Influence of richness and seeding density on invasion resistance in experimental tallgrass prairie...

RESEARCH ARTICLE

Influence of Richness and Seeding Densityon Invasion Resistance in ExperimentalTaligrass Prairie Restorations

Kristine T. Nemec, Craig R. Allen, Christopher f. Heizer and David A. Wedin

ABSTRACT

In recent years, agricultural producers and non-governmental organizations and agencies have restored thousands ofhectares of cropland to grassland in the Great Plains of the United States. However, little is known about the relation-ships between richness and seeding density in these restorations and resistance to invasive plant species. We assessedthe effects of richness and seeding density on resistance to invasive and other unseeded plant species in experimentaltaligrass prairie plots in central Nebraska. In 2006, twenty-four 55 m x 55 m plots were planted with six replicates ineach of four treatments: high richness (97 species typically planted by The Nature Conservancy), at low and high seedingdensities, and low richness (15 species representing a typical Conservation Reserve Program mix, CP25), at low and highseeding densities. There was a significant negative relationship between richness and basal cover of unseeded perennialforbs/legumes and unseeded perennial/annual grasses, abundance of bull thistle (Cirsium vulgäre), and the number ofinflorescences removed from smooth brome (Bromus inermis) transplants. Invasion resistance may have been higher inthe high richness treatments because of the characteristics of the dominant species in these plots or because of greaterinterspecific competition for limiting resources among forbs/legumes with neighboring plants belonging to the samefunctional group. Seeding density was not important in affecting invasion resistance, except in the cover of unseededgrasses. Increasing seed mix richness may be more effective than increasing the seeding density for decreasing invasionby unseeded perennial species, bull thistle, and smooth brome.

Keywords: bull thistle, grasslands, invasibility, sweet clover

Invasive, normative plant speciescan profoundly impact ecosystems

and communities, altering ecosys-tem structure and function, decreas-ing native plant species richness andphylogenetic diversity, and disrupt-ing reproductive mutualisms of nativeplant species (Pysek and Richardson2010). These impacts can reduce acommunity's resilience, or its capac-ity to absorb disturbance withoutfundamentally changing its essentialstructure and functions (Holling1973, Folke et al. 2002). Anotherconcept applied to a community'sresponse to disturbance is resistance,or the ease or difficulty of changing a

Ecological Restoration Vol. 31, No. 2, 201 3

ISSN 1522-4740 E-ISSN 1543-4079

©2013 by the Board of Regents of the

University of Wisconsin System.

system (Walker et al. 2004). In termsof invasions, resistance refers to thebiotic and abiotic factors that enhancea community's capacity to limit thespread of an invading species (Levineet al. 2004, D'Antonio and Chambers2006). Understanding the factors thatincrease community resistance to inva-sion is important if land managersand conservationists are to reduce thespread and impact of invasive plantspecies in natural and restored areas.

In recent decades, the role of plantspecies richness in resisting invasiveplant species has been a major focusof invasion research. Experimentaland observational studies conductedat small scales (< 20 m'^), particularlyin North American grasslands, haveprimarily found negative relation-ships betv^een plant species richnessand invasibility (Naeem et al. 2000,

Symstad 2000, Dukes 2002, Fargioneand Tilman 2005), while observa-tional studies conducted at large scales(> 1 km^) in a variety of ecosystemsworldwide have mostly reported posi-tive relationships between plant spe-cies richness and invasibility (Stohl-gren et al. 1999, White and Houlahan2007). The contradictory resultsbetween fine- and broad-scale stud-ies may be explained by the differentroles of environmental heterogeneityand biotic interactions in structuringplant communities across spatial scales(Fridley et al. 2007).

Because plant species that arrivefirst at a disturbed site can stronglyinfiuence the trajectory of succession,"priority effects" are an importantconsideration in establishing seededspecies that may reduce the growth oflater-arriving unseeded, or invading.

168 June 2013 ECOLOGICAL RESTORATION 31.2

species (Körner et al. 2008, Martinand Wilsey 2012). Although varyinglevels of seeding density can affect theestablishment success and plant den-sity of seeded species (Burton et al.2006, Frances et al. 2010), the role ofseeding density has received less atten-tion than richness in grassland inva-sion resistance research. Martin (2006)examined the effect of four seedingdensity treatments on prairie establish-ment. The number of unseeded stemsdid not show a strong relationshipwith seeding density and the numberof unseeded nonnative species did notvary significantly among treatments.Dickson and Busby (2009) studied theeffect of varied grass density on forbestablishment. The percent cover ofunseeded species declined over threeyears across all treatments and in twoof three years they found no signifi-cant treatment effect on the cover ofunseeded species. Peters and Schottler(2011) tested five seeding rates andaltered the ratios of grass to forb seedto study prairie establishment, find-ing unseeded species density to havea significant negative correlation toforb seeding rate.

Although all of these studies incor-porated seeding densities that are typi-cally used in North American grass-land seed mixes, to our knowledgeno study has manipulated the seedingrates of low and high richness seedmixes that are often used by practitio-ners within a particular area to test therelative effects of seeding density oninvasion resistance. Although studiesconducted under realistic restorationconditions have found that increasingthe richness of grassland seed mixesis often associated with increasedinvasive plant resistance (Young etal. 2009, Institute for Applied Ecol-ogy 2011, Oakley and Knox 2013),none have compared the resistanceof The Nature Conservancy (TNC)and Natural Resources Conserva-tion Service (NRCS) ConservationReserve Program (CRP) seed mixes,two common sources of seed mixesfor restoration efforts in the GreatPlains. In addition, few studies have

established research plots at an inter-mediate scale (between 20 m^ and1 km^). Research that incorporatescommonly used seeding methodscan provide valuable information forimproving the success of restorationprojects and advance knowledge aboutthe relationship between biodiversityand ecosystem functioning and ser-vices (Symstad 2008).

This study compared invasion resis-tance of 55 m X 55 rn research plotsseeded with a low richness tallgrassprairie seed mix commonly used incentral Nebraska (15 species represent-ing a typical Conservation ReserveProgram mix, the CP25 mix), at lowand high seeding densities, and ahigh richness mix (97 species typi-cally planted by The Nature Conser-vancy), at low and high seeding densi-ties. We assessed the degree to whichthe four treatments resisted crop fieldweeds and three non-native plants ofdifferent reproductive strategies: bullthistle {Cirsium vulgäre) and whiteand yellow sweet clover {Melilotusofficinalis), which reproduce by seed;and smooth brome {Bromus inermis),which reproduces by both seed andrhizomes (Stubbendieck et al. 2003).These species were selected becausethey are aggressive and commonlyencountered in Nebraska grasslandrestorations. All are undesirable innatural areas because they can becomewidespread and reduce the cover andgrowth of native plant species (For-cella and Randall 1994, Wolf et al.2003, Vinton and Coergen 2006,Otfinowski et al. 2009, Dillemuthet al. 2009, Van Riper and Larson2009). In addition, the sharp spines ofbull thistle can interfere with livestockgrazing (Forcella and Wood 1986) andsweet clover may facilitate the growthof other invasive plant species (Wolf etal. 2003, Van Riper and Larson 2009).

We tested four null hypotheses:1) the basal cover of seeded nativeplant species will not differ among thedifferent density and richness treat-ments; 2) the basal cover of unseededplant species will not differ amongthe different density and richness

treatments; 3) the abundance of bullthistle and sweet clover will not differamong the different density and rich-ness treatments; and 4) the abundanceof smooth brome tillers that havespread from transplants and seed andthe number of infiorescences removedfrom transplants will not differ amongthe different density and richnesstreatments.

