Productivity and Subordinate Species Response to
Dominant Grass Species and Seed Source during
Restoration
Brian J. Wilsey1,2
Abstract
Grasses can be important regulators of species diversity
and ecosystem processes in prairie systems. Although C4
grasses are usually assumed to be ecologically similar
because they are in the same functional group, there may
be important differences among species or between seed
sources that could impact restorations. I tested whether
C4 grass species identity, seed source, or grass species
richness scales to influence aboveground net primary
productivity (ANPP), resistance to weed invasion, or
establishment of subordinate prairie species during restoration. Plots in western Iowa, United States, were
planted with equal-sized transplants of one of five common grass species (Panicum virgatum L., Sorghastrum
nutans (L.) Nash, Andropogon gerardii Vitman, Schizachyrium scoparium (Michx.) Nash, and Bouteloua curtipendula (Michx.) Torrey) either from local seed or from
cultivar seed sources. These plots were compared to
plots containing all five species in mixture and to nonplanted plots. Differences in ANPP were found
Introduction
Native grasslands provide a multitude of benefits to society including forage production, wildlife habitat, and
nutrient and CO2 sequestration. Additionally, they can
have very high biodiversity when properly managed.
Recently, there has been interest in establishing native
grassland plantings to conserve biodiversity while simultaneously enhancing ecosystem services such as biomass for
cellulose-based biofuels (Parrish & Fike 2005; Tilman
et al. 2006; Jordan et al. 2007). Tilman et al. (2006) found
that biomass production increased with species richness in
plantings on a sandy soil, suggesting that we can successfully manage for high biomass production and species
richness on at least some cases. However, in more fertile
soils, there may be trade-offs between biomass production
and biodiversity such that production is highest in low-
1
Department of Ecology, Evolution and Organismal Biology, Iowa State
University, Ames, IA 50011, U.S.A.
2
Address correspondence to B. J. Wilsey, email bwilsey@iastate.edu
Ó 2008 Society for Ecological Restoration International
doi: 10.1111/j.1526-100X.2008.00471.x
628
among species but not between cultivars and noncultivars or between monocultures and mixtures. Panicum
virgatum, S. nutans, and S. scoparium were more productive than A. gerardii and B. curtipendula. Weed
invasion was much higher when plots were not planted
with grasses. Schizachyrium scoparium allowed greater
establishment of subordinant prairie species than all
other focal grass species. There were two separate mechanisms by which grasses suppressed prairie species establishment either (1) by growing tall and capturing light
or (2) by quickly filling in bare space by spreading horizontally through rhizome growth in short species. These
results suggest that high ANPP can be found with noncultivar plantings during the first 2 years after planting
and that subordinate species establishment is most likely
when shorter bunchgrasses such as S. scoparium are
dominant.
Key words: biodiversity-ecosystem functioning, biofuels,
C4 grasses, cultivars, restoration ecology, tallgrass prairie.
diversity plantings (Martin et al. 2005). For restored systems, there is little information on how biomass production might vary among plantings dominated by different
native grass species, whether diverse mixtures can outproduce monocultures, and how the high dominance
found with management for biomass production might
affect seedling establishment. Establishment of subordinate species from seed is the key to establishing high
biodiversity.
Prairie communities are typically dominated by grass
species and the abundance of these species can suppress
the establishment of rare forb species to reduce species
diversity (Howe 2000; Baer et al. 2003; Martin & Wilsey
2006; Williams et al. 2007). The proportion of biomass
production from C4 grasses within and among prairies can
vary greatly (Martin et al. 2005), but it usually makes up
a substantial portion of any given area (Turner & Knapp
1996; Wilsey & Polley 2003). However, plant species
diversity is determined more by the richness of subordinate forb species (Howe 1994; Turner & Knapp 1996),
and establishment of diverse plant communities is a common goal of restorations (e.g., Palmer et al. 1997; Sluis
2002). Big bluestem (Andropogon gerardii) can greatly
Restoration Ecology Vol. 18, No. 5, pp. 628–637
SEPTEMBER 2010
Productivity and Subordinate Species Response to Dominant Grass Species and Seed Source
suppress plant diversity in the Flint Hills prairie region of
the United States. When dominance of A. gerardii is
reduced by grazing or a reduced fire frequency, the diversity of forbs and cool season grasses and indeed the entire
plant community increases (Hartnett et al. 1996; Collins
et al. 1998). In a more mesic grassland, Williams et al.
(2007) found that frequent mowing of C4 (warm season)
grasses led to a higher forb establishment from seed additions in a grass-dominated planting.
Although C4 grasses are usually assumed to be ecologically similar because they are in the same functional
group, there may be important differences among species
and between seed sources that could impact restoration
projects. For example, Silletti & Knapp (2001, 2002) found
that the C4 grasses A. gerardii and Sorghastrum nutans
responded differently to water and nitrogen additions. A
better understanding of which traits underlie species differences will enable us to develop a general understanding
of dominant grass effects that will apply to multiple systems (McGill et al. 2006).
