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DISPLACEMENT OF RARE PLANTS
BY EXOTIC GRASSES
Roger Rosentreter
ABSTRACT
alyssum and filaree more typically occur on coarse sandy
soils; and Russian thistle, kochia, halogeton, tumblemustard, and perfoliate peppergrass are common on alkaline sites.
The domination of a site by a particular species is not
wholly dependent on soil chemistry, as these species may
proliferate rapidly in an area following a soil-disturbing
event. The species that stochastically has the greatest
abundance of seed present in the seed bank is often the
species to dominate.
Exotic grasses are displacing both rare and common
plant species on western rangelands. Interspecific competition displaces valuable genetic resources in many parts
of the world, including the Intermountain shrub-steppe
communities. This problem is especially bad in the Intermountain West because of the invasion of exotic species
preadapted to the arid conditions. Case studies of several
rare plants, including Aase's onion (Allium aaseae
Ownbey), Davis peppergrass (Lepidium davisii Rollins),
slick-spot peppergrass (L. papilliferum [Henderson]
A Nels. & J.F. Macbr), inch-high lupine (Lupinus uncialis
S. Wats), gymnosteris (Gymnosteris nudicaulis [Hook. &
Arn.] Gray), and an annual buckwheat (Oxytheca dendroides Nutt.), illustrate the challenge these species have
in surviving in environments modified by exotics. Even if
grazing were properly controlled or regulated, rare plants
would still be threatened unless exotic grasses and the resulting increased fire frequency were controlled.
Watershed Values
Sites converted to annuals have lost genetic, species,
and structural diversity. They represent low-quality watersheds with increased susceptibility to soil erosion and
are prone to desertification (Buckhouse 1985). Annual
vegetation is more susceptible to drought, as its biomass
accumulation fluctuates with the available soil moisture
more than perennial vegetation does.
Stewart and Young (1939) found forage production of
grasses varied much less for perennial grasses than for
cheatgrass. They demonstrated that perennials produce
twice the biomass of cheatgrass in a moist year and 12
times the biomass during a drought. Managing forage for
domestic livestock on annual grasslands can be seen as a
gamble under such production schedules (Roberts 1990).
An error in planning or grazing management can cause
habitat degradation during dry years and leave dangerous
flash fuels in moist years. Rangeland users need a more
dependable source of forage for proper grazing management (Roberts 1990).
INTRODUCTION
The displacement of rare plants by annual weeds is a
major problem on Intermountain rangelands. Cheatgrass
<Bromus tectorum L.) is now the dominant species on
more than 40 million ha (100 million acres) of the Intermountain West (Mack 1981). Competition between rare
plants and annual weeds is both direct and indirect. Indirect impacts caused by annuals include increased fire frequency and the associated conversion of shrub-steppe to
nonshrub annual grasslands, and the associated increased
plant litter (Billings 1990; Evans and Young 1970) and
changes in nutrient availability. Prior to the arrival of
white settlers, fire-return intervals in the sagebrush
steppe probably varied between 60 and 110 years, but
much of the region now burns at intervals of less than
5 years (Whisenant 1990).
Fire Frequency
Sites dominated by annuals typically have higher fire
frequencies than sites where perennials dominate (Pellant
Table 1-Common exotic weedy annual species on Intermountain
rangelands
The Exotic Annuals
The most prevalent exotic annual weeds in the Intermountain West are listed in table 1. These and other annuals may occur alone or in combination. Each year, depending on weather conditions, timing of precipitation
events, soil texture, soil pH, and stochastic fluctuations
in the availability of seed, exotic annual species may rapidly dominate a disturbed site. For example, medusahead wild.rye is more prevalent on heavy clay soils; pale
Common name
Cheatgrass
Medusahead wildrye
Filaree
Bur buttercup
Perfoliate peppergrass
Pale alyssum
Tumblemustard
Tansymustard
Halogeton
KochIa
Russian thistle
Paper presented at the Symposium on Ecology, Management, and Restomtion of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
Roger Rosentreter is Botanist, Bureau of Land Management, U.S.
Department of the Interior, 3380 Americana Terrace, Boise, ID 83706.
170
Scientific name
Bromus tectorum L.
Elymus caput-medusae L.
