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Journal of Vegetation Science && (2012)
Limited effects of dominant species population sourceon community composition during communityassembly
David J. Gibson, Sara G. Baer, Ryan P. Klopf, Lewis K. Reed, Ben R. Wodika & Jason E. Willand
Keywords
Community assembly; Dominant species;
Extended phenotype; Grassland; Restoration;
Seed pool; Tallgrass prairie
Nomenclature
Mohlenbrock (2002)
Received 6 March 2012
Accepted 3 August 2012
Co-ordinating Editor: Sandor Bartha
Gibson, D.J. (corresponding author,
djgibson@siu.edu), Baer, S.G. (sgbaer@siu.
edu), Klopf, R.P. (rklopf@gmail.com),
Reed, L.K. (lewiskreed@gmail.com),
Wodika, B.R. (BenWodika@gmail.com) &
Willand, J.E. (jwilland@siu.edu):
Department of Plant Biology and Center for
Ecology, Southern Illinois University
Carbondale, Carbondale, IL, 62901-6509, USA
Abstract
Question: To what extent do dominant species population sources and subordi-
nate species pools affect diversity and composition of an assembling grassland
community?
Location: Illinois, USA.
Methods: Percentage cover of all species were recorded annually in 36 1-m2
quadrats assigned to a factorial combination of dominant species population
source (functionally distinct cultivar or non-cultivar seed source) and designed
species pool (three levels varying in species identity, but with equal functional
group representation and richness) during the first 4 yr of community assembly
in an experimental grassland restoration.
Results: Univariate and multivariate analyses showed that individual species
abundance, life form and community composition differed significantly among
designed species pools, but were not strongly affected by population source of
the dominant species (cultivar or non-cultivar). There were fewer C4 species in
cultivar plots but only in one of three designed species pools during one of 4 yr
of community assembly. The number of legume and forb species was higher in
cultivar plots, but also only in one of the 4 yr of study. Other changes in species
richness and abundance were solely related to successional change.
Conclusions: Non-dominant species introduced to restore plant communities
strongly affects plant community composition, and composition can show fidel-
ity to designed species pools. Only marginal or temporary effects of dominant
species seed source were observed in the assembling plant community. Thus, we
found no strong evidence that the source of dominant species, in this case culti-
vars compared to local ecotypes, has consequences for community assembly in
the early stages of restoration (1–4 yr). The absence of a strong dominant spe-
cies source effectmaybe exacerbatedby the assembly of diverse plant communities,
resulting in a stronger effect of subordinate species seedmixture in restoration.
Introduction
There is increasing evidence that community assembly is
related to abiotic and biotic filters that constrain success of
species that are able to colonize from the regional species
pool (Baer et al. 2004; Hobbs & Norton 2004; Chase et al.
2005; Gibson et al. 2011). Niche-based models recognize
environmental (abiotic) filters and processes of limiting
similarity (i.e. biotic filters such as competition, facilitation
and mutualism) as being important determinants of com-
munity assembly (Abrams 1983). Within a site, where
environmental conditions are fairly homogeneous, com-
munity heterogeneity can arise from local species sorting,
partly determined by functional traits and phylogeny
(Houseman & Gross 2011; Pavoine et al. 2011; Weiher
et al. 2011).
The importance of intra-specific variation within the
dominant, matrix species on community assembly repre-
sents a poorly understood aspect of limiting similarity.
Dominant species are generally abundant, relatively large
and, as individual species, make a substantial contribution
to primary production. Subordinate species are generally
Journal of Vegetation ScienceDoi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science 1
relatively smaller in stature, less abundant and occupy
more restricted microhabitats (Grime 1998), but can con-
tribute most to floristic diversity, particularly in grassland
(Gibson 2009). The mass ratio hypothesis (Grime 1998)
suggests that the traits and functional diversity of domi-
nant species largely determine ecosystem function. These
dominant species can constrain subordinate species success
and diversity (Baer et al. 2004; McCain et al. 2010), with
consequences for ecosystem processes such as primary pro-
duction (Polley et al. 2007; Sasaki & Lauenroth 2011).
Intra-specific variation in dominant species can scale to
affect associated communities, and has been referred to as
the ‘extended phenotype hypothesis’ (Dawkins 1982).
This concept suggests that high levels of genotypic and
functional phenotypic diversity of dominant species can
have considerable ecological influence on assembling com-
munities (Vellend 2006; Fridley & Grime 2010; Vellend
et al. 2010; Gibson et al. 2012) at multiple scales (Bangert
et al. 2008). For example, distinct genotypes of Populus,
raised in a common garden, differentially influenced asso-
ciated foliar insect communities, and the litter of different
genotypes supported different communities of decomposer
microorganisms (Whitham et al. 2003). However,
whether variation in dominant plant species affects
within-trophic level community structure during commu-
nity assembly offers a novel context to test whether intra-
specific variation can lead to an extended phenotype.
Grassland habitats offer a dynamic, non-equilibrium
model system in which to test hypotheses of community
assembly (Collins et al. 1998). However, much of this eco-
system has been lost due to anthropogenic influence on
the environment (largely for agriculture) in most parts of
the world (Ellis & Ramankutty 2008), necessitating resto-
ration programmes to increase the extent of grassland and
improve the quality of degraded remnants (White et al.
