John Carroll UniversityCarroll Collected

Biology

4-2012

Trait Convergence and Plasticity Among Nativeand Invasive Species in Resource-PoorEnvironmentsRebecca E. DrenovskyJohn Carroll University, rdrenovsky@jcu.edu

Albina Khasanova

Jeremy J. James

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Recommended CitationDrenovsky, Rebecca E.; Khasanova, Albina; and James, Jeremy J., "Trait Convergence and Plasticity Among Native and InvasiveSpecies in Resource-Poor Environments" (2012). Biology. 7.http://collected.jcu.edu/biol-facpub/7

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Botany American Journal of Botany 99(4): 629 639. 2012.

TRAIT CO VE RGENCE A 0 PLASTICITY lONG NATIVE NO I V IVE SPECIES I

RESOURCE-POOR E VIRONMENTS 1

R EBECCA E. D RE ov KY 2A, ALB! 'A K IIASAI OVA 2, AND J EREil t Y J. J 1\ IES·'

~Biology Departmen t. John Carroll Uni,·cr,it) 20700 1orth Pari.. Bhd .. l ' ni\cr,it) ! Ieight;, Ohio44118 SA: and ·'US DA­

Agricultura l Research Service. Ea\lern Oregon Agricultural Re!-.earch Center 6 7826-A I"') 205. Burn-.. Oregon 97720

Pre111i.1e of .\tudy: Functional trai t compari!-.Oil\ pro1 ide a framework "ith which to a\\CS\ 1nva,ion and im a.,ion re'i'tance.

However. recent '>ludic; ha1c found evidence for both trait con vergence and di1·ergencc amon!?, coc\i\ting dominant native and

ima,i vc spec ies. Few studic'> have asse,sed ho" multiple \lrc\Se<, con'>train trait value' and pla'oticit). and no ,tud) ha' in­

cluded direct mca>uremcnt<, of nutrient conservation trait <.. "hich arc critical to plants gro\\ ing in lo\\ -re,ou1-ce environment\.

Merhod.1: We c,·aluatcd how nutrient and water stressc-. affect gro\\ th and allocation. wmer potential and gas exchange. and

nitrogen (1 ) allocation and usc trail'> among a suite or six codominant specie'> from the Intermountain We'ol to determine trai t

values and plasticity. In the greenhouse. we grew our species under a full factorial combination of high and low Nand water

availabil i ty. We measur d rela ti ve growth rate (RGR) and its componcnu,. total biomaS>. bioma\\ allocation. midday \\ater

potential, photosynthetic rate. wa ter-usc efficiency ( WUE). green leaf N, scncsced leaf . tot:d 1 pool\, 1 productil ity. and

photosynthetic usc effic iency.

• Key results: Overall. soi I water avai lability constrained plant responses to a,·ailabilit) and wa' the major dri1 er of plant trait

variation in our analysis. Drought decreased plant biomass and RGR. limited N con>ervation. and led to incrca'>cd WUE. For

most trait s. native and nonnati ve species were similarly pla'>tic.

• Conclusions: Our data suggest native and inva'oivc bioma'' dominants ma) converge on functional!) 'imilar trail\ and demon­

strate comparable ability to respond to changes in resource a1ailabilit).

K ey words: forbs: gas exchange: Great Basin: knapwccd: nitrogen: resorption proficicnc).

Functional traits prov ide a va luable conceptual bas is for de­scribing vari ation in pl ant ecologica l strategies, the eli . tri bution and abundance of species. and mechani sms of coex istence and community assembl y as well as for pred icting ecolog ica l ef­fec ts and responses of pl ant commu niti es to their environment (Weiher and Keddy, 1999 : Di az and Cabido. 200 I : Lavorel and Garnier, 2002: uding et al. . 2008). A lternati ve hypoth e. cs about mechanisms of in vas ion and in vas ion res istance di ffer in their pred iction of how functi onal traits and trait pia. t ic ity are expected to vary among nati ve and in vasive species . For ex­ample, hypotheses based on habitat fi ltering and neutral pro­cesses predi ct that in vas i ve species and dominant nati ve spec ie should have similar functional trait va lues (Thompson et al. , 1995; Duncan and Willi ams, 2002; Daleo et al. . 2009). A lterna­ti ve ly, hypotheses based on limiting simil arity between nati ve and in vas i ve species as well as hypotheses based on lea f and root ti ssue economi cs predict that in va. ive species and domi­nant nati ve species should di ffer in their functional traits (Fargione et al. , 2003; Funk 2008) . A dditi onally. it has long been pro­posed that greater trait pl ast ic ity or invasi ve species compared with that of nati ve species prov ides in vas ive species a fi tness advantage under fluctuating environmental conditions (Baker, 1965). Thus, understanding functional trait convergence or diver­gence among nati ve and invas ive speci s as well as environmental

1 Manuscript received 26 August 20 I I: revision accepted 18 January 2012. The authors thank L. Z iegenhagen for laboratory and greenhouse

assistance and two anonymous rev iewers for he lpful comments on the manuscript. A Summer Re earch Fellowshi p f rom John Carroll Un iversi ty (R.E.D.) helped support this work .

4 Au thor for correspondence (e-mai l : rdrcnovsky@jcu .cdu)

doi: l 0.3732/ajb. II 004 17

constraints on trait plas ti cit y i ~ a key step toward refini ng gen­eral hypotheses of in vas ion and in as ion rcs i. lance. The~e

mechani sti c hy1 othcses. in tu rn. prov ide the ecologica l ba~ i s

for pred icting and manag ing plan t invasions (James ct al. . 20 I 0) as well as for improvi ng our understand ing or invader impacts on ecosystems.

Given the importance in understand ing functi onal trai t varia­tion, a substantial amount of research has focused on describ ing differences in trai t va lues and trai t plastici ty between nati c and invas ive species. Recent quan ti tati ve syn theses of thi s l iterature have demon. trated several strong and important pattern s of functi onal trait variation between na tive and invasive spec ies.

ommunity- and global-sca le comparisons of nati ve and inva­si ve leaf tra its as we ll as meta-analys is have demonstra ted that in vas ive specie. tend to producer thinner and less dense lea es than nati ve spec ies. resulti ng in a higher spec i fic leaf area (SL ) (Leishman et al. . 2007: van Klcunen ct al. . 20 I 0). Wi th respect to carbon ass imilation and allocation. a higher LA allows in­vas ive species to achieve a grea ter return on biomass invested in leave. and allows invasi c species to achieve greater root and shoot growth rates than native species CLambers and Poortcr, 1992). U lti mately, these and other trai ts. uch as high leaf nutri ­ent concentration and assimilation rates posi tion invas ive species further along the lea f economic spectrum toward an eco log ica l strategy that favors resource caplllre ove r resource conserva ­ti on (Wri ght et al. . 2004: Leishman et al. , 20 I 0).

Recent meta-analys is and phy logeneti ca ll y con tro ll ed com­pari sons also support the long-held notion that in vas i ve species have greater trait plasti city than nati ve species when resources increase (Funk. 2008; David. on et al. . 20 II ). However. some key examples run counter to these general trend . For example. broad-. ca le com pari . ons of trait di fference between in vas i c and nati ve pec ies acros. contrasting cl imati c and land-usc

American Journal of Botany 99(4 ): 629-639. 20 12: http://www.amjbot.org/ © 2012 Botanical Society of Ameri ca

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630 \ILRIC \' JOL R'AL 01 801A ,y I Vol. 99

regime'> ~uggest envi ronmental filtering has led to similar di'>­tributions of functional traits between native and in va~ i ve her­baceous plants (Tecco et al.. 20 I 0). Likewise. a recent study found lillie ev idence for difference<, in plasticity among native and invasive forbs (Scharfy et al. . 20 11 ). Previous swdies that support increased pla<,ticity of in va<,ive over native species also suggest pla<,ticity can vary depending on resource type and that plasticity may not necessarily confer a fitness advantage (Funk. 2008: Davidson et al. . 20 11 ). Key knowledge gaps limit ou r understanding of trait va lues and trait plasticity differences be­tween native and inva<>ive <.pecies. particularly in resource-poor environments.

