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Species Compatibility and Successional
Processes Affecting Seeding of PinyonJuniper Types
Scott C. Walker
Abstract-Introduced perennial grasses have long been used in
rangeland revegetation efforts. These species have gained dominance in some communities. The competitiveness of these species
have reduced native vegetative components. Through evaluation of
long term ecological studies, the competitiveness of some introduced
seeded species have become apparent. -Our findings reveal that
differences in species composition can be attributed to the competition of the seeded introduced species.
Techniques have been developed that effectively remove
trees while retaining the understory and creating the necessary seedbed. Plant research and development has resulted
in considerable progress in improving availability of native
grass, forb and shrub seed and in improving their establishment. These plant materials allow us to now go beyond
revegetation to restoration. Ideally, ecologically speaking,
our effort now is to restore sites and help return them back
to a situation where natural processes can function.
When restoring areas that have been inundated by
pinyon-juniper trees, seeding is often necessary where
desirable understory species are absent or too sparse to
respond to treatment. Seeding efforts involved with pinyonjuniper treatment have been directed toward increasing
forage production for livestock and restoring plant communities and wildlife habitat.
Seed mixtures have included various introduced and native
grasses, forbs, and shrubs (Stevens 1983). Over the past 30
to 40 years, introduced grass species have dominated most
seed mixtures. These grasses were selected for a number of
reasons incl uding availabili ty of seed, ease of establishment,
forage production, and soil stabilization characteristics.
The compatibility of these exotic species with native herbs
and shrubs is now becoming apparent through long term
ecological studies. Many exotic species have competitive
and aggressive establishment, and consequently have a
direct adverse affect on native species and communities.
These affects are often detrimental and have an adverse
impact on natural functioning plant communities and ecosystems. Competition for limited resources may determine
the presence, absence, or abundance of species within a
community as well as their spatial arrangement (Pyke and
Archer 1991). Mter reviewing research investigating competition in semiarid plant communities, Fowler (1986) concluded that competition does occur in these systems, involves different species, and is an important determinant of
community structure. Rapid changes in plant communities
occur on disturbed areas, such as chained and seeded
pinyon-juniper sites (Stevens 1986; 1987).
Effects of Introduced Grasses _ _
In: Monsen, Stephen B.; Stevens, Richard, comps. 1999. Proceedings:
ecology and management of pinyon-juniper communities within the Interior
West; 1997 September 15-18; Provo, UT. Proc. RMRS-P-9. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Rocky Mountain Research
Station.
Scott C. Walker is a ResearchIWildlife Biologist, Division of Wildlife
Resources, Great Basin Research Center, Ephraim, UT 84627.
USDA Forest Service Proceedings RMRS-P-9. 1999
This paper will address the ideas of species compatibility.
It is important to consider the effect that seeding has on
the natural recovery of species that are released when an
area is treated, and the influences of seeding exotics with
natives. The following case studies demonstrate effects of
introduced grasses on native and other seeded species.
Case Study 1-Gambel Oak Control by
Intermediate Wheatgrass, Smooth Brome
and Fairway Crested Wheatgrass
(Plummer and Others 1970)
Large areas of Gambel oak were burned to remove overstory oak stems. These areas were then seeded to a mixture
of intermediate wheatgrass (Thinopyron intermedium),
smooth brome (Bromus inermis), and Fairway crested wheatgrass (Agropyron cristatum) in equal amounts at a total
seeding rate of 12-15 lb per acre. A like area that burned
was not seeded with the grass association.
Figure 1 shows the accumulative height of oak after
15 years on the area where, Fairway crested wheatgrass,
intermediate wheatgrass and smooth brome were established as an understory after a burn. This is contrasted
with an adjacent similar area where oak was burned but
where no seeding occurred. The accumulated growth of oak
on sites that were seeded has been maintained at an average
height of about 40 inches and has remained open; whereas
on nonseeded sites the oak has grown to an average height
of 105 inches, and the clumps have again become impenetrable thickets. When well established, the seeded grasses
have restricted the height regrowth of oak on treated sites
by more than 60 percent.
Findings of this study were that, "After 14 years, it continues to be confirmed that competitive herbs can be seeded
and established for controlling Gambel oak thickets, as
well as thickets of other shrubs. Intermediate wheatgrass
and smooth brome (two sod formers), and Fairway crested
331
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100
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l_~ithou~ exotic gr: association
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--
II
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----- - - - - - - - - - - - - - - - - - - - - - j
20
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o -t-~T--r--r-~T-,---~---,------r--T~~r-i--I
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16
Years Following Treatment
Figure 1-Mean accumulative height of Gambel oak
plants in treatment areas with and without seeded
exotic grass association.
wheatgrass have been found to be particularly useful for this
purpose." Also, "Grazing by deer, as well as by livestock,
appears to keep brush from growing beyond animal's reach,
but most important is the competition from the established
understory plants."