2000; Gibson 2009). Addition of propagules (e.g. seeds) is
the most common approach to restoring and steering the
recovery of grassland communities degraded through agri-
culture. The species mix should influence composition of
the assembling community, and dominant species (partic-
ularly matrix grasses) are likely to modulate resultant
community diversity (Baer et al. 2004). However, the role
of dominant species in community assembly, especially
aspects of limiting similarity (Abrams 1983) and the exclu-
sion of subordinate species, is not well understood. Two of
these aspects that are likely to be of particular importance
are the population source of the dominant species (Bangert
& Whitham 2007), and inter-specific interactions arising
from propagules in the regional species pool (Zobel 1997)
introduced to restore a community. In the context of
restoration, this means that the seed source of dominant
species and composition of the sown species mix could sig-
nificantly influence restoration trajectory, diversity and
ecosystem function (Lesica & Allendorf 1999; Zedler 2005;
Wilsey 2010; Larson et al. 2011). Through competitive
and facilitative interactions, both the genetic identity (pop-
ulation source) of dominant species and the local species
pool can act as biotic filters constraining community
assembly (Whitlock et al. 2007; Gibson et al. 2012).
The objective of this study was to conduct an experi-
mental test of the applicability of the ‘extended pheno-
type hypothesis’ in assembling grassland communities
sown with different population sources of commonly
dominant grass species. Preliminary evidence demon-
strates variation in genetic structure and functional traits
between population sources of the dominant grasses.
Specifically, we investigated whether dominant species
sources differentially affect diversity and composition of
an assembling grassland community. We have demon-
strated that cultivars of the dominant grasses used in this
experiment were genetically distinct from local popula-
tion sources (Gustafson et al. 2004b). Further, we have
documented that cultivar and non-cultivar sources of the
dominant grasses varied in relevant functional traits,
including enhanced leaf-level photosynthetic rates (Lam-
bert et al. 2011) and increased root length and root
surface area (Klopf & Baer 2011). The cultivar and non-
cultivar population sources affect inter-specific competi-
tive interactions (Gustafson et al. 2004a). Specifically, we
contrasted the effect of sowing cultivars of the dominant
grasses selected and recommended for use in the region
of our study site with locally collected, non-cultivar dom-
inant grass sources on the developing structure of plant
communities using three distinct pools of subordinate
species. The three species pools sown into the source of
dominant grasses was intended to elucidate whether
dominant grass source effects were a general phenome-
non (consistent across species pool) or contingent upon
inter-specific interactions. We measured species composi-
tion during the first 4 yr of community assembly in
response to dominant grass source and designed species
pool (DSP) to test three hypotheses: Our first hypothesis
(H1) was that prairie seeded with cultivars of the domi-
nant grasses would obtain a different (i.e. more grass
dominated and less diverse) community composition
over time compared with prairie seeded with locally col-
lected, non-cultivar seed sources. We expected a differ-
ence to arise because the cultivars have been selected for
agronomic traits of high productivity, seed viability, ger-
mination, as well as drought tolerance and disease resis-
tance (Alderson & Sharp 1995). We expected cultivars to
be more competitive than wild-collected sources (Gustaf-
son et al. 2004a), allowing them to attain greater domi-
nance than local ecotypes of the native grasses. Second,
we hypothesized (H2) the effect of dominant species
source would be consistent across DSPs of subordinate
Journal of Vegetation Science2 Doi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science
Grassland assembly D.J. Gibson et al.
species if the effect predicted in H1 is a general phenome-
non. Lastly, we hypothesized (H3) that the effect predicted
in H1 (greater grass dominance in cultivar- relative to
non-cultivar-sown plots) would reduce divergence in
community composition among the DSPs seeded with cul-
tivars relative to DSPs seeded with non-cultivars. More
specifically, we predicted differences in community com-
position would diminish among DSPs over time as the
contrasting effect of cultivar vs native population sources
of the dominant species increased over time and seeded
species became more prominent in the community.
Methods
In March 2006, we established an experimental tallgrass
prairie restoration in a former agricultural field at the
Southern Illinois University Agronomy Research Center in
Jackson County, Illinois, USA (37°41′ N, 89°14′ W). Soil
at the field site was a silty clay loam (Herman 1979). The
climate has had an average annual temperature of 13.4 °C(average minimum 7.4 °C, average maximum 19.4 °C),with average annual rainfall of 1212 mm, of which an
average of 52% fell during the growing season from 1 April
through 30 September (1990–2009 record, Carbondale,
IL). Total precipitation received each year of this study
(2006, 2007, 2008 and 2009) was 1475, 1084, 1492 and
1545 mm, of which 45, 42, 47 and 61% of total precipita-
tion was received during the growing season, respectively.
The restoration experiment was a split-plot design
(Appendix S1), with dominant grass population source
(cultivar or non-cultivar) assigned as the whole-plot factor
addressing H1 according to a randomized complete block
design (six whole plots per source population; n = 12).
We blocked according to former agricultural field use;
each block contained six 7m 9 23 m whole plots. The
source population treatment comprised three dominant
tallgrass prairie species: Andropogon gerardii Vitman,
Sorghastrum nutans (L.) Nash. and Schizachyrium scoparium
(Michx.) Nash. These three grasses were chosen because
they are the most frequent and dominant species in the
tallgrass prairie region (Weaver 1954; Dodd 1983).
Hypothesis 2 was addressed through a subplot factor of
DSP consisting of three pools containing unique species
randomly assigned to one of three 5m 9 5 m subplots
within each whole plot (n = 36, 12 subplots per DSP).
Each DSP consisted of 15 species and the same number of
species represented in general functional groups: C4 grass,
C3 grass, legumes and non-legumine herbaceous forbs
(Table 1). All of the sown species were native to the
region and occur in tallgrass prairie (Mohlenbrock 2002).
Each of the three DSPs was equivalent in richness and
coarsely defined functional groups, but varied in terms of
species identity, and hence any emergent functional group
traits. The experiment was monitored for the first 4 yr of
community assembly, allowing a test of H3.