First. it is unclear how multiple stresses inOuence differences in trait va lues and plasticity between nat ive and invas i ve spe­cies. Recent meta-analysis and literature rev iews indicate that most work on plasticity and trait va lues of nati ve and in vas ive species has largely been based on single- resource manipul a­ti ons (Ri chards et al. , 2006; Dav idson et al.. 20 11 ; but see Leishman and Thomson, 2005; Funk. 2008) . Single-resource manipulations ignore the important ecologica l constraints that limit or alter the adapti ve va lue of trait pl asti city (Valladares et al. , 2007). Specifically, in resource-poor environments. plant growth and plasticity often arc limited by multiple abiotic stre<.scs (Valladares and Pearcy, 2002). Thus, in these environ­ments phenotypes that di splay a fitness advantage under ma­nipulation of a single factor may be maladaptive or constrained when pl ants are exposed to other abiotic stressors (Vall adares et al. , 2007). We propose nutrient and drought stres., together, may have one of the strongest effects on differences in trai t va lues and trait plasticity between native and invasive species. Invas ion has long been tied to increases in nutrient availability (1-luenneke et al.. 1990: Thompson et al.. 200 I ). However, inva­sive species recentl y have been found to be ucces ful in both high- and low-nutrient so il s (Funk and Vitousek, 2007). Their success has been linked to greater resource-u. e effic iency (e.g .. Drenovsky et al.. 2008) and to their abi lity to construct cheaper (less thick and dense) lea f and root ti ssue in both hi gh- and low­fertility so il s, prov iding in vasive species an initial growth ad­vantage (J ames et al. , 20 I I ). However, the rapid growth and size advantage that in vas i ve species achieve through the con­struction of cheaper tissues comes at a co. tin terms of decreased tissue life span and a reduced ability to maintain physiologica l function under drought stress (Dale and Causton, 1992; Ramirez­Valiente el al. , 2010: Scheepens et al. , 2010). Construction of tougher tissues by native species can increase resource conserva­tion by decreas ing ti ssue loss due to ab iotic and biotic stress and can allow native species to maintain growth as so ils dry. Thus, whi le development of more expensive tissues may limit native plant plasticity in response to variati on in nutrient availabi lity. under drought stress it may allow native specie to maintain greater fitness than in vas ive species .

Second , we know very lilli e about variat ion and plasti city of resource conservation traits of nati ve and invas ive species. The bulk of comparati ve work has focu sed on traits related to re­source capture, resource-use efficiency, growth, and biomass allocation (van Kleunen et al., 20 I 0: Davidson et al. , 20 I I ). Resource conservation is inOuenced by traits uch as SLA and resource-use efficiency . Lower SLA va lues genera lly increa. e leaf lifespan and therefore the duration of return on resource invested in leaves: likewise. greater re ource-use efficiency means a plant can construct more bioma s per unit of resource acquired (Wright et al. , 2004; Funk and Vitou. ek, 2007). Al ­though these traits have been studied in detail for nati ve and

invasi\'e ~pecies for perennial plants. they inOuence only pan of a plant"s nutrient budget. The abilities to re'>orb nutrients from senescing tissues and to store these nutrients for fu ture u<>e in stem and rootti<>sue<> are key nutrient conservati n mechanisms for plants from low-nu trient environments (Killingbeck, 1996: Killingbeck and Whitford. 1996: van Heerwaarden et al.. 2003). For native species, hi gher leaf nutrient resorpti on has been cor­related with greater whole-p lant nutrient retention and increa<,ed plant fitness (M ay and Killingbeck. 1992: Aerts. 1996) . The patterns of resorption and storage between nati ve and invasive species and the degree to whi ch multiple environmental stresses inOuence plasticity in these traits have not been examined.

The broad objecti ve of thi s study was to examine how the interactions of water and nutrient stress influence key growth. resource capture. and resource conservati on traits as we ll as trait plasticity among codominant nati ve and in vas i ve species from the Intermountain Wes t of the United States, where both water and nutrients co limil producti vity . nder the expected trade-offs as ociated w ith ti ssue economics, we hypothesized that in vas ive species would have hi gher values for traits related to resource ca pture. utili zati on, and growth. wherea. nati ve spec ies wou ld show greater va lues for traits related to nutrient conservati on. We predicted that nutrient conservation trait. of nati ve species coupled with construction of leaves wi th lower SLA would all ow them to maintain greater biomass as nutrient and water availability simultaneous ly declined. In addition , on the basi of thi s ex pected trade-off between SLA and responses to changes in resources availability , we also predicted that in va­sive species would demonstrate a hi gher SLA and greater bio­mass plasticity in response to simultaneous changes in nutrien t and water ava ilabi lity.

MATERIALS A D METHODS

Study species-The specie-. ;,e lected included a suite of na1ivc gras<,es and forbs common ly u<,ed in restoration programs in the illlermountain Wesl of the United tales. The nalivc perennial gra,se<, included Elymus elymoides (bott le­bnl'>h <>qui rreltai l: Poaccac) and P.1eudoroegneria spicata (b lucbunch wheatgra'>s: Poaccae). and the nat ive perennial forbs included Achillea mill~folium (common yatTow: Asteraceae) and Splweralcea 11111nroana (Munro·s globcmallow: Malva­ceae). Their responses were compared with those or two key nonnative forbs.

entaurea stoehe (spotted knapweed: Asteraceae) and Linaria dalmatica (Dalma­tian toadnax: Scrophu lariaccae). Both nonnative species arc li sted as noxiou" weeds in the lntermounlain West.

Experimental design- ceds of the various pecies were germinated on filler paper. and then seedl ings were transplan ied into 12-L pot<, fi lled '' ith a I : 2 fritted clay to sandy field so il mix. Large pots were used 10 minimi7c plant effects on resource avai lab ility. Two weeks after production of fin-~ true leaves. the planls were randomly a;signcd to a factorial combina1ion of one of two nitrogen ( 1) treatments (hi gh : 2 g of; low release 10-10-10 NPK: low :no fcrtilit.er add ition) and one of two water treatments (well watered vs. droughl). At !his time. six replicates of each species were harvested for determination or initi al variabl es for relat ive growth raiC calculalions (see nex t sccli on). Remain­ing plants were arranged in six bloch with one replicate per treatment per block. utrient treat ments were initiated a! thi s time. Water >IreS!. was initi ated gradually 6 wk after produc1ion of first true leaves to simu late more appropri ­aicly the field pattern; or water stress. Volumetric soi l water con tent (SWC) was measured every 3 d on all pots 11 ith a so il moi>lure probe (Hydro ense. Campbel l Scien1ific. Logan tah. A). Well-watered planls ' ere main­tained at field capaci ty ("'22- 25 'k W C). whereas droughted plants were allowed to dry down to a WC of "'8'k. When droughtetl plan ts reached thi; threshold. pot were watered back to field capacity and allowed to dry down through another drought cycle. On average. drought cyc les spanned 5 d th rough the 65-d experiment : therefore. mos1 droughted planls experienced a total or 12 drought cyc les.

pril20121 D RI '0Y'>K \ L I AI .-TRAil Pl.;\STICII y \ \;[) 1'\VASI\'1 . PI Cll s 631

Growth and allocation-To a"e" relat11·c gro11th rate and It'> componcnh.

11 hole plan!'> 11 ere han e'ted. -.eparating leal. -.tcm. and root material. Root'.

11 ere ,cparatcd from 'oil b) 11 a-.h1ng them 01 cr a fine mc'h 'creen. Lea1c' were ,canned on a na1bed -.canner. and leaf area w<l'> mea-.uretl u-.ing the image anal ­''" program WIN RHIZO (Regent' ln-.trument'. Quebec. Canada). II plant ;nalerial wa' dried in an oven at 65 C anti then weighed. Relative gro11th rate (RGR). net a\\imilation ra te ( 1AR). leaf area ratio (LAR). <opecific leaf area (SLA). and leaf ma" ratio (LMR) were calculatetl by w,ing data from the Initial and final han·e-.h. alculation-. of mean-.. E. and 95'k confidence interval-. followed Cau-.ton and Venu-. ( 19 I) for ungraded and unpaired har1e<,l\. Root nul'>'> ratio <RM R) wa-. calculaicd a\ the proportion of total bioma" allocated to root<,. For \lati\lical compari-.on-.. RGR 11 a<, calculated a-. In (final bioma" per repl1cate) - In (mean initial bioma-.-. per -.pccie\)/65 d (duralion of time between the initial and final harYe\t\).

