Great Basin Naturalist Memoirs
Volume 2 Intermountain Biogeography: A Symposium
Article 12
3-1-1978
Problems in plant endemism on the Colorado
Plateau
Stanley L. Welsh
Life Science Museum and Department of Botany and Range Science, Brigham Young University, Provo, Utah 84602
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Welsh, Stanley L. (1978) "Problems in plant endemism on the Colorado Plateau," Great Basin Naturalist Memoirs: Vol. 2, Article 12.
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PROBLEMS
IN
PLANT ENDEMISM ON THE COLORADO PLATEAU
Stanley L. Welsh
Abstract.— The problem
of distribution
of plant
endemics
explored, especially as regards distribution of
is
Endemics per unit area are greater
data and a list of references is presented.
these plant species on elevational and substrate bases.
1980
m
and on
A summary
finely textured soils.
of
The Colorado Plateau has long been
known as a center for endemic plants. The
basis for that
volves
endemism most
certainly in-
unique features of the Plateau
the
Province, especially as regards climate, geo-
and position along migratory
Each of these features has played a
role, and the interpretation of the nature of
endemism in the region must reflect the
logical history,
routes.
part each has played. Climate
as
arid
for
region
the
is
considered
generally,
but
the
ince
stand as
moist
discontinuities
in
this
drainage system. Geology
is,
in
large part,
displayed in technicolor, with Mesozoic and
Cenozoic
thin
strata
dominating, and with the
cover of vegetation hardly obscuring
the geology.
Only
at
higher elevations
is
the
mantle of vegetation sufficiently dense as to
cloak the geological substrate. Volcanic intrusives
and extrusives are limited
in
area.
above
Main migratory routes into the plateau
the mountain sequences in the
Rocky Mountains along both the eastern
and western margins. The great river system
has provided a lane for movement of propagules both up and down stream. Great
Plains influence appears to have been
involve
mainly from the southwest. Hence, the flora
of the plateau
of
plants;
its
a function of the sources
is
Mohavean, Chihuahuan, Great
Rocky Mountain, Great
Plains,
My own
mountains within and bordering the Provarid middle portion of the Colorado River
at elevations
interest in plant
Basin, etc.
endemics within
more than two
began research leading
to a thesis on the vegetation of the Utah
portion of Dinosaur National Monument
(Welsh 1957). The geological control of
vegetation at lower elevations in the
plateau is readily apparent, but nowhere so
overwhelming as along the margins of the
Split Mountain anticline at Dinosaur Nathe Colorado Plateau began
decades ago
tional
when
I
Monument. The interaction of various
precipitation
to low annual
Sedimentary formations constitute the main
substrates
features of the plateau region.
through long periods of time has led to the
Canyons
into
the
of
grand
surfaces
of
size
are
entrenched
uplifted
the
plateau,
demonstration of edaphic differences on a
grand
scale.
Soil
formation
in
a
traditional
where formations are displayed along the
sense
margins
often merely residual parent material only
of
plateaus.
horizontal
or
gently
dipping
Broad anticlines, synclines, or
is
slightly
unknown. The substrate surface
is
modified from that a few inches be-
gentle to steeply plunging monoclines have
low the surface.
been eroded to expose hogbacks or cuestas
where vast stratigraphies are exposed in rel-
function differentlv from residual or parent
Museum and Department
and colluvium
materials.
Conditions for growth on the substrates
atively short distances.
'Life Science
Alluvium
of
Botany and Range Science, Brigham Young University, Provo, Utah 84602
191
GREAT BASIN NATURALIST MEMOIRS
192
available
often
are
rigorous
at
best.
Fre-
portion
No. 2
water which can be extracted
of
capable of establishment and reproduction are few in number. Competition is therefore limited, with
the harsh environments supporting very few
from
higher plant species.
soil
by a statement ascribed to the venerable Walter P. Cottam
concerning vegetation of the Mancos Shale
formation. While camped on that formation,
he is reported to have stated that, "there
were only four species of plants growing
here, and I don't know three-fourths of
them.'' This statement, whether true or not,
is indicative of the paucity of vegetation on
some of the formation, and of the peculiar
temperature, humidity, and opportunity for
quently,
those
This point
plants
illustrated
is
at
soil
capacity
field
greater.
is
creased species diversity on sands
is
In-
appar-
ently a function of the greater ease of es-
tablishment and better all-around water and
given
relationships,
equal
elevation,
establishment.
