Responseoftheinvader Cortaderia selloanaandtwo coexistingnativestocompetition andwater stress
RoserDome`nechÆ MontserratVila`
Abstract Alienspecies’resistanceandadjustment towater stress andplant competitionmightlargely determinethesuccessofinvasions inMediterranean ecosystems becausewateravailabilityisoftenlimit- ingbiomassproduction.Twooutdoorpotexperiments wereconductedtotestthehypotheses thatatthe recruitmentstagetheinvader perennialtussockgrass Cortaderiaselloanais a superiorcompetitor,andthat itismoreresistant towaterstress thanthetwo coexisting native species of the same functional group,FestucaarundinaceaandBrachypodiumphoe- nicoides.C.selloanareducedabovegroundbiomass oftargetnativespecies, butnotmorethantarget nativespeciesoneachother.Moreover,C.selloana didnotresistinterspecific competitionmorethan target native species. Under control conditions, C.selloana did not have larger specificleaf area (SLA)androot–shoot ratio(R/S)ratiothantarget native species,contradicting the general statement that these traits are associated to invasiveness.
R.Dome`nech
CREAF(CenterforEcologicalResearchandForestry Applications)andUnitofEcology,DepartmentofAnimal andPlantBiologyandEcology,AutonomousUniversity ofBarcelona,08193Bellaterra,Spain
M.Vila`()
Estacio´nBiolo´gicadeDon˜ana-CentroSuperiorde
InvestigacionesCient´ıficas(EDB-CSIC),Avda/Mar´ıa
Luisas/n,Pabello´ndelPeru´,41013Sevilla,Spain e-mail:
F.arundinaceawasthe specieswhichperformedbest butalsotheonemostaffectedbywaterstress.Both C.selloana and B.phoenicoides performed in a similarwayunderwaterstressconditions. However, thealienspecies’capacitytoadjusttowaterstress, indicatedbythe increaseinthe root–shootratiounder moderate andseverewaterstress,wasslightly better than thatof B.phoenicoides.Overall,atearlyrecruit- mentstages,C.selloana isnotabettercompetitor thanthecoexistingnativespecies. However,itseems tobemoreresistant towaterstressbecauseaswater becomesscarceC.selloanamaximizeswateruptake andminimizeswaterlosses morethanthenative species.
Keywords Alienplant
Brachypodiumphoenicoides Festucaarundinacea
Intra-andinterspecificcompetition
Perennialgrasses Relativeinteractionindex
SLA Root–shootratio
Introduction
Theintentionaloraccidentalintroductions ofalien speciesare thecausesofbiologicalinvasionsthat poseamajorthreattobiodiversityworldwide (Vito- usek1994;KeaneandCrawley2002;Davis2003), yetonlyafewintroducedspeciessucceedinestab- lishingintherecipientcommunity(Holdgate1986;
ParkerandReichard1998).Thus, invasivesuccessof alienspeciesdependsonthebioticandenvironmental characteristicsoftherecipientcommunity aswellas onbiological attributesrelatedtoitspotentialto colonize and expand (i.e., invasiveness)(Lonsdale
1999).
Somealienplant species havecharacteristicsthat seemtomakethem successfulinvaderssuchasa largeproduction ofviableseedswhichdisperse widely, theabilitytogerminateandgrowinabroad rangeofenvironmental conditions,andbeingagood competitor(Baker1965;Noble 1989; Roy1990; Gordon1998). Whenanalienplantisintroduced, competitionforlimitedresourcesisoneofthefirst interactionsthespecies haswiththerecipientcom- munity(Vila`andWeiner2004). Fieldobservations andexperimentshaveproved thatthethreatalien speciesposetothepersistence ofnativespeciesis usually driven bythecompetitioneffectofthealien speciesonnatives(ParkerandReichard1998;Levine etal. 2003). However, most studies are biased becausetheyhavefocusedonhighly aggressive invaderscompetingwithlessvigorous speciesof dissimilarlifeformorlife-historystage(Vila` etal.
2004).Furthermore, whenanalyzingcompetitive abilitiesofalienspecies,thecompetitiveeffectthat thenativespecieshasonthealienshould be simultaneouslycompared(Vila`andWeiner2004).
