Differential Establishment of Perennial
Grass and Cheatgrass Following Fire on
an Ungrazed Sagebrush-Juniper Site
Robin J. Tausch
Jeanne C. Chambers
Robert R. Blank
Robert S. Nowak
enough that the plant community crosses a threshold
from a perennial-dominated to an annual-dominated community. This dramatic shift in species composition alters
competitive and fire dynamics to maintain annual dominance on the affected sites (Billings 1990; Tausch and others, 1994a; Young and Evans 1973; Young and others
1987). Such major changes in community composition to
new stable communities are examples of a largely unappreciated component of global change (D’Antonio and
Vitousek 1992).
It is important for managers to understand the processes and factors behind the largely irreversible conversion of range vegetation from perennial dominance to
cheatgrass dominance. Cheatgrass currently dominates
more than 41 million hectares in the Intermountain West
(D’Antonio and Vitousek 1992; Mack 1981; Young and
others 1987) and is continually expanding (Young and
Tipton 1990). The conversion has occurred on extensive
areas of western and northern Nevada rangelands (Morrow
and Stahlman 1984). Cheatgrass is also invading undisturbed sites (Morrow and Stahlman 1984; Young and
Evans 1973) where a slow conversion to annual dominance can then occur (Svejcar and Tausch 1991; Tausch
and others, 1994b).
Is the barrier to the return of perennial dominance controlled by the presence of cheatgrass? What has been the
role of fire or other environmental factors, either alone or
in combination, in the establishment and persistence of
cheatgrass? Many factors such as soil physical, microbiological, and nutrient characteristics, are potentially involved in the change from perennials to annuals following
fire. These factors are all modified by the level and duration of disturbance (Klopatek and others 1988).
However, some known characteristics of cheatgrass are
definite factors. Once present, the dominance of cheatgrass seems related to its ability to effectively compete
with native perennials for limited soil moisture (Melgoza
and others 1990). Cheatgrass is also an example of an
invading species for which close climatic similarity to its
region of origin is not necessary (Roy and others 1991).
In Nevada the dominance of cheatgrass following disturbance varies with the environmental conditions and
plant species composition of the site (Tausch and others
1994b). Higher elevation sites and sites with a high cover
of perennial grass are more likely to return to perennial
dominance following fire than shrub-dominated lower elevation sites. This pattern, however, can be modified by
Abstract—After one fire in a bunchgrass-dominated juniper
woodland, cheatgrass became dominant under the remnant
crowns of burned juniper, but native bunchgrass dominated the
inter-spaces. In a second burn across a ridge bunchgrass dominated both under burned juniper crowns and interspaces. Vegetation variables of species crown cover, average perennial plant
size, and perennial plant and cheatgrass densities did not differ
between interspace areas for the two burn locations or between
interspace areas and the area under juniper crowns of the second
burn. Several soil surface variables, and total soil organic carbon
and organic nitrogen differed between tree and interspace areas
but not between burns within tree or interspace areas. The variables analyzed did not explain why two stable communities with
a threshold between them occurred on this site following fire.
The existence of several stable states of plant species
composition with thresholds, or barriers to change, between them is now known to exist within many plant
communities (Friedel 1991; Laycock 1991). The reasons
for the stable states and intervening thresholds can involve many biotic and abiotic factors. Understanding the
mechanisms that permit multiple stable states in community composition, and that can cause the thresholds between them, is important for managing rangelands
(Johnson and Mayeux 1992; Tausch and others 1993).
For many communities, thresholds were crossed during
the late 19th and early 20th centuries as a result of heavy
livestock use. The introduction of cheatgrass (Bromus
tectorum L.) in many areas has formed an additional
stable state community and a threshold between it and
the former plant composition. If present in a community,
cheatgrass usually remains a part of the herbaceous layer
until a fire occurs (Young and Tipton 1990; Young and
others 1987). Fire alters the biotic and abiotic factors
In: Roundy, Bruce A.; McArthur, E. Durant; Haley, Jennifer S.; Mann,
David K., comps. 1995. Proceedings: wildland shrub and arid land restoration symposium; 1993 October 19-21; Las Vegas, NV. Gen. Tech. Rep.
INT-GTR-315. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Research Station.