Many restoration projects are currently being planted
with cultivars (Jones 2003). In addition to concerns about
the possibility of cultivars hybridizing with remnant individuals (Lesica & Allendorf 1999; Gustafson et al. 2004;
Selbo & Snow 2005), dominance by cultivars may be
higher than what would be found for locally collected genotypes. Genetic differences were found by Gustafson
et al. (1999, 2004) between cultivars and remnant populations and between two commonly used cultivars in
A. gerardii. Cultivars are usually selected for high seed
germination rates and increased ‘‘vigor,’’ but whether
these traits are truly enhanced over local genotypes and
whether these traits are important to production or invasion resistance of developing prairies is largely unknown
or undocumented.
I suggest that basic ecological and evolutionary theory
(reviewed by Lesica & Allendorf 1999) predicts three possible outcomes for studies that compare native and cultivar genotypes in planted prairies. The ‘‘cultivar vigor
hypothesis’’ predicts that human selection for increased
vigor will lead to increased resource capture and aboveground biomass production in cultivars compared to
locally collected genotypes. In this scenario, cultivarplanted prairies would have more productive grasses and
a lower recruitment of other native species (e.g., forbs).
Conversely, local adaptation may be especially prevalent
and strong. In this latter case, the ‘‘local adaptation
hypothesis’’ predicts that cultivars will capture fewer
resources and will be less productive than locally collected
genotypes. This is because the original cultivar seed was
typically collected from a more distant location than local
seed. In this scenario, the cultivar genotypes would be
less productive regardless of any human selection for
increased vigor. Both hypotheses received partial support
by Gustafson et al. (2004): the Rountree cultivar of
A. gerardii had higher biomass and heights than did plants
from local seed and plants from a distant remnant source
SEPTEMBER 2010
Restoration Ecology
had lower biomass than plants from local seed. However,
a second cultivar (Pawnee) did not differ from plants from
local seed sources. A final possibility is the null hypothesis, which predicts that there will be little or no differences
between cultivars and local genotypes. This is a possibility
if the two processes (human selection for increased vigor,
local adaptation) cancel each other out or if neither process has an effect.
The cultivar vigor and local adaptation hypotheses
provide predictions for relationships between species
diversity and productivity. If cultivars were humanselected to be vigorous then they might have greater
interspecific:intraspecific competition ratios compared
to plantings with locally collected genotypes. This destabilizing effect (Chesson 2000) might lead to greater declines
in diversity over time in cultivar-dominated than in non–
cultivar dominated grasslands. In either case, ecological
theory predicts that productivity will be higher in mixtures
if species use resources differently in time or space
(i.e., have greater niche partitioning) (Tilman et al. 1997).
Dominant species in tallgrass prairies are all C4 grasses.
However, there are differences in growth form (e.g., rhizomatous vs. bunchgrass) and heights among these species,
which could have consequences for biomass production
between monocultures and mixtures due to differences in
resource uptake in space or time. If the functional differences seen among C4 grasses are important then we would
predict that productivity in mixtures will be higher on average than productivity in their corresponding monocultures.
Furthermore, these differences are predicted to be larger in
grassland plantings dominated by locally collected seed
than in plantings dominated by cultivars because local genotypes are more likely to be coevolved.
Huston (1994) hypothesized that the highest species
diversity occurs with intermediate amounts of disturbance
and growth rates of constituent species. Low growth rate
is predicted to increase diversity by limiting the rate of
competitive exclusion. Because cultivars are usually
selected for rapid growth rate (high vigor) (Gustafson
et al. 2004), then primary productivity may be higher but
forb recruitment and species diversity might be lower in
plots dominated by these species compared to plots dominated by slower growing native genotypes. Although cultivar status was not the focus of their restoration study,
Baer et al. (2005) found that a cultivar of the lowland species Panicum virgatum attained very high dominance and
suppressed local diversity. If cultivars do indeed dominate
plots more than locally collected plants then management
objectives of high productivity and high species diversity
would be in conflict if cultivars are used. Alternatively, if
there is no difference in species diversity between cultivarand native-genotype plantings then managers may opt to
use cultivars due to cheaper, more readily available seed
(Jones 2003).
In the present study, different C4 grass species were
planted in monocultures or in mixtures with local or cultivar seed sources in experimental plots in the Loess Hills
629
Productivity and Subordinate Species Response to Dominant Grass Species and Seed Source
of Iowa. I measured trait differences among grass species
(McGill et al. 2006) in both the field and the greenhouse
and then determined if traits of species and sources scale
to influence aboveground net primary productivity
(ANPP), weed invasion resistance, and/or recruitment of
subordinate prairie species during restoration.