Ithasalsobeenargued thatresource pulses(e.g., soilnutrientsandwater)provide thetriggering conditionsforinvasions(Davisetal.2000).Inthe caseofbothinvadersandnative speciesbeinglimited bythesameresources,invasionwouldtakeplaceif theinvaderhas ahigherresourceacquisitionrateora lower maintenance requirement than that of the nativespecies(SheaandChesson 2002).Ithasbeen postulated thatalienspecies mighthaveasuperior responseto particularresources,to resourcesfoundin certainplacesortimes,ortocertainabundancesof resources comparedtonativeplants(Sheaand Chesson2002).Superiorresponses toresource acquisition requireplanttraitsrelatedtophysiolog- icalprocesses.Inparticularspecificleafarea(SLA) hasbeenshown toberelatedtoinvasiveness (Grotkopp etal. 2002; Grotkopp and Rejma´nek
2007) inphylogeneticallyrelatedspecies. Plantswith largeSLAhavethinner orlighterleavesthatcan interceptmorelightandcontribute toplantfast growthespeciallyinshadedenvironments.
InMediterranean ecosystemswheretheclimateis characterizedbyhot,drysummers, wateravailability isoften limitingbiomassproduction.Competitionfor waterisverycommon inMediterraneanecosystems (Vila`andSardans1999).Moreover, Mediterranean ecosystems arepredictedtobeverysusceptibleto wateravailabilityfluctuations causedbyclimate change (Lavorel etal. 1998), which will possibly modifyspeciesdistribution andplant–plantinterac- tions.In this scenario,alien species’resistanceto plantcompetitionandadjustment towaterstressare importantforinvasion. Inordertodeterminethe importanceofcompetitiononplantinvasions in Mediterranean ecosystemsitisnecessarytoquantify theimportance ofthecompetitiveabilitiesofalien speciessimultaneouslywiththatofnativespecies;in addition,knowledgeoftheresistanceandadjustment towaterstressofbothalienandnativeisrequired.
Cortaderiaselloana(SchultesetSchultesfil.)Asch. etGraebnerisaperennialtussockgrassnativetoSouth AmericanowinvadingMediterraneanoldfields and grasslands ofCatalonia(NESpain)dominatedby perennialnativegrassessuchasFestucaarundinacea (Schreber,F.elatiorL.)andBrachipodiumphoenic- oides(Roem.etSchultes) (Masalles etal.1982).We suspectthatC.selloana isabettercompetitorthan coexistingnativespeciesofthesamefunctionalgroup andthatitpossesses traitsthatallowittotake advantageoflowwaterresources.Weconductedtwo outdoorpotexperimentstotest thefollowinghypoth- esesrelatedtoplantrecruitmentstage:(1)C.selloana is a superior competitor than F.arundinacea and B.phoenicoides;consequently,weexpect C.sello- ana’seffectonnativespeciestobestrongerthan vice versa,and(2)C.selloana seedlingtraitsrelatedto resourceacquisition (e.g.,SLAandR/Sratio)are largerthanin F.arundinaceaandB.phoenicoides.
Materialandmethods
Studyspecies
C.selloana (Pampas grass) is a South American long-living perennialgrasswhichisconsidered invasiveworldwide. ItwasfirstintroducedtoEurope between1775and1862(Bossardetal.2000).This speciesisplantedformanypurposes suchasa windbreak or to prevent erosion but due to the
attractivenessofitsplumesithasmainly beenusedas anornamental. Ithasescapedfromcultivationandis invadingabandoned agriculturallands,ruderalareas, shrublands, grasslands, and wetland communities. C.selloana threatensnativevegetationandposesa firehazardduetotheaccumulationofdryleavesand floweringstalksontheplant(Bossardetal.2000).It flowers frommidtolatesummer;theinflorescences consistofshowyplume-likeheadsattheendofastiff stem.Plumesproducecopiousamounts ofsmall, wind-dispersed seeds(asmanyas106 seedsper matureplantforfemalesand105forhermaphrodites) (ConnorandEdgar1974;Lambrinos2002).