Robin J. Tausch is Project Leader and Jeanne C. Chambers is Research
Ecologist, Intermountain Research Station, Forest Service, U.S. Department of Agriculture, Reno, NV 89512. Robert R. Blank is Soil Scientist,
Conservation Biology of Rangelands Unit, Agricultural Research Service,
U.S. Department of Agriculture, Reno, NV 89512. Robert S. Nowak is Associate Professor, Department of Environmental and Resource Sciences, University of Nevada, Reno, NV 89512.
252
the grazing history of the site (Tausch and others, 1994a).
To control these changes, a better understanding of the
environmental and vegetation conditions that allow the
conversion is needed. Particularly needed is information
on how community composition, precipitation amount and
seasonality, nutrient dynamics, and microbiological relationships of a site affect the competitive balance between
cheatgrass and perennial species.
In a mid-elevation Utah juniper (Juniperus osteosperma
[Torr.] Little) woodland with a bunchgrass-dominated
understory, cheatgrass dominance appeared to vary by
micro-site following fire. Areas under remnant juniper
crowns (crown sites) on one burned area were cheatgrass
dominated; the interspace areas (interspace sites) were
dominated by bunchgrasses. On a second burn across a
ridge both crown and interspace sites were dominated by
bunchgrasses. We hypothesized that some combination
of the presence of juniper, characteristics of the fire, soil
conditions, and effects on nutrient conditions was sufficient to push the plant community across a threshold to
a stable community dominated by cheatgrass. The fire, in
combination with the other possible factors, did not result
in the environmental conditions necessary for cheatgrass
dominance in the interspace areas of the first burn or on
any part of the second burn.
The two burns offered a rare opportunity to study the
microscale conditions that alter cheatgrass and perennial
grass dominance following fire. The objectives of this
study were to document possible vegetation differences,
and to examine some of the potential abiotic factors that
may lead to cheatgrass dominance following fire in a relic
juniper community. Efforts were focused on answering
two questions. First, were there biotic and edaphic factors that differed under the crowns of unburned juniper,
or in unburned interspace areas adjacent to the two burns,
to explain the post-fire differences? Second, were there
biotic or edaphic factors that differed between crown and
interspace areas after burning that coincided with
cheatgrass dominance?
remnant juniper crowns, burned in 1974 and the east side
burned in 1981. All of the burned area on the east side,
and the upper elevation portion of the burn on the west
side, are on the same parent material, a residuum and
colluvium from basalt. They also have the same soils—
loamy-skeletal, mixed, mesic, aridic argixerolls (SCS 1983).
Only the part of the west side burn that has the same soil
type as the east side burn was used for the study. Both
burns were patchy with unburned areas surrounded by
burned area.
The studied area of the west side burn covers an elevation range from just under 1,900 m to 1,950 m. Most of
the site is on northwest- to north-facing slopes with limited south slope involvement. Sampling on the east side
burn was over an elevation range of 1,950 m to 2,100 m
and on mostly east-facing slopes.
Data Collection
Vegetation and soils were sampled in August 1993 using circular study plots 10 m2 in area (3.57 m diameter).
The woody portions of the crowns of the burned juniper
are still standing on both burns. All burned juniper trees
on both burns with crowns large enough to contain the
circular plot were located and mapped. Five of these trees
were randomly selected on each burn. Three additional
plots were then located for each burned tree plot. A
burned interspace plot was randomly located adjacent
to each burned tree plot. The unburned tree of sufficient
size nearest to each selected burned tree, on the same
slope and aspect, and similar in elevation to the burned
tree plot, was selected as a paired unburned tree plot.
Last, an unburned interspace plot was randomly located
adjacent to each unburned tree plot. This gave a total
of five sets of four plots (burned and unburned tree and
burned and unburned interspace), or a total of 20 plots
on each burn. Plots located under burned or existing juniper crowns were centered within the area covered by the
crown. Interspace plots adjacent to each burned or unburned tree plot were located at least 2 m, but not more
than 5 m, away from the edge of the nearest burned or
unburned tree crown.
Plant crown cover was estimated in four 0.5 m2 quadrats that were randomly selected from eight possible
quadrats. One quadrat was randomly selected from each
of four pairs located on the uphill, downhill, and on either
side of the circular plot. An estimation guide representing
1 percent of the quadrat was used to determine cover.