This study demonstrates that species interaction and
competition can be beneficial depending on the desired
outcome and management objective. It also shows how
very competitive some introduced species can be. There
was no information in the study to indicate how other
plant species on the study area were affected. It is reasonable to assume that if these grasses were able to effectually
reduce oak regrowth through competition for resources,
then other understory species were likely adversely affected.
Case Study 2-The Competitive Influence
of Seeded Smooth Brome (Bromus
Inermis) and Intermediate Wheatgrass
(Thinopyron Intermedium) Within AspenMountain Brush Communities of Central
Utah (Monsen And Others 1996)
Fourty years after seeding the seeded areas showed a mean
percent ground cover for introduced grasses was 33 percent,
native grasses 1 percent, native perennial broadleaf herbs
11 percent, and shrubs 15 percent. In the nonseeded areas
the mean percent cover for introduced grasses was 4 percent,
native grasses was 17 percent, native perennial broadleaf
herbs 21 percent, and shrubs 41 percent (fig. 2).
One hundred-nine species were encountered within the
study sites. Of all species present, 86 were found in the
seeded areas and 82 in the nonseeded areas, with 61 species
common to both areas. Results of Sorensen's index of similarity, (Sorensen 1948) for all areas combined within a
treatment, shows only a 41 percent similarity of species
between the two treatments. This indicates the dissimilarity
ofthe species composition between the two treatments, even
though the number of species in each area is nearly equal.
Prevalent species ranking by frequency value shows considerable difference between seeded and nonseeded areas
(taole 1). Smooth brome, intermediate wheatgrass and mountain big sagebrush (Artemisia tridentata vaseyana) were the
most prevalent species in the seeded areas, while mountain
snowberry (Symphoricarpos oreophilus), mountain big
sagebrush and bluebunch wheatgrass (Pseudoroegneria
spicata) were the most prevalent in the nonseeded areas.
Considerable difference in forb species occurrences and
numbers were evident between treatments. The numbers of
forb species between treatments were nearly equal with
58 in the seeded and 57 in the nonseeded area of these
species 40 were common to both treatments. Native forb
species accounted for a greater percent of the ground cover
in the nonseeded than in the seeded area (fig. 2). This would
indicate that native perennial forbs are doing better in the
nonseeded area and did not fair well growing in combination
with the exotic grasses
Considerable variation in summed frequency was evident
between seeded and nonseeded areas (fig. 3). In the seeded
-]---
50 ~----- .. --
Species Groups
•
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Ecological relationships of smooth brome and intermediate wheatgrass with native species were investigated through
comparison of seeded and nonseeded sites in aspen-mountain brush communities, on the Great Basin Research Area,
Manti-La Sal National Forest. These sites were adjacent to
each other, and vegetatively comparable prior to seeding.
Within a 40-year period, the two sod-forming seeded grasses
gained dominance and reduced native herbs and shrubs.
Both introduced grasses are commonly planted to stabilize
wildlands, but they are proving to be noncompatible with
most native species and ultimately dominate seeded sites.
All lifeform groups (introduced grasses, native grasses,
native perennial broadleaf herbs, and shrubs) showed a
statistically significant difference in the percent ground
cover values between the seeded and nonseeded areas.
332
oa:
(!)
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u
a:
w
Il.
10
o
S
S
NS
S=SEEDED
NS=NONSEEDED
NS
S
NS
S
NS
TREATMENTS
Figure 2-Percent ground cover of lifeform groups
for seeded and nonseeded areas, 40 years after
seeding. *Within species groups, columns with different letters indicate significant differences (p < 0.05).
USDA Forest Service Proceedings RMRS-P-9. 1999
Table 1-Ranking of most prevalent species for area seeded with
introduced grasses and nonseeded areas.
Seeded Area
I;.
Nonseeded Area
1. Bromus inermis
2. Thinopyron intermedium
3. Artemisia tridentata
4. Lupinus sericeus
5. Symphoricarpos oreophilus
6. Vicia americana
7. Agoseris glauca
B. Aster chilensis
9. Agropyron crista tum
10. Taraxacum officinale
25 ................................................................................................................ ..
