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DISTRmUTION OF TWO EXOTIC
GRASSES ON INTERMOUNTAIN
RANGELANDS: STATUS IN 1992
Mike PeDant
Christi Hall
elevational forests (7 ,000 ft elevation), which receive up
to 22 inches of precipitation annually (Hull and Pehanec
1947).
Mack (1986) reported that cheatgrass was introduced
into the Intermountain West in the 1880's and reached its
present distribution by 1930. He included a map of cheatgrass distribution in his publication; however no acreages
of cheatgrass infestation were included. A C. Hull (1965)
described a "surveY' indicating that cheatgrass occurred on
at least 60 million acres in 11 western states. No source of
information was cited for this survey.
Several authors discuss cheatgrass distribution on a
state basis. Stewart and Hull (1949) report cheatgrass to
be a "dominant" species (75 to 95 percent of the herbage
production) on about 4 million acres of Idaho rangeland.
They described another 10 to 15 million acres of Idaho
rangeland as infested with cheatgrass (trace to 20 to 25
percent of the herbage production).
Fleming and others (1942) reported "several million
acres of rangeland" to be infested with cheatgrass making
it one of Nevada's "predominant grasses in both abundance and distribution."
Platt and Jackman (1946) indicated that cheatgrass
was introduced into Oregon around 1900 and began a
rapid expansion around 1916. They stated that cheatgrass was "in command on about 10 million acres" by
1942.
ABSTRACT
Alien annual grosses have dranuzticaUy altered ecosystem functions on rangelands throughout the Intermountain area in the
Western United States. Cheatgrass (Bromus tectorum) and
medusahead wildrye (Taeniatherum asperum) are the primary
alien gross invaders of fire altered or disturbed rangelands. In
1992, the current distribution and potential for upansion of these
two grosses were delineated on maps by Bureau of Land Management resource specialists in Nev. Oregon, Utah, Washington,
and Idaho. These maps indicate that 3.3 million acres of rangeland administered by the Bureau of Land Management in those
States are dominated by cheatgrass and medusahead wildrye.
Additionally, another 76.1 mUlion acres ofpublic rangeland are
classifi«l as infested or susceptible to invasion by the two alien
grasses.
INTRODUCTION
Alien annual grasses complicate the management of
public rangelands in the Intermountain West. Cheatgrass
and medusahead wildrye have altered ecosystem functions causing a significant increase in the frequency and
extent of wildfires (Stewart and Hull1949; Young and
others 1972, 1987; Whisenant 1990). In tum, wildfires
have adversely affected many resource values and are responsible for rising fire suppression and rehabilitation
costs (Pellant 1990).
Although growth and physiological requirements of
cheatgrass and medusahead wildrye are relatively well
known, there is little information on the current and potential distribution of these two annual grasses. Consequently, resource managers are hindered in their efforts
to plan and fund management actions and improve fire
suppression strategies.
A brief summary of historical references pertaining to
cheatgrass and medusahead wildrye distribution follows.
Medusahead Wildrye
Medusahead wildrye was introduced into the Intermountain West from the Mediterranean area in the late
1800's and is adapted to a wide range of climatic conditions (Young and Evans 1970). It grows best on soils with
a high clay content and its distribution often overlaps
rangelands infested with cheatgrass.
Several references describe medusahead wildrye
distribution on a state basis in the Intermountain West.
Hironaka (1963) reported that medusahead wildrye
spread from isolated areas to 750,000 acres in just 15
years in southern Idaho. Over 2 million acres of rangeland in Oregon are within the boundary of known infestations of medusahead wildrye (Turner and Poulton 1963).
Medusahead wildrye infestations occur on 120,000 to
150,000 acres in eastern Washington (Goebel and others
1969). Young and Evans (1970) reported medusahead
wildrye invading seral plant communities in northeastern
California, northern Nevada, and western Utah. No acreages were reported.
Cheatgrass
Cheatgrass is adapted to a wide range ofhabitats from
salt desert shrub communities (2,000 ft elevation), which
receive around 7 inches of annual precipitation, to lower
Paper pJese.tted at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, m, May 19-21, 1992.
Mike Pellant and Christi Hall. U.S. Department of Interior, BUJeau of
Land Management, Idaho State Office, 3380 Americana Terrace, Boise, ID
83706.
109
Recent surveys documenting cheatgrass and medusahead
wildrye distribution and extent in the Intermountain
West have not been conducted. A survey to update the
status of cheatgrass and medusahead wildrye infestations
in a five-state area was initiated in 1991 by the Idaho
State Office ofthe Bureau of Land Management (BLM),
U.S. Department of the Interior.
METHODS
Resource specialists at the BLM field office level identified and mapped cheatgrass and medusahead wildrye distribution on large scale maps (generally 1/100,000 acre).
