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PATTERNS OF ANNUAL GRASS
DOMINANCE ON ANAHO ISLAND:
IMPLICATIONS FOR GREAT BASIN
VEGETATION MANAGEMENT
Robin J. Tausch
Tony Svejcar
J. Wayne Burkhardt
ABSTRACT
in the absence of significant mammalian herbivory or human disturbance.
Cheatgrass CBromus tectorum L.) was introduced to North
America in the late 1800's, reached a stasis in its expansion
by the 1930's (Mack 1986), and may have reached Anaho
Island during this period. Later, red brome {B. rubens L.)
probably followed a similar but less well-documented establishment pattern. Despite the absence of herbivory
or human disturbance on Anaho Island, the presence and
dominance of cheatgrass and red brome are similar to that
observed for other areas in the Great Basin (Billings 1990;
Young and others 1987). More recently the ranges of both
species have again been expanding (Hunter 1990; Young
and Tipton 1990). Recent vegetation trends on the island
(Svejcar and Tausch 1991) appear similar to the increasing
occurrence of these species and increasing dominance of
red brome in other disturbed and undisturbed communities
(Hunter 1991) over the last two decades.
In a Master's thesis done on the island in the early 1960's
Woodbury (1966) reported red brome to be present on less
than half of the island. On most of the areas where he found
red brome, it was not the dominant in the herbaceous layer.
Svejcar and Tausch (1991) observed that large areas where
Woodbury (1966) listed cheatgrass as dominant and red
brome as absent were red brome dominated. The possible
climatic, genetic, and competitive processes behind these
patterns of change on Anaho Island, and throughout the
Great Basin, have significant implications for management
(Baker 1986; Bazzaz 1986; Roberts 1991).
This paper used aerial photography for a more detailed
look at the 1991 distributions of annual grass species and
the patterns of perennial plant replacement over the island.
We have also used a series of aerial photographs of the island dating from 1954 to 1991 to estimate possible patterns
and rates of change from perennial to annual dominance.
Anaho Island National Wildlife Refuge, northeast of
Reno, NV, is dominated by annuals despite a general absence of human-caused disturbance and fire. Four species
ofannual grass dominate areas that are segregated by elevation, aspect, and slope. A previous study and aerial photographs show annual grasses were present throztghout the
island well before the early 1960's and beginning to dominate in scattered areas by the early 1970's. By the early
1980's areas where annual grasses had replaced perennials
were larger in size, and by 1991 covered about a third ofthe
vegetated areas.
INTRODUCTION
In a previous paper Svejcar and Tausch (1991) reported
on the dominance of annual invader species on the Anaho
Island National Wildlife Refuge. Anaho Island is located in
Pyramid Lake northeast of Reno, NV. It is currently about
200 ha in size and is the largest island in Pyramid Lake.
Pyramid Lake is one of three remnants of the former pluvial Lake Lahontan. The island was established as a wildlife refuge in 1913 to protect breeding populations of primarily pelicans but also cormorants and California gulls.
Anaho Island has a long history of minimal disturbance by
humans.
The only resident mammal on the island is the deer
mouse, and the primary predator is the Great Basin rattlesnake (Woodbury 1966). There are reports of limited attempts to use the island for raising sheep, goats, and pigs,
but such attempts ended early in the 19th century (Henry
1990; Janik and Anglin 1991). The island has been under
protection for nearly 80 years. Visitors to the island are
accompanied by U.S. Department of the Interior Fish and
Wildlife Service personnel. As a result, vegetation competition on the island for the last several decades has existed
STUDY SITE
Anaho Island rises to 1,334 m above sea level and
about 180 m above the level of Pyramid Lake (fig. 1). Over
half the current area of the island has been exposed since
1911 when Derby Dam was constructed upstream of the
Truckee River to divert water to the Fallon area for irrigation. Prior to the construction of Derby Dam, Pyramid
Lake was at the level of Mud Slough where it overflowed
into the Winnemucca Lake Basin to the east. The area of
Anaho Island above this level is about 90 ha in size and is
Paper presented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
Robin J. Tausch is Project Leader, Intermountain Research Station,
Forest Service, U.S. Department of Agriculture, Reno, NV 89512. Tony
Svejcar is a Range Scientist, Eastern Oregon Agricultural Research Center, Agricultural Research Service, U.S. Department of Agriculture, Bums,
OR 97720. J. Wayne Burkhardt is an Associate Professor of Range Management, Department of Environmental and Resource Sciences, Univenity
of Nevada, Reno, NV 89512.
