The Impact of Seeding Method on Diversity and
Plant Distribution in Two Restored Grasslands
Kathryn A. Yurkonis,1,2 Brian J. Wilsey,1 Kirk A. Moloney,1 and Arnold G. van der Valk1
Abstract
Previous studies have compared grassland restoration
techniques based on resulting species richness and composition. However, none have determined if different techniques generate different plant distributions in space, which
may further impact restoration success. This study tests if
there are quadrat-scale (1 m2) differences between paired
drilled and broadcast plantings in diversity, composition,
and plant distributions. Higher competition intensity in
and more contiguous spaces between rows in drill-seeded
restorations were hypothesized to result in larger patches
of native grasses and exotic species. Two paired drill- and
broadcast-seeded plantings were sampled in June 2007 in
Iowa, U.S.A. Within 10 quadrats in each planting, we
measured species abundance with point intercept sampling and plant distributions by dividing the quadrat into
64 cells and recording the most abundant species in each
cell. Drilled and broadcast plantings at both sites had simi-
Introduction
Equipment and methodologies used for grassland restoration can strongly influence planting success (Wilson 2002).
Vegetation structure is most commonly used as an indicator of restoration success (Ruiz-Jaen & Aide 2005) and is
typically measured by species richness and composition
(Martin et al. 2005; Polley et al. 2005). An additional
aspect of vegetation structure, plant distributions, may
also be important in determining future diversity and
resource use (Tilman & Kareiva 1997; Stoll & Prati 2001;
De Boeck et al. 2006). Plant distributions may be important in two ways. First, plant distribution can refer to seed
positions relative to one another in space at planting. Second, plant distribution can describe the ways in which conspecific individuals or ramets are arranged in space as
plantings develop. Both aspects of plant distribution may
be important in determining restoration success and longterm diversity maintenance (Stoll & Prati 2001; Bolker
et al. 2003; Bartha et al. 2004; Lortie et al. 2005; De Boeck
et al. 2006). However, we know very little about how the
spatial relationships among seeds at planting might affect
1
Department of Ecology, Evolution, and Organismal Biology, Iowa State
University, Ames, IA 50011 U.S.A.
2
Address correspondence to: K. Yurkonis, email yurkonis@iastate.edu
Ó 2008 Society for Ecological Restoration International
doi: 10.1111/j.1526-100X.2008.00461.x
MAY 2010
Restoration Ecology Vol. 18, No. 3, pp. 311–321
lar Simpson’s diversity and evenness. However, the effect
of planting type on species richness, composition, and
plant distribution was site dependent. Native warm-season
grasses in one site, and exotic species in the second, occupied more space and were distributed in larger patches in
drilled plantings. Furthermore, drilled canopies consistently captured more light than broadcast canopies. This
suggests that initial differences in seed placement can
affect resulting plant distributions, resource use, and
potentially long-term species turnover. Mechanisms structuring vegetation in these communities need to be further
investigated to determine if this approach can provide
more information on long-term diversity maintenance in
restorations than traditional measures.
Key words: broadcast seeding, drill seeding, exotic species, point intercept sampling, spatial pattern, tallgrass
prairie.
final plant distributions and the impact of those distributions on future dynamics. This study tests if seed distributions at planting have an effect on diversity, species
composition, and fine-scale plant distributions in two
restored grasslands.
Planting methods such as drilling and broadcasting,
which vary in the ways seeds are spread on the soil surface,
provide an excellent context to study the influence of seed
distribution on restorations. These methods differ in how
seeds are planted in two ways. First, drill seeding plants
seeds deeper than broadcast seeding, which can affect germination (Redmann & Qi 1992; Ambrose & Wilson 2003)
and subsequent diversity. Second, drilled and broadcast
plantings presumably differ in the intensity of interactions
among germinating and establishing individuals due to their
arrangement at planting. In drilled restorations, seeds are
planted in equally spaced rows where all seeds presumably
have short mean nearest neighbor distances. In broadcast
restorations, seeds are generally spread across the landscape with potentially longer mean nearest neighbor distances than in a drilled planting, although local clumping may
still occur. Of these methods, broadcasting may more
closely mimic natural seed dispersal, which can be rather
variable through space (Rabinowitz & Rapp 1980).