Methods
Study Site and Field Preparation
The study was conducted on Iowa State University–owned
lands in the Loess Hills region of Iowa (Western Research
Farm, lat. 42°039N, long. 95°499W). The official weather
station on site receives an average of 762 mm of precipitation per year. The soil type is Ida silt loam, which is
a well-drained calcareous loess with 14–20% slopes and
approximately 2.5% organic matter. Experimental plots
were located on a hilltop in a 16-ha abandoned pasture
formerly dominated by Bromus inermis Leysser (smooth
brome, nomenclature follows Eilers & Roosa 1994). The
area was grazed by cattle until 2002 and was not fertilized
for many years. Blocks were established by disking three
areas during fall 2004 and again in early spring 2005 just
prior to planting. Blocks were located on three slopes differing in aspect: southwest-, north-, or east facing. The
2005 growing season had precipitation (658 mm) that was
slightly below the 30-year mean with a wetter than normal
April to June and drier than normal July to August. At
the neighborhood- and patch scales, prairies in this area
can be dominated by a variety of C4 (warm season) grasses
in addition to Andropogon gerardii, including Indian grass
(Sorghastrum nutans), Little bluestem (Schizachyrium
scoparium), or Side-oats grama (Bouteloua curtipendula)
in upland locations (Brudvig et al. 2007) and Switchgrass
(Panicum virgatum) in lower areas (Novecek et al. 1985).
Field Experimental Design
The experiment consisted of planting equal-mass seedlings of one of five native grass species (A. gerardii,
S. nutans, P. virgatum, S. scoparium, or B. curtipendula),
mixtures of all five species, or no grasses at all into experimental plots during early May 2005. These treatments
were crossed with seed source treatments, with seedlings
being either from remnant-collected seed or from cultivars. Remnant-collected seed was either hand collected
from remnants in Monona County (P. virgatum) or collected from remnants and grown in Pottawattamie
County, Iowa, by Custom Seeds Inc. (other species). Cultivars were selected because they were recommended for
use in this area (western Iowa; Table 2). Treatments were
randomly assigned to plots within each of the three blocks.
The main experiment used a 6 (each of the five species in
monoculture plus mixtures of all five species) 3 2 (plants
from local or cultivar seed) factorial design within each
block. There were two replicate monocultures within each
630
block for 5 species 3 2 seed source 3 3 blocks 3 2 reps ¼
60 monoculture plots total. There were four replicate mixtures within each block for a total of 2 seed source 3 3
blocks 3 4 reps ¼ 24 mixture plots total. Twelve companion bare ground plots (four within each block) were also
included to test if subordinate and weed species establishment would be greater in grass-free plots (Shirley 1994).
Transplants were used instead of seed to control the
rate of establishment and plant density, which enables
more careful comparisons across species. Using transplants also speeds up establishment of grasses by up to
2 years (previous observations). Seedlings were planted in
each 1-m2 plot at a density of 72 plants per plot. As
a result, this study is most relevant for understanding
local, neighborhood-scale processes and less relevant to
understanding larger-scale species turnover and other processes that affect diversity.
Plots were watered for 1 week to facilitate establishment of grasses and were weeded until the grass canopy
had established (i.e., until 13 July 2005). Thereafter, weeds
were allowed to colonize and grow into plots. Grass transplant survival rate was greater than 95% in all plots and
no replanting was done. Alleyways were seeded with
Agropyron smithii Rydb. between plots, which was
mowed bimonthly during the duration of the study. No
A. smithii was found invading the plots.
Plant Traits: Greenhouse Measurements
Plant traits were measured in a controlled-temperature
greenhouse at Iowa State University to test for differences
under conditions that were meant to provide plants with
optimal conditions for growth in 155-mm-diameter round
pots. Temperatures fluctuated naturally, but never dropped below freezing and were never greater than 2°C
above outside temperatures on hot days. Light was 1,522
lmolm22s21 above the plants during May. Stem density
and maximum relative growth rate (Grime 1974) were
measured for each grass species (noncultivars) in pots
filled with potting soil. Pots were watered three times per
week and received full strength Hoagland’s solution once
per week. Pots were thinned to one plant per pot after
seedlings emerged and plants were then allowed to grow
for 3 months (July to September 2005), with three replicates per species. Most species had flowered by the end of
the experiment.
Seed germination rates were estimated in two trials
using local genotype and cultivar seeds of each of the C4
grass species. Each of the two trials had three replicates
per treatment per trial. Trials were conducted in fieldcollected soil in well-watered pots (50 seeds per pot). Soil
was collected from the field site (Wilsey & Stirling 2007).
Plant Traits: Field Measurements
Restorations are often initiated on bare soil with high
light. Grasses among the treatments were predicted to
Restoration Ecology
SEPTEMBER 2010
Productivity and Subordinate Species Response to Dominant Grass Species and Seed Source
differentially fill-in during canopy development in the
three dimensions of volume. To test this, estimates were
made of traits associated with resource capture to determine whether grasses differed across species and between
locally collected and cultivar plants within each monoculture plot. Measurements were made on easily measured
traits (McGill et al. 2006) associated with total resource
uptake (light capture and total percent cover as proxies)
in both upward (height) and lateral directions (basal area).