Cover data were averaged by species for each circular plot.
Shrub and perennial grass density and average crown
area were sampled in the full 10-m2 circular plot. Two
crown diameters were measured on each perennial plant:
the longest and the one perpendicular to the longest.
Measurements for each plant were converted to crown
area using the equation for an ellipse; average crown area
was determined for each species in each circular plot. Soil
surface characteristics of litter, bare ground, gravel, cobble,
stone, and cryptograms were determined from six points
along each side of each quadrat (48 points per plot). Percent of the total number of points was computed for each
category for each circular plot.
METHODS
Study Site Description
Two sites in an area of ungrazed relic vegetation were
used for this study. These sites are on the upper elevations of the south end of the Virginia Mountains west of
Pyramid Lake in Nevada. Patterns of cheatgrass dominance following fire in this area are variable with location
(Tausch and others, 1994b). The relic communities are
dominated by an overstory of scattered Utah juniper.
Trees cover the full range of age classes from seedlings to
trees several hundred years old, but total tree cover is less
than 15 percent of the area. The understory for most of
the area is dominated by bluebunch wheatgrass (Pseudoroegneria spicatum [Pursh] A. Young). Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis Bettle &
A. Young) is scattered throughout the area. The two
burns are separated by the main north/south ridge of
the mountains. The west side, with cheatgrass under
253
Four soil samples of the 0-5 cm surface mineral layer
were randomly sampled within each 10-m2 circular plot
and composited for analysis. The Walkley-Black procedure was used to quantify soil organic carbon (Nelson and
Sommers 1982). Total organic nitrogen was quantified
with the Kjeldahle method (Bremmer and Mulvaney 1982).
Nitrogen is possibly the most limiting nutrient in juniper
woodlands (Klopatek 1987).
Table 1—Tree, shrub, grass, and forb species sampled on two
burns on the Virginia Mountains, NV
Species
West
East
Trees
Juniperus osteosperma (Torrey) Little
X
X
X
X
X
X
X
X
X
X
X
X
X
Shrubs
Artemisia nova Nelson
A. tridentata wyomingensis Beetle & A. Young
Chrysothamnus viscidiflorus (Hook) Nutt.
Ephedra viridis Cov.
Eriogonum microthecum Nutt.
Ribes velutinum E. Greene
Tetradymia canescens DC.
Data Analyses
Two areas of possible differences were the focus of the
analyses. First, were there differences between east and
west unburned tree plots that might explain the postburn differences in vegetation composition of the burned
tree plots? A supplemental analysis was a comparison
of east and west differences between unburned interspace
plots. Second, were there differences among the east and
west side burned interspace plots plus the east side burned
tree plots, and differences between these three sets of plots
and the west side burned tree plots as a separate group?
Vegetation and soils data were analyzed by dividing the
data into burned and unburned sets to directly focus on
these differences. Each set was analyzed separately with
one-way analysis of variance. Each AOV analysis contained four treatments. Burned site treatments were east
side burned tree (EBT), east side burned interspace (EBI),
west side burned tree (WBT), and west side burned interspace (WBI) plots. Unburned treatments were east side
unburned tree (EUT), east side unburned interspace (EUI),
west side unburned tree (WUT), and west side unburned
interspace (WUI) plots. Significant differences for all
analyses were at the P ≤ 0.05 level. Differences between
treatments within each analysis were by Tukey’s method
of pairwise comparisons.
The classification program TWINSPAN from the Cornell
Ecology package (Ludwig and Reynolds 1988) was used to
evaluate the degree of similarity among the 40 sampled
plots. Classification was based on the percent vegetation
cover for the individual species.
X
Grass
Achnatherum hymenoides (Roemer &
Schultes) Barkworth
A. thurberianum (Piper) Barkworth
A. webberi (Thruber) Barkworth
Bromus tectorum L.
Elymus elymoides (Raf) Swezey
Melica stricta Bolander
Poa secunda J.S. Presl.
Pseudoroegneria spicata (Pursh) A. Love
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Forb
Allium sp.
Antenaria sp.
Arabis holboellii Hornem.
Astragalus sp.
Aster sp.
Balsamorhiza hookeri Nutt.
Castilleja chromosa Nelson
Calochortus bruneaunis Nelson & J.F. Macbr.
