INVASION OF CHEATGRASS INTO
ARID ENVIRONMENTS OF THE
LAHONTAN BASIN
James A. Young
Frosty Tipton
ABSTRACT
1930's (papers summarized in a paper published in 1951).
Cheatgrass (bronco grass) was recognized as a major portion of the forage base for the Nevada livestock industry
by the 1940's (Fleming and others 1942). Robertson and
Pearse (1945), in what has become a benchmark paper,
reported that the presence of cheatgrass greatly reduced
the chance of seedlings of native perennials becoming
established. This paper was closely followed by the
Daubenmire (1946) paper on plant succession due to
overgrazing in the wheatgrass/bunchgrass vegetation
of the Columbia Basin.
It was well into the 1970's before public land management agencies recognized cheatgrass as a seasonal part
of the forage base on rangelands (Young and others 1987).
An exhaustive literature review of cheatgrass was published by Klemmedson and Smith in 1964. The status
of alien plants in the Great Basin was updated by Young
and others (1972) and again for the West by Mack (1981).
In 1975, Young and others (1975) estimated that 1 percent or less of the big sagebrush (Artemisia tridentata)l
bunchgrass rangeland of Nevada had been converted to
cheatgrass dominance.
During the last decade we have become aware that
the distribution and abundance of cheatgrass in the more
arid portions of the Lahontan Basin has greatly increased.
This increase has occurred in two distinct ecosystems.
One of these is areas of sand that support big sagebrush
in low-elevation areas that are atmospherically dry compared to the normal shadscale (Atriplex confertifolia)
zone. This spread has had a tremendous influence on
these ecosystems. The aspect that initiates these changes
is that the presence of cheatgrass provides enough fuel
for wildfires to spread in an environment that was previously considered to be fireproof. Burning in wildfires sets
off population dynamics that: (a) allow for at least an
initial increase in cheatgrass dominance; (b) allow for an
increase in other alien annuals such as Russian thistle
(Salsola iberica); and (c) set off dynamic increases in
stands of the native perennial grass, Indian rice grass
(Oryzopsis hymenoides ). The combination of the first
three processes results in a vast increase in herbage
and seed production per unit area for herbaceous species,
and greatly reduces the presence of woody species for prolonged periods.
During the last decade cheatgrass (Bromus tectorum),
an alien annual grass, has become obvious in certain of
the more arid portions of the Lahontan Basin. This paper
discusses aspects of this increase in range for cheatgrass
and speculates on the various interacting factors that may
have influenced this biological event.
INTRODUCTION
Cheatgrass (Bromus tectorum) was not collected in
western Nevada at the turn of the century (Kennedy and
Doten 1901). A new introduction must initially be rare
and the chances of cheatgrass being found soon after initially establishing in the Great Basin were slight, especially with few trained botanists visiting the area. Based
on the observations ofP. B. Kennedy, a trained botanist,
cheatgrass also was not present in northeastern Nevada
at the same time period (Kennedy 1903). Some students
of historical natural history interpret Griffith (1902) as
indicating cheatgrass was present in Humboldt County,
NV, in 1900. There is no question that some species of
alien annual grass was present in the mining camps he
visited, but it is difficult to positively identify cheatgrass.
Based on the memory of an elderly observer, cheatgrass
was first reported in Elko County, NV, in 1906 (Anonymous 1906). The presence of the grass was blamed on
the presence, the year before, of a band of tramp sheep
from the southern San Joaquin Valley of California. The
junior author's family considered cheatgrass to be abundant only on the margins of hay fields well into the 1930's
in Humboldt County.
By 1919, trained range scientists recognized that
cheatgrass was a major forage component on degraded
sagebrush/bunchgrass ranges (Jardine and Anderson
1919). Fleming (1922) recognized that Nevada ranges
were being severely overgrazed, especially by the large
range sheep industry. The relationship of cheatgrass,
overgrazing, promiscuous burning, and environment
was ·crystallized in a classic paper by Pickford (1932).
R. L. Peimeisel conducted a series of brilliant studies
on the successional stages that led to cheatgrass dominance on the Snake River Plains of Idaho during the
PURPOSE
Paper presented at the Symposiwn on Cheatgrass Invasion, Shrub DieOff, and Other Aspects of Shrub Biology and Management, Las Vegas, NV,
April 5-7, 1989.
