RECENT INCREASES IN BROMUS
POPULATIONS ON THE NEVADA
TEST SITE
Richard Hunter
ABSTRACT
METHODS
Two introduced grass species, red brome (Bromus rubens
L.) and cheatgrass (Bromus tectorum L. ), have invaded the
Mojave and Great Basin Desert vegetation of the Nevada
Test Site. During the last two decades they have become
nearly ubiquitous. One or the other dominates most ephemeral populations. They occur in densities up to thousands
per square meter. The ecosystem changes resulting from
the grasses include an increased tendency for propagation
of fire and a possible decrease in diversity of the native
ephemeral flora.
Methods of assessing winter annual density have varied
only slightly over the years. Shields and others (1963) estimated cover, rather than density, and their density data
are too few to provide many comparisons to later data.
(In these very plastic species, cover relates more to rainfall than density.) Beatley (1966) placed fifty 2- by 5-dm
permanent quadrats every 2 feet along lines within square
plots 100 feet on a side.
ffiP studies, done by T. L. Ackerman, used 2- by 5-dm
permanent quadrats placed in groups of four around permanent, randomly selected locations (Turner and Randall
1989). Neither Beatley nor Ackerman harvested plants
from within their quadrats.
Monitoring of annuals under BECAMP (1987-89)
(Hunter and Medica 1989) was done with twenty 0.025-m2
randomly placed quadrats within single 1,000-m2 plots.
Harvesting of all annual plants within each quadrat was
followed by weighing the dried plants to determine biomass. New quadrat locations were selected every year.
In other locations Turner and others (1979) used fifty
2- by 5-dm randomly placed quadrats, without harvesting;
Romney and others (1978) used 2- by 5-dm quadrats, with
harvesting, in groups of eight randomly placed on each of
thirty-two 10-m2 plots.
INTRODUCTION
Hunter (in press), using information collected from
herbaria and historical references, showed that red brome
(Bromus rubens L.) was introduced into the western United
States in the mid-nineteenth century, but did not spread
into the Mojave Desert until the early twentieth century.
By 1930 it was present in the Mojave along roadsides and
at disturbed areas and springs. It was common in some
locations at 4,000-5,000 feet on the Nevada Test Site (NTS)
in 1963-64 (Beatley 1966). In contrast, cheatgrass (Bromus
tectorum 1.) invaded at higher elevations on the NTS, in
big sagebrush (Artemisia tridentata)-dominated vegetation.
Cheatgrass was still quite sparse when Beatley (1966)
summarized the status of Bromus species.
Quantitative botanical data have been collected on the
NTS since 1957. Shields and others (1963) compiled the
first data. J. Beatley and several other researchers worked
on specific programs in succeeding years. The Rock Valley
area, located near the southwest comer of the NTS at an
elevation of 3,400 feet, has been the site of several ecological studies since 1963. It is best represented in the available data.
Important studies in Rock Valley include Heatley's, those
of the U.S. International Biological Program (IBP) Desert
Biome during the early. 1970's, and those of the present
Basic En\jronmental Compliance and Monitoring Program
of the U.S. Department of Energy (BECAMP) since 1987.
RESULTS
Available data are most complete for densities of
red brome and other ephemerals in and near Heatley's
Plot 3 in Rock Valley. They show an increase from
less than 10 plants/m2 in the 1960's to more than 1,000
plantslm2 in the late 1980's. Over the same 27-year period, native winter ephemerals fluctuated widely in density, but showed no trends either toward decreased or increased densities (table 1). There was a decrease in red
brome density from 1971 through 1973 resulting from
two very dry years, and large increases in 1974 and 1976,
following the excellent growth year 1973 and good year
1975. The driest year of record (since 1962) was 1989,
when virtually no winter ephemerals germinated, but
consequences to the Bromus populations of the failure
to germinate are not yet known.
