DRAFT - July 1999
BIOGEOGRAPHIC COMPARISON OF WESTERN NORTH AMERICA AND IRAN
Barbara Ertter
Curator of Western North American Flora
University and
Jepson Herbaria
University of California at Berkeley, USA
In May 1999, the unique opportunity presented itself to travel to
Iran, essentially off
limits to Americans for the last two decades, with invitations to speak at
the agricultural
colleges at several universities. The following essay is adapted from the
talk I chose to present,
comparing the biogeography of western North America (my personal area of
expertise) with
Iran, and ending with an illustrated overview of the biogeographic regions
of western North
America.
BIOGEOGRAPHIC SIMILARITIES
To fully understand the evolution and relationships of a floristic
area, it helps
considerably to make comparisons with similar floras in other parts of the
world. Iran is
uniquely suitable for comparing with western North America, as a result of
numerous
geographic, climatic, and geological similarities:
- Both regions are near the same latitude and about the same size, but
half a world apart.
- Both have large central plateaus with interior-draining basins, in
the rain shadows of
large mountain ranges.
- Both have Mediterranean-type climates, with the majority of
precipitation falling in the
winter.
- Both have complex geological histories, resulting in a great
diversity of current
geological and topographic features.
- Both owe much of their geological diversity to the actions of plate
tectonics, such that
both areas are still subject to frequent earthquakes. Much of western
North America is
in fact formed of "accreted terranes": former ocean bed and oceanic land
masses that
collided with the western edge of the original continent as the Pacific
Plate was subducted
under the continental margin.
HISTORICAL BIOGEOGRAPHY:
Along with climate, geology, and topography, which are primary factors
that determine
plant distribution, similarities between the floras of Iran and western
North America are
increased by historical biogeography. Both regions contain remnants
(refugia) of the broad
leaf forests that once covered most of the north temperate zone 25 - 50
million years ago, in the
early Cenozoic Era. As the climate and land forms changed, this forest
disappeared from most of
its former range, being replaced by shrublands, grasslands, deserts, and
conifer woodlands. The
two main areas where the mesophytic forest has persisted are southeastern
Asia and
southeastern North America, but smaller pockets also exist in northern Iran
(Hyrcanian forest) and along the west coast of North America.
During the Pleistocene ("Ice Ages"), glaciation had minimal impact on
either western
North America or Iran. Instead of the massive ice sheets that covered much
of northeastern
America, glaciers were largely restricted to high elevations in western
North America. The
more extensive Pleistocene event was the formation of large lakes in the
interior-draining
basins of the central plateau, most of which have now disappeared, in some
cases leaving behind
extensive alkaline flats. Furthermore, lowered sea levels resulted in a
land bridge between
Siberia and Alaska, allowing for the intercontinental migrations of cold
desert floras.
CURRENT VEGETATION:
As a result of the many similarities in geography, geology, climate,
and biogeographical
histories, over 250 genera of vascular plants occur as native in both
western North America
and Iran
(Appendix 1).
Prominent examples include Astragalus, Allium,
Potentilla, Fritillaria,
Viola, Eryngium, Epilobium, Trifolium, and Senecio. Several
species also are apparently native
in both areas, such as Galium aparine, Oligomeris linifolia, Hydrocotyle
ranunculoides, and
Oxyria digyna
(Appendix 2).
Both regions are also noteworthy for their overall floristic
richness and high degree of
endemism. According to Takhtajan (1986), endemism in the Irano-Turanian
floristic region is
probably no less than 25%, with the richest flora being that of the Iranian
Plateau. Endemism
in the Madrean Region, which includes the arid and semi-arid portions of
western North
America, is even higher, probably well over 50%. Plants new to science are
still being
actively discovered and described from both areas, at an average rate of 40
vascular plants per
year in western North America (Hartman & Nelson, 1998).
DIFFERENCES BETWEEN WESTERN NORTH AMERICA AND IRAN
Although the focus of this essay is on the biogeographic
similarities between western
North America and Iran, it is equally important to appreciate the many
differences and unique
biogeographic features of both regions. For example, the interior basin of
Iran is as a
generality hotter and drier than that of western North America, with larger
expanses of sandy
deserts and salt basins. The rainfall is on the average higher in western
North America, due to
the proximity of the Pacific Ocean and slightly higher latitude. As a
result, Iran lacks the
extensive conifer forests and numerous high mountain lakes that
characterize higher elevations
in western North America.