James A. Young is a Range Scientist, U.S. Department of Agriculture,
Agricultural Research Service, Reno, NV 89512. Frosty Tipton is a
rancher, T Quarter Circle Ranch, Winnemucca, NV 89445.
Our purpose in this study was to examine some of the
interacting factors influencing the spread of cheatgrass
into this new environmental setting and to assess some
of the consequences of this spread.
37
This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
Climate Change
after observing "second crop" establishment of cheatgrass
from late rains.
If you accept that climate change played a role in
the spread of cheatgrass to more arid environments in
the Lahontan Basin then you must also accept: (a) the
changes in climate are subtle, and (b) the spread of cheatgrass must have been preconditioned by some other factor
because the magnitude of climatic variability observed
indicates that changes as large or larger have already
occurred in the recent past.
In this era of concern over global climate change, it is
popular to point to the changing climate to explain biological events. We do not totally discount the possibility
of climate change influencing the spread of cheatgrass
into the more arid portions of the Lahontan Basin, but
we have a hard time establishing a correlation between
available climatic data and such changes. For example,
compare precipitation at Winnemucca, NV, for the decades 1930-1940 and 1970-1980 (table 1).
The 40-year average precipitation for Winnemucca
for 1910 to 1940 was 8.20 inches (20.83 em) (Anonymous
1941). The 1930's have the reputation as a period of
drought in the Great Basin, but the average precipitation
for the decade was higher than for the 1970's. One would
expect that a prolonged period of above-average precipitation might condition the spread of a plant species into a
more arid environment. The 1970's must have been the
formative decade for the spread of cheatgrass into the
more arid environments of the Lahontan Basin, but this
prelude to population expansion occurred during slightly
drier than normal average precipitation conditions.
One reviewer of this manuscript was struck by the
variability in precipitation during the 1930's. The dry
years were drier and the wet years tended to be wetter
than the decade of the 1970's.
Average annual precipitation is a gross parameter for
relating to growth of cheatgrass. Experience in Nevada
has suggested that years of maximum production for
cheatgrass occur when the first effective rains in the fall
occur early enough in the season so that temperatures
are sufficiently warm for germination before the onset
of winter cold. This happens about once in 4 years. There
is no ·clear pattern showing that the frequency of fall germination has increased during the past decade. The variability in annual precipitation should be an index of the
variability in other parameters that govern the establishment and growth of cheatgrass.
Cheatgrass can greatly increase forage production and
presumably seed production from late-season rains. Both
the timing and amount of these late rains appear to produce few benefits for native herbaceous vegetation. The
original concept of cheatgrass seeds being environmentally induced to dormancy was made by the senior author
Table 1-Precipitation data for Winnemucca, NV,
Genetic Change
The thought of a sudden mutation that sets a killer
weed loose on the western range is a subject that allows
the imagination of newspaper reporters to run wild, but
does not seem to be supported by the facts. There is no
evidence of a point mutation with lines of spread of a
new adaptation of cheatgrass radiating out from a central
location. Point mutations are difficult to identifY and
even harder to assign functional significance. If the
plants of cheatgrass that compose the populations that
spread into the more arid environments are genetically
different from their counterparts that have occupied the
sagebrush/grasslands for a half century, then they must
be segregates from the basic gene pool that originally was
introduced to North America.
Across generations, changes in gene frequency by recombination of alleles through sexual reproduction constitute a second mechanism of genetic change in natural
populations. The most conspicuously successful alien
plants are predominately self pollinated (Stebbins 1957).