1. Symphoricarpos oreophilus
2. Artemisia tridentata
3. Pseudoroegneria spicata
4. Bromus carinatus
5. Aster chilensis
6. Astragalus conval/arius
7. Vicia americana
B. Eriogonum umbel/a tum
9. Stel/aria jamesiana
10. Agropyron trachycaulum
........ii. ........................................................................... ......................... .
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64
67
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Nat. Per. Forbs
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S=SEEDED
NS=NONSEEDED
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TREATMENTS
Figure 3-Summed frequency of lifeform groups for
seeded and nonseeded areas, 40 years after seeding.
USDA Forest Service Proceedings RMRS-P-9. 1999
S
NS
82
87
Year
1--- Introduced Grasses
area the introduced grasses were significantly more frequent than in the nonseeded area. Native grasses and
shrubs were significantly more frequent in the nonseeded
area than in the seeded areas. Though native forbs showed
no significant difference between treatments as a group,
there are difference in the species composition between
treatments.
Species, cover, composition, and frequency differences
between aspen-mountain brush communities seeded to
smooth brome and intermediate wheatgrass in the 1950's
and adjacent nonseeded areas, demonstrate the competitiveness and adverse influence of these seeded grasses on
native species. Seeded and nonseeded areas were similar at
time of seeding. Grazing pressure on these areas has been
the same. Conclusions were similar to those of Rosentreter
(1994), Davis and Harper (1990), and Walker and others
(1995), in that differences in species composition can be
attributed to the competition of the seeded species. Native
grasses, forbs, and shrub species diversity, frequency, and
cover are higher in the nonseeded areas. Few native
77
72
-+-. Native Grasses
Figure 4-Average density of introduced and native
grasses within the ungrazed treatment. Values with the
same letter are not significantly different (p < 0.05).
grasses, forbs, or shrubs within the aspen-mountain brush
communities in central Utah can compete with intermediate
wheatgrass and smooth brome. These introduced grasses
are proving to be noncompatible with most native species
and ultimately dominate seeded sites. The concomitant loss
of species diversity diminishes resource values.
Case Study 3-lnteraction Between Native
and Seeded Introduced Grasses Through
23 Years Following Chaining and Seeding
of Juniper-Pinyon Woodlands (Walker
And Others 1995)
Threejuniper-pinyon woodland sites in central Utah were
evaluated over 23 years following chaining and seeding of
introduced grasses under grazed and nongrazed conditions.
The density and production of herbaceous species was
measured at intervals for 23 years in both the grazed and
nongrazed treatments. In 1964, three to five years following initial treatment, introduced grasses had a five times
greater density than the native grasses in the ungrazed
areas (fig. 4). The density trend of the introduced grasses
has continued to increase through 1987, while keeping the
native grasses somewhat suppressed.
Under grazing pressure the introduced grasses have not
increased in density at the same rate as in the ungrazed
treatment. In the grazed treatment the introduced grasses
have generally maintained at least twice the density as the
natives (fig. 5). In the grazed treatment the introduced
grasses are dominated by Fairway crested wheatgrass and
intermediate wheatgrass. In the ungrazed treatment the
introduced grasses are dominated by smooth brome and
Fairway crested wheatgrass (table 2).
There has been a dramatic shift in the composition of
the native species following seeding in both the grazed and
ungrazed treatments. In 1964, both treatments supported
a good compliment of a number of native grasses. Bottlebrush squirreltail (Sitanion hystrix), Indian ricegrass
(Oryzopsis hymenoides), and bluebunch wheatgrass was
333
25 --------------------------------------------------------------------------------------------------------------------
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64
67
72
77
82
87
Year
1---
Introduced Grasses -+-. Native Grasses
Figure 5-Average density of introduced and native
grasses within the grazed treatment. Values with the
same letter are not significantly different (p < 0.05).
well distributed throughout the areas (table 3). In 1987
the native grass composition shifted to, and is dominated by,
a less productive sandberg bluegrass (Poa secunda) and
western wheatgrass (A. smithii). The 1987 native grass
component in the grazed treatment is comprised almost
entirely (99 percent) of these two species (table 3).
Bottlebrush squirreltail and needle-and-thread (Stipa
comata) responded favorably to removal of the juniperpinyon, but succumbed to the more competitive species
once they became established. Indian ricegrass and
bluebunch wheatgrass are perennials that would be expected to respond favorably to tree removal. They both did,
however, as the introduced species became firmly established, they were not able to compete with the aggressive
exotics, with and without grazing. Western wheatgrass
was able to compete somewhat with the introduced species,
with or without grazing. Sandberg bluegrass, an opportunist, did well growing in association with western wheatgrass and the introduced species.