Methods

Study AreaThe study area lies within the CentralPlatte River ecosystem, which includesthe Platte River channel and floodplainin central Nebraska (NGPC 2005).The region has a continental climate,with warm, wet summers and cold,dry winters. Mean annual air tem-perature is 10.4° C and mean annualprecipitation is 63.9 cm (High PlainsRegional Climate Center 2010).

The study site is located approxi-mately 10 km south of Wood River,Nebraska (Hall County; N 40M4'4l ",W 98°35' 11 ") on a 7.3 ha field ownedby TNC. Soils at the site are of loamyalluvium or sandy alluvium parentmaterial (NRCS 2010). The site is bor-dered to the south and east by countyroads and TNC prairie restorations,to the west by a cornfield that wasseeded with experimental prairie resto-ration plots in the spring of 2010, andto the north by trees and the PlatteRiver. The site was under cultivationin a corn-soybean rotation for decadesprior to the experiment, during whichit was managed with conventionaltillage and chemical inputs.

Treatments andExperimental DesignIn late March and early April 2006,the field was cultivated and dividedinto twenty-four 0.30 ha plots (55 m x55m). The plots were seeded from anall-terrain vehicle (ATV) and a JohnDeere drop spreader according to a 2 x2 factorial design, in which two levelsof plant richness (low and high) wereapplied using two different seeding

June 2013 ECOLOGICAL RESTORATION 31:2 t' 169

H1

C2

H2

C1

CI

H1

C2

H2

H2

C1

H1

C2

C2

H2

C1

H1

H1

C2

H2

C1

C1

H1

C2

H2

ÎN

Figure 1. Layout of treatments applied to 55 m x 55 m plots in thecentral Platte River floodplain, Nebraska, tJSA (CI = low richness seedmix/low seeding rate; C2 = low richness seed mix/high seeding rate;HI = high richness seed mix/low seeding rate; H2 = high richness seedmix/high seeding rate).

densities (low and high seeding rates).The experiment was arranged in a sys-tematic design, with six columns run-ning west to east across the field andeach column containing four plotsassigned to the four treatments (Figure1). The treatments were applied sys-tematically instead of randomly inorder to facilitate seeding with thedrop spreader. Treatments consistedof: Cl) a low richness CRP tallgrassprairie seed mix (CP25 mix, 15 spe-cies) used by the NRCS seeded at halfthe recommended seeding rate (lowrichness/low rate mix: grass, 148 purelive seeds {PLS)lm^; forbs, 16 PLS/m^); C2) the CP25 mix applied at therecommended seeding rate (low rich-ness/high rate mix: grass, 297 PLS/m^;forbs, 31 PLS/m^); HI) a high rich-ness tallgrass prairie mix typically usedby the local T"NC (97 species) seededwith a seeding rate typical for TNCgrassland restorations in the region(high richness/low rate: grass, 129PLS/m2; forbs, 43 PLS/m^); H2) theT N C mix applied at twice the recom-mended seeding rate (high richness/high rate: grass, 258 PLS/m^; forbs,86 PLS/m2) (Appendices A and B).The second and fourth treatments areat half and double NRCS or TNCnormal seeding rates, respectively,because the NRCS normally recom-mends rates that are about twice ashigh as TNC uses.

We designed the CP25 seed mix(Table Al) with the Grand Island,

Nebraska NRCS District Conser-vationist. We purchased grass seedused in the mix from Arrow Seed inBroken Bow, Nebraska and forb seedwas locally harvested from the PlatteRiver area. The Nature Conservancyhigh-richness seed mix (Table A2) washarvested from local prairies.

With the exception of smoothbrome, which was added in 2008, weallowed non-native species to natu-rally establish. All of the plots wereburned on March 20, 2008. In July2008, yarrow {Achillea millefiolium)and Maximilian sunfiower {Helian-thus maximiliani) that had invadedinto the edges of plots where they hadnot been seeded were sprayed withglyphosate and killed, in order toreduce the edge effect on the spreadof these aggressive species. We clippedthe inflorescences of smooth bromethat had invaded the south row ofplots and two plots in the northwest-ern corner of the field in order to limitthe spread of smooth brome that hadnot been experimentally introducedinto the plots. Following that effort,no plants were intentionally killed ormanipulated. Vegetation growing inunseeded 2 m lanes between the plotswas mowed several times during thegrowing season.

Plant Community Composition

Within each plot, we established five55 m transects located 9.1 m apart.We assessed plant species composition

along three of the transects withineach plot, the middle transect andthe two end transects, in mid- to lateJune 2007-2009. We used the line-intercept transect method because itis an efficient method of collectingcover and species richness (Bonham1989). Starting at the end of eachtransect, we stretched a measuringtape to a length of one meter close tothe ground. The transect was brokenup into these smaller, one-meter seg-ments, or "sub-transects," to keep themeasuring tape from sagging in thewind. We measured the basal coverof any plant touching the top edgeof the measuring tape by recordingthe distance that the plant coveredalong the tape to the nearest 0.2 cm(Elzinga et al. 1998). Measurementswere taken along every 12'̂ '' meter andat the opposite end of the transect fora total of six, one-meter subtransectsalong the transect (data were recordedat 0, 12, 24, 36, 48, and 55 meters).

Bull Thistle and Sweet CloverWe assessed the abundance of bullthistle and sweet clover when thesespecies were flowering in Septemberand October of 2006-2009 by walk-ing belt transects along the five 55 mtransects within each plot (Grant etal. 2004). We placed a 3 m pole withflagging tape over the rebar on one endof the transect to ensure a straight linewas walked. We recorded the numberof bull thistle and sweet clover plantsobserved within 3 meters on bothsides of the transect.

Smooth BromeIn 2008, we added smooth bromeplants and seeds to each plot in orderto compare its spread from rhizomesand establishment from seeds amongthe four treatments. We added plantsto the plots on April 13, 2008. Weused a shovel to remove blocks ofsmooth brome approximately 13 cm^in surface area and 5 cm deep fromthe ditch on the southern edge of thestudy site. We transplanted four plants


along the middle of the center transectin each plot, with each plant placed3 m apart to form the corners of asquare. We marked the east edge ofeach transplant with a fiagged 1 mstake.

We clipped the inñorescences ofeach planted smooth brome on June29, 2008, June 24, 2009, and June27-June 29, 2010 in order to preventthese plants from dropping seeds andto ensure they would spread only byrhizomes. We recorded the number ofinflorescences clipped from each plantas a measure of the vigor of the plant.On June 27-29, 2010, we placed a1 m'̂ quadrat frame in each of thefour cardinal directions around eachsmooth brome transplant or seedinglocation and recorded the number oftillers in each 1 m'̂ area encompassedby the frame to assess spread of theplant from either rhizomes or seedand rhizomes. We removed the inflo-rescences of all smooth brome plantswithin the quadrat frame and within10 meters of the area.

We obtained smooth brome seedfrom Stock Seed Farms in Murdock,Nebraska. We added seed to the plotsin late April, the time of year whenfarfners often seed smooth brome,and in early September, when smoothbrome plants naturally drop theirseed (Bruce Anderson, University ofNebraska-Lincoln, pers. comm.), todetermine if timing of seed additionaffected the species' invasiveness. OnApril 30, 2008, we added seed to thenorthwest and southeast corner ofeach plot by walking 12 paces in adiagonal line from the corner of theplot. We broadcasted seeds over a 1 m'̂area using a seeding rate of 120 PLS/m^ and lightly raked in the seed. OnSeptember 7, 2008, we added seed tothe northeast and southwest corner ofeach plot using the same seeding rateand methods. In both months, werecorded each seeding location with aTrimble GeoXT handheld GPS unitwith submeter accuracy.