Analyses of the genetic systems of certain of the most
successful alien colonizing species of the annual grassdominated ranges of California, including several species
of brome, indicate that these systems represent a compromise between the high recombinational potential of outbreeding species and the stability traditionally postulated
for self-pollinated species. These species appear to be
capable of adjusting their genetic systems to obtain variability rapidly by virtue of ready modification of levels of
outcrossing, crossover rates, and other factors which govern recombination rates. Allard (1956) postulates that
successful colonizers have genetic systems optimum both
for opportunistic settlement and enduring occupation of
diverse, complex habitats. Cheatgrass has tremendous
phenotypical variability (Young and Evans 1976), and
ecotypic varability has been reported (Hurlbert 1955),
but inherent variability is not as easy to demonstrate as
with annual grasses such as medusahead (Taeniatherum
asperum) (Young and others 1970). Mack (1981) and his
students are in the process of greatly expanding our
knowledge of heritable variability in cheatgrass through
isoenzyme analysis.
for the decades
1930-1940 and .1970-1980
- - - - - - 1930-1940 - - - - - -
- - - - - - 19 70-1980 - - - - - - -
Year
Inches
em
Year
Inches
em
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
4.87
8.70
5.67
9.07
10.59
8.76
9.47
11.96
7.77
11.69
8.86
12.37
22.10
14.40
23.04
26.90
22.25
24.05
30.38
19.74
29.69
22.50
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
9.17
6.62
7.92
6.33
8.59
7.31
8.08
8.10
8.75
7.36
7.82
23.29
16.81
20.12
16.08
21.82
18.57
20.52
20.57
22.23
18.69
19.86
Mean
Mean
Changing Grazing Management
The biological environment in which cheatgrass
exists in the Lahontan Basin has dramatically changed
in the last two decades, independent of climatic or genetic
changes. Livestock numbers have declined on many allotments on public lands; the class of livestock has changed
38
with the virtual end of the range sheep industry; and
season of use has changed through grazing management.
plants. Grazing on this type of range results in a direct
reduction in cheatgrass reproductive potential with minimum consumption of herbage.
Sheep-The range sheep industry peaked in the period
1908 to 1928 at about 1,200,000 head in Nevada (Young
and others 1976). Sheep numbers in 1970 were 227,000
and in 1988 they had dropped to 96,000 (Anonymous
1988). Cattle numbers peaked in the 1970's at around
626,000 head and in 1988 had dropped to 555,000 head.
It is difficult to visualize at this time the impact the range
sheep industry had on range condition in the Lahontan
Basin. The tremendous number of sheep that were
grazed on Nevada ranges from 1910 through the Great
Depression were largely run on ranges that supported
as many or more cattle than today. The grazing of both
classes of livestock was superimposed on the same
vegetation.
Free Roaming Horses-Horses dramatically increased on many Nevada ranges following passage of
the Wild Horse and Burro Act. The T Quarter Circle
Ranch had 2,000 horses removed from the public lands
of its grazing permit through a Federal court order. The
influence of the increase in horse populations was first to
increase utilization of forage on the rangelands, followed
by a dramatic increase in available forage once the horses
were removed.
SUGGESTED MODEL FOR THE
SPREAD OF CHEATGRASS TO
ARID ENVIRONMENTS
Single-Permittee Allotments-The breaking up
of multiple-permittee allotments into single-permittee
allotments and the fencing of these allotments to greatly
reduce trespassing animals has tremendously benefited
range conditions in Nevada. It was human nature to
try to get as much of the forage as possible before your
copermittee overgrazed the range and to have little interest in improvements that benefited your neighbor while
reducing your own harvestable forage. Remember that
most range improvements require a period of no grazing
for them to be effective.
The concept of"Uniformitarianism" as first proposed
by James Hutton and applied by Charles Lyell roughly
states that what has happened before is happening now.
This concept has become the basis of geology, but also
has great application to biological processes. If you go
out and look at shadscale and big sagebrush vegetation
growing on areas of Lahontan-age sand in salt desert
situations, it is possible to find communities. The senior
author has long proposed that cheatgrass spread in the
biological vacuum created by the virtual removal of perennial bunchgrasses from big sagebrush communities. This
was the core environment that most closely fit the genotype of cheatgrass that was introduced to North America.
As the available habitat was occupied by cheatgrass,
splinter populations were accidentally distributed to associated environments by the seasonal movements of grazing livestock. In the case of the more arid environments
in the bottoms of the basins that compose the Lahontan
basin, cheatgrass was not a spectacular success.
