The Middle Asian Element in the Southern Rocky
Mountain Flora of the western United States: a
critical biogeographical review
William A. Weber* Herbarium COLO, University of Colorado, Campus Box 265, Boulder,
CO 80309, USA
Abstract
Aim Presentation of an hypothesis suggesting that the extraordinarily similarity of the
Russian Altai and the American Southern Rocky Mountain Flora represents an Oro-
boreal 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.
Keywords
Altai, Arcto-Tertiary Flora, Asa Gray, bryophytes, lichens, Nunatak theory, Oroboreal,
phanerogams, phytogeography, Rocky Mountains, W. J. Hooker.
INTRODUCTION
Similarities in floristic composition between the Asiatic Altai
and the Southern Rocky Mountains were first noted by Sir
Joseph Dalton Hooker, whose impressions are discussed
below. In a narrative of his travel across Siberia, Cockerell
(1927) mentioned many genera and species of plants which
he found to be held in common. Holm (1923) treated the
alpine flora of the Colorado Rockies with respect to its oc-
currence world-wide, but no link has yet been claimed for a
relationship to Asa Gray’s famous connections between
eastern North America and eastern Asia.
The intuitive perception of a positive correlation between
concepts that had previously gone unrecognized gives
perhaps more intellectual pleasure and satisfaction than
any other kind of scientific discovery. (Steere, 1969),
referring to Asa Gray’s realization of the relationship
between the plants of eastern North America and Japan.
The Southern Rocky Mountain Flora in Colorado is a
complex patchwork of overlapping distribution patterns,
most of which have reasonable geohistorical explanations
(Fig. 1). The most characteristic component that we associ-
ate with the name, Rocky Mountain Flora, comprises a
wide-ranging group of taxa whose distribution patterns may
be subdivided into the following elements: Boreal-American,
Central Rocky Mountain-Pacific Northwest and Madro-
Tertiary. Smaller groups include (1) Tertiary Relict, consisting
*Correspondence: William A. Weber, Herbarium COLO, University of
Colorado, Campus Box 265, Boulder, CO 80309, USA.
E-mail: weberw@spot.colorado.edu
Journal of Biogeography, 30, 649–685

2003 Blackwell Publishing Ltd

of endemic species concentrated along the eastern flanks of
the Colorado Front Range; (2) Chihuahuan, a presently
encroaching series of species, judging from their diminishing
numbers from south-eastern Colorado up into the Denver
Basin; (3) Sonoran-Great Basin steppe desert, encroaching
from the southwest and including a large endemic
segment that has evolved on the adjoining Colorado Plateau;
(4) Alpine-Desert Vicariads, which comprise alpine taxa
presumably derived from adjacent steppe species; and
(5) Relictual Eastern Woodland-Prairie species that ranged
northwesterly from eastern North America but were isolated
in the Front Range relatively recently by the desiccation of
the high plains (Weber, 1965).
A persistent mystery surrounds the origin of the alpine and
subalpine flora of the Southern Rocky mountains and to a
lesser extent the desert-steppe flora. This is the subject of the
present essay.
HISTORICAL
J. D. Hooker and the Rocky Mountain Flora
In the summer of 1877, during their journey west to Cali-
fornia, Sir Joseph Dalton Hooker and Asa Gray accepted
F. V. Hayden’s invitation to join up with his geological
survey field party in the Colorado Rocky Mountains.
Hooker collected c. 500 numbers of vascular plants.
The party also included Sir Richard Strachey and his wife.
Strachey was accompanying Hooker primarily to inspect the
system of narrow-gauge railroads and, incidentally, to
indulge his taste for natural history. They were en route to
India where General Strachey had travelled for many years
with the Royal Bengal Engineers (Turrill, 1953). Strachey is
commemorated botanically in a number of Himalayan plant
names, of which one species, coincidentally, Allocetraria
stracheyi, a lichen, has been discovered recently in Colorado.
It is very likely that Hooker’s astonishment concerning the
flora of the Rocky Mountains was shared by Strachey, who
undoubtedly knew much about the Himalayan flora and
may have played a significant part in the following story.
The Colorado portion of the trip was described (Huxley,
1918) as follows:
The journey was broken for a day at Cincinnati and at
St Louis, where the party was joined by Dr Lambourne,
Professor Leidy (the very great zoologist whom Huxley
swears by, who wants to explore the minute animals,
Diatoms, Rhizopods, etc., of the Colorado waters), his
wife and adopted daughter; Mr Hayden, head of the
Geological Survey, and Captain Stevenson, his chief
assistant. Then followed two nights and nearly two days
on the newly made railway to Pueblo across the prairies
along the Arkansas River. At Pueblo the Leidys went
north, the others to Can˜on City. Then Hooker and the
Figure 1 The Southern Rocky Mountains.

2003 Blackwell Publishing Ltd, Journal of Biogeography, 30, 649–685
650 W. A. Weber