Non-cultivar (local ecotype) seeds of the dominant
grasses for the whole plot treatment were collected from
nearby prairie remnants within 100 km of Carbondale, IL.
A small portion (7%) non-cultivar ‘Missouri ecotype’ seed
ofA. gerardiiwas included in the non-cultivar seedmix due
to limited availability of viable non-cultivar A. gerardii seed
from local prairie remnants. We used cultivars recom-
mended and widely sown in the region: A. gerardii ‘Roun-
tree’, S. nutans ‘Rumsey’ and S. scoparium ‘Aldous’. We
seeded 100 live seeds of each dominant grass species per
square meter. Cultivar percentage live seed (PLS) was pro-
vided by a native seed supplier (Hamilton Seed Co., Elk
Creek, MO, US). Non-cultivar seed was tested for PLS at a
seed testing lab (Hulsey Seed Laboratory, Inc., Decatur, GA,
US) according to the guidelines of theAssociation ofOfficial
Seed Analysts (AOSA). Seeds of subordinate species for the
subplot DSP treatment were obtained from the same native
Table 1. Designed species pools assigned to subplots within each whole
plot seeded with either cultivars or non-cultivars of the dominant prairie
grasses.
Designed species
pool 1
Designed species
pool 2
Designed species
pool 3
Non-legume Forbs
Asclepias tuberosa Achillea millefolium Brickellia
eupatorioides
Asclepias verticillata Asclepias syriaca Aster
oolentangiensis
Aster oblongifolius Echinacea purpurea Heliopsis
helianthoides
Callirhoe involucrata Eupatorium
altissimum
Monarda fistulosa
Delphinium
carolinianum
subsp. virescens
Liatris pycnostachya Penstemon digitalis
Oenothera
macrocarpa
Oenothera biennis Rudbeckia hirta
Ratibida pinnata Oligoneuron rigidum Silphium laciniatum
Rosa arkansana Ruellia humilis Solidago speciosa
Senecio plattensis Silphium integrifolium Vernonia fasciculata
Legumes
Baptisia alba var.
macrophylla
Amorpha canescens Astragalus
canadensis
Chamaecrista
fasciculata
Dalea candida Baptisia bracteata
Psoralidium
tenuiflorum
Desmanthus illinoensis Lespedeza capitata
Schrankia nuttallii Desmodium illinoense Dalea purpurea
C3 Grass
Elymus canadensis Koeleria macrantha Agrostis hyemalis
Non-dominant C4 Grass Non-dominant C4 Grass Non-dominant C4
Grass
Sporobolus heterolepis Bouteloua curtipendula Panicum virgatum
Journal of Vegetation ScienceDoi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science 3
D.J. Gibson et al. Grassland assembly
plant supplier (Hamilton Seed Co.). Subordinate species
unique to eachDSPwere seeded at a rate of 20 seed�m�2 per
species (total = 300 seed�m�2of subordinate species) follow-
ing recommendations for mesic prairie restorations (Diboll
2005). Live seedwas not determined for subordinate spe-
cies, butwas consistent amongsubplots assigned toeachDSP.
Prior to seeding, the field site was tilled using a tandem
disk and field cultivator. Seeds of the dominant grasses
and subordinate species were mixed with wet sand and
hand broadcast into plots on 1 Mar 2006. Following seed-
ing, the plots were manually compacted to ensure seed
contact with the soil. The 6-m buffers around the whole
plots were seeded with two native grasses, Elymus canaden-
sis L. and Bouteloua curtipendula (Michx.) Torr, using a
tractor-pulled Brillion Alfalfa seeder. In subsequent years,
2-m wide pathways were mowed in the buffers to main-
tain open walkways. Following standard practice for
early-stage prairie restorations (Packard & Mutel 2005),
the entire site was burned annually in the dormant season
to control volunteers and enhance establishment of the
native prairie species.
Data collection
From 2006 through 2009, canopy cover (%) and species
richness (number per m2) were recorded from early and
late summer surveys in permanently marked 1-m2 quad-
rats (n = 36) located in the centre of each subplot. Maxi-
mum cover of each species from the two surveys was used
for all community analyses. Nomenclature and exotic spe-
cies status followsMohlenbrock (2002).
In addition to examining total cover and total richness,
individual species were categorized into the following spe-
cies groups: sown species, sown dominant grass species
(sum cover of A. gerardii, S. nutans and S. scoparium),
design pool species (i.e. species in DSPs 1, 2 and 3), volun-
teers (i.e. unsown species regardless of nativity), grasses,
forbs, legumes, exotics (i.e. non-native volunteers), C3
species and C4 species. Shannon’s diversity (H′) and Simp-
son’s evenness indices were calculated in DECODA.
Data analysis
We analysed the effects of seed source (cultivars or non-
cultivar seed), DSP (three pools), year (2006–2009) and
possible interactions among these three factors according
to a split-plot design, with the whole-plot factor assigned
according to a randomized complete block design and year
as the repeated measure using the mixed model procedure
in SAS (SAS Institute Inc., Cary, NC, US; 2002–2008).
Data were log-transformed prior to analyses. Degrees of
freedom were calculated using the Kenward–Roger
estimation recommended for repeated measures using
mixed-model analyses (Littell et al. 2006). The most
appropriate covariance structure for each model was
determined by comparing AIC, AICC and BIC model fit
statistics for unstructured, compound symmetry and first-
order autoregressive covariance structures. Least-squares
mean comparisons were used to test for differences among
sources, species design pools, year and interactions
between these factors (a = 0.05).