Statistical Analysis

Plots were the experimental units inthis study. For analyzing plant com-munity composition, we groupedplant species recorded along linetransects in each plot by summingthe basal cover of plants placed intoeight classes: all seeded species, allunseeded species, seeded perennial/annual forbs/legumes, seeded peren-nial grasses, unseeded perennial forbs/legumes, unseeded perennial/annualgrasses, unseeded annual/biennialforbs/legumes, and invasive species,as defined by the Nebraska InvasiveSpecies Council (2011) (Table A3).We used these categories because wewere interested in identifying the rela-tive success of different growth formsin becoming established and in limit-ing the spread of unseeded species.The invasive species category was usedbecause we were particularly inter-ested in the ability of the treatmentsto reduce establishment of plant spe-cies that are considered invasive inNebraska compared to less aggressiveunseeded species. We tested normalityin the response variables (basal coverof plant species aggregated in eachfunctional group, number of bull this-tle or sweet clover plants, or smoothbrome inflorescences or tillers) withthe Kolmogorov-Smirnov normalitytest (PROC UNIVARIATE, SAS Ver-sion 9.2, SAS Institute 2007). Becausethe residuals were not normally dis-tributed, we fit each set of data witha mixed-effects model using PROCGLIMMIX (SAS Version 9.2, SASInstitute 2007). Mixed-effects modelsare appropriate for data that containsboth fixed and random factors andthe GLIMMIX procedure does notrequire the response to be normallydistributed (Littell et al. 2006). Rich-ness, seeding density, year, and theirinteractions were used as fixed effectsand plot column was used as a randomeffect to account for observed spatialvariarion in soil fertility that gener-ally ran from west to east across thefield. We ran post-hoc Tukey-Kramertests comparing significant richness.

density, and year interactions. Becauseno bull thistle plants were recordedalong belt transects in 2007, weomitted this year from the bull thistlemodel. In the smooth brome modelfor the number of tillers establishedfrom seeding locations, we combineddata from the April and Septemberseeding periods because of the lownumber of tillers recorded from bothtime periods. We determined the cova-riance structure that was the best fitfor each model covering multiple yearsof data by comparing Akaike's infor-mation criterion (AIC) for the plantcommunity composition models andthe pseudo-AIC for the bull thistle,sweet clover, and smooth brome. Thedistribution and covariance srructuresused for each model were: 1) plantcommunity composition: Gaussiandistribution, unstructured covari-ance structure; 2) C vulgäre: negativebinomial distribution, autoregressivecovariance structure; 3) M. officinalis:negative binomial distribution, com-pound symmetry covariance structure;4) B. inermis inflorescences removed:Poisson distribution, autoregressivecovariance structure; and 5) B. iner-mis spread from seeds and transplants:negative binomial distribution. TheKenward-Roger (1997) degrees offreedom were used in the models.

Results

Recorded seeded plant species richnesswas approximately twice as large in thehigh richness plots compared to thelow richness plots. Over three seasonsof sampling from 2007 to 2009, werecorded a total of 27 seeded species,with 9 species observed in the lowrichness, low seeding density plots; 13species observed in the low richness,high seeding density plots; 22 spe-cies observed in rhe high richness, lowseeding density plots; and 22 seededspecies observed in the high richness,high seeding density plots.

June 2013 ECOLOGICAL RESTORATION 31:2 171

2007 2008

b)

10 -

20092007 2008 2009

à)

2009 2007 2008 2009

6 -

4 -

2 -

0 -

40

30

20

10

O •

2007 2008 2009

172

2007 2008 2009

June 2013 ECOLOGICAL RESTORATION 31.2

2007 2008 2009

2007 2008 2009

Treatment Effects on PlantCommunity Composition

Seeded SpeciesYear was the only variable that had asignificant effect on the basal cover ofall seeded species across the treatments{p < 0.0001) (Figure 2a, Table A4.1).The dominant seeded forb found inthe high richness plots was the peren-nial Maximilian sunflower, which in2009 accounted for 69% and 75%of the seeded perennial forb/legumebasal cover in the low and high seedingdensity plots, respectively, and 22%of the basal cover of all plant speciesrecorded in the high richness plots.

There were significant positiveeffects of richness {p < 0.0001),year {p < 0.0001), and the richnessX year interaction {p = 0.0036) onseeded perennial forb/legume basalcover, which increased across all threeyears of the study (Figure 2c, TableA4.1). Year had a significant effect onthe basal cover of seeded grasses {p= 0.0002), which increased over thethree years of the study (Figure 2d).

Unseeded SpeciesYear {p < 0.0001) and richness {p =0.0073) had significant effects on thecover of all unseeded species, withhigher cover of all unseeded speciesbeing recorded in the low richnesstreatments (Figure 2b, Table A4.1).

Richness {p = 0.0115), year {p <0.0001), and the year x richness inter-action {p = 0.0005) had significanteffects on the basal cover of unseeded

Figure 2, opposite. Basal cover of plantspecies aggregated as a) all seeded species,b) all unseeded species, c) seeded perennialforbs/legumes, d) seeded perennial grasses,e) unseeded perennial forbs/legumes,f) unseeded perennial/annual grasses,g) unseeded annual/biennial forbs/legumes,and h) invasives during 2007-2009. Values areleast-square means (± SE) from mixed modelanalysis and represent the basal cover in cmrecorded along eighteen 1 m sub-transectswithin each plot N = 6 plots per treatment.Low richness treatments are represented

by triangles and high richness treatmentsare represented by squares. Low seedingrate treatments are represented by filledshapes and high seeding rate treatments arerepresented by open shapes.

perennial forb/legume species, withgreater cover recorded in the low rich-ness plots (Figure 2e, Table A4.1).These effects remained significantwhen considering only those speciesthat were external to the study (notpresent in the seed mixes). Six of thetwelve unseeded perennial forb andlegume species were internal to thestudy, having spread from where theywere seeded in the high richness plotsto the low richness plots where theyhad not been seeded. Maximiliansunfiower was the most widespreadinternal unseeded perennial forb/legume species, recorded in 75% ofthe low richness plots in 2009. Dan-delion {Taraxacum officinale) was themost widespread unseeded perennialforb/legume external to the study,recorded in all of the low richnessplots and 83% of the high richnessplots in 2009. Of the eleven unseededannual forbs and legumes, one spe-cies, woolly plantain {Plantago pata-gónica), was internal to the study andhad spread to the low richness plots.The large density of unseeded annualforbs and legumes in 2007 (Figure2g) was due to the high abundance ofmare's tail {Conyza canadensis) that iscommon to central Nebraska prairierestorations in their second or thirdgrowing season. Year had a significanteffect {p < 0.0001) and the richnessX seeding density had a marginallysignificant effect {p = 0.0742) on thebasal cover of unseeded annual forbsand legumes, which decreased acrossall three years, particularly betweenthe first and second year of the study(Figure 2g).

Of the seven unseeded perennial/annual grasses, five were internal tothe study and were present in the lowrichness plots in low amounts. Rich-ness {p = 0.0118), seeding density {p =0.0110), year x richness interaction {p= 0.0068), and year x richness x seedingdensity interactions {p = 0.0158) weresignificant in explaining the basal coverof unseeded perennial/annual grasses,largely because of the high cover presentin the low richness treatments in 2008and 2009 (Figure 2f, Table A4.1).

Four invasive species, smoothbrome, Kentucky bluegrass {Poa pra-tensis), black medic {Medicago lupu-lina), and sweet clover, were recordedin the plant community line transects.The basal cover of the invasive spe-cies was low compared to plant spe-cies belonging to the other groupsand there were no significant effectsof richness, seeding density, year, ortheir interaction terms, on invasivespecies basal cover, although richnesshad marginal significant effects oninvasive species basal cover (Figure2h, Table A4.1).