F. arundinacea is a perennial grass native to Europefrequentlyfoundinhumidgrasslands. Its maximumgrowth takesplaceduringspring and autumn.Its deepandextensiveroot systemhelpsitto withstanddroughtconditions.F.arundinaceaflowers inspringandseedsmatureinearlysummer. Itis adapted to a wide range of soil and climatic conditions(Tutinetal.1980;Bolo`sandVigo2001).
Brachipodium phoenicoidesisaperennialgrass nativetotheMediterranean Basin.Itformsdense communitiesinopenhabitatssuchasfield margins, pastures,grasslands, andabandoned agricultural fields.Brachipodiumphoenicoidesisconsideredan essentialspeciesforecologicalsuccessioninpastures sinceitestablishesduringtheinitialstagesandallows establishment of other species (Tutin etal. 1980; Bolo`sandVigo2001).
Thethreespeciescoexistinmanyhabitatsandcan beconsidered asbelonging tothesamefunctional group(i.e.,tussock perennialgrass)buttheydifferin sizeandbiomasswhenmature.C.selloana iscon- siderablylargerthantheotherspecies.Itcanreach from 2 to 4m in height including inflorescences (Bossard etal.2000)anditsmaximum diametercan reach3.5mwhereasF.arundinaciaandB.phoenic- oidescanreachonlyfrom40to60cminheight and thediameter ofthetussockisfivetotentimessmaller thanthatofC.selloana(Dome`nech2005).
Plantmaterial
Insummer2003,freshplumesofC.selloana were collectedfromold fieldsinAiguamollsdel’Emporda` (NESpain).Seedswereremovedfrom inflorescences andmixed.SeedsofF.arundinaceaandB.phoenic- oides were bought from Semillas Silvestres S.L.
( seedsofthealienandthetwonativegrasses were sowninflattraysandleftoutdoorsattheUniversitat Auto`nomadeBarcelona(UAB)campustogerminate beforetransplanting.
Competitionexperiment
InApril2004,3monthsaftersowing,seedlingsof C.selloana, B.phoenicoides and F.arundinacea weretransplantedoutdoorsattheUABcampusinto
2–lpotsfilledwithPlantaflorgardeningsoilwhich
contained 200mg/l of N, 180mg/l of P2O5, and
230mg/l of K2O. The climate in Barcelona is Mediterranean withmildwetwintersandhotdry summers. Meanannualtemperatureandannual precipitationare15.0°Cand673.1 mm,respectively (
Inordertodeterminethe mean initial aboveground biomassofthethreetargetspeciesweweighedthe leavesfrom20seedlings ofeachspeciesafterair- dryingat70°C toconstant weight.Mean above- ground biomass was 0.0014±0.0001g for B.phoenicoides,0.0016±0.0001gforC.selloana, and0.0039±0.0003gforF.arundinacea. Above- groundbiomasswassignificantlydifferent (ANOVA, F2,55=38.00, P\0.0001), being the largest in F.arundinacea (Fisher’sleastsignificanttest(LSD) P\0.0001).
Ninecompetitiontreatmentswhich includedall possiblepair-wisecombinationsofintraspecificand interspecific competitionandnocompetitionwere replicated12times,asfollows:(1)twoseedlingsof C.selloana per pot (C:C), (2) one seedling of C.selloana and one of B.phoenicoides per pot (C:B),(3)oneseedlingofC.selloana andoneof F.arundinacea per pot (C:F), (4) one seedling of C.selloana per pot (C), (5) two seedlings of F.arundinacea per pot (F:F), (6) one seedlingof F.arundinacea andoneofB.phoenicoidesperpot (F:B),(7)one seedlingofF.arundinaceaper pot(F), (8)twoseedlingsofB.phoenicoidesperpot(B:B), and(9)oneseedlingofB.phoenicoidesperpot(B).
Potswerewateredevery2daysto avoidwater stressandwererandomlymovedevery15daysto ensurethat all the plantsweregrowingunderthe sameenvironmentalconditions.InJuly2004,when plantsstarted toproduce panicles, plantswere collected and weighedafter air-dryingat 70°Cto
constant weight. Atthisstage,plants hadovertopped thepotsandrootswerefillingallpotsoilvolume. Someoftheplants hadtotallyorpartiallybeeneaten bysnails,andwereexcludedfromanalysis.Inorder tocorrectfortheinitialdifferences intheseedling’s aboveground biomasswecalculatedtherelative increaseinaboveground biomassfortheexperimen- talperiod(AB)ofeachspecies as:(Bt1-Bt0)/Bt0, whereBt1 is thebiomassatharvestingtime andBt0 is theestimatedbiomassbeforetreatment.