\Vater potential and gas exchange-We a'>'>cssetl midda) water potential

11 ith a cholander-typc pre;;urc bomb. following accepted procedure' to mini ­mite transpirational water los;. Sample'> were cut ju<,t before mca\urcment and were placed in pla\tic bags on icc in a cooler until mcasuremenl. Leaf ga'> ex­change. including photm,ynthctic a;-.imi lalion and '>i0 111atal conductance. wa-. mea,urcd wilh a LI -COR 6400 Portable PhoiOsynthc, is and Fluorescence Y'>· tem (Li -COR Biosciences. Lincoln . ebra;ka. SA) with ambient sun li ght '" the light 'ource (average PAR in-.idc I he chamber> 1000 ~mol · m 1 . s 1 ). CO, concentration in '> idc the chamber wa-. '>Ct to 400 ~mol · mol 1 and now rate~ were set to 400 ~mol · s 1. Mca<,uremcnh were not recorded until condition-. had equilibrated in,idc the chamber. Three ;ub,ample mea<,urcmcnt> were made on each leaf: ihe-.e ;ub<,ample<, were a1cragcd before '> lati-.tical ana ly,;.,. Water-u-.c efficiency (WUE) wa-. defined a' photO>) nlhetic assimilation (A) di1 ided b) \lOmataJ conductance (pmoJ (01 · mol 1 11 10).

itrogen allocation and conservation- i1rogen allocation and conscrvmion traits were mca-.urcd a-. green leaf. scnesced leaf. and '> lcm and root N concen­tration" and pool sit-es: instantaneou; phmm,ynthclic ni1rogen usc efficiency (PN E): and nitrogen producti vity ( P). Ti s>uc concen1ra1ion was measured on finely ground li s>ue by usi ng mi cro Dumas combus1ion on a anal yter (Cm. tec h A naly tica l, Valencia. Ca li fo rni a. U A). lnstantaneou> photosyn­thetic nitrogen use effic iency (P 1 E) was defined a'> photosynthetic ass imilation rate per unit (p mol 0 2 ·mol 1 · s 1). itrogen protluc1ivi1y ( P) was tic­fined a; RGR divided by whole-p lan! nutrient concentration in plant li \Sue (P C). P1 C wa> calcu lmcd as a weighted average of leaf. stem. and root conccntralion. 11 ith concentrations weighted by biomass allocation to each organ. Typical!). hi gher growth is a-.socia ted with grea ter P because of high

investment in photOsyn theti c tissues and lower respiration rates (Lambers et al.. 2008).

Statistical analysis-Gi ven the number of dependent variables measured. the number of compari sons to be made. and the polcntial con·clatcd responses among vari able>. we used multi variate analysi' of variance (M A OVA) to test for lrca tmcnt effect> on our functional trai t variable!. (fo llowi ng Scheiner. 200 1 ). The main effects included . water (W ). functional group (perennial gra!.s. invasive forb. native forb: thi s caiCgoritation all ows for both difference' in morphology and ori gin to be a;.,cssed). anti block. The interactive effect<, included x W. 1 x funct ional group. W x functional group. and x W x fu nct ional group. With MA OVA, the power of 1he test decreases with the number of response vari ab les incl uded: addit ional ly. in terpretation become<, increasingly complex as more response variables are included. Thus. Scheiner (200 1) recommends constructing MANOVA moclcb around specific hypothe­ses regardi ng the response vari ab le;. Therefore. three MA OVA; were run. The first as csscd grow1h and allocation response , including the response vari ­ab les of toia l biomass. root mass ratio. RGR, and SLA. The second MA OV A assessed responses of inslantaneous phys iologica l rates. including the response variab les A. \V UE. and midday water poten tial. The third M A OVA included those variables related to nutrient alloca tion and con,ervation (green leaf 1. ;encsccd leaf . P UE. and P). Four linear contrasts following each MA OVA were used to assess ho1 native species (nat ive forbs and pe r~nn i als gra,ses) and invasive species differed in their responses at ( I ) high N. high water: (2) high . low water: (3) low . high water: and(-+) low . low water. For each MA OVA model. Roy"s greatest roo1was u. ed 10 assess the sign ifi ­cance of the MA OVA models because of its power and interpretability. as it is based on 1hc fi rst eigenva lue ( cheiner. 200 1). Addi1ionally. for each MA OV model. we present the standardi Led canonica l coefficients for the first canonical vari ate. These va lues indicme which re. ponse vari ables drive

difference' among the pret11ctnr ,·anablc' a' well a-. the correlation among the 1nclutled respon'c 'anablc'. The magnitude of the \landard11ed canomcal cocf­fil' lenh ind1cate' 11hich rc-.pon-.e 1anahle' c\plam the greatest (or lca\l) \ana­lion among the predictor 1·ariahle,, and difference' in -.ign among ihcsc 1 alue<, 111tl1cate correlation-. among re,pon'e 1anable' ( chemcr. 2001 ).

Pnncipal component\ anal) 'i' (P A) 11 <h u.,ed to (I) understand "h1ch trait-. mo-.t \trongly innuenccd d1ffcrcnces among 'pecies and treatmc1m: (2) 1n1 e'>llgate ho11 trait<- related to one anolhcr in their direc1ion of respon-.e: and (3) mdicatc 11hether ihe re-.pon'e' 11cr• -.pecic-. or functiona l group 'pcc1fic. Pnnc1pal componcnh analy-.i-. i' a multivariate an~tly-.i-. melhod !hat ortlimllc' -.ample-. on the ba'" of It near combination-. of their a'>'ociated lrcatment ,·ari­able'>. The outpul. expre">ed a-. a b11·ariatc ordination plot. enables 1 i-.uall!atlon of relation-.hip-. betw.:en trail\. 1rea1menh. \pt:cle,. anti func1ional group-.. In I hi\ ca-.e. the mean-. of the umque -.pec1e' by treatment combination' were I he '>ample<,. and the respon-.e 1·ariable-. 11 ere the treat rnent , ·ariables. \ , -.ome mea­'>urcd variable' \\ere >trongl) correlated (e.g.. and P ' L' I:: green leal anti . P). on I) one of the correlated van able' "a' u'ed in the analy"'· The follo11-1ng ,.,mabie'> "ere included in the PC : lola I bioma\\. R I R. \\' E. leaf" ater potenl.ial. Pt E. ~encsced leaf . and P. Principal component\ anal) -.i, wa-. run "1th Canoco lor W1ndow-. 4.5 (Microcomputer Po11er. Ithaca. Y. U. A).

To a\\ess functional trait pJ<t,licit). we a-.'e"ed '>pccics-lcleltrait pla,licity. a-. opposed to genotype-lei el trait pla-.ticity. Thi-. approach allo11, for greater spec1cs- and lreatmeni-lcl<:i replication becau'c replicate' arc not a\\igned to indi1idual gcnOt) pe; of the specie' ( 1-unh. 2008). We calculated the ,in~plified relative diMancc pla.,ticit) index (RDPIJ for each trait as de\cnbed by alla­darc<o et al. (2006) and Martin-. ct al. (2009). It hough man) indice' ha1e been -.ugge,tcd lo as<,e" phenotypic pJa,licil). 11 uh thi-. index. 1 alue' bet11·ecn treat ment combinalion<o can be con,idered <h replicate' and compared 'tatl\ticall\ . To calculme the replicate RDPI, 'alue-. for each -.pecie-.. we u'etlthe mean 1r~;11 ,·aluc-. for each of the s1x treatment combination-.. The a1erage 1·alue of the RDPI for each '>pecies. then. was calculatetl a-. folio" s:

NDP/.1 = '\""' Distance among mean \'alue.\ for eoch species hy treotment C0111fUJri.\on L_.,( .. ) / II

Sum l~{ mean \'alues for each .\pecil.!.\ h_,·rreatmenl companson ·

where 11 wa<; the IOtaJ number of lrcatment C0111pari,on' (in our ca ' C '>i\ UniqUe comparisons of water and N level). Values for thi-. index 1ary from() (no pJa,­ticity) to I (maximal pJa,ticity ). The replicate value' \\ere compared using a MANO A 11 ith funclional group a-. I he main effect foll<mcd b) a linear con­trast comparing native and inva-. i1 e 'pec ie,. Three 1 OVA model' were cons1ruc1ed to compare the RDPi s. <, imilar to 1he model-. con-.trucled for the functional 1rai1>. II M A OVA model-. and mu lt i1ariate contra-.t' \\ere ana­ly7cd using SAS v. 9.2 ( A Jn,titute. Car). N Y. SA).