Establishing seedlings with closer neighbors may experience more initial negative interactions than those with
farther neighbors (Lortie et al. 2005; Milbau et al. 2007).
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Impact of Seeding Method on Diversity and Plant Distribution
This effect of decreasing nearest neighbor distances may
benefit early emerging species (Ross & Harper 1972) or
strong competitors, which often limit others more than
themselves (interspecific > intraspecific competition)
(Amarasekare 2003). Strong competitors may then establish large patches and dominate drilled plantings through
competitive exclusion of close neighbors. Increased distances among strong and weak competitors in broadcast
plantings may alleviate this effect of neighbor distance on
establishment.
A final aspect that may be important within drilled and
broadcast restorations is how initial seed distributions
affect weed and exotic species establishment from the
local propagule pool (Bergelson et al. 1993; Norris et al.
2001a, 2001b). When planting into bare ground, the space
available for exotic species establishment among planted
native seeds varies between planting type and may affect
invasion. Model systems have shown that larger and more
contiguous spaces, as in drilled plantings, can facilitate
invader establishment (Silvertown et al. 1992; Rees et al.
1996). Some support for this hypothesis has been provided
in intact systems. In sandy soil grasslands, space for invasion decreased as species richness increased (Kennedy
et al. 2002). Native species recruitment (Goldberg &
Werner 1983; Aguilera & Lauenroth 1993) and exotic species invasion (Cascorbi 2007) increase with gap diameter
in grasslands. These effects may translate into larger
patches of exotic and weed species in drilled plantings.
Few studies have tested for differences in plant species
composition between paired drilled and broadcast plantings and none have examined how plants establish in
space. Bakker et al. (2003) found no differences in species
establishment but higher survivorship when a mix of five
grasses was broadcast into established exotic perennial
grasses in a semiarid system. Sheley et al. (2006) found
greater density but not biomass of three perennial grasses
drilled into pothole wetlands dominated by invasive species. Finally, Montalvo et al. (2002) found that largeseeded species had higher establishment when six species
were drilled into coastal sage scrub. Although these
results suggest that drill and broadcast seeding would generate different communities, we have no sense of what
roles plant distribution and depth of seeding are playing in
generating these communities.
This study tests for fine-scale differences in vegetation
structure in paired drill- and broadcast-seeded tallgrass
prairie plantings. We measured species diversity, composition, and plant distributions in two tallgrass prairie restorations. Within each site, the same seed mix was either
drill or broadcast seeded into equal-sized plantings. We
test the hypotheses that in drilled plantings (1) species
diversity would be lower due to increased competitive
exclusion among close neighbors during establishment; (2)
native warm-season grasses would be more abundant and
occur in larger patches due to competitive exclusion,
favoring these early-emerging strong competitors (Jackson
1999; Sluis 2002); (3) exotic species would be more abun-
312
dant and occur in larger patches due to more contiguous
places for establishment; and (4) canopy light capture
would be lower (De Boeck et al. 2006) with predicted
larger patch sizes. These results will be useful for improving restoration plans (Bartha et al. 2004) and expand our
understanding of how initial plant distributions might
affect future species diversity and invasibility (Bolker
et al. 2003).
Methods
Study Sites
A 7-year-old restoration (Peterson Park, Story County,
Iowa, U.S.A.) and a 4-year old restoration (Lakeside Lab,
Dickinson County, Iowa, U.S.A.) were sampled in June
2007. Both restorations occurred on land formerly in
annual crop production and are in the Des Moines Lobe
landform region of Iowa. Each site contains a drill- and
a broadcast-seeded planting. The drilled and broadcast
areas were planted with the same seed mix and then managed in the same way within each site. The plantings were
similarly managed and provide an excellent opportunity
to compare, with all other factors generally being equal,
the differences between drill- and broadcast-seeded grassland restorations.
The Peterson Park site (lat 42°059N, long 93°359W) was
planted in fall 1999 by the Story County Conservation
Board. The site is located in the Skunk River floodplain
and contains moderately to well-drained fine loam mixed
Cumulic Hapludolls (DeWitt 1984). Mean annual temperature is 8.8°C and mean annual precipitation is 837 mm.