Treatment Effects on Bull Thistleand Sweet Ciover Abundance

Richness {p = 0.0059), year {p <0.0001), and the richness x yearinteraction {p = 0.0191) had signifi-cant effects on bull thistle abundance,with more bull thistle recorded in thelow richness plots (Figure 3a, TableA4.2). Because bull thisde is a bien-nial, observed abundances across alltreatments were highest in 2008, whenthe plants bolted (Figure 3a).

Year had the only significant effecton sweet clover {p = 0.0492), whichgradually increased across the yearsacross treatments and was moreabundant in the low seeding den-sity treatments than the high seedingdensity treatments by 2009 (Figure3b, Table A4.2). However, standarderror was also large for sweet cloverand sampling effort may not havebeen large enough to reveal densityeffects.

Treatment Effects onPlanted and Seeded SmoottiBrome Abundance

Richness {p = 0.0057), year {p =0.0104), and the richness x year inter-action {p = 0.0300) had significanteffects on the number of inflorescencesremoved from smooth brome trans-plants in 2008 and 2009, with moreinflorescences produced by plants thathad been planted in the low richnessplots (Figure 4a, Table A4.3). Novariables had significant effects on


Ti

"3

-Oc3

J2

2Ü06 2007 2008 2009

2006 2007 2008 2009

Figure 3. Abundance of bull thistle (a) and sweet clover (b) during2007-2009. Values are least-square means (± SE) from mixed modelanalysis. N = 6 plots per treatment. Low richness treatments are rep-resented by triangles and high richness treatments are represented bysquares. Low seeding rate treatments are represented by filled shapesand high seeding rate treatments are represented by open shapes.

the number of smooth brome tillerscounted in 1 m^ quadrat frames placedaround seeded and planted areas in2010 to assess the spread of smoothbrome (Figure 4b, 4c, Table A4.3).However, there were marginal rich-ness effects on the number of tillersrecorded near plugs {p = 0.0995 forrichness and/) = 0.0772 for the rich-ness X seeding density interaction)and marginal density effects on thenumber of tillers recorded near seed-ing locations {p = 0.0625 for seedingdensity), largely driven by the numberof tillers recorded in the low richness,low seeding rate plots (Figure 4c).

Discussion

We found richness to be more impor-tant than seeding density in increas-ing invasion resistance of experimental

tallgrass prairie plots to unseededperennial forbs/legumes, unseededperennial/annual grasses, bull thistle,and smooth brome. These results sup-port other studies conducted underrealistic restoration conditions, whichhave generally found increasing rich-ness confers greater invasion resis-tance (Young et al. 2009, Institutefor Applied Ecology 2011, Oakley andKnox 2013). The high richness treat-ments may have been more successfulin resisting invasion by these speciesor in reducing the vigor of transplantsbecause of the characteristics of domi-nant species in these plots, such as theallelopathic properties and competitiveadvantage of Maximilian sunfiower, orbecause of greater interspecific com-petition among forbs/legumes withneighboring plants belonging to thesame functional group for limiting

resources in the high richness plots(see Norland et al. 2013, this issue).

Seeding density had a significanteffect only on the basal cover ofunseeded perennial/annual grasses,

. which were a minor component ofall unseeded species. Density hadno significant effect on unseededspecies. There was a marginal effectof density on the number of tillersremoved near seeding locations, withmore tillers recorded near locationswhere smooth brome was seeded inlow richness plots. Other grasslandstudies have found that seeding rateis not a factor in explaining cover ordensity of unseeded species (Martin2006, Dickson and Busby 2009).

Two groups of unseeded species,perennial forbs/legumes and peren-nial/annual grasses, appeared to sup-port Elton's biotic resistance theory(1958) that species-rich plant com-munities should be less invasible, asthere was a significant negative effectof richness on basal cover for thesegroups. This finding was infiuencedto some extent by species internal tothe experimental seed mix becauselow richness plots were more likely tobe "invaded" by seeded species fromadjacent high richness plots. Ourresults refiect other grassland studiesin which species internal to the experi-mental species pool spread and estab-lish extensively throughout the studysite, particularly in species-poor plots(Roscher et al. 2009, Petermann et al.2010). However, the negative relation-ship between richness and unseededspecies still held true when consider-ing only the unseeded perennial forb/legume species that were external tothe study species pool.

An unseeded species may be lesslikely to establish if a species withsimilar traits is already present inthe community, and high richnessseed mixes have a higher probabilityof containing a species with similarresource requirements to that of anunseeded species (Funk et al. 2008).We recorded roughly twice as manyseeded species in the high richnessplots compared to the low richness


plots, similar to other studies (Carterand Blair 2012). The high richnesstreatments may have had more peren-nial forb/legume species with similarniches that were better able to out-compete unseeded species belongingto the same functional group such asCanada milkvetch {Astragalus canaden-sis), purple prairie clover {Dalea pur-purea), giant goldenrod {Solidagogigantea), and Missouri goldenrod{Solidago missouriensis). Young et al.(2009) also found that communitieswith species functionally similar to aninvasive species had greater invasionresistance than functionally dissimilarspecies. More species-rich communi-ties may also provide greater insur-ance against environmental variabilitythan communities with fewer species,termed the "portfolio effect" (Doak etal. 1998, Tilman et al. 1998).

Richness had a negative effect onthe abundance of naturally recruitedbull thistle. The performance anddensity of bull thistle may be relatedto several factors including the avail-ability of seed, disturbance, and vigorof grass competition (Louda andRand 2003). However, the low rich-ness treatments, which had a higheramount of grass cover than the highrichness treatments, had higher abun-dances of bull thistle in 2008, suggest-ing grass competition did not reducethe spread of bull thistle, at least in theyear that it flowered. The high rich-ness treatments may have been moresuccessful in resisting invasion by bullthistle because of the dominant pres-ence of Maximilian sunflower in theseplots. Maximilian sunflower inhibitsweed growth allelopathically by exud-ing chemicals that act as an herbicide(Herz and Kumar 1981, Gershenzonand Mabry 1984, Macias et al. 1996).Dickson and Busby (2009) founda significant negative relationshipbetween the percentage canopy coverof Maximilian sunflower and otherseeded tallgrass prairie species duringone year of their study.

Richness also explained the numberof inflorescences found from smoothbrome transplants, with low richness

itc

Ldiv/

L rate

Ldiv/H rate

Hdiv/Lrate

Treatment

Hdiv/H rate

Hdiv/

L rate

Hdiv/

H rate

Treatment

Ldiv/Lrate

Ldiv/H rate

Hdiv/L rate

Treatment

Hdiv/H rate

Figure 4. a) Number of smooth brome seed heads removed per plantedsmooth brome individual in 2008 and 2009, b) Number of smoothbrome tillers recorded in quadrats adjacent to planting locations in2010. c) Number of smooth brome tillers recorded in quadrats adjacentto seeding locations in 2010. Values are least-square means (± SE)from mixed model analysis. N = 6 plots per treatment. The year 2008 isrepresented by light gray bars and 2009 is represented by black bars.

treatments containing significantlymore inflorescences per plant thanhigh richness treatments in 2009.Similarly, Hille Ris Lambers et al.(2009) found the number of smoothbrome inflorescences removed per

quadrat in experimental prairie plotsto be negatively correlated with declin-ing species richness. In our study, thenegative relationship between inflo-rescence production and richnessmay have been explained by greater


interspecific competition with neigh-boring plants for limiting resources inthe high richness plots, which reducedthe vigor of smooth brome.

No variables had a significant effecton the spread of smooth brome fromseed or rhizomes. Rhizome produc-tion is affected by similar processesto those that affect seed production,such as nutrient availability and inter-specific competition (Otfinowski et al.2007), and is also sensitive to changesin light intensity and quality, withtiller density increasing with increas-ing light intensity (Biligetu and Coul-man 2010). Light conditions mayhave differed among the treatments,although this variable was not mea-sured. There were no significant differ-ences in the number of tillers aroundseeded locations. The number of tillersproduced from seeded locations wassmall, making meaningful compari-sons among the treatments difficult.

