The Middle Asian Element in the Southern Rocky Mountain Flora of the western United States: a critical biogeographical review

Aim Presentation of an hypothesis suggesting that the extraordinarily similarity of the Russian Altai and the American Southern Rocky Mountain Flora represents an Oroboreal Flora; that had to have had an essential continuity across the northern part of the world in the Tertiary period, constituting a highland and steppe component of the better-known Arcto-Tertiary Flora of eastern and far-western North America and eastern Asia. Location North America and Middle (Altai) Asia. Methods Summarization of the author's field and herbarium studies of whole floras over a period of over 60 years, consisting of successive specializations in vascular plants, lichens, and bryophytes. Main conclusions (1) The modern alpine and associated marginal steppe and montane floras contain taxa of Tertiary age. (2) The floras of the southern mountains antedate those of the present-day Arctic. (3) The Middle Asiatic and the North American floras once enjoyed a contiguous existence over a broad area involving connections between North America and Asia across the North Pole by way of Greenland. Their present disjunctions are products of extinction and attrition of ranges, not of long-distance migration or dispersal mechanisms. (4) North-eastern North American disjunctions of so-called Cordilleran species (the Nunatak hypothesis) need not require explanations involving long-distance dispersal or migration, but represent relictual populations of the once widely distributed Oroboreal flora.