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Great Basin Native Forb
Responses to Competition with
Cheatgrass and Elevated Soil
Nutrients
Marenna Disbro, Dr. Elizabeth Leger & Dr. Sarah Barga
University of Nevada, Reno, Department of Natural Resources and Environmental Science DOI: http://dx.doi.org/10.15629/6.7.8.7.5_5-1_S-2019_3
Abstract: The Great Basin Desert is among the most difficult places for successful ecological restoration due to the
repeated effects of fire, aridity and invasive plants. The goal of this research was to determine whether four local
populations of two native annual forb species, Microsteris gracilis and Layia glandulosa, differed in the capacity to
survive competition with cheatgrass (Bromus tectorum) and whether soil nutrients affected outcomes. Seeds were
collected in the spring of 2016 and planted in two phases in the fall, allowing for maximum germination. Factorial
treatments included a competition treatment, with either a single forb or cheatgrass seed sown alone or together, and
a soil nutrient treatment, with either no nutrient addition (low) or the addition of fertilizer pellets (high). Plants were
harvested, dried and weighed in March of 2017. Biomass and flower production were compared across treatments.
The two forb species had declining performance in competition with cheatgrass, especially under high nutrient
conditions, where cheatgrass grew larger. Neither species exhibited population-level variation in reproductive
response to any treatment, but L. glandulosa populations varied in biomass responses to soil nutrient availability.
Though the presence of cheatgrass competition and high soil nutrients affected all forb populations negatively, one L.
glandulosa population was able to respond to increased soil nutrients, indicating that some populations may serve as
better sources for restoration.
Introduction
The Great Basin Desert has experienced
devastating effects from the invasion of cheatgrass
(Bromus tectorum), since it was first introduced into the
western United States in the late 1890s. Cheatgrass,
originating from Europe, northern Africa, and southwest
Asia (Zouhar, 2003), has since formed monocultures on
at least 40 million hectares of land in the Great Basin
(Evans et al., 2001). Cheatgrass has been able to
outperform native species in this system by taking
advantage of ephemeral resources and reliably
reproducing in both wet years and drought years (Mack &
Pyke, 1983; Rice et al., 1992). In addition, cheatgrass is
largely responsible for rapidly increasing fire frequencies
in the Great Basin (Zouhar, 2003), thereby facilitating the
transition of native plant communities into landscapes
dominated by exotic annuals, such as cheatgrass
(Chambers & Wisdom, 2009). The best combative
strategy against further invasion is through restoration
efforts, particularly revegetation and rehabilitation of
degraded sites (Shinneman & Baker, 2009).
There is a growing body of evidence that native
early-successional species, and annual forbs in particular,
have the ability to persist in invaded areas and even
outcompete invasive species in arid systems (Abella et al.,
2011; Abella et al., 2012; Leger et al., 2014; Uselman et
al., 2014). Some native, annual forbs may be able to
serve critical roles in the re-establishment of native
vegetation in disturbed areas (Leger et al., 2014; Uselman
et al., 2014). Additionally, native forbs are integral for
healthy ecosystems as they provide support for many
ecosystem services such as providing wildlife food and
habitat, increasing biodiversity of native flora and fauna,
and encouraging landscape resiliency after disturbance
(Shaw et al., 2012; Barak et al., 2015). However,
information is currently lacking for most native forbs
regarding their ability to grow and reproduce while in
competition with introduced species.
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24 VOLUME 5 | ISSUE 1 | SPRING 2019
Soil nutrients are crucial for plant growth and
productivity, especially for desert annual forbs, with
nitrogen often a limiting resource for plants in the Great
Basin Desert (Farrior et al., 2013). Recently, desert
ecosystems have experienced elevated levels of soil
nitrogen as a result of unsustainable land use practices,
increased pollution and atmospheric deposition (Brooks,
2003). While higher soil nutrient availability may benefit
some native species, desert ecosystems are also
susceptible to invasive plant species that are able to
efficiently exploit soil resources, allocating it to biomass
and seed production. This leads to invasive plant species
with greater plant biomass and fecundity than natives
(Evans, 2001). Plants that display strategies for
enhancing their competitive outcomes, such as efficient
soil nutrient uptake and consumption, are likely to
outperform their neighbors (Tabassum & Leishman,
2016). Research indicates that invasive species in the
Great Basin, such as cheatgrass, possess such strategies
and are expected to continue invading and dominating the
Great Basin Desert (Brooks, 2003; DeFalco et al., 2003).