RESULT

Growth and biomass allocation-All f"actors included in the MA OVA model significantly affected growth and biomass all ocation except for the three-way interacti on of x W x functional group (Table I : Fig. I ). For all fac lors. significant difference among treatment. were driven most strongly by changes in tota l biomas. (Table 18). with total biomass tending to increase with higher resource a ailability (Fig. I ). In general, water avai lability affected the ability or plants to respond to avail abi lity. as indicated by the significant x W interaction (Table I A ). Across all treatments. RMR was negati ve ly corre­lated with total biomass, with RMR tending to increase as total biomass decreased (Table 18. Fig. 18). Relationships between RGR and total biomass were more complex . being pos itively correlated for ome treatments (e.g., ) and negatively correlated for other. (e.g., water) (Table 18). Although both total bioma and RGR tended to decline with N availabi lity. RGR was similar, when averaged across water treatments. but total biomas tended to be reduced under low water (Fig. I C). In mo t cases. SLA was po itively correlated with total bioma. , (Table 18), with plants with thinner leaves tending to have greater biomass (Fig. I D).

630 \ILRIC \' JOL R'AL 01 801A ,y I Vol. 99

regime'> ~uggest envi ronmental filtering has led to similar di'>­tributions of functional traits between native and in va~ i ve her­baceous plants (Tecco et al.. 20 I 0). Likewise. a recent study found lillie ev idence for difference<, in plasticity among native and invasive forbs (Scharfy et al. . 20 11 ). Previous swdies that support increased pla<,ticity of in va<,ive over native species also suggest pla<,ticity can vary depending on resource type and that plasticity may not necessarily confer a fitness advantage (Funk. 2008: Davidson et al. . 20 11 ). Key knowledge gaps limit ou r understanding of trait va lues and trait plasticity differences be­tween native and inva<>ive <.pecies. particularly in resource-poor environments.

First. it is unclear how multiple stresses inOuence differences in trait va lues and plasticity between nat ive and invas i ve spe­cies. Recent meta-analysis and literature rev iews indicate that most work on plasticity and trait va lues of nati ve and in vas ive species has largely been based on single- resource manipul a­ti ons (Ri chards et al. , 2006; Dav idson et al.. 20 11 ; but see Leishman and Thomson, 2005; Funk. 2008) . Single-resource manipulations ignore the important ecologica l constraints that limit or alter the adapti ve va lue of trait pl asti city (Valladares et al. , 2007). Specifically, in resource-poor environments. plant growth and plasticity often arc limited by multiple abiotic stre<.scs (Valladares and Pearcy, 2002). Thus, in these environ­ments phenotypes that di splay a fitness advantage under ma­nipulation of a single factor may be maladaptive or constrained when pl ants are exposed to other abiotic stressors (Vall adares et al. , 2007). We propose nutrient and drought stres., together, may have one of the strongest effects on differences in trai t va lues and trait plasticity between native and invasive species. Invas ion has long been tied to increases in nutrient availability (1-luenneke et al.. 1990: Thompson et al.. 200 I ). However, inva­sive species recentl y have been found to be ucces ful in both high- and low-nutrient so il s (Funk and Vitousek, 2007). Their success has been linked to greater resource-u. e effic iency (e.g .. Drenovsky et al.. 2008) and to their abi lity to construct cheaper (less thick and dense) lea f and root ti ssue in both hi gh- and low­fertility so il s, prov iding in vasive species an initial growth ad­vantage (J ames et al. , 20 I I ). However, the rapid growth and size advantage that in vas i ve species achieve through the con­struction of cheaper tissues comes at a co. tin terms of decreased tissue life span and a reduced ability to maintain physiologica l function under drought stress (Dale and Causton, 1992; Ramirez­Valiente el al. , 2010: Scheepens et al. , 2010). Construction of tougher tissues by native species can increase resource conserva­tion by decreas ing ti ssue loss due to ab iotic and biotic stress and can allow native species to maintain growth as so ils dry. Thus, whi le development of more expensive tissues may limit native plant plasticity in response to variati on in nutrient availabi lity. under drought stress it may allow native specie to maintain greater fitness than in vas ive species .

Second , we know very lilli e about variat ion and plasti city of resource conservation traits of nati ve and invas ive species. The bulk of comparati ve work has focu sed on traits related to re­source capture, resource-use efficiency, growth, and biomass allocation (van Kleunen et al., 20 I 0: Davidson et al. , 20 I I ). Resource conservation is inOuenced by traits uch as SLA and resource-use efficiency . Lower SLA va lues genera lly increa. e leaf lifespan and therefore the duration of return on resource invested in leaves: likewise. greater re ource-use efficiency means a plant can construct more bioma s per unit of resource acquired (Wright et al. , 2004; Funk and Vitou. ek, 2007). Al ­though these traits have been studied in detail for nati ve and

invasi\'e ~pecies for perennial plants. they inOuence only pan of a plant"s nutrient budget. The abilities to re'>orb nutrients from senescing tissues and to store these nutrients for fu ture u<>e in stem and rootti<>sue<> are key nutrient conservati n mechanisms for plants from low-nu trient environments (Killingbeck, 1996: Killingbeck and Whitford. 1996: van Heerwaarden et al.. 2003). For native species, hi gher leaf nutrient resorpti on has been cor­related with greater whole-p lant nutrient retention and increa<,ed plant fitness (M ay and Killingbeck. 1992: Aerts. 1996) . The patterns of resorption and storage between nati ve and invasive species and the degree to whi ch multiple environmental stresses inOuence plasticity in these traits have not been examined.

The broad objecti ve of thi s study was to examine how the interactions of water and nutrient stress influence key growth. resource capture. and resource conservati on traits as we ll as trait plasticity among codominant nati ve and in vas i ve species from the Intermountain Wes t of the United States, where both water and nutrients co limil producti vity . nder the expected trade-offs as ociated w ith ti ssue economics, we hypothesized that in vas ive species would have hi gher values for traits related to resource ca pture. utili zati on, and growth. wherea. nati ve spec ies wou ld show greater va lues for traits related to nutrient conservati on. We predicted that nutrient conservation trait. of nati ve species coupled with construction of leaves wi th lower SLA would all ow them to maintain greater biomass as nutrient and water availability simultaneous ly declined. In addition , on the basi of thi s ex pected trade-off between SLA and responses to changes in resources availability , we also predicted that in va­sive species would demonstrate a hi gher SLA and greater bio­mass plasticity in response to simultaneous changes in nutrien t and water ava ilabi lity.

MATERIALS A D METHODS

Study species-The specie-. ;,e lected included a suite of na1ivc gras<,es and forbs common ly u<,ed in restoration programs in the illlermountain Wesl of the United tales. The nalivc perennial gra,se<, included Elymus elymoides (bott le­bnl'>h <>qui rreltai l: Poaccac) and P.1eudoroegneria spicata (b lucbunch wheatgra'>s: Poaccae). and the nat ive perennial forbs included Achillea mill~folium (common yatTow: Asteraceae) and Splweralcea 11111nroana (Munro·s globcmallow: Malva­ceae). Their responses were compared with those or two key nonnative forbs.

entaurea stoehe (spotted knapweed: Asteraceae) and Linaria dalmatica (Dalma­tian toadnax: Scrophu lariaccae). Both nonnative species arc li sted as noxiou" weeds in the lntermounlain West.