Differences intheABofeachspeciesgrowing alone,underintraspecific competition,andunder interspecificcompetitionwerecomparedwithafour- levelone-factoranalysisofvariance(ANOVA).Data waslogtransformedtomeettheassumptions of parametricanalysis. Onlyoneplantperpotwas randomly selectedtoevaluatetheeffectofintraspe- cific competition.Fisher’sLSDtestwasusedto establishpairwisecomparisons. Accordingtoour hypothesisofC.selloanabeinga superiorcompetitor thanthenativespecies,weexpectedC.selloanatobe lessaffectedbyinterspecificorintraspecific compe- titionthantheothernativespecies.
Furthermore, aseconddataanalysiswascon- ducted to test if C.selloana had a superior competitiveabilitythanthetwonativespecies.We consideredboththeimpactandtheresistancecom- ponents of invasion by using two different approaches proposedbyVila`andWeiner(2004). First, with regard to the alien species impact we testedwhethertheeffectofC.selloanaoneachofthe twotargetnativespecies waslargerthantheeffectof onenativespecies ontheothernativespecies. Secondly,focusingonthenativespecies’resistance presentedtothealienwetestediftheeffectofa native species on C.selloana was lower than its effectsontheothernativespecies. Therelative interaction index (RII) proposed by Armas etal. (2004)wasusedtoestimatetheintensity ofthesize effectofcompetitionineachpot.Thisindex has revealedseveraladvantages comparedtoothercom- petitionintensity indicessuchastherelative competitionintensity orthelogresponseratio(Grace
1995;Goldbergetal.1999).RIIhasvaluesranging from-1to+1anditissymmetricalaround zero.A negativevalueindicatescompetition(i.e., growth of thetargetspeciesisreduced) andapositivevalue indicatesfacilitation(i.e.,growthofthetargetspecies ispromoted).RIIisexpressedas:
RII Bw—Bo
¼
wþBo
whereBwistheobserved massofthetargetplant whengrowingwithanotherplantandBoisthemean massachievedbythetargetplantgrowing inthe absenceofinter-orintraspecificinteractions.
IfC.selloanahasa highercompetitiveabilitythan thetwo nativespecieswe wouldexpectthefollowing results.First,withregard tothealien’simpact,RII F:CandRIIB:CwouldbemorenegativethanRII B:FandRIIF:B,indicatingthatthenegative effectof C.selloanaonthegrowthofnativespeciesislarger thantheeffect ofanativeontheothernative.Second, withregardto the native’sresistance,RIIC:FandRII C:Bwouldbelessnegative thanRIIB:FandRIIF:B, indicating thatthenegativeeffectof anativespecies ontheother nativeislargerthantheeffectonthe alien.Thesecontrastsweretestedbyunpaired t-tests.
Waterstressexperiment
InApril2004,3monthsaftersowing,seedlingsof C.selloana, B.phoenicoides and F.arundinacea weretransplantedoutdoorsattheUABcampusinto
2–lpotswithgardeningsoilplacedonbenchesunder ashelterwhichconsisted ofa6-m-tallplasticcover sustained by a metallic structure. Therefore the shelterexcludedrainfall,allowing forthecontrolof watering during theexperiment.Thisshelterreduced incidentlightby20%.
Before starting theexperiment,allpotswere wateredtoexcess andallowed todrainduringone night.Species wererandomlyassignedtooneofthe followingwateringtreatmentsfollowing thesame protocol as in a previous study conducted with C.selloana and C.jubata seedlings (Stanton and DiTomaso 2004):(1)afullywateredtreatment (hereafter calledcontrol) whichwasusedasan indicatorofunstressed growth;(2)anintermediate drought stresstreatment(hereaftercalledmoderately stressed),inwhichatthebeginningoftheexperiment waterwaswithheld fromtheplantsfor6days,which were then watered every day for the following
8days,andnotwateredduringforthe successive
31days;(3)asustaineddrought treatment(hereafter calledseverelystressed), inwhichplantsdidnot receivewaterduringthecourseoftheexperiment.