Each area has also developed species-rich genera that are completely
absent from the
other area. In western North America, such characteristic components of
the vegetation as
Eriogonum (Polygonaceae), Lupinus (Fabaceae), Lomatium
(Apiaceae) Phacelia
(Hydrophyllaceae), Penstemon (Scrophulariaceae) and
Castilleja (Scrophulariaceae), as well
as the entire family Cactaceae, are not found in Iran, where one encounters
instead such
species-rich genera as Onobrychis (Fabaceae), Nepeta
(Lamiaceae), Haplophyllum (Rutaceae),
Gagea (Liliaceae), Ferula (Apiaceae), Echinops
(Asteraceae), and Cousinia (Asteraceae). These
are only some of the most conspicuous examples of genera that represent the
differences
between the two areas, differences that are as important as any
similarities.
VALUE OF COMPARATIVE STUDIES
The combination of similarities and differences of two areas as
comparable as western
North America and Iran create significant research opportunities beyond
what could be learned
by studying either region alone. Comparative studies of this kind can
sometimes result in
critical or surprising information that might not otherwise have even been
suspected.
TAXONOMIC STUDIES: At the top of the list are taxonomic studies of
genera that occur in both
North America and Eurasia, especially those that have radiated into
numerous species. Such
groups provide an excellent opportunity for comparative studies of
independent radiations from
common ancestral stock, posing potentially significant evolutionary and
ecological questions of a
general nature.
Comprehensive taxonomic studies of species throughout the entire range
of a genus are
particularly critical for determining the phylogeny (relationships) of
species within the genus.
The closest living relatives of a species do not necessarily occur in close
geographic proximity,
but could potentially be found in another part of the world entirely. Two
excellent examples
from western North America are the following:
- Stroganowia tiehmii Rollins: In 1980, only a few miles off a
well-traveled highway 20
airmiles east-southeast of Reno, a population of an unknown perennial
Brassicaceae was
found growing in undisturbed sagebrush steppe. The plant was subsequently
determined
to be a new species of the central Asian genus Stroganowia, not
previous known to occur
in North America (Rollins, 1982). Three species of Stroganowia are
currently
recorded from Iran: S. afghana (Boiss.) Pavlov, S. litwinowii
Lipsky, and S. persica
Busch (Mozaffarian, 1996).
- Paronychia ahartii Ertter: When an inconspicuous
Paronychia was first encountered in
the Sacramento Valley of California, it was assumed to represent yet one
more introduced
Eurasian annual, although which one could not be readily determined. Only
when a
worldwide monograph of the Paronychiinae was available (Chaudhri, 1968) was it
evident that an anomalous undescribed species was involved. Although
apparently
restricted to the northern Sacramento Valley, and of unclear affinities, it
is most likely
that Paronychia ahartii is most closely related to species in the
Mediterranean Region or
Near East (Ertter, 1985).
NATURALIZED SPECIES: The biogeographic similarities that have
been noted have not only
resulted in the natural occurrence of numerous genera, and even some
species, in both western
North American and Iran, but they have also created suitable conditions for
species from one
area to become naturalized in the area. At least 375 species that are
apparently indigenous in
Iran have become naturalized in California
(Appendix 3),
either deliberately or
unintentionally. Many of these have become serious pest plants in western
North America (e.g.,
Avena fatua, Brassica nigra, Bromus tectorum, Centaurea solstitialis,
Chondrilla juncea,
Euphorbia esula, Lepidium latifolium, Lythrum salicaria, Potentilla recta,
Ranunculus
testiculatus, and Tamarix ramosissima), causing significant
agricultural losses, outcompeting
many unique native species, and threatening entire ecosystems.
Although Iran is not necessarily the primary center of
distribution for any of these
species, field work in Iran might potentially lead to significant insights
into why species that
are relatively benign members of their native ecosystems become serious
pest plants in
western North America. This understanding might then lead to the
development of techniques to
control infestations. This is an especially promising possibility in the
field of biocontrol, in
which the various insects and pathogens that checked infestations in the
original range are
deliberately introduced (after rigorous testing to determine host
specificity) to provide similar
control where the species has become naturalized.
COMPARATIVE ECOLOGY: By comparing the ecologies of two similar
regions, questions present
themselves that might not have otherwise have arisen by studying one area
alone. For example,
do the different impacts of cattle vs. sheep grazing contribute to the
tendency of Bromus
tectorum to form a monoculture in western North American, displacing
the native shrub-steppe
vegetation? And why are the higher elevations in Iran not forested, as
they are in western
North America? Is this due to climatic differences, or is it an indication
of prehistoric land
management practices? The answers to these kinds of questions can
obviously have great
significance to current land use decision-making.
LAND MANAGEMENT ISSUES
As noted above, opportunities for comparative studies can have direct
implications for
land management issues in both western North America and Iran. This is
especially apparent in
agricultural practices, in which crop plants developed in one area are
subsequently grown in
the other. Because of the significantly longer history of intensive
agriculture in Eurasia than in
western North America, most of this transfer of crop plants (and associated
weeds) has been
from the former to the latter. This has recently become increasingly
complicated by the advent
of genetic engineering, which triggers a largely unresolved set of
questions with legal, political,
economic, and ethical ramifications.