Experimental design- ceds of the various pecies were germinated on filler paper. and then seedl ings were transplan ied into 12-L pot<, fi lled '' ith a I : 2 fritted clay to sandy field so il mix. Large pots were used 10 minimi7c plant effects on resource avai lab ility. Two weeks after production of fin-~ true leaves. the planls were randomly a;signcd to a factorial combina1ion of one of two nitrogen ( 1) treatments (hi gh : 2 g of; low release 10-10-10 NPK: low :no fcrtilit.er add ition) and one of two water treatments (well watered vs. droughl). At !his time. six replicates of each species were harvested for determination or initi al variabl es for relat ive growth raiC calculalions (see nex t sccli on). Remain­ing plants were arranged in six bloch with one replicate per treatment per block. utrient treat ments were initiated a! thi s time. Water >IreS!. was initi ated gradually 6 wk after produc1ion of first true leaves to simu late more appropri ­aicly the field pattern; or water stress. Volumetric soi l water con tent (SWC) was measured every 3 d on all pots 11 ith a so il moi>lure probe (Hydro ense. Campbel l Scien1ific. Logan tah. A). Well-watered planls ' ere main­tained at field capaci ty ("'22- 25 'k W C). whereas droughted plants were allowed to dry down to a WC of "'8'k. When droughtetl plan ts reached thi; threshold. pot were watered back to field capacity and allowed to dry down through another drought cycle. On average. drought cyc les spanned 5 d th rough the 65-d experiment : therefore. mos1 droughted planls experienced a total or 12 drought cyc les.

pril20121 D RI '0Y'>K \ L I AI .-TRAil Pl.;\STICII y \ \;[) 1'\VASI\'1 . PI Cll s 631

Growth and allocation-To a"e" relat11·c gro11th rate and It'> componcnh.

11 hole plan!'> 11 ere han e'ted. -.eparating leal. -.tcm. and root material. Root'.

11 ere ,cparatcd from 'oil b) 11 a-.h1ng them 01 cr a fine mc'h 'creen. Lea1c' were ,canned on a na1bed -.canner. and leaf area w<l'> mea-.uretl u-.ing the image anal ­''" program WIN RHIZO (Regent' ln-.trument'. Quebec. Canada). II plant ;nalerial wa' dried in an oven at 65 C anti then weighed. Relative gro11th rate (RGR). net a\\imilation ra te ( 1AR). leaf area ratio (LAR). <opecific leaf area (SLA). and leaf ma" ratio (LMR) were calculatetl by w,ing data from the Initial and final han·e-.h. alculation-. of mean-.. E. and 95'k confidence interval-. followed Cau-.ton and Venu-. ( 19 I) for ungraded and unpaired har1e<,l\. Root nul'>'> ratio <RM R) wa-. calculaicd a\ the proportion of total bioma" allocated to root<,. For \lati\lical compari-.on-.. RGR 11 a<, calculated a-. In (final bioma" per repl1cate) - In (mean initial bioma-.-. per -.pccie\)/65 d (duralion of time between the initial and final harYe\t\).

\Vater potential and gas exchange-We a'>'>cssetl midda) water potential

11 ith a cholander-typc pre;;urc bomb. following accepted procedure' to mini ­mite transpirational water los;. Sample'> were cut ju<,t before mca\urcment and were placed in pla\tic bags on icc in a cooler until mcasuremenl. Leaf ga'> ex­change. including photm,ynthctic a;-.imi lalion and '>i0 111atal conductance. wa-. mea,urcd wilh a LI -COR 6400 Portable PhoiOsynthc, is and Fluorescence Y'>· tem (Li -COR Biosciences. Lincoln . ebra;ka. SA) with ambient sun li ght '" the light 'ource (average PAR in-.idc I he chamber> 1000 ~mol · m 1 . s 1 ). CO, concentration in '> idc the chamber wa-. '>Ct to 400 ~mol · mol 1 and now rate~ were set to 400 ~mol · s 1. Mca<,uremcnh were not recorded until condition-. had equilibrated in,idc the chamber. Three ;ub,ample mea<,urcmcnt> were made on each leaf: ihe-.e ;ub<,ample<, were a1cragcd before '> lati-.tical ana ly,;.,. Water-u-.c efficiency (WUE) wa-. defined a' photO>) nlhetic assimilation (A) di1 ided b) \lOmataJ conductance (pmoJ (01 · mol 1 11 10).

itrogen allocation and conservation- i1rogen allocation and conscrvmion traits were mca-.urcd a-. green leaf. scnesced leaf. and '> lcm and root N concen­tration" and pool sit-es: instantaneou; phmm,ynthclic ni1rogen usc efficiency (PN E): and nitrogen producti vity ( P). Ti s>uc concen1ra1ion was measured on finely ground li s>ue by usi ng mi cro Dumas combus1ion on a anal yter (Cm. tec h A naly tica l, Valencia. Ca li fo rni a. U A). lnstantaneou> photosyn­thetic nitrogen use effic iency (P 1 E) was defined a'> photosynthetic ass imilation rate per unit (p mol 0 2 ·mol 1 · s 1). itrogen protluc1ivi1y ( P) was tic­fined a; RGR divided by whole-p lan! nutrient concentration in plant li \Sue (P C). P1 C wa> calcu lmcd as a weighted average of leaf. stem. and root conccntralion. 11 ith concentrations weighted by biomass allocation to each organ. Typical!). hi gher growth is a-.socia ted with grea ter P because of high

investment in photOsyn theti c tissues and lower respiration rates (Lambers et al.. 2008).

Statistical analysis-Gi ven the number of dependent variables measured. the number of compari sons to be made. and the polcntial con·clatcd responses among vari able>. we used multi variate analysi' of variance (M A OVA) to test for lrca tmcnt effect> on our functional trai t variable!. (fo llowi ng Scheiner. 200 1 ). The main effects included . water (W ). functional group (perennial gra!.s. invasive forb. native forb: thi s caiCgoritation all ows for both difference' in morphology and ori gin to be a;.,cssed). anti block. The interactive effect<, included x W. 1 x funct ional group. W x functional group. and x W x fu nct ional group. With MA OVA, the power of 1he test decreases with the number of response vari ab les incl uded: addit ional ly. in terpretation become<, increasingly complex as more response variables are included. Thus. Scheiner (200 1) recommends constructing MANOVA moclcb around specific hypothe­ses regardi ng the response vari ab le;. Therefore. three MA OVA; were run. The first as csscd grow1h and allocation response , including the response vari ­ab les of toia l biomass. root mass ratio. RGR, and SLA. The second MA OV A assessed responses of inslantaneous phys iologica l rates. including the response variab les A. \V UE. and midday water poten tial. The third M A OVA included those variables related to nutrient alloca tion and con,ervation (green leaf 1. ;encsccd leaf . P UE. and P). Four linear contrasts following each MA OVA were used to assess ho1 native species (nat ive forbs and pe r~nn i als gra,ses) and invasive species differed in their responses at ( I ) high N. high water: (2) high . low water: (3) low . high water: and(-+) low . low water. For each MA OVA model. Roy"s greatest roo1was u. ed 10 assess the sign ifi ­cance of the MA OVA models because of its power and interpretability. as it is based on 1hc fi rst eigenva lue ( cheiner. 200 1). Addi1ionally. for each MA OV model. we present the standardi Led canonica l coefficients for the first canonical vari ate. These va lues indicme which re. ponse vari ables drive

difference' among the pret11ctnr ,·anablc' a' well a-. the correlation among the 1nclutled respon'c 'anablc'. The magnitude of the \landard11ed canomcal cocf­fil' lenh ind1cate' 11hich rc-.pon-.e 1anahle' c\plam the greatest (or lca\l) \ana­lion among the predictor 1·ariahle,, and difference' in -.ign among ihcsc 1 alue<, 111tl1cate correlation-. among re,pon'e 1anable' ( chemcr. 2001 ).