Non-metric multidimensional scaling (NMDS) ordina-
tion analysis was conducted on the species cover by plot
matrix using DECODA software. NMDS is considered to be
the most robust method of ordination (Minchin 1987).
Species cover values were standardized to unit maxima,
and species occurring in <15 plots (i.e. 10% of the plots
over all years) were excluded from the analysis. NMDS
was run using 100 random starting configurations on the
Bray–Curtis dissimilarity measure. The relationship of the
abundance of species to the retained ordination solution
was assessed by calculating species scores for each species
in the NMDS space. The species scores were calculated as
the weighted average of the abundance scores of the sam-
ples in which the species occurred in each dimension.
These weighted averages were used to plot species as
points in the NMDS ordination and are referred to as spe-
cies centroids because they show the centre of the species’
distribution with respect to the ordination axes. The rela-
tionship between species functional groups (Table 2) and
the ordination solution was tested by fitting vectors of
maximum correlation (Faith & Norris 1989; Kantvilas &
Minchin 1989). Vector significance was assessed following
permutation tests to generate correlation values. Vectors
significantly correlated with the ordination were retained
for plotting in ordination space relative to the ordination
centroid. Analysis of similarity (ANOSIM) was used to test
for the relationship between seed source, DSPs and block
and the ordination solution retained for interpretation.
Results
Cover, richness and diversity
Over the 4 yr of study, 126 species of vascular plant were
recorded in the plots, with a decline from 85 species in
2006 to 53 in 2009. In the first year of establishment, the
plots were dominated by agricultural weeds, including Pa
nicum dichotomum (frequency = 75% of plots, mean cover
= 16%), Bromus inermis (frequency = 69%, cover = 8.8%)
and Digitaria sanguinalis (frequency = 100%, cover =7.0%). The dominant grasses established at a high fre-
quency (occurrence in sampling plots) in 2006, albeit with
lower cover than expected (Andropogon gerardii:
frequency = 78%, cover = 1.1 ± 0.2%; Sorghastrum nutans:
frequency = 86%, cover = 0.9 ± 0.1%; Schizachyrium scop
arium: frequency = 92%, cover = 1.2 ± 0.1%). By 2009,
Journal of Vegetation Science4 Doi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science
Grassland assembly D.J. Gibson et al.
the plots were dominated by sown prairie species, includ-
ing the dominant grasses (S. nutans: frequency = 89%,
cover = 29.3 ± 3.0%; A. gerardii: frequency = 83.3%,
cover = 16.9 ± 2.2%; and S. scoparium: frequency = 42%,
cover = 3.1 ± 0.8%) and sown forbs (e.g., Rudbeckia hirta:
frequency = 50% despite being seeded into only 33% of
the plots, cover = 5.6 ± 1.4%; Solidago canadensis: fre-
quency = 64%, cover = 4.5 ± 1.0%; and Silphium inte-
grifolium: frequency = 36%, cover = 7.4 ± 2.0%). Some
agricultural weeds persisted in the plots into 2009, e.g. Stel
laria media (frequency = 75%, cover = 1.3 ± 0.2%) and
Trifolium repens (frequency = 61%, cover = 12.8 ± 3.8%).
There was no effect of seed source on the cover of any
plant groups examined. Change in cover was significant
over time (i.e. main effect of year) for most plant categories
(Table 2). Cover of the dominant grasses, with the excep-
tion of S. scoparium, increased from 2006 through 2009 in
all of the DSPs and as a group, as did total cover (Fig. 1a,b),
and cover of sown species, forbs, legumes, all C4 and all C3
species (Fig. 1c). In contrast, cover of volunteers decreased
by more than 50% between 2006 and 2009 (57.4 ± 5.4%
to 25.7 ± 5.1%) and the cover of exotics decreased from
18.2 ± 2.5% in 2006 to 13.4 ± 3.7% in 2007 (with no
significant change thereafter; Fig. 1c).
Table 2. Repeatedmeasures mixed-model analysis of species functional groups based on (a) cover, (b) richness and (c) diversity and evenness.