Each treatment was replicated 12times. The final number of pots was 108 (3 water stress treat- ments93 species912 replicates). Pots were randomlymovedonceaweekinordertoguarantee thatalltheplantsweregrowing underthesame conditions.
At the end of the experiment, 45days after planting,inordertodetectifthewatering treatments hadbeenhomogeneous amongspecies,wemeasured soil moisture with a time-domain reflectometer (TDR)inasubsampleofsevenpotspertreatment andperspecies.Attheendoftheexperimentthree leavesofthreeplantsofeachspeciespertreatment wererandomlychosen todeterminemeanleafarea (LA). Leaveswereimmediatelytakentothelabora- toryandtheirareawasmeasured with adeskarea meter(Li-COR, Lincoln,NE,USA).Leaveswere weighedafterair-drying at70°Ctoconstantweight, andthemeanSLAwascalculatedforeachplantas theratiobetween meanleafareaandmeanfoliar weight.
Alltheplants werecut,air-dried at70°C to constantweight,andweighedtomeasurefinalabove- andbelow-ground biomass.Rootswerewashedvery carefullyinorder toseparate themfrom thesoil without losingmaterial.Asforthecompetition experiment,inordertocorrectfortheinitialdiffer- encesinabove-andbelow-ground biomasswe calculatedtherelativeincrease inaboveground biomass(AB)andbelow-ground biomass(BB)of eachspeciesas:(Bt1-Bt0)/Bt0, whereBt1 isthebio- mass at harvesting time and Bt0 is theestimated biomass beforetreatment.Wealsocalculatedthe root–shootratio(R/Sratio)foreachplantastheratio betweenbelow-andabovegroundbiomass.
Differences betweenwaterstresstreatmentsand species on SLA, AB, BB, and R/S ratio were analyzedwithatwo-wayANOVAwithspeciesand waterstresstreatmentasfixedfactors.IfC.selloana isless affectedbywater stress than thenativespecies aspredicted,wewouldexpectB.phoenicoidesand F.arundinacia torespondmoredrasticallytomod- erateandseverewaterstressinallthemeasuredplant parameters than C.selloana. Pairwise differences between stresstreatmentsandbetweenspecieswere analyzedwithaFisher’s LSDtest.AB,BB,andR/S ratiowaslogtransformedandSLA1/logtransformed tomeettheassumptionsofhomogeneityofvariances andtofitanormaldistributionofdata.
Results
Competitionexperiment
Therelativeincreaseinabovegroundbiomass(AB) ofC.selloanaandF.arundinaceaseedlingswasnot significantly affectedbyanyofthefourcompetition treatments: (F3,40=1.81, P=0.16, b=0.43 and F3,31=0.41, P=0.75, b=0.75; respectively) (Fig.1).However,significantdifferenceswerefound for B.phoenicoides (F3,42=5.04, P=0.005, b=0.9).ABofB.phoenicoidesseedlingsincom- petition with C.selloana (Fisher’s LSD test, P=0.006)andwithF.arundinacea (Fisher’sLSD test,P=0.01)wassignificantlylowerthanunder intraspecificcompetition(Fig.1).
Becauseoursamplesizewassmall,toreducetypeII errorwealsoanalyzeddataforeachspeciesbypooling thetwointerspecificcompetitiontreatments.Wefound interspecificcompetitionnottoreduceABinC.sello- ana(Fisher’sLSDtest,P=0.20)andF.arundinacea (Fisher’sLSDtest,P=0.105)butonlyinB.phoe- nicoides(Fisher’sLSDtest,P=0.005).