Table 2-Rank of introduced grasses by frequency and by density for grazed and ungrazed treatments. The rank is based on the
average of three study areas, in the first data collection year (1964) and in the last data collection year (1987), showing
species composition changes
Rank by
frequency
Frequency of
occurrence
Rank by
density
Density'"
(per hal
Ungrazed 1964
Intermediate wheatgrass'
Fairway wheatgrass"
Smooth brome
Orchard grass
Russian wildrye
Bulbous bluegrass
1
2
3
4
5
6
15
10
5
4
4
3
2
1
4
3
6
5
1,990
5,779
565
684
24
24
--9,066
Ungrazed 1987
Fairway wheatgrass"
Intermediate wheatgrass'
Russian wildrye
Smooth brome
1
2
3
4
15
11
8
5
2
3
4
5,913
402
24
11,745
18,084
Grazed 1964
Fairway wheatgrass"
Intermediate wheatgrass'
Smooth brome
Bulbous bluegrass
Russian wild rye
Orchard grass
1
2
3
4
5
6
15
14
10
4
4
3
1
2
3
5
6
4
5,360
1,918
1,091
62
29
67
8,527
Grazed 1987
Fairway wheatgrass"
Intermediate wheatgrass'
Russian wild rye
1
2
3
10
7
6
1
2
3
10,635
646
178
11,459
Species
'Combination of intermediate and pubescent wheatgrass.
"Combination of fairway and standard wheatgrass.
'''Number of individual culms with rhizomatous species. Number of individual plants with bunchgrasses.
334
USDA Forest Service Proceedings RMRS-P-9. 1999
Table 3-Rank of native grasses by frequency and by density for grazed and ungrazed treatments in the first data collection year
(1964) and in the last data collection year (1987), showing species composition changes
Species
Rank by
frequency
Frequency of
occurrence
Rank by
density
Ungrazed 1964
Bottlebrush squirreltail
Indian ricegrass
Bluebunch wheatgrass
Western wheatgrass
Sandberg bluegrass
Needle-and-thread
1
2
3
4
5
6
15
11
9
4
4
2
1
2
3
4
5
6
Density'"
(per ha)
837
598
378
148
29
19
2,009
Ungrazed 1987
Sandberg bluegrass
Western wheatgrass
Indian ricegrass
Bluebunch wheatgrass
Bottlebrush squirreltail
1
2
3
4
5
10
9
3
2
1
2
3
4
5
2,29
1,512
24
19
5
---
3,856
Grazed 1964
Bottlebrush squirreltail
Indian ricegrass
Sandberg bluegrass
Bluebunch wheatgrass
Western wheatgrass
Needle-and-thread
1
2
3
4
5
6
15
13
5
5
2
1
2
4
5
3
6
723
698
72
48
101
5
1,647
Grazed 1987
Sandberg bluegrass
Western wheatgrass
Indian ricegrass
Bluebunch wheatgrass
Needle-and-thread
Bottlebrush squirreltail
1
2
3
4
5
6
10
10
5
5
1
1
2
3
4
5
6
9,989
4,758
75
32
22
11
---
14,887
'Number of individual culms with rhizomatous species. Number of individual plants with bunchgrasses.
Results show that after 23 years following the introduction of exotic grass species, the communities, though changing in density and cover, have not yet stabilized in plant
dominance. The introduced grasses are increasing in density, cover, and production at a greater rate than are the
native grasses that have shown a reduction in diversity.
Case Study 4-Long-Term Harmful Effects
of Crested Wheatgrass on Great Plains
Grassland Ecosystems (Lesica and
Deluca 1996)
Lesica and DeLuca (1996) have done a comprehensive
review of literature related to crested wheatgrass effects
on Great Plains ecosystems. In summary:
Invasions by exotic plants are occurring at an increasing
rate and are considered a serious threat to both agricultural
systems as well as native communities (Drake and others
1989). Not only are exotics invading large areas, exotics
have and are being planted extensively. Crested wheatgrass
(A. cristatum and A. desertorum) is the most commonly
planted exotic grass in western North America.
USDA Forest Service Proceedings RMRS-P-9. 1999
Crested wheatgrass has many desirable characteristics.
These include, good forage yields, ease of establishment,
good nutritional values, and it resists invasion by weeds.