120
D
Bromus rubens L
E3
E3
Bromus rigidus Roth.
ETI]
Hordeum /eporinum Unk
rsJ
Area exposed since 1911
Bromus tectorum L
Figure 1-Map of Anaho Island showing locations where four annual grasses dominate the forb layer.
in 1991 each dominated the herbaceous layer on different
parts of the island.
the subject of this paper. The area below the 19th century
shoreline also has some interesting vegetation dynamics of
colonization and community development, but these will
not be discussed here.
Anaho Island receives 12 to 18 em precipitation annually.
Native vegetation is composed of plant species common to
the salt-desert shrub zone throughout the Great Basin.
The four species of annual grass found on Anaho Island are
cheatgrass, red brome, rip-gut brome (B. rigidus Roth), and
annual foxtail (Jlordeum leporinum Link). All four species
were present when Woodbury (1966) studied the island and
METHODS
Two types of aerial photographs were used to map the
patterns of annual grass dominance on Anaho Island. First,
a 1:24,000 vertical color infrared aerial photograph taken
in July 1991 was enlarged, and second a series of oblique
color aerial photos were taken in December 1991. Both
types of photos were used to estimate the distribution and
121
relative dominance of annual grasses and dominant perennial species. These comparisons were confined to the area
of the island above the 19th century water line (fig. 1) and
excluded rock outcrops and pelican rookery areas.
The aerial photographs were first used to map the distributions of dominance by the four species of annual grass.
Field reconnaissance was used to verify the mapped distributions. The photographs were then used to divide the vegetation into three cover types. These cover type designations
were also verified with field reconnaissance. The first cover
type is areas where we estimated the dominant perennial
cover to have a consistent distribution or presence on twothirds or more of the area. Gaps in the pereBDial cover from
one to a few dead individuals for this first cover type were
estimated to total less than one-third of the area.
The second cover type is areas where the dominant perennial cover had a consistent distribution or presence on
less than one-third of the area. In these areas perennials
occurred as scattered individuals with annual species the
only cover on over two-thirds or more of the area. The third
cover type is the remaining area that was estimated to be
between the first two categories.
Aerial photographs from various sources for 1954, 1956,
1970, 1971, 1973, 1980, and 1991 were used to estimate the
pattern over time of the conversion of island vegetation to
dominance by annuals. These included black-and-white,
color, and color infrared types. Quality and scale varied
considerably among the photos, and none was equal to the
quality or resolution of the 1991 photographs. Because of
the limited quality of these photographs, and the small size
and scattered nature of areas showing loss of perennials,
they were not useful for mapping.
ANNUAL GRASS DISTRIBUTION
In 1991 annuals grew on all parts of the island. Red
brome was the dominant herbaceous species on about 44 ha
(60 percent) of the area above the 19th century water line,
other than rock outcrop (fig. 1). It was dominant on areas
where Woodbury (1966) indicated cheatgrass to be dominant and red brome to be absent. Red brome dominated
most of the southerly aspects and lower elevation areas of
the island.
Cheatgrass was the dominant herbaceous species on about
25 ha or over 35 percent of the area (fig. 1). Cheatgrass
dominance was largely confined to the upper elevation and
northerly aspects of the island. Where dominant on lower
elevations, cheatgrass was on steeper northerly facing slopes
of beach terraces. Filaree (Erodium cicutarium [L.] L'Her)
and barbwire Russian-thistle (Salsola paulsenii Litv.) were
also common throughout the island on both red bronie- and
cheatgrass-dominated areas.
Together, annual foxtail and rip-gut brome occupied
slightly over 1 percent of the area (fig. 1). The annual
foxtail-dominated area was located on a level area near
the top of the island. This area was reported by Woodbury
(1966) as the possible location of a pelican rookery earlier
in this century. The two rip-gut brome locations were both
on sites adjacent to large tufa-covered rock outcrops that
may increase effective site moisture.