However, information is lacking with regards to how native
species vary in their ability to utilize soil nutrients and
whether these differences may lead to variation in their
ability to compete with cheatgrass when soil nutrients are
more abundant (Booth et al., 2003; James, 2008).
While many researchers have explored species-
level variation in performance in invaded arid systems
(e.g. Abella et al., 2012; Herron et al., 2013), very little is
known about the degree of population-level variation
exhibited by desert forbs. Variation among populations is
largely controlled by exposure to varying environmental
factors, such as year-to-year differences in climate or
spatial variation in soil nutrients (Ackerly et al., 2000), and
could result in populations that are adapted to their
specific local environmental conditions (Meyer et al.,
1995; Wright et al., 2006). For example, Collinsia
sparsiflora, an annual forb native to California, has
displayed population-level variation in its ability to grow on
two different soil types (Wright et al., 2006) and
populations of Penstemon differed in the timing of
germination depending on environmental characteristics
such as elevation (Meyer et al., 1995). Desert plants may
have a higher propensity for local adaptation due to their
lack of adaptations for long-range seed dispersal (Fllner
& Shmida, 1981), leading to infrequent transfer of genetic
material across populations, and thus populations will
differ in their responses to disturbance and competition.
Identifying appropriate restoration species will
require an understanding of how native species respond
to multiple stressors, which can include both competition
with invasive species and higher nutrient availability
(Abella et al., 2012). This study investigates how the
growth and reproduction of two native annual forbs are
affected by both nutrient availability and competition with
cheatgrass, asking whether these species exhibit
population-level differences in their performance. Two
annual forbs were selected for this experiment,
Microsteris gracilis (Hook.) Greene (slender phlox) and
Layia glandulosa (Hook.) Hook. & Arn. (tidytips). Both
species are known to be important food sources for
Greater sage-grouse (Centrocercus urophasianus), a
species of particular conservation concern in the Great
Basin and beyond (Gregg & Crawford, 2009; Pennington
et al., 2016). Both forb species were observed to be
prevalent in cheatgrass-invaded sites throughout the
Great Basin. The factorial experimental design of this
study includes both a competition treatment and a nutrient
addition treatment, in an effort to test the separate and
combined effects of resource availability on the capacity
of these native forbs to grow with and potentially suppress
cheatgrass. In order to identify whether there were
population-level differences in the growth and
reproduction of these forbs when competing with
cheatgrass, seeds were collected from four populations
for each species. We asked whether a) competition with
cheatgrass, b) nutrient addition, or c) both competition
and nutrient addition affect the performance of the focal
species, asking the following research questions:
1) How is the growth and reproduction of native
annual forbs affected by the above experimental
treatments?
2) Are there population-level differences in the
ability of forbs to grow and reproduce when
experiencing our different experimental
treatments?
3) How is the growth and reproduction of cheatgrass
affected by nutrient availability?
4) Are there differences in cheatgrass response to
competition with different forb species?
We hypothesized that L. glandulosa and M. gracilis would
exhibit a positive response in biomass and flower
production to high nutrient conditions. We predicted that
both native forbs would exhibit population-level variation
in their response to treatments but would generally
decline in performance when exposed to competition with
cheatgrass, with greater reductions under conditions of
greater nutrient availability. Finally, we expected
cheatgrass to exhibit differences in response to
competition with different native forb species.
Methods
Natural History Background
The native species studied here differ in their
seed dispersal ability and reproductive strategies, which
may affect population differentiation. M. gracilis can self-
pollinate, resulting in less genetic diversity (Grant & Grant,
1965; Levin, 1978), while L. glandulosa is incapable of
self-pollinating and requires out-crossing to produce
viable seeds. M. gracilis is ballistically dispersed, meaning
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VOLUME 5 | ISSUE 1 | SPRING 2019 25
that ripe seeds are enclosed in a capsule that bursts and
catapults seeds within the vicinity of the parent plant
(Levin, 1978), while L. glandulosa has a feather-like
pappus that enables wind dispersal, leading to greater
dispersal distances than M. gracilis.