In conclusion, the results fromthese 3,025m^ plots support studiesconducted at smaller (< 20 m^) scalesin which negative relationships havebeen observed between plant speciesrichness and invasibility (Naeem etal. 2000, Symstad 2000, Dukes 2002,Fargione and Tilman 2005). Environ-mental heterogeneity was controlledfor by a systematic block design inour study and similar to other studies,smaller-scale competitive biotic inter-actions appeared to be most importantin contributing to invasion resistance.Further research is needed on the bioticand abiotic factors that control theseed production and spread of invasiveplant species vegetatively in low or highrichness plant communities to betterunderstand the effects of various seedmixes on invasion resistance. However,based on our results, increasing theplant richness of seed mixes for res-toration efforts may be more effectivethan increasing the seeding density fordecreasing invasion by unseeded peren-nial species and bull thistle. Increas-ing the richness of a seed mix mayreduce the establishment of unseededspecies due to species complementingeach other in their resource use or by

a portfolio effect. In the long run, theinitially higher costs of higher richnessseed mixes may be offset by reducedtime and effort in managing unseededspecies.

AcknowledgementsWe thank Katy Dornbos, Michelle Hellman,Elizabeth Manley, John Nemec, Chris Kelly,Trie Heger, Thad Miller, Phil Phipps, andJustin Williams for assistance with field-work and data collection, RaeAnn Powers,Nanette Wliitren, and Nelson Winkel forassistance with plot maintenance, andThe Nature Conservancy for site accessand lodging. We thank Erin Blankenship,John Brejda, and Andrew J. Tyre for helpwith statistical analysis, Gerry Sreinauer forassistance with plant sampling methods,and Tim McCoy for information on theConservation Reserve Program in Nebraska.The manuscript was greatly improved by thecomments of two anonymous reviewers.This study was supported by rhe James S.McDonnell Foundation and a State Wild-life Grant from rhe Nebraska Game andParks Commission. K.T.N. received supportthrough the NSF IGERT on Resilience andAdaptive Governance of Stressed Watershedsat the University of Nebraska-Lincoln. TheNebraska Cooperative Fish and WildlifeResearch Unit is jointly supported by acooperative agreement between the U.S.Geological Survey, the Nebraska Gameand Parks Commission, the University ofNebraska-Lincoln, the United Stares Fishand Wildlife Service, and the Wildlife Man-agement Institute. Reference to trade namesdoes not imply endorsement by the authorsor the U.S. government.

References

Biligeru, B. and B. Coulman. 2010.Responses of three bromegrass(Bromus) species to defoliation underdifferent growth conditions. Interna-tionaljournal of Agronomy 2010:1—5.

Bonham, C D . 1989. Measurements forTerrestrial Vegetation. New York: JohnWiley and Sons.

Burton, CM., P.J. Burton, R. Hebda andN.J. Turner. 2006. Determining theoptimal sowing density for a mix-ture of native plants used to revege-tate degraded ecosystems. RestorationEcology 14:379-390.

Carter, D.L. and J.M. Blair. 2012. Highrichness and dense seeding enhancegrassland restoration establishmentbur have little effect on drought

response. Ecological Applications22:1308-1319.

D'Antonio, C. and J.C. Chambers. 2006.Using ecological theory to manage orrestore ecosystems affected by inva-sive plant species. Pages 260-279in D. Falk, M. Palmer and J. Zedler(eds). Foundations of RestorationEcology. Covelo, CA: Island Press.

Dickson, T.L. and W.H. Busby. 2009.Forb species establishment increaseswith decreased grass seeding densityand with increased forb seeding den-sity in a northeast Kansas, U.S.A.,experimental prairie restoration.Restoration Ecology 17:597-605.

Dillemuth, F.R, E.A. Rietschier andJ.T. Cronin. 2009. Patch dynam-ics of a native grass in relation to thespread of invasive smooth brome{Bromus inermis). Biological Invasions6:1381-1391.

Doak, D.F., D. Bigger, E.K. Harding,M.A. Marvier, R.E. O'Malley andD. Thompson. 1998. The statisti-cal inevitability of stability-diversityrelationships in community ecology.American Naturalist 131:264-276.

Dukes, J.S. 2002. Species compositionand diversity affect grassland sus-ceptibility and response to invasion.Ecological Applications 12:602-617.

Elton, C.S. 1958. The Ecology of Inva-sions by Animals and Plants. London:Methuen.

Elzinga, C.L., D.W. Salzer and J.W Wil-loughby. \^'i8. Measuring and Mon-itoring Plant Populations. Denver,CO: Bureau of Land ManagementTechnical Reference 1730-1.

Fargione, J.E. and D. Tilman. 2005.Diversity decreases invasion via bothsampling and complimentarity effects.Ecology Letters 8:604-611.

Folke, C , S. Carpenter, T. Elmqvist,L. Gunderson, C S. Holling,B. Walker, J. Bengtsson, F. Berkes,J. Colding, K. Danell, M. Falkenmark,L. Gordon, R. Kasperson, N. Kautsky,A. Kinzig, S. Levin, K-G. Maler,F. Moberg, L. Ohlsson, P Olsson,E. Ostrom, W. Reid, J. Rockström,H. Savenije and U. Svedin. 2002.Resilience and sustainable develop-ment: Building adaptive capacity ina world of transformations. AMBIO31:437-440.

Forcella, E and J.M. Randall. 1994. Biol-ogy of bull thistle, Cirsium vulgäre(Savi) Tenore. Review of Weed Science6:29-50.

176 % June 2013 ECOLOGICAL RESTORATION 31.2

Forcella, F. and H. Wood. 1986. Demog-raphy and control of Cirsium vulgäre(Savi) Tenore in relation to grazing.Weed Research 26:199-206.

Frances, A.L., C.R. Adams and J.G. Nor-cini. 2010. Importance of seed andmicrosite limitation: Native wildflowerestablishment in non-native pasture.Restoration Ecology 18:944-953.

Fridley, J.D., J.J. Stachowicz, S. Naeem,D.R Sax, E.W. Seabloom, M.D.Smith, TJ. Stohlgren, D. Tiiman andB.V. Holle. 2007. The invasion para-dox: Reconciling pattern and processin species invasions. Ecology 88:3—17.

Funk, J.L., E.E. Cleland, K.N. Sudingand E.S. Zavaleta. 2008. Restorationthrough reassembly: Plant traits andinvasion resistance. Trends in Ecologyand Evolution 23:695-703.

Gershenzon, J. and Mabry, T.J. 1984. Ses-quiterpene lactones from a Texas pop-ulation oí HeUanthus maximiliani.Phytochemistry 23:1959-1966.

Grant, T.A., E.M. Madden, R.K. Murphy,K.A. Smith and M.P. Nenneman.2004. Monitoring native prairie veg-etation: The belt transect method.Ecological Restoration 22:106—112.

Herz, W. and Kumar, N. 1981. Helian-golides from HeUanthus maximiliani.Phytochemistry 20:93-98.

High Plains Regional Glimate Center.2010. Grand Island WSO AP,Nebraska (253395). www.hprcc.unl.edu/data/historical

Hille Ris Lambers, J., WS. Harpole,S. Schnitzer, D. Tilman and P.B.Reich. 2009. CO2, nitrogen anddiversity differentially aiïect seed pro-duction of prairie plants. Ecology90:1810-1820.

HoUing, C.S. 1973. Resilience and sta-bility of ecological systems. AnnualReview of Ecological Systems 4:1—23.