Functional group Covariance
structure
Source (S) Designed
Species
Pool (DSP)
Year (Y) S*DSP S*Y DSP*Y S*DSP*Y R-value
(a)
df 1,9 2,20 3,90 2,20 3,90 6,90 6,90
Total cover cs 0.00 0.06 42.52*** 0.02 0.70 0.42 0.78 0.62***
Sown spp. cs 0.76 1.59 480.63*** 0.50 1.83 0.60 1.24 0.86***
Grasses un 0.4930 3.6730* 18.2128*** 0.4030 2.2128 0.8735.9 0.4335.9 0.48***
Forbs un 0.0211.7 1.9917.7 41.1228*** 0.5617.7 1.1228 1.8335.9 0.6835.9 0.67***
Legumes un 0.5527.6 2.2327.6 30.4228*** 0.0927.6 0.4528 1.4535.9 0.4535.9 0.42***
C4 un 1.0727.8 0.2727.8 19.6228*** 0.4227.8 1.9928 1.5735.9 0.6335.9 0.72***
C3 ar(1) 0.079.11 0.1621 8.1881.1*** 0.6721 0.6581.1 1.2184.1 0.4584.1 0.50***
Pool species ar(1) 3.78† 0.329.5 123.0588.2*** 0.0525 2.5388.2† 4.9590.3** 0.7390.3 0.62***
Volunteers un 0.4611.9 1.1215.1 17.1928*** 1.0415.1 0.6828 0.5835.9 0.2435.9 0.59***
Annuals un 0.2430 3.2830† 49.8828*** 0.3330 0.6228 3.0635.9* 0.9335.9 0.49***
Exotics un 2.210.1 2.378.5 6.5428** 1.408.5 1.7928 0.3435.9 0.5735.9 0.51***
Andropogon gerardii un 0.9518.3 1.2925.7 39.4328*** 1.1225.7 0.3728 1.0435.9 1.2435.9 –
Sorghastrum nutans un 0.2030 0.4330 56.2628*** 0.2430 1.7628 0.6135.9 0.7035.9 –
Schizachyrium scoparium un 0.6324.2 0.1821.5 7.5628** 1.2921.5 0.1528 0.7135.9 0.4135.9 –
Total dominant grasses un 0.6019.6 0.2719.7 67.7528*** 0.7019.7 2.4728† 1.1435.9 1.3635.9 0.83***
(b)
df 1,9 2,20 3,90 2,20 3,90 6,90 6,90
Total richness cs 0.17 7.17** 121.33*** 1.73 0.48 0.62 0.33 0.80***
Number sown species ar(1) 1.8836.3 5.5236.3** 4.2987.1** 1.7536.3 0.8187.1 0.4589.9 1.0889.9 0.27**
Grasses un 3.7720.2† 2.2922.7 88.028*** 0.9022.7 2.0228 0.5535.9 0.4235.9 0.72***
Forbs cs 1.91 14.95*** 91.04*** 0.64 2.72* 1.02 0.41 0.75***
Legumes ar(1) 0.4636.4 5.7536.4** 3.4590.3** 1.0536.4 3.5890.3* 0.7191.9 0.3791.9 0.41***
C4 cs 2.71 3.91* 34.57*** 4.75* 3.75* 0.69 0.69 0.65***
C3 ar(1) 1.109.01 12.3636.6*** 94.1683.9*** 0.4936.6 2.0883.9 1.0088.1 0.4388.1 0.77***
Pool species ar(1) 3.1432.7† 5.532.7** 2.6887.4* 0.6232.7 1.5087.4 0.8889.2 0.7589.2 0.30**
Volunteers un 0.479.51 1.1625.6 274.2928*** 0.9525.6 0.4128 1.6735.9 0.7435.9 0.87***
Annuals cs 0.63 9.17** 160.13*** 1.98 0.19 1.74 0.91 0.87***
Exotics un 4.1215.3† 0.0917.5 113.5628*** 1.6517.5 0.1025 0.5035.9 2.2735.9
† 0.70***
(c)
df 1,9 2.20 3,90 2,20 3,90 6,90 6,90
H′ un 0.0310 1.2220.3 34.6528*** 0.3120.3 0.4428 0.3835.9 0.8635.9 0.55***
Evenness cs 0.00 0.22 0.76 0.12 0.25 0.82 0.12 0.06
Values are F-statistics for species functional groups, and R correlation values for vector analyses of species functional groups with the NMDS ordination.†P � 0.1, *P � 0.05, **P � 0.01, ***P � 0.001. Covariance structure, cs = compound symmetry, un = unstructured, and ar(1) = first-order autore-
gressive. Degrees of freedom (df) calculated according to the Kenwood–Roger (KR) correction, which uses an invariant denominator df for each effect with
a compound symmetry covariance structure, but is variable for un and ar(1) covariance structures (shown as subscript following F-statistics).
Journal of Vegetation ScienceDoi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science 5
D.J. Gibson et al. Grassland assembly
Designed species pools differentially affected total grass
cover across all years (Table 2a). There was significantly
higher cover of grasses in DSP 1 (50.2 ± 3.9%) than DSP 2
(38.7 ± 3.4%) over all years resulting from the establish-
ment success of S. nutans and Elymus canadensis seeded in
DSP 1 (Cover in 2009, DSP 1 = 32.3 ± 5.8% and
3.3 ± 1.5%, respectively, DSP 2 = 26.3 ± 5.2 and
0.2 ± 0.2%, DSP 3 = 27.1 ± 5.4% and 0.3 ± 0.3%). Cover
of designed pool species and annuals exhibited an interac-
tion with year resulting from differences in cover of
designed pool species in 2008 and 2009, respectively,
which did not occur in other years. More specifically, in
2008 the cover of designed pool species was significantly
higher in DSP 3 (54.4 ± 5.6%) compared to DSP 1
(36.7 ± 4.4%); DSP 2 was intermediate at 40.4 ± 4.9%. In
2009, the cover of annual species was highest in DSP 1
(17.1 ± 7.6%) compared with both DSP 2 (2.8 ± 0.7%)
and DSP 3 (1.9 ± 0.4%).
Source of dominant grasses did not influence total rich-
ness, but weakly (0.10 > P > 0.05) affected the richness of
grasses, designed pool species and exotics over all years
(Table 2b) and richness of forbs, legumes and C4 grasses in
some years. Total grass richness was higher with the non-
cultivar seed source (5.3 ± 0.3 grass species�m�2) com-
pared to growing with the cultivar seed source
(4.7 ± 0.3 spp.�m�2), whereas richness of designed pool
species was lower in the presence of non-cultivar grasses
(5.9 ± 0.2 spp. m�2) than cultivars (6.7 ± 0.2 spp. m�2).
The number of exotic species was also marginally affected
by seed source, with fewer (2.9 ± 0.3 spp. m�2) exotic
species in plots with cultivars than the non-cultivar seed
sources (3.4 ± 0.3 species m�2). The number of legume,
forb and C4 species exhibited an interaction between
source and year resulting from different levels of richness
in association with cultivar sources in only 2007, the sec-
ond year of community assembly. In 2007, legume rich-
ness and forb richness were higher in the presence of grass
cultivars than non-cultivars (Fig. 3a,b). In contrast, the
number of C4 species was higher in plots seeded with a
non-cultivar source compared with a cultivar seed source
in this year (Fig. 2c).