Seed Collection
Seeds were collected from four populations for
each species: B. tectorum, L. glandulosa, and M. gracilis
(Table 1). Seeds were collected on two occasions at each
location, late May and early June of 2016, with the
exception of only collecting once from the Dutch John
population of M. gracilis in Winnemucca, NV along Dutch
John Creek. The Hoge Rd. populations were collected off
of Hoge Rd, Reno NV. The Patagonia populations were
collected on a hiking trail behind the Patagonia
warehouse in Reno, NV, and the Hidden Valley
populations were collected from Hidden Valley Regional
Park in Reno, NV. The Susileen population of L.
glandulosa was collected on Susileen Rd. in Reno, NV.
Collecting seeds at multiple points in time creates a more
representative sample of the seeds from the entire
population, given that the timing of seed maturation may
be staggered across individuals within a population.
Cheatgrass seeds from all populations were mixed to
create a sample that would be representative of diversity
across all sites, to avoid confounding competition
treatments with any population differences in cheatgrass.
All seeds were cleaned and stored in a dry, room
temperature environment until sowing.
Greenhouse Experiment
Phase 1 (early planting)
On October 2, 2016, we filled 640 pots (Ray
Leach - 158 ml Cone-tainers) with soil local to the Reno,
NV area, and placed them into trays with 80 pots per
tray. High nutrient treatments were given four pellets of
fertilizer (Osmocote Smart-Release Plant Food Plus
Outdoor and Indoor Food - 15% N, 9% P2O5, 12% K2O)
per pot. Both treated and untreated pots were watered
for five days to increase soil water content and to allow
nutrients to release from the fertilizer pellets into the soil
prior to planting.
Table 1. Distance between sites and sample sizes for treatments for A) Layia glandulosa, B) Microsteris gracilis, and C)
Bromus tectorum.
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To quantify differences in soil nutrients in the low and high
soil nutrient treatments, 20 pots were filled with soil at the
beginning of the experiment, 10 of which were given four
fertilizer pellets, and were watered along with the pot
planted for experimentation. At the end of the experiment,
soil from these pots was sent to A & L Western
Agricultural Lab in Modesto, CA (www.al-labs-west.com)
for analysis of soil nutrient composition, including:
nitrogen, phosphorus, and potassium. All of these
nutrients increased with the addition of the fertilizer pellets.
The soil samples contained 3 ppm of nitrogen in low
nutrient and 52 ppm in high nutrient treatments, 18 ppm
of phosphorus (weak bray) in low and 21 ppm in high, and
291 ppm potassium in low and 334 ppm in high.
On October 7, 2016 all seeds were sown and
factorial treatments assigned (control, competition, and
nutrient modification). Control treatments for forbs were
represented by a single forb seed planted in the center of
a pot without fertilizer pellets. Each forb species was
represented by 20 individuals from each population,
resulting in a total of 80 forb individuals per treatment,
though not all individuals germinated (Table 1).
Competition treatments were sown with one cheatgrass
seed and one forb seed approximately ½ inch apart in the
center of a pot. Nutrient modification treatments were
sown with either one forb seed alone or one forb seed
sown with one cheatgrass seed assigned to either high or
low soil nutrients. After treatments were applied, pots
were re-distributed among trays, maintaining equal
representation of all populations and treatments within
each tray. For the first month, plants were watered every
day on a mist setting, to avoid seed displacement, and
trays were randomly re-distributed along the greenhouse
tables every week.
Phase 2 (late planting)
A second round of sowing took place on
November 9th and 10th. Pots without forb germination and
pots with solely cheatgrass germination were removed
from every tray. All removed pots were then re-sown with
their respective treatment and randomized in the same
fashion as in round one. Plants from phase-1 were kept in
separate trays for the following four weeks and watered
every other day, while plants from phase-2 were watered
every day. At the beginning of December, pots from both
phases of planting were combined and watered on an
every-other day schedule for the duration of the
experiment. In late January, the number of trays was
doubled and the pots were spread randomly among each
tray to provide extra room for above ground biomass.
Data Analysis
On March 10th all aboveground biomass was
collected, labeled, and placed in individual 5 x 7 coin
envelopes to be dried for two days in a drying oven at
48ºC. Biomass was then weighed and the number of
flowers per plant were counted for inclusion in the
analyses. Additionally, the biomass and flowers of all
cheatgrass that were involved in the competition
treatment were measured to determine differences in the
competitive effects on cheatgrass among the forbs.