Institute for Applied Ecology. 2011.Restoring diverse, invasion-resis-tant, wedand prairies project update,appliedeco.org/news/restoring-diverse-invasion-resistant-wetland-prairies

Kenward, M.G. and J.H. Roger. 1997.Small sample inference for fixed effectsfrom restricted maximum likelihood.Biometrics 53:983-997.

Körner, C , J. Stocklin, L. Reuther-Thiébaud and S. Pelaez-Riedl. 2008.Small differences in arrival time influ-ence composition and productivity ofplant communities. New Phytologist177:698-705.

Levine, J., PB. Adler and S.G. Yele-nik. 2004. A meta-analysis of biotic

resistance to exotic plant invasions.Ecology Letters 7:975-989.

Littell, R.C., G.A. Milliken, W.W.Stroup, R.D. Wolfinger and O. Scha-benberger. 2006. SAS for MixedModels. 2nd edition. Cary, NC: SASPublishing.

Louda, S. M. andT A. Rand. 2003.Native thistles: Expendable or integralto ecosystem resistance to invasion?Pages 5-15 /'« S. Levin and P. Kareiva(eds). The Importance of Species.Princeton, NJ: Princeton UniversityPress.

Macias, F.A., Torres, A., Molinillo,J.M.G., Várela, R.M. and D. Castel-lano. 1996. Potential allelopathic ses-quiterpene lactones from sunflowerleaves. Phytochemistry 4'b:\2Çi'i-\2\^.

Martin, A. 2006. The ecological and socialimplications of varying seeding den-sity in dry mesic prairies: Satisfyingcorresponding values. M.S. Thesis,University of Wisconsin, Madison.

Martin, L.M. and B.J. Wilsey. 2012.Assembly history alters alpha and betadiversir)?, exotic-native proportionsand functioning of restored prairieplant communities. Journal of AppliedEcology 49:1436-1445.

Naeem, S., J.M.H. Knops, D. Tilman,K.M. Howe, T. Kennedy and S. Gale.2000. Plant diversity increases resis-tance to invasion in the absence ofcovarying extrinsic factors. Oikos91:97-108.

Nebraska Game and Parks Commission(NGPC). 2005. The Nebraska NaturalLegacy Project: A Comprehensive Wild-life Conservation Strategy. Lincoln,Nebraska.

Nebraska Invasive Species Council. 2011.Invasive Plants of Nebraska, snr.unl.edu/invasives/invasiveplantslist.htm

Natural Resources Conservation Service(NRCS). 2010. Web Soil Survey, HallCounty, Nebraska. U.S. Departmentof Agriculture, National CooperativeSoil Survey, websoilsurvey.nrcs.usda.gov/app/

Norland, J.E., S. Fasching, C. Dixon,K. Askerooth, K. Kelsey and G. Wang.2013. Reduced establishment ofCanada thistle {Cirsium arvense) usingfunctionally similar native forbs.Ecological Restoration 31:143-146.

Oakley, C.A. and J.S. Knoz. 2013. Plantspecies richness increases resistance toinvasion by non-resident plant speciesduring grassland restoration. AppliedVegetation Science 16:21-28.

Otfînowski, R., N. C. Kenkel and P. M.Catling. 2007. The biology of Cana-dian weeds. 134. Bromus inermisLeyss. Canadian Journal of PlantScience ?,7:\?,3-\9?,.

Pecermann, J. S., A. J. E Fergus,C. Roscher, L. A. Turnbull, A. Wei-gelt and B. Schmid. 2010. Biol-ogy, chance, or history? The predict-able reassembly of temperate grasslandcommunities. Ecology 91:408-421.

Peters, M. and S. Schotder. 2011. The roleof forb seeding rate in enhancing flo-ristic diversity. Pages 70-78 in D. Wil-liams, B. Butler and D. Smith (eds).Proceedings of the 22nd North Amer-ican Prairie Conference, August 1—5,2010, Cedar Falls, Iowa.

Pysek, P and D. M. Richardson. 2010.Invasive species, environmentalchange and management and health.Annual Review of Environment andResources 35:1—31.

Roscher, C , H. Beßler, Y. Oelmann,C. Englels, W. Wilcke and E-DSchulze. 2009. Resources, recruitmentlimitation and invader species iden-tity determine pattern of spontaneousinvasion in experimental grasslands.Journal of Ecology 97:32—47.

Stohlgren, T. J., D. Binkley, G. W.Chong, M. A. Kalkhan, L. D. Schell,K. A. Bull, Y. Otsuki, G. Newman,M. Bashkin and Y. Son. 1999. Exoticplant species invade hot spots of nativeplant diversity. Ecology Monographs69:25^6.

Stubbendieck, J., M. J. CofEn andL. M. Landholt. 2003. Weeds ofthe Great Plains. Nebraska Depart-ment of Agriculture and University ofNebraska-Lincoln.

Symstad, A.J. 2000. A test of the effects offunctional group richness and compo-sition on grassland invasibility. Ecology81:99-109.

Symstad, A.J. 2008. Does biodiversity-ecosystem function science applyto prairie restoration? EcologicalRestoration 26:100-102.

Tilman, D., C.L. Lehman and C.E. Bris-tow. 1998. Diversity-stability relation-ships: Statistical inevitability or eco-logical consequence? The AmericanNaturalist \5\:277-29,2.

Van Riper, L.C. and D.L. Larson. 2009.Role of invasive Melilotus ofßcinalis intwo native plant communities. PlantEcology 200:129-139.

Vinton, M.A. and E.M. Goergen. 2006.Plant-soil feedbacks contribute to


the persistence oí Bromus inermis intaligrass prairie. Ecosystems 9:967-976.

Walker, B., C.S. Holling, S.R. Carpen-ter and A. Kinzig. 2004. Resilience,adaptability and transformability. Ecol-ogy and Society 9:5 www.ecologyandsociety.org/vol9/iss2/art5

White, P.J.T. and J. Houlahan. 2007. Tlierelationship between native and non-native species differs among taxa inCanadian national parks. Ecoscience14:195-204.

Wolf, J.J., S.W. Beatty and G. Carey.2003. Invasion by sweet clover {Mel-ilotus) in Montane grasslands. RockyMountain National Park. Annals of theAssociation of American Geographers93:531-543.

Young, S.L., J.N. Barney, G.B. Kyser,T.S. Jones and J.M. DiTomaso. 2009.Functionally similar species confergreater resistance to invasion: Impli-cations for grassland restoration.Restoration Ecology 17:884-892.

Kristine Nemec (corresponding author),Nebraska Cooperative Fish and WildlifeResearch Unit, 422 Hardin Hall,3310 Holdrege Street, University ofNebraska, Lincoln, NE 68583,kristine. nemec@huskers. uni. edu.

Craig R. Allen, U.S. Geological Survey,Nebraska Cooperative Fish and WildlifeResearch Unit, 423 Hardin Hall, 3310Holdrege Street, University of Nebraska,Lincoln, NE 68583, [email protected].

Christopher J. Heizer, The Nature Con-servancy, Eastern Nebraska Project OJfice,P.O. Box 438, Aurora, NF68818,chelzer@tnc. org.

David A. Wedin, School of NaturalResources, 411 Hardin Hall, 3310

. Holdrege Street, University of Nebraska,Lincoln, NF 68583, [email protected].

Table A l . Low richness seed mix used in the study for experimental restoration of taligrass prairie in Nebraska, USA.