Richness of C3 species, exotic and volunteer species
was strongly and only affected by year (Table 2b). Total
species richness, number of volunteers and annuals
decreased each year (Fig. 2d,e,g), and the number of
exotics decreased from 6.3 ± 0.3 species m�2 in 2006 to
2.2 ± 0.3 in 2007 (with no significant change thereaf-
ter; Fig. 2f). Richness of sown species, grasses, C3
species and designed pool species were affected by year
(C3 species and grasses decreasing; Fig. 3i,k), but also
by either seed source of dominant grasses or DSP (see
below).
Designed species pool strongly affected total richness
and richness of all species groups except the number of
grasses, exotics and volunteers (Table 2b). The total
number of species, number of sown species, forbs,
legumes, pool species and C3 species was highest in DSP 2
(Fig. 3). The number of annuals was highest in DSP 1.
There was an interaction between DSP and seed source
on the number of C4 species, with the lowest number of
Cov
er (%
)
0
10
20
30
40
50
Andropogon gerardiiSorghastrum nutansSchizachyrium scopariumTotal Dominant grasses
Cov
er (%
)
0
20
40
60
80
100
120
140Total cover Sown Designed Pool Volunteers
A
AB
C
DCB
D C
BA
B
A
A
B
A
B
AA
A
A
B B
B
C
C
D
D
C
Year2006 2007 2008 2009
Cov
er (%
)
0
20
40
60
80
ForbsLegumes C3 C4 Annuals Exotics
B
BA
A
C
BB
A
A A
BB
A
B BB
C B
AA
(a)
(b)
(c)
Fig. 1. Temporal changes in percentage cover (mean ± SE) of (a) all
species, sown, designed species pool (pool) and volunteer species, (b) the
dominant grasses collectively and by species, and (c) species groups from
2006 to 2009, corresponding to the first through fourth year of community
establishment. Means within a cover group accompanied by the same
letter were not significantly different (a = 0.05; letters not shown for cover
of designed pool species and annuals because these species groups
exhibited an interaction between time and designed pool (Table 2a).
Journal of Vegetation Science6 Doi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science
Grassland assembly D.J. Gibson et al.
these species in DSP 2 seeded with cultivar compared
with non-cultivar seed (Fig. 3h).
Diversity (H′) decreased through time but not differen-
tially among DSPs or seed sources of dominant grasses
(Table 2c). In 2006, 2007, 2008 and 2009 H′ was
2.62 ± 0.09, 2.21 ± 0.06, 2.11 ± 0.05 and 1.84 ± 0.04,
respectively, and significantly different between all years
except 2007 and 2008. Evenness was unrelated to DSP,
source or time. Average evenness of the restored commu-
nity was 0.37 ± 0.01.
Community composition
A three-dimensional NMDS ordination solution was
retained for interpretation of community change over time
and in the response to the biotic factors that were manipu-
lated (stress = 0.18). The distribution of plots in the 3-D
solution was related to year of sampling (ANOSIM
R = 0.63, P < 0.0001; Fig. 4a), DSP (R = 0.45, P < 0.0001)
and weakly to sample block (R = 0.08, P < 0.0001), but
not seed source of the dominant species (R = 0.01,
P = 0.25). Sample plots showed a trajectory through ordi-
nation space with a clear separation of DSPs (Fig. 4b). The
change in composition from 1 yr to the next decreased
from 2006 through 2009 (decreasing distance between
plots from 1 yr to the next in multivariate space; Fig. 4a).
Species centroids (data not shown) showed that the plots
were initially dominated by a high frequency of volunteer
species, including Poa annua (33% of plots in 2006, 2% in
2009) and Veronica peregrina (34% and 0%, respectively),
Year
2
4
6
8
10
0
4
8
12
16
20
24
4
8
12
16
20
24
0
2
4
6
8
10
(a) Legumes
0
2
4
6
8
10CultivarNon-cultivar
*
(b) Forbs
4
8
12
16
20
24 CultivarNon-cultivar*
(c) C4 species
Year2006 2007 2008 2009
No
spec
ies
. m–2
No
spec
ies
. m–2
No
spec
ies
. m–2
No species . m
–2N
o species . m–2
No species . m
–2N
o species . m–2
0
2
4
6
8
10 CultivarNon-cultivar
*
A
B
CD
(g) Annuals
(f) Exotics
(e) Volunteers
(d) Total richness
A
A
A
B BB
B
B
C
C
D
D
0
4
8
12
16
20
24(h) Sown species
AA BBA
4
8
12
16
20
24(i) C3 speciesA
BC
D
0
2
4
6
8
10(j) DSP species
AB A A B
Year2006 2007 2008 2009 2006 2007 2008 2009
2
4
6
8
10(k) Grasses
B B
C
A
A B AB AB
AB
CD
A
BB C
Fig. 2. Richness (mean ± SE) of (a) legumes, (b) forbs and (c) all C4 species from 2006 to 2009 by dominant grass seed source (to illustrate the source x
time interaction for these groups), (d) total richness and richness of (e) volunteers (i.e. unsown plant species), (f) exotics (i.e. non-native volunteer species),
(g) annuals, (h) sown species, (i) C3 species, (j) design pool species, and (k) grasses by year. Cultivar vs non-cultivar patterns for each year were the same
for (d–k) (i.e. non-significant interaction between seed source and year; year effect only; Table 2b). Asterisks indicate significant differences between
cultivar and non-cultivar seed sources within a year (a = 0.05) (a–c). Means accompanied by the same letter were not significantly different (a = 0.05).