Pnncipal component\ anal) 'i' (P A) 11 <h u.,ed to (I) understand "h1ch trait-. mo-.t \trongly innuenccd d1ffcrcnces among 'pecies and treatmc1m: (2) 1n1 e'>llgate ho11 trait<- related to one anolhcr in their direc1ion of respon-.e: and (3) mdicatc 11hether ihe re-.pon'e' 11cr• -.pecic-. or functiona l group 'pcc1fic. Pnnc1pal componcnh analy-.i-. i' a multivariate an~tly-.i-. melhod !hat ortlimllc' -.ample-. on the ba'" of It near combination-. of their a'>'ociated lrcatment ,·ari­able'>. The outpul. expre">ed a-. a b11·ariatc ordination plot. enables 1 i-.uall!atlon of relation-.hip-. betw.:en trail\. 1rea1menh. \pt:cle,. anti func1ional group-.. In I hi\ ca-.e. the mean-. of the umque -.pec1e' by treatment combination' were I he '>ample<,. and the respon-.e 1·ariable-. 11 ere the treat rnent , ·ariables. \ , -.ome mea­'>urcd variable' \\ere >trongl) correlated (e.g.. and P ' L' I:: green leal anti . P). on I) one of the correlated van able' "a' u'ed in the analy"'· The follo11-1ng ,.,mabie'> "ere included in the PC : lola I bioma\\. R I R. \\' E. leaf" ater potenl.ial. Pt E. ~encsced leaf . and P. Principal component\ anal) -.i, wa-. run "1th Canoco lor W1ndow-. 4.5 (Microcomputer Po11er. Ithaca. Y. U. A).

To a\\ess functional trait pJ<t,licit). we a-.'e"ed '>pccics-lcleltrait pla,licity. a-. opposed to genotype-lei el trait pla-.ticity. Thi-. approach allo11, for greater spec1cs- and lreatmeni-lcl<:i replication becau'c replicate' arc not a\\igned to indi1idual gcnOt) pe; of the specie' ( 1-unh. 2008). We calculated the ,in~plified relative diMancc pla.,ticit) index (RDPIJ for each trait as de\cnbed by alla­darc<o et al. (2006) and Martin-. ct al. (2009). It hough man) indice' ha1e been -.ugge,tcd lo as<,e" phenotypic pJa,licil). 11 uh thi-. index. 1 alue' bet11·ecn treat ment combinalion<o can be con,idered <h replicate' and compared 'tatl\ticall\ . To calculme the replicate RDPI, 'alue-. for each -.pecie-.. we u'etlthe mean 1r~;11 ,·aluc-. for each of the s1x treatment combination-.. The a1erage 1·alue of the RDPI for each '>pecies. then. was calculatetl a-. folio" s:

NDP/.1 = '\""' Distance among mean \'alue.\ for eoch species hy treotment C0111fUJri.\on L_.,( .. ) / II

Sum l~{ mean \'alues for each .\pecil.!.\ h_,·rreatmenl companson ·

where 11 wa<; the IOtaJ number of lrcatment C0111pari,on' (in our ca ' C '>i\ UniqUe comparisons of water and N level). Values for thi-. index 1ary from() (no pJa,­ticity) to I (maximal pJa,ticity ). The replicate value' \\ere compared using a MANO A 11 ith funclional group a-. I he main effect foll<mcd b) a linear con­trast comparing native and inva-. i1 e 'pec ie,. Three 1 OVA model' were cons1ruc1ed to compare the RDPi s. <, imilar to 1he model-. con-.trucled for the functional 1rai1>. II M A OVA model-. and mu lt i1ariate contra-.t' \\ere ana­ly7cd using SAS v. 9.2 ( A Jn,titute. Car). N Y. SA).

RESULT

Growth and biomass allocation-All f"actors included in the MA OVA model significantly affected growth and biomass all ocation except for the three-way interacti on of x W x functional group (Table I : Fig. I ). For all fac lors. significant difference among treatment. were driven most strongly by changes in tota l biomas. (Table 18). with total biomass tending to increase with higher resource a ailability (Fig. I ). In general, water avai lability affected the ability or plants to respond to avail abi lity. as indicated by the significant x W interaction (Table I A ). Across all treatments. RMR was negati ve ly corre­lated with total biomass, with RMR tending to increase as total biomass decreased (Table 18. Fig. 18). Relationships between RGR and total biomass were more complex . being pos itively correlated for ome treatments (e.g., ) and negatively correlated for other. (e.g., water) (Table 18). Although both total bioma and RGR tended to decline with N availabi lity. RGR was similar, when averaged across water treatments. but total biomas tended to be reduced under low water (Fig. I C). In mo t cases. SLA was po itively correlated with total bioma. , (Table 18), with plants with thinner leaves tending to have greater biomass (Fig. I D).

632 A \II· RIC\'\ J Ol R'\1\L 01 B OlA '\Y I o l. 99

T \Ill! I. Statl\tical analy\ is of grov. th and allocation trait\ . (A) Overall multivariate analysis of variance (MAi\:0 A) results for gro,qh and h!oma-.., all ocation trai l '>. including total hioma'>s. root mas\ ratio (R:vlR J. relative grov. th rate <RGRJ. and specific leaf area (. LA) . Significant factor' arc 111 hold. (B ) Standardi1ed canonica l coefficient\ arc prc\cntcd to indicate the amount of variat ion described by each response ,·ariablc in the model prc-,e!Hcd. (C) Roy\ grcale\l root for speci fied linear contrasts indicates differences among native and m,a.,ive specie\ at '>pccific rc\ourcc availabilitie'>. Degree' of freedom for both the numerator (drn) and denominator (dfd) arc pre\ented.

(A) 0\l:rall MA OVA: Roy·s greate\l root

Source Value /· dfn dfd p

N 1.3 17 62.58 4 190 <0.0001 w 0.387 18.40 .:1 190 <(J.OOO I Functional group 0.479 22.88 4 19 1 <0.0001 Block 0. 145 5.61 5 193 <0.0001

X 'r\' 0. 142 6.73 4 190 <0.0001 'x functional group 0.087 4. 15 4 191 0.003

W x functional group (1.()77 3.67 4 191 0.007 N x W x functional group 0.047 2.23 4 191 0.067

(BJ Overall MA OVA: Standanhtcd canonical coefficient'> Total

Source biOITI<I\\ RMR RGR SLA 1.328 0.5-IR 0.327 0.33R

w 1. 733 - 0.700 -0.254 0.608 Functional group 1.737 -0.37-1 - 1.-186 0.896 Block 1.3 12 0.978 0.551 0.883

x W 1.916 0.054 -0.576 0.456 N x functional group 1.170 - 0.420 0.522 0.308 W x functional group 1.537 0.188 -0.325 0.996

x W x functi onal group 0.989 0.325 0.571 -0.140

(C) Linear contra-. ts: Roy\ grcatc'>t roo! Colllra\l: ativc v~.

invasive '>pccie> Value F dfn dfd p

lli gh . high water 0.135 6.41 4 190 <0.000 1 ll igh .low water 0.119 5.(rl -1 190 0.0003 Low . high water 0.219 10.38 -1 190 <0.000 1 Low . lov. water 0.08-1 4.01 4 190 0.0038

For all four contras ts, significa nt differences ex isted among nati ve and in vas i ve spec ies (Table I ). A t high and water avail ability, nati ve spec ies tended to have lower va lues for most growth-related traits than did in vasi ve species (i.e. , total bi o­mass, RMR. and SL A). thigh , low water avail abilit y, the trend was reversed, w ith nati ve species having equi va lent or sl ightl y higher va lues fo r total biomass, RMR, and RGR; only

LA tended to be higher for the in vas i ve species at high , low water ava ilability. Simil ar pauerns were observed at low high water, with in vas ive species tending to have hi gher va lues than nati ve species fo r most growth -related traits ( i.e., bi omas., RMR, and LA). At low , low water avail ability, in vas ive species tended to have hi gher biomass and higher SLA , but na­ti ve spec ies tended to have slightl y hi gher RGR and RMR .