Species

Grasses

Andropogon gerardii

Bouteloua curtipendula

Elymus canadensis

Elymus virginicus

Panicum virgatum

Pascopyrum smithii

Schizachyrium scoparium

Sorghastrum nutans

Total grass

Forbs

Amorpha canescens

Astragalus canadensis

Dalea purpurea

Desmanthus illinoensis

Liatris punctata

Ratibida columnifera

Solidago missouriensis

Total forbs

Low seeding densityPure live seeds/m^

36.0

16.1

13.4

7.5

13.4

14.5

26.9

20.4

148.2

1.1

4.3

1.1

5.4

0.5

1.1

2.2

15.7

High seeding densityPure live seeds/m^

72.1

32.3

26.9

15.1

26.9

29.0

53.840.9

297.0

2.28.62.2

10.8

1.12.2

4.331.4

% of seed mix

22.0

9.8

8.24.68.28.8

16.4

12.5

90.5

0.672.6

0.67

3.3

0.33

0.671.39.5


Table A2. High richness seed mix used in the study for experimental restoration of tallgrass prairie in Nebraska,USA. Because the entire mix was not planted, the % forb column best describes the relative amounts of each forbspecies seeded. Messy: includes stems and seed heads mixed in with the seed.

SpeciesLow seeding density

Dry liters (L)High seeding density

Dry liters (L) % forb

Crass mix

Andropogon gerardii

Bouteioua curtipendula

Calamagrostis canadensis

Digitaria cognata

Elymus canadensis

Elymus trachycauius

Elymus virginicus

Eragrostis spectabilis

Eragrostis trichodes

Koeleria macrantha

Panicum virgatum

Paspaium setaceum

Sorghastrum nutans

Spartina pectinata

Sphenophoiis obtusata

Sporobolus compositus

Sporobolus cryptandrus

Tridens fiavus

Forbs Included in grass mix


Heiianthus maximiliani

Forbs

Achiiiea miiiefolium

Allium canadense

Amorpha canescens

Anemone canadensis

Artemisia ludoviciana

Asclepias speciosa

Asclepais syriaca

Asclepias verticillata


Brici<ellia eupatorioides

Callirhoe involucrata

Calylophus serrulatus

Carex brevior

Carex duriuscuia

Carex grávida

Crépis runcinata

Cyperus lupulinus

Cyperus schweinitzii

Daiea candida

Dalea purpurea

Delphinium carolinianum


Desmodium illinoense

Eleocharis elliptica

Eupatorium aitissimum

Eustoma grandiflorum

Euthamia graminifolia

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

4.4

2.2

1.1

0.36

0.36

2.2

2.2

0.18

0.55

2.2

3.3

3.3

0.24

2.2 (messy)

0.06

0.06

0.06

0.24

0.47

11.0

0.0074

2.2

0.24

0.12

6.6

0.08

6.6

N/AN/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

8.8

4.4

2.2

0.71

0.71

4.4

4.4

0.36

1.14.4

6.6

6.6

0.47

4.4 (messy)

0.12

0.12

0.12

0.47

0.9522.0

0.00744.4

0.47

0.24

13.2

0.16

13.2

2.71.40.690.220.221.41.40.110.341.42.12.10.151.4

0.037

0.037

0.037

0.15

0.306.9

0.0023

1.4

0.15

0.075

4.1

0.050

4.1


SpeciesLow seeding density

Dry liters (L)High seeding density

Dry liters (L) % forb

Caura parvi flora

Geum canadense

Ceum vernum

Clycyrrhiza lepidota

Helianthus grosseserratus

Helianthus pauciflorus

Helianthus petiolaris

Helianthus tuberosus

Heliopsis helianthoides

Hesperostipa comata

Hesperostipa spartea

Heterotheca villosa

¡uncus dudleyi

Lespedeza capitata

Liatris lancifolia

Liatris punctata

Liatris squarrosa

Lithospermum caroliniense

Lithospermum incisum

Lotus unifoliolatus

Mimosa nuttallii

Mirabilis nyctaginea

Monarda fistulosa

Oenothera biennis

Oenothera rhombipetala

Oligoneuron rigidum

Onosmodium bejariense

Packera plattensis

Penstemon digitalis

Penstemon gracilis

Penstemon grandiflorus

Plantago patagónica

Potentilla norvegica

Prunella vulgaris

Pycnanthemum virginianum


Rosa arkansana

Rudbeckia hirta

Silphium integrifolium

Sisyrinchium campestre

Solidago gigantea


Symphyotrichum ericoides

Symphyotrichum novae-angliae

Symphyotrichum lanceolatum

Teucrium canadense

Tradescantia bracteata

Tradescantia occidentalis

Verbena hastata

Verbena stricta

Vernonia fasciculata

Grasses included in forb mix

Hesperostipa comata

Hesperostipa spartea

0.12

0.060.060.360.084.40.12

0.08

0.360.470.12

4.4

0.022

13.2

6.64.44.40.47 (messy)

0.12 (messy)

0.360.240.12 (messy)

4.4 (messy)

0.12

0.55

11.0

4.4

4.4

0.71

0.015

0.602.20.00740.00740.900.80.700.36

15.4

0.0074

0.12

1.7

4.4

4.4

2.2

0.12

0.24

2.2

0.12

0.12

4.4

0.47

0.12

0.240.12

0.12

0.71

0.16

8.8

0.24

0.16

0.71

0.95

0.24

8.8

0.044

26.4

13.2

8.8

8.8

0.95 (messy)

0.24 (messy)

0.71

0.47

0.24 (messy)

8.8 (messy)

0.24

1.1

22.0

8.8

8.8

1.4

0.030

1.2

4.4

0.015

0.015

1.8

1.6

1.4

0.71

30.8

0.015

0.24

3.3

8.8

8.8

4.4

0.24

0.47

4.4

0.24

0.24

8.8

0.95

0.24

0.0750.037

0.037

0.22

0.050

2.7

0.075

0.050

0.22

0.30

0.075

2.7

0.014

8.2

4.1

2.7

2.7

0.30

0.075

0.22

0.15

0.075

2.7

0.075

0.34

6.9

2.72.70.44

0.0094

0.37

1.4

0.0094

0.0094

0.56

0.50

0.44

0.22

9.6

0.0094

0.075

1.02.72.7

1.4

0.075

0.15

1.4

0.075

0.075

2.7

0.30

0.075


Table A3. Plant species recorded in plots by seeded/unseeded status andgrowth form and life cycle. Species marked with * were seeded or unseededin high richness plots, and species marked with ** were seeded or unseededin low richness plots; a species may appear in multiple lists if it was foundboth in plots where it was seeded and where it was not.

Species Common Name

Seeded perennial forb/legume

Achillea millefolium


Dalea purpurea


Ceum canadense

Helianthus maximiliani



Rudbeckia hirta

Solidago gigantea


Symphyotrichum ericoides

Verbena stricta

Seeded perennial grass

Andropogon gerardii

Bouteloua curtipendula

Dichanthelium oligosanthes

Elymus canadensis

Elymus trachycaulus

Elymus virginicus

Koeleria macrantha

Panicum virgatum

Pascopyrum smithii

Schizachyrium scoparium

Sorghastrum nutans

Spartina pectinate

Sphenopholis obtusata

Sporobolus compositus

Unseeded perennial forb/legume

(internal to study, from a seed mix)

Achillea millefolium

Dalea candida

Eupatorium altissimum

Helianthus maximiliani

Solidago gigantea

Verbena stricta

Unseeded perennial forb/legume

(external to study, not from seed mix)

Ambrosia psilostachya

Equisetum arvense

Physaiis longifolia

Physalis virginiana

Soiidago canadensis

Taraxacum officinaie

yarrow*Canada milkvetch

purple prairie clover

Illinois bundleflower

white avens*

Maximilian sunflower*

woolly plantain*

upright prairie coneflower

black-eyed Susan*

giant goldenrod*

Missouri goldenrod

heath aster*

hoary vervain*

big bluestem

sideoats grama

Scribner's panic grass*

Canada wildrye

slender wheatgrass*

Virginia wildrye

junegrass*

switchgrass

western wheatgrass**

little bluestem

Indiangrass

prairie cordgrass*

wedge grass*

tall dropseed*

yarrow**white prairie clover**

tall white joe pye**

Maximilian sunflower*

giant goldenrod**

hoary vervain**

western ragweed

common horsetail

common groundcherry

Virginia groundcherry

Canada goldenrod

dandelion


Species

Unseeded annual forb/legume

(internal to study)


