their niche breadths in future scenarios, and their niche breadths would determine their distribution variation with climate change. The Ostrya species having a broader niche currently would be further widespread in future scenarios while narrowly distributed Ostrya species having a narrower niche currently would further reduce their distribution range under changing climate and might be subjected to a high risk of extinction in future scenarios. Our results support the range size–niche breadth hypothesis both at present and future climate scenarios, and they provide a useful reference for the conservation of rare species like O. rehderiana.