Acreages of infestation were then calculated from maps
using the appropriate dot acreage grid. Only public lands
administered by BLM are included in the survey; although it is likely that some state and private lands
were inadvertently included in acreage determinations.
States included in this survey are Idaho, Oregon,
Washington, Utah, and Nevada. This survey relied upon
the experience and knowledge of local field personnel
along with pertinent monitoring and inventory data to
identifY areas of cheatgrass and medusahead wildrye infestations. Criteria provided to field personnel to assist
in mapping annual grass distribution are:
Figure 2-Area representative of the "understory"
category of cheatgrass and/or medusahead
wildrye infestation. Between 10 to 59 percent of
the species composition by weight is composed
of one or both of these annual grasses.
1. Delineate three categories of infestation based
upon the species composition by weight of cheatgrass or
medusahead wildrye in the plant community (fig. 1, 2,
and 3). Categories are "monoculture," "understory," and
"potential."
2. Exclude all units of cheatgrass and medusahead
wildrye from the mapping process that are less than 5,000
acres in area.
3. Map cheatgrass arid medusahead wildrye separately
whenever possible.
This qualitative approach minimized the time spent
by field personnel to complete the survey. However, this
approach also resulted in several obvious discrepancies in
Figure 3-Area representative of the "potential"
category of cheatgrass and/or medusahead
wildrye infestation. less than 10 percent of the
species composition by weight is composed of
one or both of these annual grasses. As shown
by the cheatgrass in the background of this
photo; such areas are at risk of invasion by annual grasses following disturbance.
vegetation mapping units along state and BLM district
boundaries (fig. 4). Unfortunately, no reconciliation of
these discrepancies were made.
RESULTS
According to this survey, 3.3 million acres of rangeland
are classified as cheatgrass/medusahead wildrye monocultures; nearly 14 million acres are infested with one or
both of these grasses; and 62.1 million acres are at risk
of invasion by the two grasses if a disturbance occurs
(table 1).
Figure 1-Area representative of the "monoculture"
category of cheatgrass or medusahead wildrye infestation. Over 60 percent of the species composition
by weight is composed of one or both of these
annual grasses.
110
,...
\
Oregon
,------
Table 1-Cheatgrass and medusahead wildrye status on
rangelands administered by BLM in Idaho, Oregon,
Washington, Utah, and Nevada. Total public lands
in the five-States are approximately 98 million acres
I
Idaho
Category'
Acres
Public lands
affected
Percent
Cheatgrass:
Monoculture {>60%)
Understory {1 0-59%)
Potential {<10%)
2,908,000
13,987,000
62,098,000
14
63
414,000
<1
79,407,000
>80
3
Medusshead wlldrye:
Monoculture (>60%)
Total
'Percent values refer to the estimated species composition by weight of
cheatgrass or medusahead wildrye in the plant community.
•
Monocu~uro
•
Understory
Differences in the application of the survey criteria and
inventory/monitoring data by the BLM personnel at different field offices diminishes the accuracy of this qualitative survey. The "monoculture" and "understory" values
in tables 1 and 2 are generally more accurate than the
values displayed in the "potential" category. Predicting
the potential for alien grass invasion is more subjective
and therefore more difficult, than identifying known
infestations of cheatgrass and medusahead wildrye.
Acres of rangeland infested with cheatgrass and
medusahead wildrye are displayed by State in table 2.
The authors consider that the acreage estimated to have
cheatgrasslmedusahead "potential" in southern Nevada
to be high. Conversely, the acreage designated as cheatgrass and medusahead "understory" in southern Idaho,
especially in the southeastern portion of the State, is
underestimated.
Comparison of historical reports about cheatgrass and
medusahead wildrye extent and distribution with this
survey is not feasible because different criteria were used
(only BLM-administered rangelands were considered in
this survey). However, since 80 percent of the public lands
in the five States are either infested or at risk of invasion
by these two alien grasses, new resource management
lg[J Potential
Figure 4-Distribution of cheatgrass and
medusahead wildrye in the Intermountain West.
Category boundaries represent broad types that
include private and state land in addition to public land. Vegetation units of 5,000 acres or less
are not included in this map. Original maps prepared by the field office personnel are available
for inspection at the BLM's Idaho State Office.
The geographic distribution of these exotic grass infestations are portrayed graphically in figure 4. Maps for
cheatgrass and medusahead wildrye distribution were not
provided for Washington; therefore this State is not represented in figure 4. However, acreages of exotic grass infestation were provided for Washington and are included
in tables 1 and 2.