Journal of Vegetation ScienceDoi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science 7
D.J. Gibson et al. Grassland assembly
but through time became increasingly dominated by the
sown species, including the three dominant grasses
(Fig. 1b). The most frequently occurring species character-
izing the plots in each designed pool were increasingly
those included in the DSP mix. By 2009, the designed pool
species with the highest cover were Baptisia alba var macro-
phylla (19.5 ± 6.6% cover), Chamaecrista fasciculata (14.0 ±7.8%) and Ratibida pinnata (14.0 ± 4.5%) in DSP 1, Silphi-
um integrifolium (19.8 ± 3.6%) and Desmanthus illinoensis
(7.9 ± 3.5%) in DSP 2, and Rudbeckia hirta (14.4 ± 2.7%)
and Silphium laciniatum (12.3 ± 1.3%) in DSP 3. Vector
analysis showed that species functional groups were signif-
icantly related to the ordination solution (Table 2; all
P < 0.01). A plot of these vectors (not shown) reflects a
contrast in composition from plots with the highest num-
ber of species overall, and in most species functional
groups, including volunteer species, to plots with fewer
species but higher cover in all groups. The vectors indicated
that while sown species increased in both richness and
cover through time, volunteers decreased in richness and
cover. Evenness was unrelated to the distribution of plots
within the ordination (vector correlation R = 0.06,
P = 0.91).
Discussion
Counter to other studies that have supported the ‘extended
phenotype hypothesis’ (Whitham et al. 2003; Bailey et al.
2009), but consistent with Wilsey (2010), we found no
strong evidence in support of our hypotheses that popula-
tion source of dominant species consistently affected plant
community composition (H1 and H2, this study) or ecosys-
tem processes (S.G. Baer, D.J. Gibson, A.M. Lambert, L.K.
Reed, R.E. Campbell, R.P. Klopf, J. Willand & B.Wodika in
(a) Total richness
No
spec
ies
. m–2
0
5
10
15
20
25
B A B(b) Pool species
0
5
10
15
20
25
BA B
(d) Sown species
No species . m
–2
5
10
15
20
BA
B
(c) Forb species
0
5
10
15
20
BA
B
(e) C3 species
0
5
10
15
20
(f) Legume species
0
5
10
15
20
BA
B
A A B
(g) Annual species
0
2
4
6
8
BBA
Designed species pool1 2 3 1 2 3
0
2
4
6
8CultivarNon-cultivar
AA AAAB B
(h) C4 species
Fig. 3. Richness (mean ± SE) of species life-form groups significantly affected by designed species pool (a–g), and (h) the interaction between designed
species pool and seed source of dominant species. Mean values with the same letter were not significantly different (a = 0.05).
Journal of Vegetation Science8 Doi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science
Grassland assembly D.J. Gibson et al.
review) during development of a diverse grassland assem-
blage. The use of DSPs reinforced the generality of the role
of this anticipated biotic filter in structuring assembling
plant communities. Dominant species population source
had only a weak effect across all DSPs on the richness of
some species groups. An overwhelmingly consistent effect
of DSP with no divergence through time in response to
population source on developing community composition
through 4 yr refuted our third hypothesis. This finding
supports models of community assembly in which individ-
ual species or a particular mix of species are given primacy,
allowing species niche partitioning or facilitative effects to
drive the development of species associations (Fridley
2001).
As expected, we observed rapid changes in community
composition during the early stages of community assem-
bly of a prairie restoration (Cottam & Wilson 1966; Camill
et al. 2004). In contrast to some restorations (Kindscher &
Tieszen 1998), we observed a decrease in diversity as the
sown, native prairie species came to dominate the commu-
nity at the expense of the early volunteer colonists. Most
importantly, the weak effect of dominant species seed
source that we observed during these first 4 yr of commu-
nity assembly does not support the commonly held dogma
regarding the importance of the seed source on commu-
nity assembly in grassland restorations, discouraging the
use of cultivars (Lesica & Allendorf 1999; Gustafson et al.
2005). Concern regarding appropriate germplasm for rein-
troduction into communities is largely based on the
genetic/population principles that founder effects deter-
mine genetic variability among colonizing species, particu-
larly dominant matrix species (Montalvo et al. 1997;
Hufford & Mazer 2003). However, this concern has not
been tested, as far as we know, with species in a restoration
and in a community common garden setting with the pres-
ence of inter-specific interactions. Further, the hierarchical
effects of different genetic structure on community assem-
bly and ecosystem processes in multi-species settings is
poorly understood. We contrasted the effects of seeding
with locally adapted cultivars vs locally collected non-culti-
var seed, which we have documented to contain a differ-
ent genetic structure (Gustafson et al. 2004b). Cultivars
have been selected for plant traits associated with ‘vigour’
and we have documented enhanced traits in cultivars
(i.e. physiological performance, root architecture and
below-ground net primary productivity) relative to non-
cultivars of the dominant prairie grasses in a common gar-
den setting (Klopf & Baer 2011; Lambert et al. 2011).
Despite trait differences documented between the popula-
tion sources, the outcome of any inter-specific interactions
between the dominant grasses and subordinate species did
not lead to any consistent differences in species composition
during the first 4 yr of community assembly. The occur-
rence of such an effectwas perhaps unlikely during the first
2 yr of community assembly because the dominant grasses
were not the most abundant species (i.e. dominant grasses
cover was <20%). However, in the third and fourth year of
restoration, native grass cover exceeded 40%, principally
due to high cover of S. nutans. Moreover, in an additional
field experiment in Illinois, seed source effects of dominant
grasses did not scale to affect species diversity (R.P. Klopf, S.