Regardingthealienspeciesimpact,onaveragewe found negativevaluesofRIIeitherwhen C.selloana wasgrowingwithB.phoenicoidesorwithF.arun- dinacea,indicatingthatthealienspecies competed withthetwonativespecies(Fig.2).However,the effect of C.selloana on B.phoenicoides was not significantlylargerthantheeffectofF.arundinacia onB.phoenicoides(t20 =1.07,P=0.30).Similarly, theeffectofthealienspeciesonF.arundinaciawas notsignificantlylargerthantheeffectofB.phoenic- oides on F.arundinacea (t16 =0.81, P=0.43). Whenfocusingonthealienspeciesresistance tothe natives,asignificantdifferenceappeared:theeffectof B.phoenicoidesonC.selloana wasnegativewhile theeffectofB.phoenicoidesonF.arundinacia was positive (t20 =-5.06, P=0.001). However, no significantdifferenceswerefoundbetweentheeffect ofF.arundinacea onC.selloana andtheeffectof F.arundinacea on B.phoenicoides (t16 =0.86, P=0.40).
Waterstressexperiment
Droughttreatmentssignificantlyreducedsoilmois- ture with respect to the fully watered control treatment (F2,54=321.12, P\0.0001). However,
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Fig.2 Relativeinteractionindices(RII)(mean+SE)com- parisonusingthealien’sspeciesimpactandnative’sspecies resistanceapproaches.Positive RIIindicateafacilitativeeffect betweenspeciesandnegativeRIIindicatecompetitionbetween
species.Theasteriskindicatesignificantdifferences(P\0.05).
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Theinteractionbetween thesoilmoisture stress treatment and species was not significant (F4,54=2.54,P=0.05),indicating that the water stresstreatmentwashomogeneousacrossspecies.
TheSLAwassignificantlydifferentbetweenthe threetargetspecies(F2,18=25.31,P\0.0001)and between water stress treatments (F2,18=20.44,
0
BB:BB:CB:F
Competitiontreatments
Fig.1 Relative increase in aboveground biomass (mean+SE) of C.selloana (C), F.arundinacea (F), and B.phoenicoides(B)infourtreatments:growingalone,under intraspecific competition,andunderinterspecific competition. Different lettersabove columnsindicatesignificantdifferences betweencompetitiontreatmentsaccordingtoFisher’stest
therewerenosignificant differencesinsoilmoisture between the three target species (F2,54=2.75, P=0.07). As expected, the highest soil moisture was measured in control/nonstressed pots (36.30±1.43%), followedby moderately stressed pots (9.72±0.81%), and severely stressed pots (5.98±0.51%).Consequently,moderateandsevere waterstresscaused73.22%and83.5%reductionin soilmoisturewithrespecttothecontroltreatment.
P\0.0001)(Fig.3a).TheSLAforF.arundinacea
wassignificantlyhigherthanintheothertwospecies (Fisher’sLSDtest,P\0.0001)andtherewereno significantdifferencesbetweenB.phoenicoidesand C.selloana (Fisher’s LSD test, P=0.15). The highestSLAwas measuredincontrolplants(Fisher’s LSD test, P B0.0001). There was a significant interactionbetween speciesandwaterstresstreat- ments (F4,18=6.94, P=0.001), indicating that water stress affectedinadifferentway thetarget species:theSLAofF.arundinaceawasonlyreduced by severe water stress. In contrast, the SLA of C.selloanaandB.phoenicoidesstartedtobereduced under moderate water stress. Under severe water stresstherewerenosignificantdifferencesintheSLA betweenthethreespecies.
TheABwasalsosignificantlydifferentbetween species(F2,98=17.57,P\0.0001)andtreatments
(F2,98=427.87,P\0.0001)(Fig.3b).F.arundin- aceaproduced significantly more AB than B.phoenicoides(Fisher’sLSDtest,P=0.02)and C.selloana(Fisher’sLSDtest,P=0.0006).ABof C.selloana was significantly lower than that of B.phoenicoides (Fisher’s LSD test, P\0.0001). ABwasthehighest inthecontrolwaterstress treatmentsandthelowestintheseverewaterstress treatments (Fisher’s LSD test, P\0.0001). The interactionbetween speciesandwaterstresstreat- ments was also significant (F4,98=8.90, P\0.0001): AB of all species was reduced by moderateandseverewaterstress,yetF.arundinacea plantswerethemost negativelyaffectedbecausethe reductioncausedbymoderateandseverestresson ABwasconsiderably largerthanthatexperiencedby C.selloanaandB.phoenicoides.