Chorizanthe brevicornu Torrey
Crepis acuminata Nutt.
Cryptantha micrantha (Torrey)
Eriastrum sparsiflorum (Eastw.) H. Mason
Erigeron sp.
Galium sp.
Gayophytum sp.
Gilia inconspicua (Smith) Sweet
Lactuca sp.
Lomatium sp.
Lupinus caudatus Kellogg
Lygodesmia spinosa (Nutt) Tomb.
Machaeranthera canescens var.
leucanthemifolia (E.Greene) Welsh
Mentzelia albicaulis Hook.
Phacelia bicolor S. Watson
Phlox stansburyi (Torrey) A.A. Heller
Senecio multilobatus A. Gray
Sisymbrium altissimum L.
Results and Discussion
Seven shrub, seven grass, and 22 forb species were
sampled on the east side burn (table 1). The west side
burn had five shrub, seven grass, and 17 forb species. A
total of 42 species were identified with seven more species
sampled on the east side than the west side.
Vegetation cover differences between east and west
side unburned tree plots were not significant. Unburned
interspace plots on the west side, however, had significantly higher total vegetation cover than the east side
plots because of higher sagebrush and total perennial
grass cover (table 2). Over 97 percent of the total plant
cover on burned tree plots on the west side was cheatgrass or annual forbs; these species comprised less than
9 percent of the cover on burned tree plots on the east
side. Cheatgrass plus annual forbs comprised 17 and 11
percent of the burned interspace plots on the west and
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
east sides, respectively. The west side fairly consistently
had the highest total plant cover across burned and unburned plots compared to east side plots.
Bluebunch wheatgrass density on the west side unburned tree plots was only slightly higher than on the
east side (table 3). All unburned interspace plots and east
side burned tree plots were similar in bluebunch wheatgrass density. The burned tree plots on the west side had
254
Table 2—Average percent vegetation crown cover for unburned
and burned, east and west, and tree and interspace plots.
Means with different letters across rows are significantly
different (P ≤ 0.05). Species with less than 0.1 percent
cover are indicated with ‘t’
Total cover
A. tridentata wyomingensis
Total shrub
Pseudoroegneria spicata
Elymus elymoides
Poa secunda
Total perennial grass
Bromus tectorum
Forbs
Burned
Total cover
A. tridentata wyomingensis
Total shrub
Pseudoroegneria spicata
Elymus elymoides
Poa secunda
Total perennial grass
Bromus tectorum
Forbs
Interspace
East
West
side
side
2.2a
0.2a
0.2a
0.8a
ta
0.3a
1.1a
0.2a
0.8a
6.5a
0.1a
1.3a
1.2a
0.3a
0.7a
2.2a
0.2a
2.8a
26.0b
5.7b
10.7b
13.4b
ta
0.5a
13.9b
0.6a
1.4a
39.5c
11.0c
16.8b
19.9b
0.6a
1.2a
21.7c
0.2a
1.0a
47.5ab
2.3a
4.2a
39.0a
0.0a
0.1a
39.1a
1.1a
66.3a
0.0a
0.6a
0.9b
0.2a
0.0a
1.1b
47.7b
40.4b
ta
0.6a
35.4a
t4a
ta
35.4a
2.6a
50.2ab
ta
2.4a
37.7a
0.5a
0.9b
39.2a
0.8a
3.1ab
16.9a
1.9b
7.8ab
.5
Eigenvalue
Unburned
Tree
East
West
side
side
.6
Tree
Unburned
A. tridentata wyomingensis
Pseudoroegneria spicata
Elymus elymoides
Total perennial grass
Bromus tectorum (no./m2)
West
side
1.6ab
17.0a
9.0b
26.8ab
14.7a
5.0ab
69.6b
0.2a
70.6c
22.4a
5.6b
54.6b
0.8a
57.2bc
9.5a
No. of plots
0.4a
49.8a
0.0a
49.8ab
42.9a
0.0a
3.2b
1.8a
16.0b
284.6b
0.0a
72.6a
0.2a
72.8a
74.2a
0.2a
50.8a
1.2a
53.2ab
79.3a
Total perennial grass
Bromus tectorum (no./m2)
Tree
Unburned
A. tridentata wyomingensis
Pseudoroegneria spicata
Elymus elymoides
593a
4a
16a
1,187a
9a
5a
1,313a
47b
6a
880a
54b
80a
1,891a
124a
0a
0a
23b
19a
0a
104a
19a
113a
122a
35a
WBI
5
WUI
5
EBT
1
WUI
5
EUT WUT
5
5
Table 5—Total soil organic nitrogen (percent), total soil organic carbon (percent) and C/N ratios for unburned and burned, east
and west, and tree and interspace plots. Means with different letters across rows are significantly different (P ≤ 0.05)
Tree
Burned
A. tridentata wyomingensis
Pseudoroegneria spicata
Elymus elymoides
EBT
4
less than a third of the bluebunch wheatgrass density and
were an order of magnitude greater in cheatgrass density
than all the other plots.