The increase in red brome densities during the years
missing from table 1 (1977 -82) can be partially reconstructed from data of Turner and others, who collected
data at Jackass Flats a few miles north of the Rock Valley
Paper presented at the Symposium on Cheatgrass Invasion, Shrub DieOff, and Other Aspects of Shrub Biology and Management, Las Vegas, NV,
April 5-7, 1989.
Richard Hunter is Principal Ecologist, Reynolds Electrical and Engineering Co., Inc., Environmental Health Division, P.O. Box 98521, Las
Vegas, NV 89193-8521.
22
This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
Table 2-Bromus rubens densities (n/m 2) and number of
species per 0.01-m2 quadrat on irrigated and control
plots in Mercury Valley on the Nevada Test Site
Table 1-Densities of Bromus rubens and native ephemerals at
Beatley's Plot 3 in Rock Valley, 1963-89, from Hunter
(in press). Rainfall data (inches, September-April) are
from NOAAINWS
Bromus rubens
n!tif
Natives
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
5.0
6.2
2.2
3.2
3.6
7.6
14.0
19.8
.2
0
.4
11.2
13.2
90.9
10.0
27.6
2.4
69.6
7.0
99.6
109.8
14.0
2.0
3.0
118.0
101.0
216.0
327.0
1983
1984
1985
1986
1987
1988
1989
89.0
167.0
156.0
108.0
19.0
111.0
Year
754.0± 298
2,034.0 ± 632
0.0± 0.0
nttif
100.0 ± 22
86.0 ± 33
0.0± 0.0
Precipitation
1974
1975
1976
5.7 ± 0.5
124.0 ± 48
3.3 ± 0.2
82.0 ± 21
6.6 ± 0.4
557.0 ± 77
Control plots
Inches
Number of species
B. rubens density
3.00
3.57
5.50
2.91
3.69
8.36
>2.68
1.84
1.70
8.66
2.76
4.42
>4.95
Irrigated plots
Number of species
B. rubens density
7.1 ± 0.5
4.4 ± 0.2
~;3± 0.2
146.0 ± 61 203.0 ± 42 1,795.0 ± 232
in press). In the late 1980's, it was present widely on the
NTS, but was especially dense on disturbed areas at middle elevations (table 3).
The relative importance of the introduced species is demonstrated by the percent of ephemeral biomass consisting
of introduced species (table 4). The data in table 4 include
species besides red brome and cheatgrass, particularly
Russian thistle (Salsola spp.), storksbill (Erodium cicutarium), and Jim Hill mustard (Sisymbrium altissimum).
Although there were exceptions, introduced species made
up an average of 81 percent of the biomass in disturbed
areas and 61 percent in pristine areas. The primary exception to dominance by introduced species on the eight disturbed areas was a roadside in Frenchman Flat. That road
was closed to traffic and maintenance for many years prior
to censusing and probably did not reflect normal roadside
conditions.
6.84
1.98
>4.64
>3.88
6.58
7.99
.70
study site (Turner and Edney 1977; Turner and others
1979; Turner and Vollmer 1982). They measured densities
of red brome on plots treated with coal-precipitator ash.
The control plot values for 1977 through 1981 were 9.0,
2.4, 16.0, 121.2, and 132.2 plants/m2, respectively, suggesting a tenfold increase over those years. Rock Valley
precipitation (September-April) for those years was 4.12,
11.26, 6.36, 6.24, and 3.62 inches (NOAA/NWS). The tenfold increase was not apparent in the 1983 Rock Valley
data, suggesting there was a decrease in 1982 or 1983.
An IBP study of the effects of irrigation and nitrogen
applications was performed in 1974, 1975, and 1976
(Romney and others 1978). The final year's results were
never published, but an analysis of the ephemeral data
shows a significant decrease in species richness on irrigated plots in the third year (table 2; p < 0.0001, T-test).