The site was divided into two sections, each planted with
a seed mix containing 20 native species collected in bulk
from three locations in Story County, Iowa. The northern
3.5 ha was planted at 15.6 kg pure live seed/ha with
a broadcast seeder and cultipacked after seeding. The
southern 1.9 ha was drilled with the same seed mix, mixed
from the three sites in a similar ratio, at 16.8 kg pure live
seed/ha. The most abundant species within the bulk mix
were Stiff goldenrod (Solidago rigida), Yellow coneflower
(Ratibida pinnata), Canada wildrye (Elymus canadensis),
Big bluestem (Andropogon gerardi), Indian grass (Sorghastrum nutans), and Virginia wildrye (E. virginicus).
The entire site was burned in 2004, 2005, and 2006 springs.
The western half of the site was burned (0.5 of the drilled
and 0.5 of the broadcast planting) in fall 2006 and broadcast interseeded to increase species diversity (B. Gleason
2007, Story County Conservation Board, personal communication). As a result, sampling for this study was restricted to the eastern portion of the site.
The Lakeside Lab site (lat 95°109N, long 43°239W) is
a 9.3-ha planting located on a south facing slope at the Iowa
Lakeside Laboratory. Mean annual temperature is 7.2°C
and mean annual rainfall is 725 mm. Soils are predominantly fine loam mixed Typic Hapludolls on 2–9% slope
with some Cumulic Hapludolls (Dankert 1983). Soil series
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Impact of Seeding Method on Diversity and Plant Distribution
run east–west across the site and plantings were established
with an equal proportion within each soil type (north–
south). Sections (1.0 ha) that were drilled or broadcast with
pure live seed during spring 2003 were sampled for this
study. The site was disked twice and leveled with a cultipacker before planting. Both drilled and broadcast areas
were drilled with the annual Oat (Avena sativa) as a cover
crop (17.4 kg/ha) in 2002 spring. A seed mix consisting of
37 forbs and 9 grasses was added at 12.0 kg pure live seed/
ha within both plantings. The most abundant forbs (>10
seeds/m2) in the mixture were Yellow cone flower (R. pinnata), Black eyed susan (Rudbeckia hirta), Stiff goldenrod
(Solidago rigida), and Purple prairie clover (Petalostomum
purpurea). The most abundant grasses were Little bluestem
(Schizachyrium scoparium), Junegrass (Koelaria macrantha), and Indian grass (S. nutans). Both plantings have been
mowed twice yearly (spring and late summer) to control
thistles, primarily Canada thistle (Cirsium arvense) and
Musk thistle (Carduus nutans).
Vegetation Sampling
In each of the four plantings we sampled ten 1-m2 quadrats. Quadrats were located randomly along and away
from a transect through the longest portion of the planting. We used a 70-m transect at Peterson Park and a 100m transect at Lakeside Laboratory. All species were
recorded and species relative abundance was determined
through point intercept sampling in each quadrat (Jonasson 1988; Frank & McNaughton 1990). A 1-m2 sampling
frame was placed over each plot and a pin dropped at 20cm intervals (alternating odd and even holes) in rows
spaced 10 cm apart for a total of 40 pins/m2. The identity
of each leaf and stem touching the pin was recorded for
each pin drop. This method captured approximately 80%
of the species in the plot. To account for species that were
not captured, a small value (0.5 hit) was added for each
species with no hits when calculating diversity measures
(Bowman et al. 2006). Species relative abundance was
determined by dividing the total touches for species i in
a quadrat by the total touches in the quadrat. These data
were used to determine species
P richness (S), Simpson’s
diversity (1/D), where D ¼ pi2 and pi ¼ relative abundance of species i, and evenness ([1/D]/S) at the quadrat
scale (Smith & Wilson 1996; Wilsey et al. 2005).
We took a fine-scale cell-based approach (Herben et al.
1993; Purves & Law 2002) to quantify plant distributions
in each quadrat. Each 1-m2 quadrat was divided into sixtyfour 12.5 3 12.5–cm cells using metal rods passed through
the vegetation. This cell size corresponds to the average
plant size in restored grasslands (Losure et al. 2007) and
thus was an appropriate scale to capture individual plants.