Plant traits (cover, basal area, and height) were measured
in July and September 2005 in the first growing season,
when individual plants could be differentiated. The two
dates correspond to the period of maximum seedling
establishment (July; Losure et al. 2007) and peak canopy
cover (September).
Canopy light capture was estimated by comparing light
above and below the canopy during midday (10:00 to
2:00 p.m. standard time) using a 1-m Decagon (Pullman,
WA, U.S.A.) ceptometer. The ceptometer was placed
diagonally into each plot in two locations below the canopy at the soil surface. The end of the light bar was
always at least 10 cm from the corner of the plot. Soil surface light values were compared to light values above the
canopy (below/above) to estimate the proportion of light
that reached the soil surface and this value was subtracted from one for estimates of capture. Percent vegetation cover was visually estimated separately in each of
the four-quarters of each plot (i.e., for each 0.25 m2).
This was done to improve the accuracy of plot-level estimates by sampling a smaller area. These values were
then averaged across the four estimates per plot to
obtain one cover estimate per plot. Small sheets of calibration paper of known cover of 0.1, 1.0, 5, and 10%
were used to initially calibrate the cover estimates, and
all estimates of cover were done by the same person to
reduce observer bias.
Height was measured from the soil surface to the base
(where the blade joins the sheath) of the upper most leaf
on three plants per plot. The basal area of each plant
approximated a circle. Therefore, basal area was estimated by measuring two plant diameters between the farthest tillers at the base of three plants per plot. These
values were converted into one estimate of basal area per
plot with the standard equation for the area of a circle
(area ¼ pr2) using the mean radius. Measurements of each
variable were averaged across the three plants per plot to
prevent pseudoreplication.
Aboveground biomass was harvested at peak to estimate ANPP during the second growing season (2006).
ANPP was estimated by clipping biomass to 2 cm on 22–
23 September 2006. Live material was sorted by species,
dried at 65°C for 48 hours until dry, and weighed.
Seed Additions of Subordinate Prairie Species
Subordinate species, which were mostly prairie forbs
found in remnants (Table 1), were added to field plots in
SEPTEMBER 2010
Restoration Ecology
a seed mix after grasses had established. This addition
tested how grass species, sources of each species, and richness of grass species influence recruitment of subordinate
prairie species. Seeds from 26 native species (Table 1)
were added to each plot in two additions: one on 15 June
and the other on 16 December 2005. A total of 520 seeds/
m2 (20 per species) were added to plots during the two
additions.
Statistical Analyses
Total ANPP (grass 1 weeds 1 subordinate species from
the seed mix), weed ANPP, and seeded species ANPP
were analyzed according to randomized complete block
analysis of variance (ANOVA) to test for dominant species effects (six levels), seed source effects (two levels, cultivar vs. local), and their interaction. Block by treatment
interactions (which were tested and found to be nonsignificant) were pooled into the error term (Steel & Torrie
1980; Sokal & Rohlf 1995). Main effect differences among
species were tested with Tukey’s post-ANOVA test. The
species 3 cultivar interaction was further tested with the
SLICE option (Littell et al. 2002). The SLICE option
tested cultivar versus noncultivars for each species when
the interaction was significant (p < 0.05). Germination
rates were analyzed with a similar approach and model,
except that blocking was done on trial.
Resource capture data were analyzed first with principal components analysis (PCA) to test whether trait variables were highly correlated with one another. There were
no zeros in the dataset, relationships were linear, and
there were no ‘‘dust bunny’’ distributions found (McCune
& Grace 2002), which makes PCA an appropriate technique to determine how many components of variation
were found in the dataset. There were two major principal
components of variation in the data (i.e., two axes with
eigenvalues >1.0). Light capture (0.62), percent cover
(0.61), and height (0.48) all loaded heavily on axis 1, which
accounted for 54.4% of the variation in the data. Basal area
had a low loading of 0.10 on axis 1. Axis 2 was explained by
a trade-off between basal area, with a loading of 0.86, and
height, which had a loading of 20.47. Loadings of other
variables were <0.22. Axis 2 accounted for 29.8% of the
variation. Because height (axis 1) and basal area (axis 2)
were largely independent (univariate correlation of 20.22),
but other variables were highly correlated with one or the
other, I simplified the analyses and analyzed how these two
key trait variables varied among treatments with univariate
ANOVAs. These two variables were then regressed against
light capture to determine if they were related to overall
resource capture. Regressions with percent cover gave similar results so they will not be presented.
Variables were compared between the bare ground and
the vegetated plots with a Dunnet’s test in a one-way
ANOVA. One-way ANOVA was used because the
bare ground treatment was not crossed with other treatments. Dunnet’s test compares a control, in this case the
631
Productivity and Subordinate Species Response to Dominant Grass Species and Seed Source
Table 1. List of native prairie species used in the subordinate species seed mix*.