Erodium cicutarlum (L.) L'Her.
Ranunculus testiculatus Crantz
Lepidium perfoliatum L.
Alyssum alyssoides L.
Sisymbrlum altissimum L.
Descurainia pinnata (Walt.)Britt
Halogeton glomeratus Meyer
Kochiaspp.
Sa/sola kali L.
1990). Exotic annuals produce a continuous bed of flash
fuels that perpetuates an abbreviated fire cycle, a cycle
unsuitable for persistence of most native plants (Wright
1985). The resulting loBS of genetic and structural diversity is important to the functional dynamics of the system
and as an indicator of ecosystem health.
Studies of rare species not only deepen our knowledge
of these species, but they also deepen our understanding
of common species, offering insightS to questions of fundamental importance in ecology and evolutionary biology.
LoBSes in vegetation diversity cause collateral declines in
faunal diversity and shorten the time period of active
plant growth (Parmenter and MacMahon 1983).
It is the reduced period of active growth and the associated reduction in community biomass that causes the decline in many animal populations. For example, loss of
shrubs used as winter food and thermal cover for jackrabbits caused a decline in the use of the Snake River Birds
of Prey Area by golden eagles (Nydegger and Smith 1986;
Steenhof and Kochert 1988). A reduced period of active
green vegetation means a reduced period when wildlife
and domestic livestock can properly utilize an area.
Ecosystem Stability
Loss of diversity generally causes ecosystem instability
and, in portions of the Intermountain West, increases fire
frequencies (Whisenant 1990). In addition to the biological "cost" of increased fire frequencies, the enormous
monetary cost of fire in terms of suppression and rehabilitation provides a major incentive to maintain native perennial rangelands.
Annual fire suppression costs on Bureau of Land
Management-administered lands in the State of Idaho
alone averaged $4,339,000 per year between 1987 and
1991. In 1992 the costs increased to $10,253,000 (Bill
Mitchell: personal communication). Without a conversion
from the annual grassland vegetation back to a perennial
type, these costs will recur as areas repeatedly burn, and
traditional rehabilitation costs of these sites can be much
higher than the appraised value of the land (Roberts
1990).
STUDY AREA
The results discussed later are based on several studies
of rare plant populations in the Snake River Plain and adjacent foothills. The Snake River Plain is of basaltic origin, while the foothills are a mixture of granitic alluvium
and fine silts and clays. Elevations range from 671 m
(2,200 ft) to 1,676 m (5,500 ft) at midmountain. Soil
moisture& are classified as aridic; soil temperatures are
claBSified as mesic. Cooler soil sites that are frigid soil
temperature regime or cooler are not addressed in this
treatment.
Landscape Patterns
Annual grasslands that burn frequently are highly uniform, interspersed with very few patches of unburned vegetation remaining (Whisenant 1990). Large, species-poor
sites have a strong isolating effect on the remaining native vegetation patches. The new fire regime results in
areas with poor species richness, low landscape patchiness, and altered successional patterns. Mature sagebrush steppe is eliminated, as are many earlier successional species that occupy a site after minor disturbances
within the mature community.
Loss of a dominant species such as sagebrush may
cause the loss of sympatric species dependent on it, such
as twisted moBS (Tortula ruralis [Hedw.] Gaerth.), which
grows in the shade of sagebrush. Likewise, spiny hopsage
(Grayia spinosa [Hook.] Moq.) becomes established beneath canopies of mature shrubs (Nancy Shaw: personal
communication). Some sagebrush grassland species are
mutualistic, sharing mycorrhizal associations as well as
complementing each other ecologically (Wicklow-Howard
1989). As one species is lost, associated species may also
be threatened.
ABIOTIC FACTORS AND LANDSCAPE
PA'ITERNS
Shrubs increase abiotic microsite variability by providing shade, decreasing wind, and capturing snow and rain
on a local scale. This is especially important in areas like
the Intermountain West where precipitation comes primarily in the winter as snow.