Analysis of variance (ANOVA) was used to compare
performance within and among populations of forbs, as
well as differences in the performance of cheatgrass in
response to nutrient addition and competition with both
native forbs (Table 2). Because planting took place in two
phases, planting date was incorporated as a variable in
the analyses. All data were analyzed using program R (R
Development Core Team, 2016). The car package was
used to analyze the percent change in trait values for the
factorial treatments using type 2 ANOVAs (Fox &
Weisberg, 2011), the agricolae package to perform
Tukey’s tests for significant differences in mean values
between groups (de Mendiburu, 2016), and the ggplot2
package to create figures 2, 3, and 4 (Wickham, 2009).
Results
Native forb responses to competition and nutrient
treatments (Question 1)
Our predictions were supported in that both L.
glandulosa and M. gracilis plants were smaller and
produced fewer flowers when exposed to cheatgrass
competition (P < 0.001), and were larger and produced
more flowers when nutrient levels were high (P < 0.001)
(Table 2, Fig. 1, Fig. 2). For both L. glandulosa and M.
gracilis there was a significant interaction between
competition and nutrient addition (L. glandulosa – P <
0.01, M. gracilis – P < 0.001) (Table 2) where plants
differed more dramatically in size in response to nutrient
availability when there was a lack of competition. L.
glandulosa exhibited no interaction between nutrient
addition and competition on the number of flowers
produced. In contrast, M. gracilis exhibited a significant
interaction between nutrient addition and competition,
where plants differed more dramatically in flower
production in response to nutrient availability when there
was a lack of competition (P < 0.001) (Table 2, Fig. 2B).
Native forb population-level responses to competition and
nutrient treatments (Question 2)
Our predictions for the existence of population-
level variation in response to our treatments was partially
supported for L. glandulosa but unsupported for M.
gracilis (Table 2). Populations of L. glandulosa did not
vary in their response to competition with cheatgrass but
did vary in biomass in response to nutrient availability (P
< 0.05) (Table 2). The Hidden Valley, Hoge Road, and
Patagonia populations of L. glandulosa exhibited a much
greater difference in biomass in response to differences
in soil nutrient availability. In contrast, the Susileen
population exhibited much less variation in biomass
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VOLUME 5 | ISSUE 1 | SPRING 2019 27
Table 2. ANOVA table of results for biomass and flower response to each factor and interaction (indicated with “:”),
including degrees of freedom (df; numerator, denominator), F values, and P values, with “NS” indicating P>0.05.
relative to nutrient availability, relative to the other
populations (Fig. 2A). Neither L. glandulosa nor M. gracilis
exhibited population-level variation in flower production in
response to competition with cheatgrass or nutrient
addition.
Effects of nutrient availability on cheatgrass growth and
reproduction (Question 3)
The growth and flower production of cheatgrass
was significantly affected by nutrient availability, with
cheatgrass plants growing 50-70% larger and producing
25-40% more flowers under high nutrient conditions (P <
0.001) (Table 2, Fig. 3A, Fig. 3B).
Cheatgrass responses to competition with native forbs
(Question 4)
Our predictions for cheatgrass were unsupported
as we did not observe variation in cheatgrass biomass or
flower production when competing with either native forb
species in competition treatments (Table 2, Fig. 3A, Fig.
3B).
Discussion
Because cheatgrass has an annual lifecycle and
germinates in the winter, it has the ability to efficiently
28 VOLUME 5 | ISSUE 1 | SPRING 2019
consume ephemeral resources and therefore excels at
outperforming native vegetation, (Mack & Pyke, 1983;
Rice et al., 1992; Leger et al., 2014). This competitive
advantage results in the formation of dense monocultures
across the Great Basin (Evans et al., 2001). As predicted,
both competition with cheatgrass and low soil nutrient
availability resulted in declining performance for both forb
species. Populations of L. glandulosa exhibited variation
in biomass response to differences in soil nutrient
availability. Finally, cheatgrass was found to be smaller
and produced fewer flowers under low nutrient conditions,
but its performance was not differentially affected by
competition with either native forb species. Because
cheatgrass experienced significant growth in biomass
and flower production in response to high nutrient
environments but was unaffected by native forb
competition, elevated levels of soil nutrients are likely to
enhance the success of this invasive weed, as has been
seen in other systems (Brooks et al., 2003).