Cell identity was determined by the species occupying
50% or more of the aboveground space in the cell. This
method generates a fine-scale map of the species using the
most resources and having the strongest influence
throughout the quadrat (reviewed in Schwinning & Weiner
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1998). Although other metrics and approaches may better
characterize pattern within this system (Glenn & Collins
1990; Bartha et al. 1995), we focus on cell- and patchbased metrics as an easily quantifiable indicator of local
plant extent and thus potential species interaction and
resource use patterns.
The program QRULE (Gardner 1999; Gardner &
Urban 2007) was used to determine the number and size
of patches within each quadrat. A patch was defined as
a group of neighboring conspecific cells using an eightneighbor rule (Turner et al. 2001). With this approach the
four cells immediately adjacent to and the four cells on
the diagonal from a focal cell were considered neighboring
cells. We used an eight-neighbor rule because adjacent
conspecifics and conspecifics on the diagonal may be
ramets of the same individual sharing above- and belowground space. With these data we computed several simple metrics of landscape composition: number of species
per map, proportion of the quadrat covered by a focal species, mean patch area, and patch mean-squared radius.
The number of species per map refers to how many species
within each quadrat occupy one or more 12.5 3 12.5–cm
cells. Because some species may not occupy the majority
of even one cell, this value could be equal to or smaller
than the actual quadrat species richness. The number of
species per map and proportion of the quadrat covered by
focal species are nonspatial but give a sense of how species
generally occupy space within each quadrat (Turner et al.
2001). Mean patch area (m2) and patch mean-squared
radius, a measure of patch dispersion in meters (Gardner
1999), were used as patch-based measures of spatial
configuration (Turner et al. 2001). Larger mean-squared
radius values indicate that a patch is more dispersed;
a larger area is needed to encompass the patch, than one
with smaller values (Gardner 1999). These metrics are
generally uncorrelated, with the possible exception of
patch size and patch mean-squared radius, hereafter dispersion, for maps with a large spatial extent and capture
different aspects of plant distribution (Riitters et al. 1995;
Gardner & Urban 2007).
We used two approaches to assess plant distributions in
the quadrats. The first analysis summarized plant distributions by calculating the mean size (m2) and dispersion (m)
of all patches within each quadrat, irrespective of their
species identity. A second analysis focused on how two
groups of species, native warm-season grasses and exotic
species, were collectively distributed within each quadrat.
Exotic species were defined as those that are introduced
to North America. Both groups of species can dominate
restorations despite efforts to promote realistic native species composition (Sluis 2002; Martin et al. 2005). For this
analysis, each quadrat map was simplified into three classes: native warm-season grass, exotic species, and ‘‘other.’’
Heterospecific native warm-season and exotic species
patches were then summarized with QRULE. We determined the proportion of the quadrat covered by each
species group and the mean size and dispersion of the
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Impact of Seeding Method on Diversity and Plant Distribution
heterospecific patches within each group. Whereas the
first analysis tests for differences in general patch structure, the second analysis tests if dominant species groups
occupy space, and potentially utilize resources, within
each planting in different ways.
Photosynthetic active radiation (PAR) captured by the
canopy was also measured as an estimate of resource use
within each quadrat. A Decagon AccuPAR LP-80 sensor
light meter (Pullman, Washington, D.C., U.S.A.) was used
for the below-canopy measurement, with a Li-Cor external point sensor (Lincoln, Nebraska, U.S.A) for the abovecanopy measurement. Above- and below-canopy midday
(10-2 CST) PAR was measured twice, in a north-south
and east-west direction, within each quadrat and the
results averaged. From the PAR data we determined what
percent of the available light was captured by the canopy
(1 minus % PAR at soil surface) as a proxy for overall
resource capture in the quadrat.
Although sites differed in some aspects of diversity, there
was no main effect of planting type on quadrat Simpson’s
diversity, evenness, or species richness (Table 1; Fig. 1).
Because there was a site 3 planting–type interaction for
species richness (Table 1; Fig. 1), we also separately considered the effect of planting type on species richness within
each site. At Peterson Park, there were no differences in
quadrat species richness (F[1, 18] ¼ 0.07; p > 0.05) between
plantings, whereas quadrat species richness was higher
in the broadcast planting at Lakeside Lab (F[1, 18] ¼ 5.27;
p ¼ 0.03).