The junior author proposes that the idea of cheatgrass
spreading in a biological vacuum created by excessive
grazing may be somewhat misleading. In southeastern
Washington, Daubenmire (1940) and Harris (1967) observed that cheatgrass had inserted itself successfully
into climax perennial grass/shrub communities that
had been protected from fire or grazing for as long as
50 years. Ecologists such as Neil West of Utah State
University view this as evidence that the "biological
vacuum from overgrazing" theory may be overstated
(personal communication from Dr. West). Cheatgrass
may partially grow in environmental potential that native
plants never evolved to occupy. This may have great
application on the margins of the more arid plant communities within the sagebrush/bunchgrass zone.
The second stage of this adaptive process involves
chance hybridization and recombination. Time would
allow for the incorporation of genetic material from
repeated introduction of cheatgrass. Remember the
principle that a single introduction often results in an
extremely narrow genetic base (Young and others 1972).
Evidence for repeated introductions is scanty, but remember that halogeton (Halogeton glomeratus), which is a
restricted-distribution species of far central Asia, was
introduced to Nevada, so a cosmopolitan worldwide weed
Grazing Management-Since the 1960's, the public
land management agencies in the Lahontan Basin have
made a concerted effort to establish grazing management
systems on all range allotments. These systems are designed so that the vegetation is not grazed during the
spring growing period every year so that the herbaceous
plant species have the opportunity to produce seed and
the resulting seedlings have a chance to establish. Depending on the grazing system employed, portions of the
range either have grazing deferred until after seedripe or
are deferred for an entire season. For the first time in a
century, herbaceous biomass was allowed to accumulate
on some Nevada rangelands.
Obviously, the annual growth habit requires seed production, seed germination, and seedling establishment
annually for the species to persist. The caryopses of
cheatgrass cannot take up moisture from the surface of
a bare seedbed faster than the moisture is lost to the atmosphere (Evans and Young 1970, 1972). This means
that the caryopses must be buried by soil or litter to germinate. Heavy grazing reduces the seed production of
cheatgrass and reduces the potential of seedbeds to support the germination of seeds of cheatgrass. Such grazing
on a sustained basis is even harder on perennial grasses
and does not lead to the eradication of cheatgrass. But
many individuals, ranchers, land managers, and scientists fail to appreciate that heavy grazing cannot help
but partially suppress cheatgrass.
Down on the margins of salt desert ranges where cheatgrass has made its spectacular increases, caryopses of this
species tend to stay in inflorescence much longer than at
higher elevations in the sagebrush zone. In this type of
rangeland, which is often grazed in the winter, cattle become, through experience, masters of picking seeds from
39
Fleming, C. E. 1922. One-night camps versus established
bed grounds on Nevada sheep ranges. Nev. Agric. Exp.
Stn. Bull. 103. Reno, NV: University ofNevada. 21 p.
Fleming, C. E.; Shipley, M. A.; Miller, M. R. 1942.
Broncograss (Bromus tectorum) on Nevada ranges.
Nev. Agric. Exp. Stn. Bull. 159. Reno, NV: University
ofNevada. 21 p.
Griffith, D. 1902. Forage conditions on the northern border of the Great Basin. Bull. 15. Washington, DC: U.S.
Department of Agriculture, Bureau of Plant Industry.
59p.
Harris, G. A. 1967. Some competitive relationships
between Agropyron spicatum and Bromus tectorum.
Ecological Monographs. 37: 89-111.
Hulbert, L. C. 1955. Ecological studies of Bromus tectorum and other annual bromegrasses. Ecological
Monographs. 25: 181-213.
Jardine, J. T.; Anderson, M. R. 1919. Range management
in the National Forests. Agric. Bull. 74. Washington,
DC: U.S. Department of Agriculture. 6 p.
Kennedy, P. B. 1903. Summer ranges of eastern Nevada
sheep. Nev. Agric. Exp. Stn. Bull. 55. Reno, NV: University of Nevada. 55 p.
Kennedy, P. B.; Doten, S. B. 1901. A preliminary report
on the summer ranges of western Nevada sheep. Nev.
Agric. Exp. Stn. Bull. 51. Reno, NV: University of
Nevada. 57 p.
Klemmedson, J. 0.; Smith, J. G. 1964. Cheatgrass
(Bromus tectorum L.). Botanical Review. 30: 226-262.