G. Baer, & D.J. Gibson unpubl.). Dynamic patterns of com-
munity change during the early phases of community
assembly are important to characterize as they may persist
much later in succession (Ejrnæs et al. 2006; Roscher et al.
2009) in the absence of disturbance (Bartha et al. 2003).
These early successional transient prairie communities are
important in the context of landscape diversity.
The different DSPs led to differences in overall commu-
nity composition (indicated by the multivariate analysis)
demonstrating that sown species influence local species
–1.5
–1.0
–0.5
0.0
0.5
1.0
1.5
–1.0
–0.50.0
0.51.0
1.5
–1.5–1.0
–0.50.00.51.0
NM
DS
3
NMDS 1
NMDS 2
2006200720082009
(a) Year of sampling
–1.5
–1.0
–0.5
0.0
0.5
1.0
1.5
–1.0
–0.50.0
0.51.0
1.5
–1.5–1.0
–0.50.00.51.0
NM
DS
3
NMDS 1
NMDS 2
123
(b) Designed species pools
Fig. 4. Three-dimensional NMDS plots (stress = 0.18) with sample plots
labelled by (a) year of sampling and (b) designed species pool.
Journal of Vegetation ScienceDoi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science 9
D.J. Gibson et al. Grassland assembly
composition, which has important implications for restor-
ing plant community heterogeneity across a landscape that
can be fraught with difficulty in restoration (Baer et al.
2005). Although the dominant grasses characterize the
system generally as tallgrass prairie, subordinate species
mixtures represented in our DSPs characterize local inter-
actions and local heterogeneity. The DSPs also differen-
tially influenced richness of several functional groups
(i.e. forbs, legumes, annuals, C3 and C4 species), despite
that the cover of each group was not affected by the DSPs.
The implications are that ‘species matter’ in this system, or
at least in the mixtures that we sowed, supporting argu-
ments about species function (Lawton 1994; Giller &
O’Donovan 2007). Even though species mixtures may
contain the same initial representation of functional
groups, as they did in our DSPs, other trait differences
among species can affect community composition (Wein-
berg 1984; Helsen et al. 2012) and drive ecosystem func-
tion (Isbell et al. 2011). In our study, four legume species,
for example, were seeded equally into each DSP, but after
4 yr came to be abundant in only two of the three pools,
likely resulting from non-equivalency in establishment
among this group. In contrast, one or more composite spe-
cies characterized each of the three DSPs after 4 yr, reflect-
ing similar patterns of colonization and perhaps increased
functional equivalency among these species (Ejrnæs et al.
2006; Harpole & Tilman 2006).
Although DSP effects on richness were predominant, a
small number of these effects did occur in the context of
the seed source of the dominant grasses (e.g. the DSP by
seed source interaction), albeit not every year. These inter-
actions are consistent with the operation of a hierarchical
species filter (Hobbs & Norton 2004; Gibson et al. 2012),
where species sorting occurs through multiple abiotic and
biotic filters. In this case, our DSPs were representative of
initial abiotic filtering via local seed pool limitations, with
later biotic filtering imposed in a weak manner by the seed
source (cultivar or non-cultivar source) of the dominant
grasses. Longer-term study of species composition is
needed to determine if weak seed source effects become
more or even less prevalent as the communities mature.
These results have relevance for practitioners and land
managers because there is an on-going debate regarding
the source of seed to use in restoration (Hufford & Mazer
2003; Gustafson et al. 2005). General concerns are that
cultivar seed sources may introduce non-local germplasm
into native communities and cause genetic swamping. We
suggest that these concerns may needmore context depen-
dency, particularly if native communities are nearly non-
existent on the landscape (e.g. grassland widely converted
to agriculture). In many instances, we do not know ‘how
local is local’ or how well suited (locally adapted) remain-
ing populations are for use in restoration (McKay et al.
2005). Less is understood about the hierarchical ecological
consequences of the genetic structure of sources used in
restoration. If seed sources of dominant species are not well
adapted to the local environment, then increased estab-
lishment success and growth of subordinate species should
promote diversity (Mijnsbrugee et al. 2010). This study
suggests that during initial community assembly, the com-
position of the subordinate species seed mixture is more
important than the seed source of the dominant species,
particularly in diverse restored communities.
Acknowledgements
Funding for this research was provided by the National Sci-
ence Foundation (DEB 0516429). We thank Ryan E.
Campbell, Allison M. Lambert, Elizabeth M. Bach and
many undergraduate student workers for their assistance
with data collection, and the Southern Illinois University
Carbondale Agronomy Research Center for site prepara-
tion andmaintenance.
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Supporting Information
Additional supporting information may be found in the
online version of this article:
Appendix S1. The split-plot design used to test
whether intraspecific functional variation of dominant
species affects community composition and structure over
the first 4 yr of community assembly. Six whole plots were
randomly assigned to cultivar (n = 3) and non-cultivar
(n = 3) source of three dominant grasses within each of
two adjacent agricultural fields (blocks). Three unique
designed species pools (A–C) of subordinate prairie species
were randomly assigned to 5 9 5 m subplots within each
whole-plot. This figure also appears in Baer et al. (in
review).
Please note: Wiley-Blackwell are not responsible for
the content or functionality of any supporting materials
supplied by the authors. Any queries (other than missing
material) should be directed to the corresponding author
for the article.
Journal of Vegetation Science12 Doi: 10.1111/j.1654-1103.2012.01475.x© 2012 International Association for Vegetation Science
Grassland assembly D.J. Gibson et al.