122
ANNUAL GRASS DOMINANCE
PATtERNS
Rock outcrops comprised about 20 ha or 22 percent of the
total area above the 19th century water line (fig. 2). Active
pelican rookeries covered about 6 ha or 7 percent of the
same area.
Areas where the dominant perennials were estimated to
have two-thirds or more of their original cover represented
about 13 ha or 18 percent of the nonrock outcrop area above
the 19th century water line (fig. 2). The most common community in this type was mixed desert shrub with shadscale .
(.Atriplex confertifolia [Torr. & Frem.] Wats.), spiny hopsage
(Grayia spinosa [Hook.] Moq.), rubber rabbitbrush (Chrysothamnus nauseosus [Pallas] Britt.), bud sagebrush (Artemisia spinescens D.C. Eaton), and winterfat (Ceratoideslanata [Pursh] J. T. Howell). Communities of pure stands of
four-wing saltbush (Atriplex canescens [Pursh] Nutt.) and
winterfat also were recorded. Even in these communities
a dense understory of annuals was evident.
The areas where the dominant perennial cover had a consistent distribution or presence on less than one-third of
the area covered 21 ha or about 30 percent of the nonrock
outcrop area (fig. 2). In these areas perennials occurred
as scattered individuals with annual species the only cover
on over two-thirds of the area. This cover type was most
common on the southerly and lower elevation areas of the
island but occurred on all aspects and elevations. The loss
of perennials has not been confined to shrub-dominated
·communities. A former desert needlegrass (Stipa speciosa
Trin. & Rupr.)-dominated community on the west side of
the island has been largely replaced by red brome.
The remaining cover type where dominant perennials
covered between one-third and two-thirds of the area was
approximately 36 ha (fig. 2), or about 51 percent of the nonrock outcrop area of the island above the 19th centurY water
line. Throughout the island existing perennial vegetation,
both shrubs and grasses, was composed mostly of mature
to decadent plants. Perennial seedlings or juveniles were
largely absent. The reduction in perennial dominance, combined with the absence of herbivory on the island, has resulted in robust growth by some individuals of the remaining perennials.
CHANGES OVER TIME
The series of 1954 to 1991 aerial photographs of the island
(not shown) allowed a general estimation of the pOSSlble patterns and rates of conversion from perennials to annuals.
Because the quality of the photographs prior to 1991 was
not sufficient for mapping, direct estimation of the areas
involved was not possible.
Pictures of Anaho in the thesis by Woodbury (1966)
showed annual grasses to be prevalent in the understory
in the early 1960's. From the 1954 through to the 1973
aerial photographs the perennial cover appeared generally
uniform. Areas that appeared to be annual dominated were
small and scattered and more frequent on the south slopes.
By 1973 the scattered areas of possible annual dominance
appeared to be about twice the size of those in 1956. The
total area in 1973 that was not perennial dominated was
~ Perennials present on % or more of the area
fZI
Perennials present on less than ~of the area
D Perennials on more than ~ and less than %of the area
~ Rock outcrops
ffi)
Pelican rookery areas
0
Area exposed since 1911
Figure 2-Map of Anaho Island showing three classifications of the relative dominance of perennials
and annuals.
still small, accounting for less than about 20 percent of the
vegetation cover on the island above the 19th century water level.
Although much of the 1980 aerial photograph had limited
usefulness because of a low sun angle, some large areas
that appeared to be dominated by annuals were present.
These annual-dominated areas were possibly up to half
or more the size of areas present in the same locations in
1991. Most of the replacement of perennials by annuals
on Anaho Island appears to have occurred in about the last
20 years. As much as half or more of the change may have
occurred in the last decade. The pattern of change appears
to be one where many small areas of annual dominance
have enlarged over time and coalesced into fewer large
areas. The timing of these changes appears to coincide
with the increasing dominance of annual grasses in dry
sagebrush and salt-desert communities in other areas of
the Great Basin, including protected areas such as the
Nevada Test Site (Hunter 1990, 1991; Young and.Tipton
1990).
123
DISCUSSION
wyomingensis Beetle) and mountain big sagebrush (A. t.
vaseyana [Rydb.] Beetle).