Glaciation has
major role
apparently not played a
the
in
highlands along
except
plateau,
its
plants through the Pleistocene has
possibility
in
at
in
of
been a
lower elevation
the
least
and indeed
portions,
the
in
Persistence
fringes.
most of the Plateau
Province.
there.
With this background, it seems both feaand necessary to attempt to quantify
the data on endemism. The data have been
Worst possible substrates available are the
derived from saline shales such as
members of the Mancos Formation and its
generated from information recorded on
herbarium labels of specimens deposited at
Brigham Young University and at the Na-
counterparts such as the Tropic Shale (Tu-
tional
nunk equivalent).
nian
nature of some of the entities growing
clays
ppm
are not
uncommon.
water
is
of
30,000
Additionally, the
manner when
added. A rainstorm of some six
perform
clays
contents
Salt
in
a
special
sible
Museum
of Natural History, Smithso-
Further
Institution.
from
taken
various
of references). Despite
(see
list
the
information
have been
data
taxonomic
available
is
treatments
all
attempts,
fragmentary,
few plants do grow on this substrate,
some of them continuing to flower and to
mature fruit at soil moisture levels below
and the conclusions derived from that information must be regarded as tentative. A
portion of the problem involves the nature
of endemics generally. Some of them are
highly restricted and they have been collected only once or a very few times. Information on substrate, elevation, geological
formation, and other pertinent information
is not available either on the herbarium materials or in the literature. Further, most
the arbitrary 15 atmosphere level accepted
plant
hours' duration supplied from 2 to 6 inches
of water in June 1972 to the Tropic Shale
east of
Glen Canyon City
in
Kane County,
Utah. Penetration into the clay of the Tropic
Shale
when wet
vide very
Still,
to
not
field
little
exceed
2
inches.
Even
capacity the clays pro-
moisture for plant growth.
a
as the
At
did
permanent wilting
higher
elevations,
where
taxonomists (and others
plants)
point.
rainfall
is
even these substrate types are overwhelmed by vegetation and a soil mantle is
developed which eventually is effective in
fication
not
are
of substrate
some
who
prepared
well
types.
in
The
collect
identi-
clays
and
investigators will be the sands
greater,
silts
insulating the vegetative cover from the ef-
and gravels of others. Despite the shortcomings of the information, there seem to
be trends which can form the basis for fur-
fects
of high
salinity
and from such sub-
stances as selenium.
Sandstones and their derivatives form the
other extreme in edaphic situations. Soluble
are fewer, selenium is more restricted,
water penetration is greater, and the prosalts
of
ther investigations.
The data summarized from
the
investigated are presented in Table
sources
I.
Only
34 families of vascular plants are known to
have endemic species
tal
number
of
The tosome 340,
in the region.
endemic
entities
is
INTERMOUNTA1N BIOGEOGRAPHY: A SYMPOSIUM
1978
an average of about 10 per family. The en-
demies are
far
from equally divided among
the various families.
The Boraginaeeae
(20),
Compositae (58), Leguminosae (86), Polygonaeeae (28), and Scrophulariaeeae (29) contain some 221 species or about 65 percent
of all endemic taxa. The greatest number of
taxa occurs in the
Table
1.
Summary
Legume
of plant
family.
The 86
entities constitute
193
some 25 percent of
all
of
those in the region.
The genus
taxa,
is
Astragalus,
with 62 endemic
not only the largest single contrib-
utor to the
a greater
list
of endemics, but contributes
number
of taxa than any family
beside the legumes. Most of those taxa oc-
cur at low elevations, contributing to the
endemics on the Colorado Plateau
194
GREAT BASIN NATURALIST MEMOIRS
idea
endemism
that
is
especially
great
elevation;
m
1980
a
quarter of them occur above
12 percent overlap, and
unknown. However, the total land area above 1980 m in elevation
represents only about 30 percent of the region. When endemics are expressed in per
unit area figures there is 1 endemic per
1170 km for the higher elevations and 1
per 1318 km below that elevation. Thus,
the incidence of endemics generally is
(6500
feet),
8.5 percent are
2
2
greater at the higher elevations.