Family
Cool season (C3) grasses
1. Canadian wild rye (Elymus canadensis L.)
2. June grass (Koeleria macrantha (Ledeb.) Schultes)
3. Porcupine grass (Stipa spartea Trin.)
Forbs
4. Wild bergamot (Monarda fistulosa L.)
5. Black-eyed Susan (Rudbeckia hirta L.)
6. Bottle gentian (Gentiana andrewsii Griseb.)
7. Butterfly milkweed (Asclepias tuberosa L.)
8. Dotted blazing star (Liatris punctata Hooker)
9. Ground plum (Astragalus crassicarpus Nutt.)
10. Hoary vervain (Verbena stricta Vent.)
11. Illinois bundle flower (Desmanthus illinoensis MacM.)
12. Lead plant (Amorpha canescens Pursh)
13. Purple coneflower (Echinacea angustifolia DC.)
14. New Jersey tea (Ceanothus americanus L.)
15. Ox-eye (Heliopsis helianthoides (L.) Sweet)
16. Partridge pea (Chamaecrista fasciculata (Michx.) Greene)
17. Prairie phlox (Phlox pilosa L.)
18. Prairie larkspur (Delphinium virescens Nutt.)
19. Prairie rose (Rosa arkansana Porter)
20. Purple prairie clover (Dalea purpurea Vent.)
21. Red root (Ceanothus herbaceus Raf.)
22. Round-headed bush clover (Lespedeza capitata Michx.)
23. Smooth aster (Aster laevis L.)
24. Stiff goldenrod (Solidago rigida L.)
25. White prairie clover (Dalea candida Willd.)
26. Yellow coneflower (Ratibida pinnata (Vent.) Barnh.)
g/120 Seeds
Poaceae
Poaceae
Poaceae
0.66
0.02
3.43
Lamiaceae
Asteraceae
Gentianaceae
Asclepiadaceae
Asteraceae
Fabaceae
Verbenaceae
Fabaceae
Fabaceae
Asteraceae
Rhamnaceae
Asteraceae
Fabaceae
Polemoniaceae
Ranunculaceae
Rosaceae
Fabaceae
Rhamnaceae
Fabaceae
Asteraceae
Asteraceae
Fabaceae
Asteraceae
0.49
0.04
0.01
0.80
0.49
0.66
0.12
0.77
0.21
0.49
0.37
0.54
1.27
0.18
0.10
1.74
0.19
0.30
0.43
0.06
0.08
0.18
0.11
* Nomenclature follows Eilers and Roosa (1994).
grass-free plots, to each of the other planted treatments in
turn while controlling the type I error rate.
Results
Plant Traits: Greenhouse Studies
Germination rates varied significantly among species
(F[1, 49] ¼ 47.0, p < 0.0001) and were different between
cultivars and local genotypes in every species pair (cultivar
F[1, 49] ¼ 44.7, slice by species, all p values < 0.01). However,
differences were not consistently in the same direction.
Cultivars had higher germination rates in general than local
genotypes with differences ranging from a 32-fold higher
germination rate in Sorghastrum nutans to a 1-fold higher
rate in Schizachyrium scoparium cultivars (Table 2). However, there was an exception to this pattern in that Andropogon gerardii noncultivars had sixfold higher germination
than cultivars (species 3 genotype interaction, F[4, 49] ¼ 61.0,
p values for each species pair <0.0001, slice p < 0.0001).
Table 2. Species and seed source studied, original site of seed collection, seed mass, and emergence rates in field soil in greenhouse trials.
Emergence Rate (%)
C4 Grass Species
Andropogon gerardii
Sorghastrum nutans
Schizachyrium scoparium
Bouteloua curtipendula
Panicum virgatum
632
Seed Source
Seed Mass (mg)
Trial 1
Trial 2
Local
Rountree Cultivar (Iowa)
Local
Holt Cultivar (Nebraska)
Local
Camper Cultivar (Kansas)
Local
Butte Cultivar (Nebraska)
Local
Pathfinder Cultivar (Kansas)
2.82
2.12
2.73
2.00
1.95
1.88
0.86
1.24
1.42
1.82
52.0 (42–62)
6.0 (2–10)
0.7 (0–2)
32.7 (24–38)
19.3 (12–28)
36.0 (24–46)
6.7 (4–10)
29.3 (28–30)
22.0 (20–24)
71.3 (66–82)
48.0 (44–54)
8.0 (2–14)
1.3 (0–2)
32.7 (30–34)
17.3 (14–20)
38.7 (30–48)
8.0 (4–12)
20.0 (14–24)
43.3 (36–48)
66.0 (58–74)
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Productivity and Subordinate Species Response to Dominant Grass Species and Seed Source
The number of stems (culms) produced in pots was
much higher in Bouteloua curtipendula than other species
(F[4, 10] ¼ 131.1, p < 0.01). Species fell into four significantly
different groups. Bouteloua had stem densities of 47.3,
which was significantly higher than S. nutans (10.3), S.
scoparium (8.7), and Panicum virgatum (5.3). Andropogon
gerardii had the lowest stem production with 1.3 (pooled
SE ¼ 1.62, significance determined with a Tukey’s test).