Water potential, photosynthetic rate, and water-use effi­ciency- itrogen ava ilability, water avail ability, functional group, and the interacti on of and water all significantl y af­fec ted water potential, photo. yntheti c rate, and water-use effi ­ciency (Table 2A , Fig. 2). II other effects were not . igni ficant. For all pred ictor vari ables, water potenti al and W E were in ­verse ly correlated w ith photosyntheti c rate (Table 2B); a. water potential became increas ingly negati ve and plants became more

\\.ater-use efficient. photo'>ynthetic rate declined ( ig. 2A- ). The response .~ to and the interaction. of and water were most strongly driven by W E. wnh slightly h1gher W E at hi oher N availability: additionally. W E tended to be greater un~ler the combination of high and low water than low and low water avail abi l ity (Fig. 28). In con tras t. the responses to water availabi l ity and functional group were most strong ly driven by water potential (Tab le 2B). Plan ts grown at low water avai lability tended to have lower water potenuals than plants grown at high water availabilit~ . and perennia l grasses tended to operate at lower water pote~uals than the forbs ( F1g . . 2C).

In oeneral, native and invas1ve spec1es responded Slmtl arl y to changes in resource availabi li t) ,. ith respect to water potential. photosynthetic rate, and W E (T~b l e 2C). Only the con trast comparing native and invas ive spec1es at low . h1 gh water was maroinal ly significant. Native spec ies tended to operate at lower wat~· potemials and higher W E than in vas ive spec ies in thi s treatment combinati on. but 1n vas1ve spec1es tended to mm nta1n higher photosymhctic rates.

Nitrogen allocation and conservation-Tra its related to all ocation and conservation were signi fican tl y affected by water. functional group, x W , and x functi onal group: all other factor. were not signi ficant (Table 3A, Fig. 3). Di ffer­ences in N P descri bed the greatest proporti on of the vari ation in trails due to . functional group, x W. and x functional group (Tab le 3B). There was a trend for lower . P w ith reduced Nand water avail ability, w ith N P 1.5- to 2.5- fo ld h1gher under hi gh N, high water avail ability compared w ith all other trea t­ments in most species (F ig. 3A). Differences in P E described the oreate ·t proporti on of the vari ati on in traits due to water availability (Table 3B), w ith P UE declining 1.9-fold under low water availability (Fig. 38 ). Overall , green and senesccd leaf N tended to be higher under lower and water avail ability (Fig. 3C- D).

Few di fferences were observed between nati ve and invas ive species for all ocation and con. ervati on traits (Table 3C). The only signi ficant contrast was comparing nati ve and in vas ive specie. at high , low water avail ability . ati ve species tended to have higher green leaf and P than in vas ive spec ies but lower P UE and senesced leaf than in vas ive species under thi s treatment (Fig. 3A- D).

itrogen pool data indicate that both and water availabilit y inAuenced pool . ize in most species (Fig. 4). In general, plants grown at high N tended to have greater total pools than plants grown at lower . However. water availability limited pool size. even under high conditions. Thu . although green leaf tended to increase at lower water availability, the decrease in bio­mass at lower water availability limited total pool size. Root. and green leaves accounted for the maj ority of the total pool across treatments. However, as resource availability decreased, root pools accounted for a greater propo1tion of the total pool. These changes in root pool can be linked to increased root biomass allocation under resource-poor conditions.

Relationships among functional traits, species, and resource availability-In general, the average species scores were ar­ranged along the fir.t ax is w ith re. pect to so il water availability: thi s ax i explained 80.0% of the variati on in the data. Samples associated w ith high water avail ability were located on the I fl side of the fi rst ax is, and amples a .. ociated w ith low water availability were located on the ri ght . ide of the first ax is (Fi g. 5) . Thus, plants grown at high water availability were a_. oc iated

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Fig. I. Growth and all oca tion trai t~ of native and invasive perennia l SJ ecics, includ ing (A) total bionl<IS\. (8 ) roo t ma>s ratio. ( ) re lative gro" th rate (RG R). and (D) specific leaf area (SLA). Data are means ±SE (N = 8-9). Figure abbre•'illlirms: ELEL, £/nnu.1· elrmoides: PSSP. Pseudoroegneria 1pimw: ACM I, Achillea millefolium: SPM . Sphaeralcea Tlllmroww: CEST. Ce111a11rea stoebe: LI DA. Li11aria dalmmica . l nvasi\ e species arc indicated by an asterisk preced ing the species abbrev iation.

with hi gher P UE. P, and total biomass. In contrast, plants grown at low water avail ability were a. soc iated with higher se­nesced leaf N, more negati ve water potenti als, greater W E, and greater RMR. The second ax is explained 12.6% of the variation in the data and was most strongly assoc iated w ith RMR. Thi s ax is was associated w ith neither nor water avail ability. A l­though some diffuse grouping could be observed for some . pe­cies (e.g., S. IIIW7roana and P spica/a). neither ax is wa associated with either morphology or ori gin . w ith strong overl ap in trait responses between nati ve and invas ive species. as well as among perennial grasses and perennial Forbs.

Plasticity in functional traits-As assessed by RDPI. , plas­ticity did not differ between nati ve and in vas ive species for the suite o f growth and alloca tion traits measured (F4.30 = 0.60. P = 0.66). Of the four traits, total biomass was the most pl asti c. w ith RDPI. ranging from 0.24-0.39 (T able 4). Relati ve growth rate vari ed lillie across and water ava ilabilit y, and RMR and SLA were fairl y constant across treatments, w ith mean RDPI va lue as low as 0.0 I ca lculated for LA (T able 4A ). Overall. SLA was the least pl asti c in re. ponse to vari ation in re ource avail ­ability of the II traits evaluated.

In contrast, pl asti city va lues for water potent ial, photosyn­theti c rate. and W E were sign ificanll y different between na­ti ve and in vas i ve species (Fu 1 = 5. 12. P = 0.005) . Across all three traits. in asive spec ies were sign ificantl y more plas tic than nati ve species. O f a lithe functional trai ts measured . photo­syntheti c rate wa. the mo. t plastic in response to variation in resource availability (Table -+ B).

dditi onall y . a marginall y signi fican t di fference in plasti cit y was found between nati ve and in vas i ve species for t ra it ~ re lated to all ocation and conservati on (F4.:10 = 2.73 . P = 0.048). A l­though plasti city va lues for senesced lea f and P E were similar between nati ve and in vas ive s pec i e~. pl asti c ity in gree n lea f and P was greater for in vas i e compared w ith nati ve specie. (T able 4C) .

DISCUSSIO

In parti al supp011 of our fir t hypothe i . in va ive species achieved greater biomass than nati ve species under both low and high , when water uppl y was high. For example, at hi gh high water availability, invas ive pec ies had higher biomass than

632 A \II· RIC\'\ J Ol R'\1\L 01 B OlA '\Y I o l. 99

T \Ill! I. Statl\tical analy\ is of grov. th and allocation trait\ . (A) Overall multivariate analysis of variance (MAi\:0 A) results for gro,qh and h!oma-.., all ocation trai l '>. including total hioma'>s. root mas\ ratio (R:vlR J. relative grov. th rate <RGRJ. and specific leaf area (. LA) . Significant factor' arc 111 hold. (B ) Standardi1ed canonica l coefficient\ arc prc\cntcd to indicate the amount of variat ion described by each response ,·ariablc in the model prc-,e!Hcd. (C) Roy\ grcale\l root for speci fied linear contrasts indicates differences among native and m,a.,ive specie\ at '>pccific rc\ourcc availabilitie'>. Degree' of freedom for both the numerator (drn) and denominator (dfd) arc pre\ented.