L. glandulosa and M. gracilis produced slightly
contrasting responses in population-level variation,
suggesting that these species may have different
strategies for handling variation in soil nutrients. The
results of this study support current research showing that
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Layia glandulosa performance
Figure 1. Mean biomass (A) and mean number of flowers (B) of L. glandulosa grown alone
and in competition with cheatgrass under high and low nutrients. Error bars show the
standard error across individuals.
Microsteris gracilis performance
Figure 2. Mean biomass (A) and mean number of flowers (B) of M. gracilis grown alone and
in competition with cheatgrass under high and low nutrients. Error bars show the standard
error across individuals.
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VOLUME 5 | ISSUE 1 | SPRING 2019 29
Bromus tectorum performance
Figure 3. Bromus tectorum (cheatgrass) performance. Mean biomass (A) and mean number
of flowers (B) of cheatgrass grown in competition with L. glandulosa and M. gracilis under
high and low nutrient conditions. Error bars show the standard error across individuals.
nutrient addition enhances plant growth and reproduction
for both native forbs and cheatgrass (Evans et al., 2001,
Farrior et al., 2013). These results also suggest that there
are population-level differences in response to nutrient
availability for L. glandulosa, which may lead to
population-level variation in their ability to persist in
disturbed systems.
Because nitrogen deposition is projected to
negatively affect the Great Basin (Brooks, 2003), it is
crucial that managers take into account the response of
species and populations to enhanced levels of soil
nutrients. This information may be used to select
appropriate species, or populations, for restoration
purposes. For example, though not statistically significant,
the Hidden Valley population of L. glandulosa may be an
appropriate seed source for areas with high levels of bare
ground and high soil nutrient availability as it
outperformed other populations in biomass production
(Fig. 1A, Fig. 1B).
One goal was to determine how populations
differed in competition with cheatgrass, with the hope of
identifying promising populations for use in restoration.
Though not statistically significant, the Susileen
population of L. glandulosa may be an appropriate seed
source for areas with high soil nutrient levels and
cheatgrass, as it outperformed other populations in
biomass production when experiencing both competition
and high soil nutrient conditions (Fig. 1A, Fig. 1B). Further
sampling of additional forb species populations could
identify populations that are more tolerant of competition
with this highly-competitive species.
Differences in performance among populations
may be the result of local adaptation to site-specific
environmental conditions (Meyer et al., 1995, Wright et al.,
2006) or to genetic drift (Loveless & Hamrick, 1984).
Another explanation for differences in performance
among populations may stem from spatial variation in the
resources that were available to the parent plant, also
known as maternal effects (Roach & Wulff, 1987). For
example, maternal effects could influence the size and
mass of offspring seeds potentially resulting in variation in
performance (Roach & Wulff, 1987). Maternal effects
were not controlled for in this study, but they could
potentially have been factors contributing to the
differences observed between L. glandulosa populations.
One limitation must be address regarding the soil we used
to conduct this experiment. Soil was only collected from
one site in Reno, NV and may have differentially affected
our plant populations.
This research supports the importance of
selecting seed mixes that are sourced from populations of
appropriate restoration species that are well adapted to
site conditions, whether those conditions be enhanced
levels of soil nutrients, intense invasions of exotic plant
species, or other factors. Although there was no
population-level variation for M. gracilis in response to
competition or nutrient addition, previous research
suggests that variation among populations does in fact
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30 VOLUME 5 | ISSUE 1 | SPRING 2019
exist: M. gracilis exhibited population-level variation in
seed germination in response to temperature and
moisture cues (Barga et al., 2017). This research
suggests that it may be important to test population
performance prior to restoration, as native forb seeds are
expensive and difficult to procure (Shaw et al., 2005).
Finally, there is demand for research that can inform
species selection and improve restoration success in the
Great Basin (Oldfield et al., 2015). These results provide
further support for the importance of considering both
species and population-level responses to multiple
environmental factors when formulating restoration
protocols.
Acknowledgements
I would like to thank Dr. Elizabeth Leger for
helping design this experiment and providing thoughtful
feedback on the manuscript and Dr. Sarah Barga for
mentoring me through this process, conducting the
analyses and providing key feedback and contributions to
the manuscript. I would also like to thank Dr. Tara de
Quiroz for supplying a population of M. gracilis seeds, as
well as Donald Lovejoy, Jennafer Disbro, Jonathan Disbro,
and Sage Ellis for their help with greenhouse work.
Funding was provided from the Nevada Undergraduate
Research Award.
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