Species also differed in relative growth rate (F[4,10] ¼
4.0, p < 0.05) over the 3-month period. Maximum relative
growth rate in greenhouse trials was higher in P. virgatum
(5.1 g) than in all species but B. curtipendula (4.6 g) and
S. scoparium (3.1). Andropogon gerardii (2.5 g) and
S. nutans (2.0 g) had lower relative growth rates (pooled
SE ¼ 0.67) than P. virgatum (both species) and B. curtipendula (S. nutans).
Plant Traits: Field Plots
There were large differences in height among species (species main effect, F[4, 47] ¼69.9, p < 0.01), but no consistent
difference in height between cultivars and noncultivars
(main effect, F[1, 47] ¼ 2.9, p ¼ 0.095). As expected, P. virgatum, A. gerardii, and S. nutans were much taller than
S. scoparium and B. curtipendula (Fig. 1). Height was different between cultivars and noncultivars in three of five
cases, but the difference varied among species (species 3
cultivar interaction, F[4, 47] ¼7.9, p < 0.01; species 3 cultivar 3 time, F[4,47] ¼4.1, p < 0.05) and with time (p < 0.01).
Cultivars were shorter than noncultivars for S. nutans
(July; p < 0.01) and P. virgatum (September; p < 0.01),
whereas the S. scoparium cultivar was significantly taller
than noncultivar in July only (p ¼ 0.04).
Basal area varied among species (Fig. 2A). The shortest
species B. curtipendula had the greatest basal area (species
main effect, F[4, 47] ¼ 4.87, p < 0.01) and this difference
between B. curtipendula and other species increased over
time (species 3 time, p < 0.01). Bouteloua curtipendula had
significantly greater basal area than S. nutans in July and
P. virgatum and A. gerardii in September. Cultivars had
18–19% wider bases than noncultivars (cultivar main effect,
F[1, 47] ¼ 4.44, p ¼ 0.04), and this difference was consistent
across time periods (Fig. 2B) and species (i.e., there was no
cultivar 3 species interaction).
Canopy light capture, which served as a proxy for
total resource capture, was not affected by seed source
(F[1, 70] ¼ 1.2, p > 0.28) but was positively related to height
and basal area during the early (July) sampling period
(height slope ¼ 0.012, area slope ¼ 0.009, combined r2 ¼
0.36, p < 0.01 for both variables) but was only related to
height during the later (September) sampling date (height
slope ¼ 0.009, r2 ¼ 0.15, p < 0.01).
richness levels (i.e., monoculture vs. mixture plots; Fig. 3A).
Sorghastrum nutans, P. virgatum, and S. scoparium were
more productive on average (mean across species of 661.5 g/
m2) than A. gerardii or B. curtipendula (mean 424.1 g/m2,
ANOVA, F[5, 70] ¼ 9.05, Duncan’s tests, p < 0.05; Fig. 3A).
Differences in lateral spread between seedlings from
locally collected seed and cultivar seed did not result in
greater productivity: there was no significant difference
in productivity between plots planted with seedlings from
locally collected seed and cultivar seed (F[1, 70] ¼ 0.07, p >
0.05). There was also no difference in ANPP between single-species plantings and five-species mixtures (p > 0.05).
The overall mean for monocultures was 563.5 g/m2 versus
a mean of 566.5 g/m2 for mixtures. Not surprisingly, ANPP
was much higher in every planted grass treatment than in
unplanted plots (Dunnet’s test, difference between
unplanted and all planted treatments, p < 0.05).
Aboveground Net Primary Productivity
Seedling Establishment
There were significant differences among dominant grass
treatments in ANPP but not between seed sources or grass
Weeds (species not planted or seeded) generally made up
<10% of the total biomass at harvest in planted plots, but
SEPTEMBER 2010
Restoration Ecology
Figure 1. Plant height in monocultures of Bouteloua curtipendula
(Bc), Schizachyrium scoparium (Ss), Andropogon gerardii (Ag),
Sorghastrum nutans (Sn), or Panicum virgatum (Pv) during time of
peak seedling establishment (A, July; Losure et al. 2007) and peak
biomass (B, September) during the first growing season.