Abiotic factors change when a site is converted from
shrubs to exotic annual grasslands, and the loss of shrubs
is the major factor affecting abiotic changes (Billings
1990). With shrub loss, there is a decline in (1) forage
production, (2) soil stability, (3) diversity, (4) consistent
annual biomass production, (5) mineral cycling, (6) thermal and escape cover for wildlife, and (7) esthetic values
(Parmenter and MacMahon 1983; Rosentreter and
Jorgensen 1986). These local influences on the abiotic
conditions are reflected in the vegetative cover at every
level of scale on the landscape. For example, the composition of the local vegetation is simplified and the degree of
landscape patchiness decreases with an increased prevalence of annual grasses, biotic changes which can persist
for long periods due to the changes in the abiotic environment (Billings 1990; Whisenant 1990). In northern Nevada, Billings (1990) followed a sagebrush- steppe site for
41 years after it burned. He found that sagebrush did not
return to this cheatgrass-dominated site.
Shrubs
Biological relationships and their structural dependency are critical in the shrub-steppe of the Intermountain West. Shrubs intercept moisture, provide shade,
recycle deep soil nutrients, and modify soil-surface temperatures by decreasing wind velocity (Murray 1975;
Rosentreter and Jorgensen 1986). These structural features of steppe shrubs are comparable to values attributed to trees in forests. Conversion of shrub-steppe to
annual grasslands represents a drastic and detrimental
change in a community.
171
In a climate where most of the annual precipitation
comes in the form of snow, shrub presence or absence may
determine the effective soil moisture throughout the year.
At low and mid-elevations, the snow melts periodically,
saturating the ground beneath the snowbank. Repeated
occurrences of this snow-capturing and melting process
contribute to a patchy vegetation pattern.
SPREAD OF EXOTICS
Initially, annual grasses may occur only where a fire or
other major disturbance has taken place, but they may
spread to adjacent sites. Exotic, nongrass species increase slowly after the disturbed site is dominated by
cheatgrass. After a second fire, other exotics, in addition
to cheatgrass, quickly invade the areas adjacent to the
original disturbance. I believe this is largely determined
by the "stochastic occurrence of seed."
STRUCTURAL DIVERSITY
The structural diversity of a site is dramatically lowered as the shrub component declines. Shrubs provide
important thermal and escape cover for wildlife, just as
they provide favorable microsites for establishment of
grass and forb seedlings.
The structure of the shrub community influences the
pattern of snow accumulation and melting process and
moderates soil temperatures both beneath individual
shrubs and at the stand level. These differences influence
animal and plant composition, dispersal, and vigor. Animals dependant on forbs and grasses in a shrub community find green forage for up to a month longer than in
an adjacent annual grassland lacking structural diversity
(Rosentreter, personal observation).
CASE STUDIES
Direct Competition
Native Annuals-In portions of the Intermountain
West cheatgrass has replaced native annuals such as
gymnosteris (Gymnosteris nudicaulis [Hook. & Am.]
Gray), inch-high lupine (Lupinus uncialis S. Wats.),
oxytheca ((hytheca dendroides Nutt.), and langloisia
(Langloisia punctata [Torr.] Greene). Most of these native
species are now on the Idaho State sensitive plant list
(Moseley and Groves 1990). Exotic annuals compete directly with native plants for space, moisture, and light.
Gymnosteris is found in basin big sagebrush (Artemisia
tridentata Nutt. ssp. tridentata) habitat (DeBolt and
Rosentreter 1988). Label data from specimens collected
20 to 40 years ago state that it was "abundant" or "common" (College of Idaho Herbarium, Caldwell, ID [CIC]).
Today this plant is rarely seen, with small colonies found
only on good-condition rangelands when spring moisture
is abundant. I collected extensively in two above-average
moisture years in southwestern Idaho and found few
populations of this species.
The inflorescences of gymnosteris plants collected in
Idaho in the last 10 years are predominately yellow or
white. Older collections exhibited a wider range of colors
at each collection site, including white, yellow, pink, violet, and orange. This decline in color variation may represent a loss of genetic diversity caused by reduced genetic
mixing of the remnant populations. The isolated patches
of gymnosteris may have become reproductively isolated
and may be experiencing genetic bottlenecks (Falk and
Holsinger 1991). This decline in genetic diversity may
decrease the vigor of individuals and populations.