Although crested wheatgrass has desirable characteristics
there are several often overlooked characteristics that
may create a significant long-term decline in biological
diversity and soil resource sustainability.
It is not uncommon to have considerable soil loss in crested
wheatgrass stands. The strong competitiveness of crested
wheatgrass creates a situation that results in high amounts
of exposed soil. Grasslands with more exposed soil experiences higher rates of erosion (Wilson 1989; Dormaar and
others 1995; McWilliams and Van Cleave 1960).
Perhaps a more serious effect on soil properties than the
potential for increased soil erosion is the effect crested
wheatgrass has on biochemical soil quality. Crested wheatgrass has a higher above-ground productivity than many
native grasses. However, the below-ground biomass in the
surface horizon is significantly lower (Dormaar and others
1995; Retente and others 1989; Smoliak and Dormaar 1985;
Smoliak and others 1967). This lower below-ground biomass
in crested wheatgrass reflects a reduction in both root
detritus and root exudates that would otherwise be available
335
for microbial use in the formation of soil organic matter. Ai:,
a result, below-ground biomass under stands of crested
wheatgrass has a higher carbon to nitrogen ratio than native
grass species and only supplies about half as much organic
N to the soil as does native grasslands (Biondini and others
1988; Klein and others 1987, 1988; Redente and others
1989). The small quantity and lower quality of organic
matter in the upper soil horizons under stands of crested
wheatgrass also results in a lower energy input to these soils
as compared to native ranges (Dormaar and others 1978)
and alters physical and biochemical processes in the soil.
Stands of crested wheatgrass are associated with higher
bulk density, fewer water stable aggregates, and lower
levels of organic matter and nitrogen when compared to
native grasses (Biondini and others 1988; Dormaar and
others 1978; 1995; McHenry and Newell 1947; Redente and
others 1989; Smoliak and others 1967Y. Crested wheatgrass
provides the soil with a relatively high concentration of
carbohydrates and little organic nitrogen (that is, the socalled "priming effect") as quantities of readily degraded
carbohydrates in the presence of limited nitrogen often
result in a net demand on soil organic nitrogen (DeLuca and
Keeney 1993; Jansson and Persson 1982; Mortensen 1963).
It has been suggested that these alterations to soil quality
may prevent native species from invading crested wheatgrass monocultures (Klein and others 1988).
We presently lack the knowledge to know the long-term
effects of crested wheatgrass. However, there is a growing
body of knowledge that suggests that crested wheatgrass
alters the environment in many undesirable ways. Further
research into the changes in soils and plant and animal
diversity associated with crested wheatgrass in the Great
Plains as well as the Intermountain west are needed to
assess its impact. Nonetheless, the continued conversion of
native range and planting of crested wheatgrass in large
stands as mono cultures or other exotic species seems ill
advised.
Discussion --------------------------------These four case studies help us understand that plant
communities are continually changing in plant composition,
density, and cover due to the effect directly caused by seeded
species, precipitation cycles, and impacts from grazing or
other disturbances. Introduced grasses have became more
dominant in the communities, especially in the absence of
grazing. The competitiveness of these grasses have caused
a decline in native vegetation, reducing desirable species
and changing plant composition. The data indicate that
most native grasses do not appear to compete well with the
species that were introduced. These results are Similar to
those of Bock and others (1986) who reported that stands of
exotic grasses support significantly lower variety and abundance of indigenous grasses. Davis and Harper (1990) report
that planting a mixture of introduced and native species
may produce artificial plant associations in which species
mayor may not be fully compatible with each other; and that
it is difficult to maintain a stand of specified composition
because each species responds differently to natural and
imposed environmental factors that affect competitiveness.
336
There is an obvious shift in composition over time, the
exotics are favored generally at the expense of native communities. Trends indicate that over a 30 or 40 year period
of time introduced grasses are going to increase until
they become dominant. For wildlife habitat, for biological
diversity and for ecological integrity this scenario is not
acceptable.
Acknowledgments
Funds were provided through Federal Aid in Wildlife
Restoration Project W82R, Studies 4,5, and 7; and Rocky
Mountain Research Station, Forest Service, U.S. Department of Agriculture, Provo, Utah.
References _ _ _ _ _ _ _ _ __
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losses through root exudation by Agropyron spicatum. A. smithii
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Davis, J. N., Harper, K. T. 1990. Weedy annuals and establishment
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Tueller, compilers. Proceedings-symposium on cheatgrass
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arid and semiarid regions. Annu. Rev. Ecol. Syst. 17:89-110.
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