633
Productivity and Subordinate Species Response to Dominant Grass Species and Seed Source
Figure 2. Basal area in monocultures of Bouteloua curtipendula (Bc),
Schizachyrium scoparium (Ss), Andropogon gerardii (Ag), Sorghastrum nutans (Sn), or Panicum virgatum (Pv) during the first growing
season (A) and in monoculture plots planted with cultivars or noncultivars (B, letters denote differences among species and between
cultivars).
there were significant differences among species treatments (F[5, 70] ¼ 2.8, p < 0.05; Fig. 3B) but not between
cultivars and noncultivars (F[1, 70] ¼ 0.07, p > 0.05). Schizachyrium scoparium plots had more weed biomass at
54.1 g/m2 than did P. virgatum at 13.1 when averaged
across cultivar–noncultivar groups. Bare ground plots had
much higher weed biomass than planted plots. Bare
ground plots had between 53 (S. scoparium noncultivar at
56.4 vs. 263.5 g/m2) and 353 higher weed biomass (P. virgatum cultivars at 7.6 vs. 263.5 g/m2 in bare ground plots)
than planted grass plots (Dunnet’s test, all p values < 0.05).
Seeded-species biomass was dominated by Verbena
stricta, which made up 98% of the total (Fig. 4). Biomass
of seeded species varied significantly among the species
treatments (F[5, 70] ¼ 5.5, p < 0.05) but not between seed
source treatments (F[1, 70] ¼ 0.7, p > 0.05). Schizachyrium
scoparium had higher seeded species biomass (44.7 g/m2)
than did all other species (5.2–14.9 g/m2) (ANOVA and
Tukey’s test, p < 0.05).
In general, unplanted plots did not have greater ANPP
of seeded species than planted plots (data not shown). In
only one case, unplanted versus P. virgatum cultivar plant-
634
Figure 3. Peak C4 grass biomass (A) and weed biomass (nonplanted
species, B) in experimental plots planted with monocultures of Bouteloua curtipendula (Bc), Schizachyrium scoparium (Ss), Andropogon
gerardii (Ag), Sorghastrum nutans (Sn), Panicum virgatum (Pv), with
a mixture of all five of these species (MIX), or with no grasses at all
(BARE). Different letters signify differences among species treatments. There were no significant differences between cultivars and
noncultivars in either variable.
ings, was there a significant difference in seeded species
biomass (Dunnet’s test, p < 0.05, all other comparisons nonsignificant), with P. virgatum cultivars having less seeded
species biomass than the unplanted controls. In each of the
other 11 cases, there was no difference in seeded species
biomass between planted and unplanted control plots.
Discussion
Many prairie restoration projects have addressed factors
influencing species diversity (e.g., Howe 1994; Kindscher &
Tieszen 1998; Sluis 2002; Baer et al. 2003; Blumenthal
et al. 2003; Prach 2003; Polley et al. 2005; Martin et al.
2005; Martin & Wilsey 2006). At the same time, there is
renewed interest in developing plantings that enhance
ecosystem services such as biomass production (e.g.,
Tilman et al. 2006). In this study, C4 grass species identity,
but not species richness or seed source, affected ANPP,
weed resistance, and subordinate species establishment
during the first 2 years after planting. Some cultivars of
Restoration Ecology
SEPTEMBER 2010
Productivity and Subordinate Species Response to Dominant Grass Species and Seed Source
Figure 4. Biomass of subordinate species that established from prairie seed mix (primarily Verbena stricta) in monocultures of Bouteloua
curtipendula (Bc), Schizachyrium scoparium (Ss), Andropogon gerardii (Ag), Sorghastrum nutans (Sn), Panicum virgatum (Pv), or a mix
of all five grass species.
the dominant grasses differed from noncultivars in their
heights, but the response was not consistent across species.
However, basal area was consistently higher in cultivars.
These differences suggest that cultivars are different from
noncultivars, which could result from human selection for
increased vigor. Nevertheless, differences in height and
basal area between cultivars and noncultivars did not
scale to affect ANPP or subordinate species establishment
within the first two growing seasons. These two variables
differed much more among grass species than they did
between seed sources. Schizachyrium scoparium had productivity levels that were similar to other grass species,
but it allowed seeded species (Verbena stricta) to establish
at a higher rate.
The lack of a seed source effect on light capture, ANPP,
or subordinate species recruitment suggests that processes
underlying both the ‘‘cultivar vigor’’ and the ‘‘local adaptation’’ hypotheses were operating, but possibly canceling
each other out for these variables and cultivars. For most
species, cultivars had traits that appeared to make them
more vigorous, for example, higher basal area and/or
higher seed germination. However, for other important
traits like height, differences were inconsistent. The fact
that the cultivars are from distant seed sources and are not
adapted to the area could have caused a corresponding
reduction in biomass (reducing fitness) that counteracted
the human selection for increased vigor, although this
hypothesis needs to be tested directly in future studies.
Future studies should also test these ideas with multiple
cultivars per species (Gustafson et al. 1999, 2004).
There is growing interest in using biomass for biofuels
(Tilman et al. 2006; Adler et al. 2007). Prairie grasses can
be highly productive on marginal lands (Baer et al. 2002).