The same pattern was observed for BB: there weresignificantdifferencesbetweenspecies
(F2,98=122.75, P\0.0001) and treatments (F2,98=277.35, P\0.0001) and the interaction was also significant (F4,98=17.19, P\0.0001). Severewaterstresssignificantlyreducedmorethe BBwithrespecttothecontrol treatmentthanthe moderatetreatment(Fisher’sLSDtest,P\0.0001). F.arundinacea producedthehighestBB,especially inthecontrol waterstresstreatment,butthisspecies wasagainthemost affectedbymoderateandsevere waterstresstreatments(Fig.3c).
Finally,theR/Sratiowassignificantly different between species (F2,98=256.29, P\0.0001). F.arundinaceahadthehighest R/Sratio(Fisher’s LSDtest,P\0.0001)anditwassignificantlylower inB.phoenicoidesthaninC.selloana(Fisher’sLSD test,P\0.0001).Waterstresstreatmentsalsohada significant effect (F2,98=5.22, P=0.0007). Fur- thermore,therewasasignificantinteractionbetween speciesandwaterstresstreatments(F4,98=10.47,
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Fig.3 Effectofwaterstresson(A)specificleafarea,(B) relativeincreaseinabovegroundbiomass,(C)relativeincrease in below-ground biomass, and (D) root–shoot ratio
(mean+SE) in the alien C.selloana and in the natives
B.phoenicoidesandF.arundinacea
P\0.0001).BothC.selloana andB.phoenicoides increased theirR/Sratioundermoderateorsevere droughtconditions,andtheincreaseexperiencedby C.selloanawashigherthanthatofB.phoenicoides. However,F. arundinacea behavedinanopposite manner(Fig.3d).
Discussion
Our competitionexperimentbetweenseedlingsofthe alienC.selloanaandthetwonativegrassesF.arun- dinaciaandB.phoenicoidesrejectedthehypothesis that at early recruitment stages C.selloana is a superiorcompetitorthanthetwocoexisting native species.Interspecificcompetitionhasbeenreported toplayanimportantroleindetermining thelikeli- hoodofplant invasions(Crawley1990).Inparticular, alienspeciesarehypothesized tobesuperiorcom- petitors thannativespeciesasaresultofdifferent evolutionaryhistories (Baker1965;Holdgate1986; Roy1990;KeaneandCrawley2002).However, this statementshouldbecarefullyconsidered sincethe nativespecieschosen toperform competitiveexper- imentswithalienspeciesusuallytendtobelong to differentfunctionalgroups(HuennekeandThompson
1994).Forinstance,alien annualgrassesinCalifornia grasslandshavebeenreportedtonegativelyaffectthe perennialbunchgrass Nassellapulchra,especially whencompetingforwater(Hamiltonetal. 1999). Similarly, astrongsuppressive effectofthealien Lythrumsalicaria hasbeenfoundontherarenative congener,L.alatum(Hager2004).
Furthermore,whentestingthehypothesisthatan alienspecies isabettercompetitorthananative species,simultaneous considerationofboththe invader’s relativeimpactandthenativespecies’ relativeresistance totheinvaderhasrarelybeen attempted(Vila`andWeiner2004).Consideringthe invader’srelativeimpact,weexpectedC.selloanato reducethegrowthofthetargetnativesF.arundinacea andB.phoenicoidesmorethanitcouldbereducedby growingwiththecoexistingnative.Conversely,we foundthattheeffectofC.selloana onbothnative specieswasnotsignificantlydifferentthantheeffect that F.arundinacea and B.phoenicoides had on B.phoenicoidesandF.arundinacea,respectively.
Withregard tonativespecies’ resistance,we expectedthatthenegativeeffectofanativespecies
ontheother would begreaterthantheeffectonthe alienC.selloana.However,ourresultsconfirmedthe opposite outcome: B. phoeincoides reduced the growthofC.selloanawhereasitfacilitatedF.arun- dinaceagrowth.Therefore,attheindividualleveland atanearlystageofrecruitment,C.selloana seems nottohavethepotentialtodisplaceanyofthetwo nativespecies andatthesametimetoresist competitionposedbythenativespecies.