In addition to agriculture, the economies of both regions depend
heavily on other land
management practices, including grazing (mostly sheep in Iran, cattle in
western North
America), mineral and petroleum extraction, timber production and harvest,
and hydroelectric
power generation. Water issues can become important in both regions, given
the competing
demands on a limited and finite water supply. Wild harvesting of medicinal
herbs, which has a
long tradition in Iran, has recently come to prominence in western North
America as well.
As the population of both western North America and Iran continues to
increase, all of
these land management practices come into increasing conflict with the
conservation of each
region's rich floristic heritage. As a consequence, it may be that the
most valuable long-term
result of comparative studies is the insights potentially gained that can
address both the needs of
the human populations and the preservation of each region's richly unique
natural heritage.
PICTORIAL OVERVIEW OF WESTERN NORTH AMERICA
The remainder of the lecture presented in Iran consisted of slides
showing the geography
and vegetation of the following regions:
- Coast Ranges of California and Oregon: coastal terrace; steep
mountains rising directly
from the sea; mixed grassland and shrubland communities; oak woodland; relict
mesophytic forest in southwest Oregon; redwood forest
- Central Valley of California: original vegetation dominated
by Atriplex and other shrub
communities, with abundant displays of spring annuals; conversion to
agriculture
- Sierra Nevada: transect from riparian communities in
foothills to high elevation alpine
zones
- Cascade Range: differs from Sierra Nevada in being of
volcanic origin
- Columbia Basalt Plateau: formed from massive outpouring of
basalt, in rain shadow of
Cascade Ranges
- Northern Rocky Mountains of Central Idaho: transect from
shrub-steppe community in
foothills, dominated by Artemisia tridentata, to open conifer
forests of mid-elevations,
to dense conifer elevations around high elevation meadows
- Great Basin: cold desert shrub-steppe in rain shadow of
Sierra Nevada; dry lake-bed
surrounded by Atriplex scrub, giving way to Artemisia;
pinyon-juniper woodlands on
mountains, with other conifers and even alpine zones on highest peaks
- Mojave Desert: hot desert south of Great Basin; characterized
by Larrea tridentata
(creosote bush) and Yucca brevifolia (Joshua tree)
- Sonoran Desert of Arizona: characterized by periodic summer
thunderstorms,
conspicuous cacti
- Colorado Plateau of Utah: characterized by dramatic eroded
landscape, forming the
backbone of several national parks (e.g., Grand Canyon, Bryce)
ACKNOWLEDGEMENTS
Full credit for the opportunity to visit Iran and to present the
lecture on which this
essay is based must be given to Dr. Fosiee Tahbaz, who solicited the
original invitations and then
handled subsequent arrangements with Iranian authorities. Significant
acknowledgments are
also earned by Dr. Brent Mishler (Director of the University and Jepson
Herbaria) for his
backing of the Iranian visit, and by Dr. Razani (Permanent Mission of the
Islamic Republic of
Iran to the United Nations) for his efforts in making the visit possible.
The encouragement by
Dr. Peter Raven at all stages of the undertaking has also been significant.
Our hosts at the
various Iranian universities, as well as the overwhelming reception by
Iranian people in
general, are deeply appreciated. The generous gift by V. Mozaffarian of
his compilation of
Iranian plant names greatly simplified comparisons of the Iranian and
California floras.
Airfare to and from Iran was funded by the Committee for Research and
Exploration of the
National Geographic Society, grant 6489-99.
LITERATURE CITED
Chaudhri, M.N. 1968. A revision of the Paronychiinae. Meded. Bot. Mus.
Herb. Rijks Univ.
Utrecht 285: 1 - 440.
Ertter, B. 1985. Paronychia ahartii (Caryophyllaceae), a new
species from California.
Madroño 32: 87 - 90.
Hartman, R.L. & B.E. Nelson. 1998. Taxonomic novelties from North America
north of Mexico:
a 20-year vascular plant diversity baseline. Monogr. Syst. Bot. Missouri
Bot. Gard. 67: 1 - 59.
Hickman, J.C. (editor). 1993. The Jepson Manual: Higher Plants of
California. University of
California Press, Berkeley.
Mozaffarian, V. 1996. A dictionary of Iranian plant names: Latin,
English, Persian. Farhang
Mo'aser (publ.): Tehran.
Rollins, R. C. 1892. A new species of the Asiatic genus Stroganowia
Cruciferae) from North
America and its biogeographic implications. Syst. Bot. 7: 214 - 220.
Takhtajan, A. (transl. by T.J. Crovello et al.). 1986. Floristic regions
of the world.
University of California Press: Berkeley.