This coincided with a marked increase in the density of
red brome. Within the irrigated treatments in 1976 there
was a significantly ttegative correlation between red brome
density and number of species/0.1-m2 quadrat (p < 0.01,
Spearman's Rho = -0.300, n = 64, Conover 1971). Within
the unirrigated plots, in contrast, there was a significant
positive correlation between red brome density and number of species (p < 0.01, Rho = +0.343).
During the 1980's, populations of cheatgrass also became
much more common above 4,000 feet. It was observed in
monoculture patches in some of the large clear areas left
from 1950's aboveground nuclear explosive tests (Hunter
Table 3-Densities of Bromus rubens and Bromus tectorum
on the Nevada Test Site in 1988, arranged by
altitude. (Extracted from Hunter in press)
Disturbance
None
None
Gophers
None
None
Shrubs removed
None
None
Ground zero
Sedan GZ
Sedan GZ
None
None
T2GZ
None
Fire-1986
None
None
23
Altitude
B.rubens
Feet
nltif
3,100
3,280
3,400
3,500
3,800
3,800
4,100
4,200
4,240
4,350
4,350
4,350
4,600
4,600
4,800
4,800
6,200
7,500
16 ± 14
34± 22
3,550 ± 366
2,034 ± 632
446 ± 130
1,912 ± 476
1,872 ± 556
188 ± 49
1,472 ± 356
19 ± 11
324 ± 154
142 ± 69
658 ± 344
774 ± 169
362 ± 84
114 ± 37
B. tectorum
nltif
+
0
0
+
2±2
8±7
20 ± 11
2±2
1,884 ± 396
1,155 ± 431
2,004 ± 424
2±2
4±4
+
414 ± 152
3,916 ± 752
+
+
0
0
approximately doubled (Romney and others 1978). A
series of several wetter years might be predicted, therefore, to favor the introduced grasses over the native
ephemerals. The data in table 1 suggest populations
increase the year following the wet year, but not reliably
(for example, compare 1969-70 with 1973-74).
My opinion is that red brome and cheatgrass will slowly
reduce populations of native ephemerals through shading,
competition for nutrients, changes in the fire regime, and
probably several other mechanisms. Because there is an
established seedbank for most species and desert plants
have very specific germination requirements (Juhren and
others 1956), I feel this will be a long-term process, rather
than a rapid one. It bodes ill for the rare, massive floral
displays for which the Mojave Desert is famous.
Table 4-Numbers of species and percent of ephemeral biomass consisting of introduced species on NTS sites
during 1988
Location
Number of
species
Percent
introduced
Disturbed
T1GZ
T2GZ
Roadside
Gopher
Sedan 1000'
Sedan 3000'
3B scraped
Burned
means
±sem
12.0
8.0
26.0
12.0
15.0
22.0
2.0
23.0
15.0
±2.9
99.8
84.2
5.2
97.6
77.0
98.8
100.0
83.7
80.8
±11.2
ACKNOWLEDGMENTS
Undisturbed
·~·
.· ':> ·:
Pristine
Beatley 3
Beatley 4
Pristine
Pristine
Control
Pristine
Pristine
T1 control
T2 control
Sedan 5000'
3B control
Burn control
Pristine
Pristine
means
±sem
13.0
38.0
29.0
32.0
20.0
31.0
21.0
15.0
16.0
20.0
23.0
30.0
23.0
16.0
16.0
22.9
±1.9
10.2
97.5
65.2
41.0
39.5
18.9
88.8
98.0
87.5
98.0
88.3
96.1
84.0
0.0
0.0
60.9
±9.9
I am greatly indebted to the several researchers who
studied and recorded information on ephemeral plants
over the years. In addition to those mentioned in the text,
P. A. Medica played a significant role in making the data
of Beatley available. Over three decades considerable
support has been directed to this research by the U.S.
Department of Energy, the National Science Foundation,
and Southern California Edison. This study was supported by the U.S. Department of Energy, Nevada Operations Office, under contract No. AC08-89NV10630.