(external to study)

Abutilón theophrasti

Ambrosia artemisiifolia

Ambrosia trifida

Cannabis sativa

Chenopodium aibum

Conyza canadensis

HeUanthus annuus

Lactuca serriola

Sonchus asper

Xanthium strumarium

Unseeded biennial forb/legume

Cirsium altissimum

Cirsium vulgäre

Conium maculatum

Unseeded perennial/annual grass

(internal to study)

Dichanthelium oligosanthes

Elymus trachycaulus

Koeleria macrantha

Spartina pectinata

Sporoboius compositus

(external to study)

Setaria pumila

Setaria species

Invasive species

Bromus inermis

Medicago lupulina

Meliiotus officinalis

Poa pratensis

Common Name

woolly plantain**

velvetleaf

common ragweed

giant ragweed

hemp

lamb's quarters

mare's tail

common sunflower

wild lettuce

prickly star thistle

cocklebur

tall thistle

bull thistle

poison hemlock

Scribner's panic grass**

slender wheatgrass**

Junegrass**

prairie cordgrass**

tall dropseed**

yellow foxtail

foxtail

smooth brome

black medic

sweet clover

Kentucky bluegrass


Table A4.1. Results of mixed-model analysis for testing the effects of richness, seeding density, and year on basalcover of seeded and unseeded plant species aggregated according to growth form and functional group. Internal =species included in the study seed mix; external = species not included in the study seed mix. Values in boldface aresignificant at p < 0.05.

Effect df

Seeded perennial forbs/legumes

Richness

Seeding density

Richness*Seeding density

Year

Year* Rich ness

Year*Seeding density

Year*Richness*Seeding density

Seeded perennial grasses

Richness

Seeding density


Year

Year*Richness



Unseeded perennial forbs/legumes (all)

Richness

Seeding density


Year

Year*Richness



Unseeded perennial forbs/legumes (external)

Richness

Seeding density


Year

Year*Richness



Unseeded perennial forbs/legumes (internal)

Richness

Seeding density


Year

Year*Richness



Unseeded perennial/annual grasses (all)

Richness

Seeding density


Year

Year*Richness



1,201,20

1,20

2,19

2, 19

2, 19

2, 19

1, 19.93

1, 19.93

1, 19.93

2, 19

2, 19

2, 19

2, 19

1,20

1,20

1,20

2,19

2,19

2,19

2, 19

1,20

1,20

1,20

2, 19

1, 19

2, 19

2, 19

0

1, 10

0

2,9

0

2,9

0

1,20

1,20

1,20

2,19

2,19

2,19

2,19

46.620.79

0.43

60.80

7.66

1.38

0.72

2.17

0.57

0.09

14.28

0.93

1.42

0.14

7.75

1.76

0.36

41.63

11.50

1.24

0.21

3.58

1.42

0.24

25.12

3.77

0.99

0.31

0.17

6.82

0.05

7.68

7.85

2.01

0.28

6.560.59

5.20

<0.00010.3832

0.5216

<0.0001

0.0036

0.2747

0.5009

0.1564

0.4576

0.7710

0.0002

0.4118

0.2658

0.8678

0.0155

0.1995

0.5561

<0.0001

0.0005

0.3118

0.8122

0.0729

0.2472

0.6289

<0.0001

0.0418

0.3911

0.7358

0.6892

0.0157

0.9541

0.0118

0.0110

0.1721

0.7557

0.0068

0.5638

0.0158


Effect

Unseeded perennial/annual grasses (external)

Richness

Seeding density


Year

Year*Rjchness


Year*Richness*Seedlng density

Unseeded perennial/annual grasses (internal)

Richness

Seeding density


Year

Year*Richness



Unseeded annual/biennial forbs/legumes

Richness

Seeding density

Richness*Seedmg density

Year

Year*Richness



Invasives

Richness

Seeding density


Year

Year*Rlchness

Year*Seedlng density

Year*Richness*Seedlng density

Seeded

Richness

Seeding density


Year

Year*Richness



Unseeded

Richness

Seeding density

Rlchness*Seeding density

Year

Year*Richness



df

1, 19.81,19.8

1,19.8

2, 19

2,192,192,19

1, 19.86

1,19.86

1, 19.86

2,19

2,192,192,19

1,201,201,202,192,192,192,19

1, 17.46

1, 17.46

1, 17.46

2,192,192,192, 19

1,20

1,20

1,202,192,192,19

2,19

1,20

1,20

1,202,20

2,20

2,202,20

0.080.46

2.51

29.49

4.73

2.45

2.42

6.00

5.57

5.02

2.36

2.95

2.15

2.73

1.61

0

3.55

159.21

0.18

0.60

1.25

3.51

0.01

0.93

0.88

1.05

1.61

1.60

0.39

0.76

0.14

20.32

0.43

2.01

0.17

8.91

0.09

2.68

55.50

0.82

1.24

0.63

0.78480.5048

0.1288

<0.0001

0.0215

0.1133

0.1161

0.0237

0.0286

0.0366

0.1213

0.0763

0.1437

0.0907

0.2195

1.000

0.0742

<0.00010.8327

0.5605

0.3084

0.0779

0.9160

0.3486

0.4299

0.3680

0.2258

0.2272

0.5418

0.3937

0.7095

<0.0001

0.6554

0.1619

0.8476

0.0073

0.7654

0.1173

<0.0001

0.4536

0.3117

0.5446


Table A4.2. Results of mixed-model analysis for testing the effects of richness, seeding density, and year onabundance of bull thistle and sweet clover. Values in boldface are significant at p < 0.05.

Effect df

Bull thistleRichnessSeeding densityRichness*Seeding densityYearYear*RichnessYear*Seeding densityYear*Richness*Seeding density

Sweet cloverRichnessSeeding densityRichness*Seeding densityYearYear*RichnessYear*Seeding densityYear*Richness*Seeding density

1,601,601, 602,602,602,602,60

1, 741, 741,74

3, 67.83, 67.73, 67.93, 58.9

8.140.011.27

112.504.230.170.40

0.562.390.142.840.181.530.64

0.00590.93210.2633

<0.00010.01910.84370.6693

0.45590.12610.70760.04920.90900.21480.5888

Table A4.3. Results of mixed-model analysis for testing the effects of richness, seeding density, and year on thenumber of inflorescences removed from planted smooth brome in 2008 and 2009 and on number of smooth brometillers recorded in 2010 from quadrats placed adjacent to locations where smooth brome had been planted andseeded. Values in boldface are significant at p < 0.05.

Effect df

Smooth brome - inflorescences removedRichnessSeeding densityRichness*Seeding densityYearYear*RichnessYear*Seeding densityYear*Richness*Seeding density

Smooth brome—tillers near planting locationsRichnessSeeding densityRlchness*Seeding density

Smooth brome—t;7/ers near seeding locationsRichnessSeeding densityRichness*Seeding density

1,22.971,22.971,22.971,22.971,22.971,22.971,22.97

1,201,201,20

1,201,201,20

9.310.700.077.785.350.270.34

2.982.263.47

0.013.890.32

0.00570.41140.79920.01040.03000.60860.5639

0.09950.14840.0772

0.93590.06250.5766


Copyright of Ecological Restoration is the property of University of Wisconsin Press and its content may not be

copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written

permission. However, users may print, download, or email articles for individual use.

Influence of richness and seeding density on invasion resistance in experimental tallgrass prairie...

Documents

Transcript of Influence of richness and seeding density on invasion resistance in experimental tallgrass prairie...