SUCCESSIONAL PATTERNS
It appears that the steppe ecosystem, once controlled
by classical successional patterns (relay floristics), is now
largely dominated by initial floristics due to the introduction of exotic annuals (Billings 1990). In southwestern
Idaho, the first exotic seeds to arrive at a site have a great
advantage over other species, often dominating the site
for long periods of time (Whisenant 1990). This may be
referred to as the "stochastic occurrence of seed." While
seasonal fluctuations in precipitation or litter may affect
the frequency of specific exotics, displaced native perennials typically do not vary seasonally in frequency (Evans
and Young 1970). For example, with fall precipitation,
cheatgrass grows rapidly and will dominate a site by the
following spring. If fall moisture is minimal or lacking,
bur buttercup (Ranunculus testiculatus Crantz) frequently dominates. Russian thistle (Salsola kaU L.)
emerges in spring and comes to dominate disturbed sites
that receive summer precipitation. These exotics take advantage of the full spectrum of the phenologic and temperature niches available in the Intermountain steppe.
While exotic annuals exploit soil moisture at different
times of the year, they do not utilize the entire soil profile.
This may explain why, without fire, grazing, or other major disturbance, shrub-steppe habitats in good condition
can exclude most exotic annuals. Most ecologists recognize the importance of competition for space above the soil
surface, but in arid regions, interspecific competition is
largely played out underground, in the soil profile. Shrubsteppe communities converted to annual grasslands generally lack deeply rooted species to utilize deep soil moisture. Shrubs recycle nutrients and moisture leached deep
into the soil profile (Caldwell and Richards 1989; Murray
1975). Without the return of these deep nutrients and
moisture, many sites may become increasingly impoverished, arid, and less productive (Billings 1990).
Indirect Competition
Aase's Ouion-Aase's onion is a rare (Federal category
1 species), small perennial that occurs on coarse, barren,
sandy soils on east-, south-, and west-facing slopes in the
Boise foothills (DeBolt and Rosentreter 1988). It is often
associated with mature bitterbrush (Purshia tridentata
[Pursh] DC.) on or near ridgetops. Aase's onion is restricted both ecologically and geographically. Its phenology is very early, with growth beginning in February and
flowering in March and April. While its habitat is not
optimal for the growth of cheatgrass, as the surrounding
vegetation is converted to annuals, cheatgrass can be
found invading Aase's onion habitat. As the occurrence
of cheatgrass increases in areas adjacent to onion habitat,
litter cover increases thereby modifying soil conditions
172
Table 2-Number of Allium umbels and cover class values of exotic
annuals per quadrat. Annual cover classes: 1 = 1-5
percent, 2 = 6-25 percent, 3 = 26-50 percent, 4 =51-75
percent
Growth of these annuals on coarse, sandy soil and adjacent areas causes an accumulation of litter and a nutrient
flush. This litter is damaging to the onion, which is
adapted to barren, nutrient-poor sites. Though no onion
sites have been totally displaced by exotics to date, the extent of the onion's suitable habitat has decreased in size.
Cover class of Mean number of
S.d. of
Quadrat sample
annual s pecies Allium umbels Allium umbels
size (N)
1
2
3
4
11 .5
8.4
5.6
4
8.9
5.9
4.9
2
Davis Peppergrass-Davis peppergrass, a Federal
candidate 2 species, is a caespitose perennial mustard restricted to hard-bottom playas (fig. 1) that are usually
barren of other vegetation (DeBolt and Rosentreter 1988).
The playas are poorly drained and often inundated with
standing water in early spring. It occurs in eastern
Oregon and southwestern Idaho.
Although cheatgrass does not grow on the playas with
Davis peppergrass, it indirectly affects peppergrass survival. Playas in the Great Basin receive runoff from adjacent areas. When these adjacent areas are converted to
exotic annual grasslands, siltation generally increases
(DeBolt and Doremus 1989, 1990). Some playas adjacent
to annual grasslands can receive over 1.5 em of siltation
in the spring following a summer wildfire (Ann DeBolt
1988: personal observation). Excess siltation may
smother and kill individual Davis peppergrass plants.
A second and more direct threat created by the degraded grasslands is the growth of other annual weeds,
.especially Russian thistle (fig. 2). After death, Russian
thistle, kochia, and tumblemustard plants dry and break
38
14
12
3
and encouraging the growth of other exotic annuals such
as filaree (Erodium cicutarium [L.] L'Her.) and pale alyssum (Alyssum alyssoides L.) (Young and Evans 1979).