The area
the
of
km
roughly 340,000
2
1000
km
1
2
Almost half (55%) of
the endemics occur on sand and gravel, and
a quarter on clays, shales, or muds. Those
that grow on both types constitute only
11.8 percent, and data is missing for some
.
8.5 percent.
It
difficult
is
derive data on the pro-
to
Atwood, and
Reveal
1975),
extinct,
endemic, and rare or restricted vas-
cular
part
of
those
threatened,
Utah are plotted by geo-
of
plants
endangered,
the
graphic subdivision of the
The
state.
large
represent endemic
plants
plants of Utah. Instructive from the plotting
is
the apparent unequal distribution of those
Main
plants in the state.
centers of distribu-
tion include the high plateaus of south central
Utah, the Canyonlands section, and the
A trend is present, however,
which indicates that the greater numbers
occur near the southern end of Utah, with
Uinta Basin.
progressively fewer taxa northward.
endemic plants are
Since
Colorado Plateau is
(133,000 sq mi), which
places the density of endemics at about
per each
In a previous paper (Welsh,
at
low elevations. Despite this apparent abundance of endemics at low elevations, only
slightly more than half of the entities are
known from below 1980 m (6500 feet) in
No. 2
likely
to
be
considered as endangered or threatened, and
since most of the lands within the Colorado
Plateau are federally controlled, then some
management
these
of
Perhaps
dicated.
is
it
specialties
possible
to
is
in-
establish
some predictive guidelines with regard to
location of endemics (i.e., where they might
be expected to occur). Outcrops of shale,
mudstone, and siltstone at all elevations
should be considered as suspect sources for
Any
portions of sands and gravels to other sub-
plant endemics.
and gravels
cover more of the land surface than do the
other substrate types. If one assumes that
as the lacustrine limestones at higher eleva-
strate types. Certainly the sands
peculiar substrate, such
western margin of the plateau,
tions in the
should be considered as important localities
only a quarter of the included region con-
for
and muds, then the endemics on clays occur at about 1 per 1000
km 2 Those on sands and gravels are present
generally be excluded from consideration in
of clays, shales,
sists
endemic
taxa.
management
Glaciated localities can
practices.
.
at a density of only
1
per each 1350
km
demics are especially rich on the finely
tured
soils
.
less
is
tex-
by the fabricated
the total area occupied by
are supported
data. Certainly
clays
References
2
impressions by collectors that en-
Initial
if
than 25 percent, then the en-
demics per unit area might be expected to
be very great indeed. Surprising is the
greater density of endemics in the montane
sections of the plateau. Distribution of en-
demics
tempt
area
is
to
is
hardly
at
random, and any
at-
provide averages or taxa per unit
likely
to
obscure
the
geographic
which result in the development
and placement of the taxa.
controls
Atuood, N. D.
1972.
group
crenulata
A
revision
of
the
(Hydrophyllaceae)
Phacelia
for
North
America. Great Basin Nat. 35: 127-190.
Barneby, R. C.
tragalus.
Beaman,
of
J.
1964.
Atlas of North
Mem. New York
H.
American As-
Bot. Card. 13: 1-1188.
The systematica and
1957.
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Townsendia (Compositae). Contr. Gray Herb.
183: 1-151.
Cronquist,
A.
J.
1947.
Revision of the North Amer-
ican species of Erigeron, north of Mexico. Brittonia 6: 121-302.
Harrington, H. D.
1954.
Manual
of the
plants of
Colorado. Sage Books, Denver. 666 pp.
1968.
New species of perennial
Higgins, L. C.
Cryptantha
195-198.
from
Utah.
Great
Basin
Nat.
28:
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INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1971.
A
revision
of
Cryptantha subgenus
Oreoearya. Brigham Young Univ.
Sci.
Bull.,
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Arizona
Flora. University of California Press, Berkeley.
Arizona.
B.
1973.
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of
Seed plants of northern
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Young
Welsh, S.
I.
1925.
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1957.
the
of
An
ecological survey of the veg-
Dinosaur National
Unpublished master's
Monument,
Brigham
thesis,
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Beveal.
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etation
Kearney, T. H. and B. H. Peebles.
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N.
1976.
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Atwood,
Endangered,
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