These results suggest that when uniformly established in
a common environment, native C4 perennial prairie grass
SEPTEMBER 2010
Restoration Ecology
species can differ greatly in biomass production early in
the restoration process. During the second year of the
study, Panicum virgatum and Sorghastrum nutans were
both highly productive. Schizachyrium scoparium was just
as productive when volunteer and seeded species were
considered. Andropogon gerardii and Bouteloua curtipendula were less productive than other species at this site.
There was also no difference in biomass production in this
time frame between cultivars and noncultivars. Thus,
using local sources should be considered as a viable alternative to using cultivars in biofuel plantings (Lesica &
Allendorf 1999; Selbo & Snow 2005).
Shirley (1994) and Dickson & Busby (2008) suggested
that having little or no C4 grass in the seed mix, at least
initially, will lead to increased forb recruitment. C4 grasses
could then be seeded in later years after forbs have established (Dickson & Busby 2008). In the current study, there
was similar forb biomass but much higher weed biomass
at the end of the second growing between unplanted and
planted plots. Adding grasses after forbs might work to
increase forb establishment if weed invasion is either very
low or weeds are irrelevant to establishment. However,
a previous seeded restoration study at this same site had
to be abandoned because annual and then perennial
weeds prevented prairie establishment. Blumenthal et al.
(2003) found high perennial weed biomass and almost
zero prairie establishment in control plots that did not
receive C additions to reduce N availability in Minnesota.
I found that plots not planted with grasses had much
higher weed biomass than grass-planted plots and this
should be taken into consideration in future studies.
The highest establishment rates from the seed mix
occurred in plantings of S. scoparium. These plantings had
much less weed biomass, but the same amount of biomass
from species in the seed mix (primarily V. stricta) as bare
ground plots. Plots with P. virgatum, S. nutans, and
A. gerardii had lower light levels and forb recruitment. On
the other hand, the shortest species B. curtipendula had
very high light levels at the soil surface but had lower forb
recruitment than plots with S. scoparium. Low forb
recruitment with Bouteloua was associated with plants
having large increases in basal areas and a large number
of stems produced during establishment. Taken together,
these results suggest that there are two mechanism by
which grasses can prevent forb recruitment: (1) by growing tall and capturing light and other resources (e.g., tallgrass species) or (2) by quickly colonizing bare ground
with high stem production and rapidly spreading basal
areas (e.g., Bouteloua). The first is widely appreciated, but
the second mechanism has not been noted previously.
Productivity can sometimes be higher in mixtures than
in monocultures (Hooper et al. 2005). Here, mixtures did
not increase ANPP over monocultures, most likely
because the grasses shared traits related to their C4 grass
functional group (Sage & Monson 1999). Moreover, mixtures had similar weed and subordinate species establishment compared to monocultures. At the same field site as
635
Productivity and Subordinate Species Response to Dominant Grass Species and Seed Source
this, we found no change in productivity (Isbell et al.
2008) or invasion resistance (Losure et al. 2007) across
plots that varied in among-plant height heterogeneity.
However, both variables increased with the proportion of
early-emerging forbs in the mix. This suggests that mixtures can have attributes such as greater pest resistance
(Kennedy et al. 2002; Wilsey & Polley 2002; Losure et al.
2007) or productivity if plantings include plant species
from several functional groups that grow at different times
of the year. However, increasing the number of C4 grass
species appears to be insufficient in providing the same
benefits.
Three caveats to consider when interpreting these
results are that (1) transplants were used in the field
experiment; (2) only aboveground responses were measured; and (3) the study was based on the first 2 years after
transplant establishment. Plant establishment is accelerated in the absence of a seedling establishment phase
when transplants are used instead of seeds, and in this
study, a closed canopy had developed by the middle of
the second growing season. Thus, the restoration process
was sped up, but the seedling establishment phase was
bypassed. The differences found in seedling emergence
between cultivars and noncultivars could also be important during early stages of prairie establishment and this
deserves further study with seeded plots. Finally, the longterm patterns in productivity of cultivars and noncultivars
of these grasses are unknown. In the longer term, it will be
important to determine how these species and cultivars/
noncultivars respond to years with abnormal precipitation
or temperature and years with pest outbreaks.
Implications for Practice
Planting native grasses reduces weed invasion in disturbed environments with bare soil.
d Having short bunch grasses as the C
4 grass is the
most likely way to achieve the objectives of having
high prairie forb recruitment while keeping weeds to
an acceptable minimum.
d Similarities between five-species mixtures and monocultures suggest that ANPP will not be higher in grass
mixtures than in their component monocultures.
d
Acknowledgments
Thanks to K. Wahl, D. Losure, A. Blong, L. Martin, A.
Loan-Wilsey, W. Roush, and D. Hummel for their help
with planting. S. Holland provided remnant Panicum virgatum seed. S. Baer, F. Isbell, K. Yurkonis, T. Dickson,
and two anonymous reviewers provided useful comments
on an earlier version of this manuscript. This project was
funded by the Iowa Department of Transportation Living
Roadway Trust Fund and a National Science Foundation
grant DEB0639417.
636
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