Data Analysis
Species Composition
We used analysis of variance (ANOVA; PROC GLM;
SAS version 9.1) to test for quadrat-scale differences in
species diversity, exotic species composition, plant distributions, and light capture between drilled and broadcast
plantings. Native planted forb and exotic species relative
abundance were arcsine square root transformed to meet
normality assumptions. An initial ANOVA model
included site, planting type, and site 3 planting type as
fixed factors tested with the residual quadrat error term.
With this analysis, we assess differences between plantings
within these specific restorations. In most cases the interaction was significant and it was unreasonable to use the
pooled error to test for model term significance. Therefore,
we also present separate one-way ANOVA’s for each site.
A multiresponse permutation procedure (MRPP; Zimmerman et al. 1985) based on Bray–Curtis distance (Vegan
package in R; Oksanen et al. 2007) was performed to test
for differences in species composition between plantings at
each site. Finally, exotic plants at Peterson Park and
native warm-season grasses at Lakeside Lab were not
recorded as occupying cells in several quadrats and normality assumptions could not be met with data transformations for their distribution metrics. A nonparametric
Of the species that established from the seed mix, the
native warm-season grasses, Andropogon gerardi, Sorghastrum nutans, and Schizachyrium scoparium were
abundant (by number of point intercept touches) at both
sites in addition to Elymus canadensis at Lakeside Lab.
The most abundant exotic species at Peterson Park were
Poa pratensis and Bromus inermis. The most abundant
exotic species at Lakeside Lab were B. inermis, P. pratensis, Elymus repens, and Dactylis glomerata. Despite similarities in the species that occurred between sites, there
were large differences in the relative abundance of native
warm-season grasses, planted forbs, and exotic species
between sites (Table 1; Fig. 2). The Peterson Park plantings were dominated by native warm-season grasses with
few exotic species and Lakeside Lab plantings were dominated by exotic species with fewer native warm-season
grasses (Fig. 2). There were also differences in the effect
of planting type within each site (Table 1; Fig. 2). At
Peterson Park there was no effect of planting type on
species composition (MRPP A ¼ –0.004715; p > 0.05
for 1,000 permutations). There were no significant differences in the relative abundance of native warm-season
grasses (F[1,18] ¼ 0.82; p > 0.05; Fig. 2), planted forbs
Kruskal–Wallis test was performed to test for differences
in plant distributions.
Results
Species Diversity
Table 1. F values from ANOVA of quadrat-scale diversity, species composition, and resource use variables for drilled and broadcast plantings in
two grassland restorations in Iowa, U.S.A.a
Source
df
Site (S)
Planting type (P)
S3P
Error
1
1
1
36
a
y
Species
richness
Simpson’s
diversity
Evenness
C4 grass
relative abundance
Planted forb
relative abundance
Exotic relative
abundance
Canopy light
capture (%)
87.84***
3.42
4.46*
16.39***
0.78
1.58
1.14
0.02
0.16
116.92***
8.97**
5.30*
5.06*
2.60
3.63y
93.05***
17.24***
14.25***
1.16
16.92***
0.44
Separate ANOVAs were performed for each site when there was a significant site 3 planting type interaction.
p < 0.10; * p < 0.05; ** p < 0.01; *** p < 0.001.
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Impact of Seeding Method on Diversity and Plant Distribution
Figure 1. Quadrat-scale diversity (Simpson’s diversity, evenness, and species richness) and percent canopy light capture (1 minus % PAR at soil
surface) for broadcast and drilled plantings in two grassland restorations, Lakeside Lab (LL) and Peterson Park (PP). Means are shown ±1 SE
from separate site ANOVAs.
(F[1,18] ¼ 0.08; p > 0.05), or exotic species (F[1,18] ¼ 0.25, p
> 0.05; Fig. 2) between plantings. However, there was an
effect of planting type on species composition at Lakeside
Lab (MRPP A ¼ 0.07654; p < 0.01 for 1,000 permutations). There were no differences in planted forb abundance (F[1,18] ¼ 4.27; p < 0.10) and the drilled planting at
Lakeside Lab had higher exotic species relative abundance (F[1,18] ¼ 18.32; p < 0.001) and lower relative abundance of native warm-season grasses (F[1,18] ¼ 8.22; p <
0.05) than the broadcast planting.
Plant Distributions
Mean patch size and dispersion per quadrat were not different between sites or planting types (Table 2; Fig. 3).