Sagebrush plants tended to be larger on the east side
while both bluebunch wheatgrass and squirreltail (Elymus
elymoides) were generally larger on the west side (table 4).
Overall, differences in the results between east and west
sides, with the exception of species composition on the
west side burned tree plots are small, indicating the two
sites share the same basic community.
Classification of the 40 sample plots based on vegetation cover indicated four groups (fig. 1). The first division
separated the unburned tree plots from the rest, and the
second separated the burned tree plots on the west side.
At no level of division were the unburned tree plots on the
east and west sides separated. The final two groups separated the remaining unburned plots from the remaining
Interspace
East
West
side
side
West
side
EBI
5
Figure 1—Dendrogram of TWINSPAN analysis results for the 40 circular plots sampled on two burns
on the Virginia Mountains, NV. Only the first three
levels of division are shown. Sample site abbreviations are: WBT = west side burned tree, EBI = east
side burned interspace, EBT = east side burned
tree, WBI = west side burned interspace, WUI =
west side unburned interspace, EUT = east side
unburned tree, WUT = west side unburned tree.
Table 4—Average plant crown area (cm2) for unburned and burned,
east and west, and tree and interspace plots. Means with
different letters across rows are significantly different
(P ≤ 0.05)
East
side
WBT
5
Twinspan Dendrogram Virginia Mountains Burn
Burned
A. tridentata wyomingensis
Pseudoroegneria spicata
Elymus elymoides
.2
0
Interspace
East
West
side
side
0.8a
9.6a
0.4a
11.0a
18.0a
.3
.1
Table 3—Average plant density (no./10 m2) for unburned east
and west, tree and interspace plots. Means with different
letters across rows are significantly different (P ≤ 0.05)
East
side
.4
255
Interspace
East
West
side
side
Unburned
East
side
West
side
Nitrogen
Carbon
C/N ratio
0.32ab
4.86a
15.56a
0.56a
7.33a
12.32a
0.10b
1.28a
13.65a
0.15ab
1.87a
13.68a
Burned
Nitrogen
Carbon
C/N ratio
0.33ab
3.86ab
11.99a
0.51a
5.80a
11.44a
0.17b
1.70b
10.64a
0.16b
1.65b
10.47a
Table 6—Percent ground cover for litter, bare ground, gravel,
cobble, stone, and cryptogram for unburned and burned,
east and west, and tree and interspace plots. Means
with different letters across rows are significantly different
(P ≤ 0.05)
Tree
(table 6). Cryptogram cover was lower in the burned plots
compared to the unburned plots, particularly in the tree
plots. Within both the interspace and tree plots, the eastern and western sides were not significantly different for
any of the ground cover variables.
An investigation of the associated precipitation patterns
at Reno, NV, revealed an interesting pattern (fig. 2). Two
years prior to the date of each burn precipitation was below
average. The year prior to the year of each burn had above
average precipitation and the precipitation of the year of
the burn was below average for both. However, the 2 years
following the burn on the west side (1974) were below average while those following the burn on the east side (1981)
were two of the wettest on record. Direct experimentation
will be necessary to determine if these differences were an
important factor in the dominance of annuals under the
burned western trees.