Table 2- Acres of public lands either infested or at risk of infestation by chsatgrass in a
five-State area. Medusahead wildrye {not included in this table) was identified
as occurring on 322,560 acres in Idaho (in combination with cheatgrass) and
as a monoculture on 92,160 acres in Oregon
State
Idaho
Utah
Oregon
Washington
Nevada
Total
Monoculture {>600k1)
Understory {10-59%1)
Potential (<10%1 )
1,082,880
297,600
437,760
85,500
1,004,000
1,751,040
1,082,880
2,004,480
142,500
9,006,000
1,221,120
11,635,200
9,169,920
72,000
40,000,000
2,822,240
13,844,400
62,026,240
'Percent values refer to the estimated species composition by weight of cheatgrass in the plant
community.
111
strategies will be required to maintain or restore ecosystem functions and accomplish resource objectives.
1965 July 27-30; Vale, OR. Portland, OR: U.S.
Department of the Interior, Oregon Bureau of Land
Management: 20-26.
Mack, R. N. 1986. Alien plant invasion into the Intermountain West: a case history. In: Mooney, H. A;
Drake, J. A., eds. Ecology of biological invasions of
North America and Hawaii. Ecological Studies Vol. 58.
Springer-Verlag, New York: 191-313.
Pellant, Mike. 1990. The cheatgrass-wildfire cycle-are
there any solutions? In: McArthur, E. D.; Romney, E. M.;
Smith, S.D.; Tueller, P. T., comps. Proceedingssymposium on cheatgrass invasion, shrub die-off, and
other aspects of shrub biology and management; 1989
April 5-7; Las Vegas, NV. Gen. Tech. Rep. INT-276.
Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Research Station: 11-17.
Platt, K.; Jackman, E. R. 1946. The cheatgrass problem in
Oregon. Bull. 668. Corvallis, OR: Oregon Agricultural
Experiment Station. 48 p.
Stewart, G.; Hull, A C., Jr. 1949. Cheatgrass (Bromus
tectorum L.) - an ecologic intruder in southern Idaho.
Ecology. 30(1): 58-74.
Turner, R. B.; Poulton, C. E.; Gould, W. L. 1963.
Medusahead-a threat to Oregon rangeland. Oregon
State University Agriculturial Experiment Station
Spec. Rep. 149. 22 p.
Whisenant, Steven G. 1990. Changing fire frequencies on
Idaho's Snake River Plains: ecological and management
implications. In: McArthur, E. D.; Romney, E. M.;
Smith, S. D.; Tueller, P. T., comps. Proceedingssymposium on cheatgrass invasion, shrub die-off, and
other aspects of shrub biology and management; 1989
April 5-7; Las Vegas, NV. Gen. Tech. Rep. INT-276.
Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Research Station: 1-7.
Young, J. A.; Evans, R. A; Major, J.1972. Alien plants in
the Great Basin. Journal of Range Management. 25:
194-201.
Young, J. A; Evans, R. A.; Eckert, R. E., Jr.; Kay, B. L.
1987. Cheatgrass. Rangelands. 9(6): 266-270.
Young, J. A.; Evans, R. A. 1970. Invasion of medusahead
into the Great Basin. Weed Science. 18: 89-97.
SUMMARY
The qualitative nature of this survey combined with the
large numbers of personnel who prepared the maps calls
for caution in the uses and interpretation of this product.
However, given the magnitude of the c:wTent cheatgrass
and medusahead wildrye infestations and potential for
future expansion in distribution and abundance, all resource managers in the Intermountain West should be
concerned and open to proactive management and restoration strategies. H Intermountain managers do not apply proactive approaches to slow the spread of exotic
weeds and break the cheatgrass-wildfire cycle; the next
alien annU:BI grass survey will paint a far more bleak picture than this survey.
ACKNOWLEDGMENTS
The authors thank all BLM resource specialists who
prepared the plant distribution maps used in this survey.
REFERENCES
Fleming, C. E.; Shipley, A; Miller, M. R. 1942. Bronco
grass (Bromus tectorum) on Nevada ranges. Nevada
Agricultural Experiment Station Bulletin 159. 21 p.
Goebel, C. J.; Nelson, J. R.; Harris, G. A 1969.
Medusahead-a threat to Washington rangeland.
Washington State University Extension Service
Circular 359. 3 p.
Hironaka, M. 1963. Plant-environment relations of major
species in sagebrush-grass-vegetation of southern
Idah?. Ph.D. Dissertation. University of Wisconsin,
Madison, WI. 124 p.
Hull, A C., Jr.; Pehanec, J. F. 1947. Cheatgrass-a challenge to range research. Journal of Forestry. 45:
555-564.
Hull, A. C., Jr. 1965. Cheatgrass-a persistent homesteader. In: Proceedings, cheatgrass symposium,
112