Although different numbers of species were recorded in
quadrat cells at each site, there was no effect of planting
type (Table 2; Fig. 3). However, there were differences in
how groups of similar species established and were distributed between planting types. At Peterson Park, native
warm-season (C4) grasses were recorded in over half of
the cells in both plantings. C4 grasses occupied a larger
proportion of cells per quadrat (F[1,18] ¼ 5.18; p ¼ 0.0352;
Fig. 4) and occurred in larger heterospecific patches per
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Restoration Ecology
quadrat (F[1,18] ¼ 6.00; p ¼ 0.0247; Fig. 4) in the drilled than
in the broadcast planting (Fig. 5). There was no effect of
planting type on patch dispersion (v2 ¼ 0.5714; df ¼ 1; p >
0.05; Fig. 4). Exotic species comprised a much lower proportion of space within quadrats at Peterson Park (Fig. 5).
Exotic species were recorded in four broadcast and one
drilled quadrat at Peterson Park and there was no effect of
planting type on exotic species proportion of space covered
per quadrat (v2 ¼ 2.2208; df ¼ 1; p > 0.05; Fig. 4), mean
patch size (v2 ¼ 1.9371; df ¼ 1; p > 0.05; Fig. 4), or dispersion (v2 ¼ 1.8054; df ¼ 1; p > 0.05; Fig. 4).
At Lakeside Lab, these suites of species were distributed
differently. Exotic species were abundant in a greater proportion of cells, in some cases comprising the entire quadrat. In the drilled planting, exotic species occupied more
cells (F[1,18] ¼ 19.82, p ¼ 0.0003; Fig. 4) and collectively
occurred in larger (v2 ¼ 8.6914; df ¼ 1; p ¼ 0.0032; Fig. 4)
similarly dispersed (v2 ¼ 3.0400; df ¼ 1; p ¼ 0.0812; Fig. 4)
patches than in the broadcast planting (Fig. 5). C4 grasses
were recorded in at least one cell in each broadcast quadrat
and in four drilled quadrats. C4 grasses occupied a larger
proportion of space (v2 ¼ 6.2325; df ¼ 1; p ¼ 0.0125; Fig. 4)
and were more dispersed (v2 ¼ 6.0142; df ¼ 1; p ¼ 0.0142;
Fig. 4) in broadcast quadrats but did not differ in patch size
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Impact of Seeding Method on Diversity and Plant Distribution
Table 2. F values from ANOVA of the number of species recorded
per map, mean patch size, and mean patch dispersion for quadrats
within drilled and broadcast plantings in two grassland restorations in
Iowa, U.S.A.
Source
df
Site (S)
Planting type (P)
S3P
Error
1
1
1
36
y
Number of
species per map
Mean
patch size
Mean patch
dispersion
15.22***
0.25
3.67y
0.22
3.73y
3.82y
4.11y
0.85
3.06y
p < 0.10; * p < 0.05; ** p < 0.01; *** p < 0.001.
Figure 2. Native warm-season grass and exotic species relative abundance (number of touches/total number of touches) from point intercept sampling ±1 SE from separate site ANOVAs.
(v2 ¼ 3.6708; df ¼ 1; p > 0.05; Fig. 4) between planting
types.
Light Capture
There were consistent differences in canopy light capture
between sites and planting types. Canopies in drilled plots
captured more light (less reached the soil surface) than in
broadcast plots (Table 1; Fig. 1).
316
Figure 3. Mean patch size and dispersion (mean-squared radius) for
plants distributed in 1-m2 quadrats (12.5 3 12.5–cm resolution) in
restored grasslands. Means are shown ±1 SE from joint site ANOVA.
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Impact of Seeding Method on Diversity and Plant Distribution
Figure 4. Native C4 grass and exotic plant distributions in two grassland restorations. Means are shown ±1 SE either from ANOVA or data
depending on statistical test used.
Discussion
This study tested whether vegetation structure differed
between drilled and broadcast plantings in two established
grassland restorations. There was no main effect of planting type on Simpson’s diversity or evenness in either site.