REFERENCES
Beatley, J. C. 1966. Ecological status of introduced brome
grasses (Bromus spp.) in desert vegetation of southern
Nevada. Ecology. 4 7: 548-554.
Conover, W. J. 1971. Practical nonparametric statistics.
New York: John Wiley & Sons. 462 p.
Hunter, Richard B. [In press]. Progress of Bromus invasions on the Nevada Test Site. Great Basin Naturalist.
Hunter, Richard B.; Medica, P. A. 1989. Status of the flora
and fauna on the Nevada Test Site in 1987. Report
DOEINV/10630-2. Springfield, VA: U.S. Department
of Commerce, National Technical Information Serivce.
103p.
Juhren, M.; Went, F. W.; Phillips, E. 1956. Ecology of
desert plants. IV. Combined field and laboratory work
on germination of annuals in the Joshua Tree National
Monument, California. Ecology. 37: 318-330.
Mack, R.N. 1981. Invasion of Bromus tectorum L. into
western North America: an ecological chronicle. AgroEcosystems. 7: 145-165.
Morrow, L. A.; Stahlman, P. W. 1984. The history and
distribution of downy brome (Bromus tectorum) in
North America. Weed Science. 32 Supplement 1:2-7.
Romney, E. M.; Wallace, A.; Hunter, R. B. 1978. Plant
response to nitrogen fertilization in the Northern
Mojave Desert and its relationship to water manipulation. In: West, N. E.; Skujins, John, eds. Nitrogen
in desert ecosystems. Stroudsberg, PA: Dowden,
Hutchinson and Ross: 232-243.
Shields, L. M.; Wells, P. V.; Rickard, W. H. 1963. Vegetational recovery on atomic target areas in Nevada.
Ecology. 44: 697-705.
DISCUSSION
The fact of invasion of North American deserts by introduced Bromus species is well established (Mack 1981;
Morrow and Stahlman 1984; Yensen 1981). The massive
invasion on the NTS is not fully documented, but can be
inferred from present and some historical records. There
are also records, at least of opinions and testimonials,
that the invasion of western North America by cheatgrass
led to increased fire frequencies (Stewart and Hull 1949;
Beatley 1966; Yensen 1981). These fire effects are almost
undoubtedly true of red brome also, as it shares the cheatgrass habits of dense growth and persistent dead stems.
Long-term records of fire frequency and sizes, however,
are not available for the NTSc.
The question of what the dense Bromus populations
have done to the native ephemeral populations is more
difficult to determine. In two instances on the NTS the
question has been addressed. Turner and Vollmer (1982)
found that diversity on plots treated with ash decreased
on plots dominated by red brome. They attributed that
decrease to the high relative abundance of red brome
rather than the ash treatment. That was likely an effect
of differential susceptibility to the ash and cannot be
extrapolated to pristine conditions. Irrigation (table 2)
was a more natural manipulation, in which rainfall was
24
Turner, F. B.; Vollmer, AT. 1982. Ecological effects of
precipitator ash on desert plants. Southern California
Edison Research and Development Series 82-RD-107.
138p.
Turner, F. B.; Randall, D. C. 1989. Net production
by shrubs and winter annuals in Southern Nevada.
Journal of Arid Environments. 17: 23-26.
Yensen, D. I. 1981. The 1900 invasion of alien plants into
southern Idaho. Great Basin Naturalist. 41: 176-183.
Stewart, G.; Hull, A. C. 1949. Cheatgrass (Bromus
tectorum L.)-an ecologic intruder in southern Idaho.
Ecology. 30: 58-74.
Turner, F. B.; Edney, E. B. 1977. Ecological effects of
coal fly ash on desert plants and animals. Southern
California Edison Research and Development Series
77-RD-114. 118 p.
Turner, F. B.; Edney, E. B.; Vollmer, A. T. 1979. Ecological effects of precipitator ash on desert plants and
animals. Southern California Edison Research and
Development Series 79-RD-101. 85 p.
25