Mack, R. M. 1981. Invasion of Bromus tectorum L. into
western North America: an ecological chronicle. AgroEcosystems. 7: 145-165.
Pickford, G. D. 1932. The influence of continued heavy
grazing and of promiscuous burning on spring fall
ranges in Utah. Ecology. 13: 159-171.
Piemeisel, R. L. 1951. Causes affects change and rate
of change in a vegetation of annuals in Idaho. Ecology.
32:33-72.
Robertson, J. H.; Pearse, C. K. 1945. Artificial reseeding
and the closed community. Northwest Science. 19:
58-66.
Stebbins, G. L., Jr. 1957. Self-fertilization and population
variability in the higher plants. American Naturalist.
91: 337-354.
Young, J. A.; Evans, R. A. 1976. Response of weed populations to human manipulations of the natural environment. Weed Science. 24: 186-190.
Young, J. A.; Evans, R. A.; Eckert, R. E., Jr.; Kay, B. L.
1987. Cheatgrass. Rangelands. 9: 266-270.
Young, J. A.; Evans, R. A.; Kay, B. L. 1970. Phenology
of reproduction ofmedusahead. Weed Science. 18:
451-454.
Young, J. A.; Evans, R. A.; Major, J. 1972. Alien plants
in the Great Basin. Journal of Range Management.
27: 127-132.
Young, J. A.; Evans, R. A.; Tueller, P. T. 1975. Great
Basin plant communities-pristine and grazed. In:
Elston, R., ed. Holocene environmental change in the
Great Basin. Res. Pap. 6. Reno, NV: Nevada Archeological Survey: 187-215.
like cheatgrass would appear to stand the chance of repeated introduction. The largely self-pollinated habit
of cheatgrass means that after chance hybridization and
recombination, cheatgrass populations grow in arid environments where there is a series of self-pollinated or
inbred lines being selected by natural selection for adaptation to a harsh environmental situation. Gradually over
time, the genetic core of the population would shift toward
adaptation to the arid environments.
If this gradual change in genetic potential is coupled
with a relaxation in grazing pressure, which has been the
case on many Lahontan Basin rangelands, then there is
the opportunity for the selected genetic material to express its potential. Such expressions free from grazing
allow the opportunity for additional hybridization and
recombination and for the expression of population
heterosis as postulated by Young and Evans (1976).
The final step in this scenario is the occurrence of 1
or 2 years with above-average conditions for the growth
of cheatgrass. Under such a train of events, the occurrence of excellent years for cheatgrass growth would result in the explosion of populations in areas where the
gradual increase in population size over time had not
been noticed. The accumulation of biomass from such
population explosions of cheatgrass conditions the potential for widespread occurrence of wildfires and the prolonged removal of shrubs from these arid environments.
Go look and draw your own conclusions. It appears that
an era of great change is occurring on the rangelands of
the Lahontan Basin.
ACKNOWLEDGMENT
The senior author is responsible for interpretations
drawn in this article, but gratefully acknowledges the
contributions of the junior author, whose deep family
roots in Humboldt County, NV, and keen perspective
on rangelands contributed in a substantial way to the
article's preparation.
REFERENCES
Anonymous. 1941. Climate and man. Yearbook of Agricul. ture. Washington, DC: U.S. Department of Agriculture,
Superintendent of Documents.
Anonymous. 1966. Water and related land resources,
Humboldt River Basin, Nevada. Carson City, NV: U.S.
Department of Agriculture, Soil Conservation Service.
R. 12.120 p.
·
Anonymous. 1988. Nevada agricultural statistics. Reno,
NV: Unlversity of Nevada, Nevada Agricultural Statistics Service.
Allard, R. W. 1965. Genetic systems associated with colonizing ability in predominantly self-pollinated species.
In: Baker, H. G.; Stebbins, G. L., eds. The genetics of
colonizing species. New York: Academic Press: 50-76.
Daubenmire, R. A.; Young, J. A. 1970. Plant litter and
establishment of alien annual species in rangeland
communities. Weed Science. 18: 697-703.
Evans, R. A.; Young, J. A. 1972. Microsite requirements
for establishment of annual rangeland weeds. Weed
Science. 20: 350-356.
40