The taxonomic status of Timmia sibirica

A critical study of the differentiating features of the North American populations of Timmia sibirica Lindb. et Arnell and T. norvegica Zett. reveals that populations with structure intermediate between these two taxa occur. Although from widespread localities, the intermediate specimens seem limited in number and T. sibirica and T. norvegica quite commnonly occur in mixed populations with no evidence of any intergradation. Therefore, it is concluded that the two taxa ought to be regarded as distinct species. The taxonomically important character-states of upper leaf cells and costa of T. sibirica. T. norvegica, and of the intermediate populations are illustrated with scanning elecron micrographs. The habitat of T. sibirica, like that of T. norvegica, is invariably on strongly calcareous substrates adjacent to waterfalls, along streams, or in low-lying tundra. The North American distribution of T. sibirica is mapped, including previously unreported localities as far south as 51° N in the Canadian Rocky Mo...