Interspace
East
West
side
side
Unburned
East
side
West
side
Litter
Bare
Gravel
Cobble
Stone
Cryptogram
80.00a
0.42a
2.08a
2.97ab
5.84a
7.92a
84.58a
0.00a
0.82a
0.84a
4.18a
8.34a
41.66b
7.50a
14.58b
7.08ab
18.76b
9.58a
37.10b
10.42a
4.16a
9.16b
20.84b
16.82a
Burned
Litter
Bare
Gravel
Cobble
Stone
Cryptogram
68.34a
0.84a
4.46a
0.84a
25.84a
0.42a
65.42ab
0.84a
0.00a
1.26a
32.50a
0.00a
50.42b
7.08a
9.18a
3.76a
26.68a
3.32a
55.00ab
6.68a
4.58a
4.16a
25.00a
4.58a
Conclusions
The existence of microscale patterns of alternate perennial bunchgrass- or cheatgrass-dominated stable communities in an area of woodland vegetation represented a rare
opportunity for investigating some of the processes involved. Similar to many areas of the Great Basin (Young
and others 1987), where either annual or perennial species dominated the site, seedling establishment by the
other group was very limited. Differences might still persist because cheatgrass dominance may be preventing
bunchgrass establishment on the soils under burned juniper, and bunchgrass dominance may be preventing cheatgrass establishment on the interspace soils. If the vegetation or soil factors sampled for this study had been modified
by the previous fire, these changes were no longer present
at sampling. While different stable states in community
composition are common, and potentially important, the
reasons for their existence and for the threshold between
them can be subtle and difficult to identify.
Several possibilities may be involved. Fire, in combination with the presence of large juniper, may have modified
the physical, chemical, or microbiological characteristics
of the soil. Nutrient islands form beneath juniper canopies and result in different nutrient dynamics between
canopy and interspace soils. Nitrogen mineralization and
nitrification can also be higher beneath the canopy than
in the interspace (Klopatek 1987; Klopatek and others
1990). Soil N had a small but significant increase in canopy
versus interspace soils following fire in woodlands (Klopatek
and others 1991). Nitrogen and other soil nutrients also
showed increases under burned debris piles (Gifford 1981).
Juniper as well as most of the understory species are dependent on vesicular-arbuscular endomycorrhizae (VAM)
species that can be substantially reduced by fire (Klopatek
and others 1988). Cheatgrass and other introduced annuals, by contrast, are not mycorrhizal dependent and are
unaffected by mycorrhizal reduction by fire. Other possibilities can include different grazing regimes, differences
in the intensity and timing of fire or other disturbance,
and other environmental differences at the time of both
the disturbance and subsequent plant reestablishment.
burned plots, with one exception. The east side burned
tree plot with the highest shrub cover ended up with the
unburned interspace plots from both the east and west
sides. Except for the burned tree plots on the west side
that had been cheatgrass dominated for 19 years, east
and west side plots did not separate at any level of the
classification analysis.
Overall, the east and west sides were not significantly
different for any carbon or nitrogen measures (table 5).
Levels were largely the same on the burned plots, even
the burned western tree plots. Carbon/nitrogen ratios
were slightly lower on the burned plots.
Litter cover in the unburned tree plots was higher than
in the interspace plots, as may be expected from tree litter
Precipitation (inches)
12
Average PPT
8
4
0
1972
1974
1976
1979
1981
Year
Annual Precipitation Reno, NV
1983
Figure 2—Histograms of annual precipitation
for Reno, NV, for the years 1972 to 1984. Precipitation histograms coinciding with years of
the two burns on the Virginia Mountains, NV,
are filled in solid.
256
More detailed studies of the changes in key soil factors
and the patterns of plant establishment that immediately
follow fire are needed to better understand the processes
involved. Future studies would also need to be conducted
in different community types and locations to verify the
processes involved.
Melgoza, G.; Nowak, R. S.; Tausch, R. J. 1990. Soil water
exploitation after fire: Competition between Bromus
tectorum (cheatgrass) and two native species. Oecologia.
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Morrow, L. A.; Stahlman, P. W. 1984. The history and distribution of downy brome (Bromus tectorum) in North
America. Weed Science. 32: 2-6.
Nelson, D. W.; Sommers, L. E. 1982. Total carbon, organic
carbon and organic matter. In: Page, A. L., ed. Methods
of soil analysis. Part 2. Madison, WI: Soil Science Society of America: 539-580.
Roy, J.; Navas, M. L.; Sonie, L. 1991. Invasion by annual
brome grasses: a case study challenging the homoclime
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