We compared the growth and physiological responses of cuttings of Populus kangdingensis C. Wang et Tung and P. cathayana Rehder originating from altitudes of 3500 m and 1500 m, respectively, when exposed to three ultraviolet-B (UV-B) radiation regimes: zero UV-B, ambient (4.5 kJ m(-2) day(-1)) and twice-ambient (9 kJ m(-2) day(-1)) biologically effective UV-B radiation for one growing season, to determine if Populus trees that are adapted to contrasting UV-B habitats exhibit different tolerances to enhanced UV-B radiation. Compared with cuttings grown without UV-B radiation, twice-ambient UV-B radiation significantly decreased total biomass, total leaf area and internode length in both species, whereas root/shoot ratio, leaf number, amount of photosynthetic pigments and ascorbate peroxidase activity were unaffected. Differences in responses were also observed between the study species. Cutting height increment, total biomass, total leaf area, free proline concentration and membrane damage assessed by electrolyte leakage were significantly more affected by the twice-ambient UV-B radiation in P. cathayana than in P. kangdingensis. However, specific leaf mass, amount of UV-B absorbing compounds and superoxide dismutase and guaiacol peroxidase activities increased more with increasing UV-B radiation in P. kangdingensis than in P. cathayana, perhaps reflecting important characteristics of species with low to moderate tolerance to UV-B radiation. Overall, the results indicated that P. kangdingensis, which originates from altitudes of 3500 m, has greater tolerance to enhanced UV-B radiation than P. cathayana originating from altitudes of 1500 m.