However, there were site-specific effects of planting type
on species richness and composition with differences
found only in the site with more exotics. A simple method
used to quantify common plant distributions revealed that
the abundant species within each site were collectively distributed in larger patches in drilled plantings. Finally,
resource use, as measured by canopy light capture, was
consistently greater in drilled plantings.
We have expanded upon previous studies of drilled and
broadcast plantings by assessing if species richness and
evenness components of plant diversity (Wilsey & Polley
2002; Ruiz-Jaen & Aide 2005), differ between planting
types. Our hypothesis that diversity would be lower in
drilled plantings due to greater competitive exclusion at
establishment was not supported. There was no main
effect of planting type on quadrat Simpson’s diversity,
evenness, and, in one site, species richness. These results
are consistent with comparisons between drilled and
broadcast plantings in other systems. Studies in less species rich plantings (three to six planted species, primarily
perennial grasses) also found no differences in species
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Restoration Ecology
richness between planting types (Montalvo et al. 2002;
Sheley et al. 2006). Thus, these planting methods appear
to be interchangeable when measuring species diversity.
However, our additional findings concerning the species
present and quadrat-scale plant distributions suggest that
diversity metrics may be inadequate for understanding
how the communities might develop in the future.
Native and exotic species composition differed between sites potentially due to differences in seed mix
composition, timing of planting, and the ways each site
was restored including use of fire (Howe 1994) versus
mowing (Williams et al. 2007). As a consequence, we
analyzed each site independently for the effect of planting type. There was no effect of planting type on species
composition at Peterson Park (fall seeded and burned),
but there was in the more invaded Lakeside Lab (spring
seeded and mowed) restoration. Native grass abundance
was lower and exotic species abundance higher in the
Lakeside Lab drilled planting. Although others suggest
drill seeding should facilitate native grass establishment
(Jackson 1999), our findings are consistent with Bakker
et al. (2003) that broadcasting may increase native grass
establishment. Under greater invasion pressure, drilling
might lead to more exotic species establishment than
broadcast seeding as previously demonstrated by Bakker
et al. (2003).
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Impact of Seeding Method on Diversity and Plant Distribution
Figure 5. Representative quadrat maps for proportion of the quadrat covered and mean patch size of native warm-season grasses (gray) and
exotic species (black) in broadcast (left) and drilled (right) plantings at two grassland restorations. Each 1-m2 quadrat was divided into sixty-four
12.5 3 12.5–cm cells and the identity of each cell determined by the species occupying a majority of the cell. These quadrats represent the median
values of proportion of the quadrat covered and patch size for both species groups.
Native warm-season grass and exotic species abundances within each planting partially reflected their quadratscale distributions. Although there were no differences in
mean patch structure between plantings in either site,
native warm-season (C4) grasses and exotic species were
distributed in larger patches in drilled plantings, when
they were abundant. Because there were no differences in
richness this was likely not due to competitive exclusion
leading to larger patch sizes as predicted. Rather, differences in distribution likely result from individuals being
placed more closely together and spreading through space
differently between plantings. Although there was no
effect of planting type on C4 grass abundance at Peterson
Park, there was an effect of planting type on C4 grass distribution. C4 grasses probably started more contiguously
in drilled plantings because they were planted deeper,
which may have increased their germination rate
(Ambrose & Wilson 2003). As the C4 grasses established,
they likely dictated what spaces could be occupied by
other species (Glenn & Collins 1990).
We could not determine to what extent the planting
method influenced invasion at Peterson Park because
318
exotic species were so infrequent. However, results from
Lakeside Lab suggest that planting method can influence
invasion when there is greater invasion pressure. Exotic
species were more abundant and occupied more space in
drilled quadrats at Lakeside Lab. Exotic plants may have
established more extensively in this drilled planting as
a result of larger spaces for establishment (Bergelson et al.
1993) and feedbacks that promoted their persistence (Bergelson 1990). Further experiments are needed to test and
determine the ubiquity of these mechanisms structuring
plant distributions in variously restored grasslands.