Withregards toresponse towateravailability,our resultsdonotsuggestabetterperformanceofC.sello- anaunderwaterstressthanthetwocoexisting native grasses.C.selloana’sinvasivepotentialinMediterra- neanecosystemshasbeenrelatedtohighwater-use efficiencyandtoahighwatercapturewhenwaterisnot limiting.Moreover, abroadtolerancetowaterstress hasbeenreported(Lambrinos 2002).Anotherstudy whichcompared thegrowthresponseofC.selloana andC.jubataseedlingstodifferentwateravailabilities in greenhouse experiments found that C.selloana toleratedwaterstressbetterthanitscongener(Stanton andDiTomaso 2004).Ourwaterstressexperiment onlypartiallysupportstheseresults.C.selloanawas notasaffectedbymoderateandseverewaterstress treatmentsasF.arundinacea.However,thereduction inabove-andbelow-ground biomassexperiencedby C.selloanaduetomoderateandseverewaterstress wassimilarto thatofB.phoenicoides.
Invasiveness hasoftenbeenrelatedtotraits associated withtheabilitytoopportunistically cap- tureavailableresources.SLAandroot–shootratioare twoparameters indicatingplantabilitytointercept solarenergyandsoilresources, respectively.The capacity to achieve a large R/S ratio as water becomes a limiting factor can determine plant survivalespeciallyinMediterranean climateswitha longsummerdrought(Spechtetal.1983;Broncano etal. 1998;Sardansetal. 2004).We foundthat under stress conditions the R/S ratio of C. selloana increasedmorethanthatofB.phoenicoides, indicat- ingthatC.selloanaseems tomaximizewater uptake byincreasingbelow-groundbiomassandtominimize waterlossesbydecreasingaboveground biomass (Matsuda etal.1989;PoorterandRemkes 1990).In contrast, the opposite response was found for F.arundinacea, indicatingthat it ismoreaffected bywaterstressthantheothertwospecies.
Similarly, weexpectedthatifaninvasivespecies hasabetterresistancetowaterstressthananative
speciesitshould havehighSLAandexperiencea lowerreduction initsSLAwhenwaterislimitingas foundinotherstudiescomparingpairsofinvasive andnoninvasivespecies(BaruchandGoldstein1999; Grottkopp etal. 2002; Lake and Leishman 2004; GrottkoppandRejma´nek2007).However,ourresults provedthatC.selloanawasnotthespecieswiththe highest SLA and in fact SLA was reduced at moderate water reduction. Garcia-Serrano etal. (2005)alsofoundSLAnottobeagoodpredictor ofinvasiveness forinvasiveandnoninvasive Senecio speciesgrowinginMediterraneanconditions.
Mostinvadersdonotalwaysperformbetterthan co-occurring native species (Daehler 2003). We foundtheinvaderC.selloanatodisplaylittlecom- petitiveadvantageorresistance tocompetitionwhen growingwiththetwonativegrassesB. phoenicoides andF.arundinacea.Inaddition,C.selloanadoesnot alwaysperformbetterundermoderateorseverewater stress;yet,thegreatestcapacitytoincreasetheR/S ratiowhenwaterisscarcecanbeanadvantageduring thedriestMediterranean seasonandseemstooffer plasticityintheabilitytocapturesoil resources. Althoughourstudywasconductedataveryearly C.selloana growthstage,theresultsareinaccor- dancewithfield observations,seedlingtransplant experimentsinthefield,andmodelingsimulations, whichhavefoundthatC.selloanapopulationsper- formbestinruderalhabitatsandbenefit from disturbances(Dome`nechandVila`2006;Dome`nech andVila`2008;Pausasetal.2006).
Acknowledgements We thank L. Marco for helping to maintain the outdoor pot experiments, and J. Sardans and twoanonymousreviewersforvaluablecommentsonanearlier draftofthismanuscript. Fundingwasprovidedbythe REN2000-0361/GLO, RINVE (CGL2004-04884-C02-01/ BOS)projectsfromtheMinisteriodeCienciayTecnolog´ıa, andALARM(GOCE-CT-2003-506675)oftheFP6oftheEU.
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