These exotics spread easily into sandy sites directly competing with the onion (table 2).
Table 2 data group cheatgrass, filaree, and pale alyssum together as annuals. Table 2 shows that the number
of Allium umbels decreases as the percentage of annuals
increases in a qua drat (unpublished data, Boise District,
BLM files). Field observations of Aase's onion show an
increase in filaree, pale alyssum, and cheatgrass on sites
that have burned (table 2), with filaree the best adapted
for sand.
..
""
....
~-
- • . ...
~~
•
Figure 1- Piaya with Davis peppergrass.
~
Figure 2-ln this playa Davis peppergrass died
due to the mulching effect of Russian thistle.
173
-
CONCLUSIONS
free from the ground to blow about as "tumbleweeds,"
occasionally coming to rest in the playas. Because playas
are depreBBions in the landscape, they may collect large
numbers of tumbleweeds to depths of a meter or more.
Tumbleweeds can act as a mulch, blocking light for extended portions of the year, and sometimes causing extensive peppergrass mortality. Several playas have been
completely covered by tumbleweeds (fig. 3). Figure 8
shows data from two playas (number 64 and 75) where
populations of peppergrass went from 114 and 65 plants
respectively to none in a 2-year period. Six other playas
(fig. 3) with Davis peppergrass have greatly decreased in
population size due to the mulching effect of tumbleweeds
(DeBolt and Doremus 1989, 1990, and unpublished 1992
data).
Throughout the Intermountain West, native plants,
both rare and common, are reduced in number and genetic diversity where exotic annuals become dominant.
This change in community structure is not a temporary
situation, but rather a shift in successional patterns to a
self-perpetuating annual community. This change in species composition promotes an increased fire frequency
that concomitantly reduces structural diversity and
patchiness of the landscape.
The loss of structural diversity, once provided by shrubs
and perennial bunchgrasses, modifies abiotic conditions in
the community. The new community of exotic annuals
competes both directly and indirectly with native plants.
Indirect competition includes the increase in fire frequency, stochastic distribution of exotic seeds, increased
litter accumulation, increased siltation, and loss of
microphytic plants.
Most native species can tolerate moderate amounts of
grazing. However, even one season of misuse may cause
degradation and subsequently invasion by exotic annuals,
thus predisposing the site to fire. After one or more
burns, the diverse native plant comm~ty is converted to
an impoverished community of exotic annuals. Maintaining healthy native communities, the suppression of fire,
and the prevention of rangeland wildfires is needed to retain rare and common native plant communities.
SUck-Spot Peppergrass-Slick-spot peppergrass is
a small biennial mustard restricted to relatively barren
"slick spots" within the Wyoming big sagebrush (Artemisia tridentata Nutt. BBp. wyomingensis Beetle and
Young) community type in southern Idaho (DeBolt and
Rosentreter 1988). Slick spots are shallow soil sites or
tiny playas also referred to as intercoppice areas (Eckert
and others 1986). These sites are variable in size, but
they are generally small (2-5 mt) and irregular in shape.
They are sparsely vegetated by vascular plants, but are
often carpeted with nonvascular microphytic plants, also
called cryptogamic crusts (Rosentreter 1986).
For slick-spot peppergrass, cheatgrass is not a direct,
but rather an indirect, competitor. The intercoppice areas
are small, and as the adjacent vegetation is converted
from shrubs to annuals conditions change~ Fires become
more frequent, altering the character of the soil's surface,
causing siltation onto the slick spots. The fires and siltation also destroy the crust of microphytic plants. In
Montana, Lesica and Shelly (1992) determined thatArabis fecunda Rollins, another rare mustard, was positively
888ociated with the growth of microphytic plants. The cumulative effects of these changes permit invasion of exotic
annual plants into the slick spots.
·
9
36
13
37
59
64
75
ACKNOWLEDGMENTS
I would like to thank Ann DeBolt for her assistance in
the field and in the review of this paper. I would also like
to thank Drs. Nancy Shaw and Wayne Owen for their review and suggestions on this paper.
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