We also found that canopy light capture was consistently higher in drilled over broadcast plantings. Within
these sites, differences in light capture between plantings
suggest that recruitment opportunities may become more
limited in drilled plantings than in broadcast plantings due
to differences in microsites for establishment (Tilman
1997; Foster et al. 2007). As a result, we would predict
higher species turnover in broadcast plantings due to
increased light and more microsites for establishment,
which may have large consequences for future vegetation
dynamics (Foster et al. 2002, 2007). Differences in mean
Restoration Ecology
MAY 2010
Impact of Seeding Method on Diversity and Plant Distribution
light capture and plant distribution may also be correlated
with fine-scale differences in nutrient cycling (McKane
et al. 1990; Foster et al. 2007), which may have consequences at broader scales (De Boeck et al. 2006).
Our results suggest that planting methods can affect
plant distributions and resource use, without affecting
diversity, which may have consequences for future vegetation dynamics (Silvertown et al. 1992; Rees et al. 1996;
Racz & Karsai 2006) via two mechanisms. Low neighborhood evenness resulting from the presence of large conspecific patches may maintain future species diversity
through the development of spatial refugia for weaker
competitors (Stoll & Prati 2001; Monzeglio & Stoll 2005;
Idjadi & Karlson 2007). In this scenario, species occurrence in large patches would be a desirable management
objective. In contrast, low neighborhood evenness may
destabilize diversity through higher invasibility (Tilman
et al. 1996; Wilsey & Polley 2002). In this scenario, species
occurrence in small patches would be a desirable management objective. The relative influence of each of these
mechanisms needs further experimental testing to determine how seed arrangement at planting may be manipulated to maximize long-term maintenance of diversity in
grassland restoration.
Incorporating a consideration of initial propagule distribution in the restoration process has been important in
wetland (Liu et al. 2004) and aquatic (Sleeman et al. 2005)
systems and should be further considered in grassland restoration. The long-term effects of varying plant distributions in space are especially important to consider when
grasslands are being reconstructed in former agricultural
lands (e.g., Muller et al. 1998; Walker et al. 2004) where
the seed bank has been depleted. This approach may also
prove useful for understanding how other aspects of the
soil biota (Viketoft 2007) and resources (Reynolds et al.
1997) develop in space.
This is the first study that we are aware of that takes
a fine-scale spatial approach to assessing grassland restoration success. We demonstrate that distributions of dominant species in space and resource capture do differ among
variously restored grasslands despite having similar levels
of diversity. These differences may have long-term effects
on vegetation dynamics. The mechanisms that generate
plant distributions and the implications of different distributions for diversity maintenance and invasibility need to
be further investigated in experimental settings. As we
examine restorations to determine what aspects are and are
not restorable (Hobbs 2007; Miller & Hobbs 2007), we
need to consider how species utilize space and how spatial
heterogeneity develops within plantings as a result of initial
conditions and/or subsequent management (Bartha et al.
2004). By taking such a fine-scale approach to assessing
restorations we may be able to more readily describe otherwise elusive (Ruiz-Jaen & Aide 2005) aspects of self-sustainability (SER 2004) within restorations.
MAY 2010
Restoration Ecology
Implications for Practice
d Our findings suggest that drill and broadcast seeding
are interchangeable when assessing restoration success through the lens of plant species diversity.
d Drill seeding may result in greater native warm-season
grass and exotic species abundance. However, seeding
method does not appear to affect forb recruitment.
d Drill and broadcast seeding produce communities
with different plant distributions and resource capture. These differences may have effects on longterm diversity maintenance within the plantings and
need to be further investigated.
d The effects of drilled versus broadcast seeding were
site specific and may be related to site preparation
and subsequent management including seed mix
composition, time of planting, local propagule pool,
or use of fire as a management tool. The ways these
factors interact to affect drilled versus broadcast
planting outcomes need to be further studied.
Acknowledgments
Many thanks to the thoughtful individuals who planned,
managed, and allowed us to sample the sites. Tom Rosburg, Daryl Smith, and Arnold van der Valk planned the
Lakeside Lab planting, and Joe Kooiker in conjunction
with the Story County (Iowa) Conservation Board
planned the Peterson Park planting. Adam Asche, Tom
Moeller, Joe Reynolds, and Kim Wahl helped with data
collection. Tom Rosburg, Stuart Allison, and two anonymous reviewers provided valuable feedback on earlier
versions of the manuscript. This project was partially
funded by the Iowa DOT Living Roadway Trust Fund
(90-00-LRTF-707), the Iowa Prairie Network, the Iowa
Native Plant Society, and the Iowa Lakeside Laboratory.
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