Interpreting the patterns of 1991 annual grass dominance
and the patterns of past change, we conclude that the majority of the island vegetation could be dominated by annual
grass by the end of the decade. For most of the island the
annual grass species involved are the same as in other areas
of the Great Basin salt desert (Hunter 1990, 1991; Young
and Tipton 1990). The annual foxtail-dominated area on
the top of the island is an exception.
The most important implication for management is the
increase in annual grass dominance despite the absence
of significant herbivory or other disturbance from human
activity or fire. Those changes appear related to the competitive superiority of annuals for the ~ted available moisture (Melgoza and others 1990). The absence of herbivory
may also be important. In California grasslands, many
areas protected from grazing can have a higher or even total dominance by annuals compared to adjacent grazed areas (Edwards 1992). The associated heavy thatch in these
California grasslands also restricts the growth of perennial
seedlings.
Annual grass-dominated areas on Anaho Island have a
substantial thatch cover and little successful perennial reproduction (Svejcar and Tausch 1991). These conditions are
not present on the adjacent shoreline that has both native
herbivory and year-long livestock use. The cryptogamic
cover of lichens and mosses on Anaho Island is also less
evident in high annual grass cover areas of the island where
the ground surface is covered by thatch. The combined effects of abundant annual grasses appear to be suppressing
the reproduction of the perennials. That many of the older
perennials have died over the last two decades is evident
from the common presence of dead plants. Without younger
plants to replace them the sites became annual dominated.
Effective management to control annual grasses will require
a better understanding of the biotic and abiotic conditions
that can result in the patterns apparent on Anaho Island.
Nesting activities of pelicans and other birds have been
confined to a small part of the eastern shore during most of
the 19th century. In the past, however, these use patterns
have apparently varied. Earlier in the century a rookery
site was reported in the area now dominated by annual foxtail (Woodbury 1966). Recently available information indicates that annuals are better able to compete with perennials under improved nutrient conditions (Harper, these
proceedings). The unusual presence of foxtail and its dominance of one site may represent a nutrient effect from past
use of the location for nesting. Other still unknown factors
may also be involved.
The dominance by annuals on Anaho Island appears related to its aridity. A more mesic site about 800 to 1,000 m
higher in elevation on the Virginia Mountains on the west
side of Pyramid Lake supports relatively large relict areas
dominated by bluebunch wheatgrass (Agropyron spicatum
[Pursh] Scribn. & Sm.). One of these areas burned about
15 years ago and has since returned to even greater wheatgrass dominance despite the presence of cheatgrass. This
area is lightly used by livestock because of rugged terrain.
The boundary for the higher moisture zone represented by
the bluebunch wheatgrass relict areas is currently unknown,
but appears to be approximated in this area by the boundary
between Wyoming big sagebrush (Artemisia tridentata ssp.
The drier Anaho Island annuals outcompete perennials
because of their ability (in this moisture-limited environment) to utilize available soil moisture before the native
perennials can complete their annual growth and reproduction cycles (Melgoza and others 1990). On the Vuginia
Mountains relict sites, soil moisture may remain after the
annuals complete their life cycle. Somewhere between
these two moisture concijtions is a transition zone where
perennials persist despite the presence of annual grass
as long as there is no disturbance by tire. Once such a
disturbance occurs in this zone, a threshold is crossed
(Laycock 1991) and an annual-dominated community results. Large areas of western Nevada appear to be somewhere in this transition zone.
Annuals do not gain dominance under all circumstances
and the patterns by which dominance occurs, when it is
possible, also vary (Mooney and others 1986). The adaptations involved in changes such as the recent range expansions and competitive interactions of cheatgrass and
red brome need to be better understood for effective management of affected areas (Baker 1986). To manage Great
Basin communities for sustainable uses requires an ability to recognize relationships between the encroachment
of annuals and important environmental controls such as
annual soil moisture patterns, the var,iation of other environmental factors associated with a site, and site differences in patterns of competitive interactions between annual and perennial species.
124
ACKNOWLEDGMENTS
Special thanks to Dr. Paul Tueller for providing from
his Remote Sensing Laboratory many of the aerial photos
used in this study and to Bill Henry and Ann Janik of the
USDI Fish and Wildlife Service, Stillwater Wildlife Management area, and Paul Wagner and Dan Mosley of the
Pyramid Lake Fisheries Unit for permission to work on
the island and for their assistance with the field work.
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