(A) 0\l:rall MA OVA: Roy·s greate\l root

Source Value /· dfn dfd p

N 1.3 17 62.58 4 190 <0.0001 w 0.387 18.40 .:1 190 <(J.OOO I Functional group 0.479 22.88 4 19 1 <0.0001 Block 0. 145 5.61 5 193 <0.0001

X 'r\' 0. 142 6.73 4 190 <0.0001 'x functional group 0.087 4. 15 4 191 0.003

W x functional group (1.()77 3.67 4 191 0.007 N x W x functional group 0.047 2.23 4 191 0.067

(BJ Overall MA OVA: Standanhtcd canonical coefficient'> Total

Source biOITI<I\\ RMR RGR SLA 1.328 0.5-IR 0.327 0.33R

w 1. 733 - 0.700 -0.254 0.608 Functional group 1.737 -0.37-1 - 1.-186 0.896 Block 1.3 12 0.978 0.551 0.883

x W 1.916 0.054 -0.576 0.456 N x functional group 1.170 - 0.420 0.522 0.308 W x functional group 1.537 0.188 -0.325 0.996

x W x functi onal group 0.989 0.325 0.571 -0.140

(C) Linear contra-. ts: Roy\ grcatc'>t roo! Colllra\l: ativc v~.

invasive '>pccie> Value F dfn dfd p

lli gh . high water 0.135 6.41 4 190 <0.000 1 ll igh .low water 0.119 5.(rl -1 190 0.0003 Low . high water 0.219 10.38 -1 190 <0.000 1 Low . lov. water 0.08-1 4.01 4 190 0.0038

For all four contras ts, significa nt differences ex isted among nati ve and in vas i ve spec ies (Table I ). A t high and water avail ability, nati ve spec ies tended to have lower va lues for most growth-related traits than did in vasi ve species (i.e. , total bi o­mass, RMR. and SL A). thigh , low water avail abilit y, the trend was reversed, w ith nati ve species having equi va lent or sl ightl y higher va lues fo r total biomass, RMR, and RGR; only

LA tended to be higher for the in vas i ve species at high , low water ava ilability. Simil ar pauerns were observed at low high water, with in vas ive species tending to have hi gher va lues than nati ve species fo r most growth -related traits ( i.e., bi omas., RMR, and LA). At low , low water avail ability, in vas ive species tended to have hi gher biomass and higher SLA , but na­ti ve spec ies tended to have slightl y hi gher RGR and RMR .

Water potential, photosynthetic rate, and water-use effi­ciency- itrogen ava ilability, water avail ability, functional group, and the interacti on of and water all significantl y af­fec ted water potential, photo. yntheti c rate, and water-use effi ­ciency (Table 2A , Fig. 2). II other effects were not . igni ficant. For all pred ictor vari ables, water potenti al and W E were in ­verse ly correlated w ith photosyntheti c rate (Table 2B); a. water potential became increas ingly negati ve and plants became more

\\.ater-use efficient. photo'>ynthetic rate declined ( ig. 2A- ). The response .~ to and the interaction. of and water were most strongly driven by W E. wnh slightly h1gher W E at hi oher N availability: additionally. W E tended to be greater un~ler the combination of high and low water than low and low water avail abi l ity (Fig. 28). In con tras t. the responses to water availabi l ity and functional group were most strong ly driven by water potential (Tab le 2B). Plan ts grown at low water avai lability tended to have lower water potenuals than plants grown at high water availabilit~ . and perennia l grasses tended to operate at lower water pote~uals than the forbs ( F1g . . 2C).

In oeneral, native and invas1ve spec1es responded Slmtl arl y to changes in resource availabi li t) ,. ith respect to water potential. photosynthetic rate, and W E (T~b l e 2C). Only the con trast comparing native and invas ive spec1es at low . h1 gh water was maroinal ly significant. Native spec ies tended to operate at lower wat~· potemials and higher W E than in vas ive spec ies in thi s treatment combinati on. but 1n vas1ve spec1es tended to mm nta1n higher photosymhctic rates.

Nitrogen allocation and conservation-Tra its related to all ocation and conservation were signi fican tl y affected by water. functional group, x W , and x functi onal group: all other factor. were not signi ficant (Table 3A, Fig. 3). Di ffer­ences in N P descri bed the greatest proporti on of the vari ation in trails due to . functional group, x W. and x functional group (Tab le 3B). There was a trend for lower . P w ith reduced Nand water avail ability, w ith N P 1.5- to 2.5- fo ld h1gher under hi gh N, high water avail ability compared w ith all other trea t­ments in most species (F ig. 3A). Differences in P E described the oreate ·t proporti on of the vari ati on in traits due to water availability (Table 3B), w ith P UE declining 1.9-fold under low water availability (Fig. 38 ). Overall , green and senesccd leaf N tended to be higher under lower and water avail ability (Fig. 3C- D).

Few di fferences were observed between nati ve and invas ive species for all ocation and con. ervati on traits (Table 3C). The only signi ficant contrast was comparing nati ve and in vas ive specie. at high , low water avail ability . ati ve species tended to have higher green leaf and P than in vas ive spec ies but lower P UE and senesced leaf than in vas ive species under thi s treatment (Fig. 3A- D).

itrogen pool data indicate that both and water availabilit y inAuenced pool . ize in most species (Fig. 4). In general, plants grown at high N tended to have greater total pools than plants grown at lower . However. water availability limited pool size. even under high conditions. Thu . although green leaf tended to increase at lower water availability, the decrease in bio­mass at lower water availability limited total pool size. Root. and green leaves accounted for the maj ority of the total pool across treatments. However, as resource availability decreased, root pools accounted for a greater propo1tion of the total pool. These changes in root pool can be linked to increased root biomass allocation under resource-poor conditions.

Relationships among functional traits, species, and resource availability-In general, the average species scores were ar­ranged along the fir.t ax is w ith re. pect to so il water availability: thi s ax i explained 80.0% of the variati on in the data. Samples associated w ith high water avail ability were located on the I fl side of the fi rst ax is, and amples a .. ociated w ith low water availability were located on the ri ght . ide of the first ax is (Fi g. 5) . Thus, plants grown at high water availability were a_. oc iated

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Fig. I. Growth and all oca tion trai t~ of native and invasive perennia l SJ ecics, includ ing (A) total bionl<IS\. (8 ) roo t ma>s ratio. ( ) re lative gro" th rate (RG R). and (D) specific leaf area (SLA). Data are means ±SE (N = 8-9). Figure abbre•'illlirms: ELEL, £/nnu.1· elrmoides: PSSP. Pseudoroegneria 1pimw: ACM I, Achillea millefolium: SPM . Sphaeralcea Tlllmroww: CEST. Ce111a11rea stoebe: LI DA. Li11aria dalmmica . l nvasi\ e species arc indicated by an asterisk preced ing the species abbrev iation.

with hi gher P UE. P, and total biomass. In contrast, plants grown at low water avail ability were a. soc iated with higher se­nesced leaf N, more negati ve water potenti als, greater W E, and greater RMR. The second ax is explained 12.6% of the variation in the data and was most strongly assoc iated w ith RMR. Thi s ax is was associated w ith neither nor water avail ability. A l­though some diffuse grouping could be observed for some . pe­cies (e.g., S. IIIW7roana and P spica/a). neither ax is wa associated with either morphology or ori gin . w ith strong overl ap in trait responses between nati ve and invas ive species. as well as among perennial grasses and perennial Forbs.

Plasticity in functional traits-As assessed by RDPI. , plas­ticity did not differ between nati ve and in vas ive species for the suite o f growth and alloca tion traits measured (F4.30 = 0.60. P = 0.66). Of the four traits, total biomass was the most pl asti c. w ith RDPI. ranging from 0.24-0.39 (T able 4). Relati ve growth rate vari ed lillie across and water ava ilabilit y, and RMR and SLA were fairl y constant across treatments, w ith mean RDPI va lue as low as 0.0 I ca lculated for LA (T able 4A ). Overall. SLA was the least pl asti c in re. ponse to vari ation in re ource avail ­ability of the II traits evaluated.

In contrast, pl asti city va lues for water potent ial, photosyn­theti c rate. and W E were sign ificanll y different between na­ti ve and in vas i ve species (Fu 1 = 5. 12. P = 0.005) . Across all three traits. in asive spec ies were sign ificantl y more plas tic than nati ve species. O f a lithe functional trai ts measured . photo­syntheti c rate wa. the mo. t plastic in response to variation in resource availability (Table -+ B).

dditi onall y . a marginall y signi fican t di fference in plasti cit y was found between nati ve and in vas i ve species for t ra it ~ re lated to all ocation and conservati on (F4.:10 = 2.73 . P = 0.048). A l­though plasti city va lues for senesced lea f and P E were similar between nati ve and in vas ive s pec i e~. pl asti c ity in gree n lea f and P was greater for in vas i e compared w ith nati ve specie. (T able 4C) .

DISCUSSIO

In parti al supp011 of our fir t hypothe i . in va ive species achieved greater biomass than nati ve species under both low and high , when water uppl y was high. For example, at hi gh high water availability, invas ive pec ies had higher biomass than

634

T\HII 2. Stati-.t ical analy-.i-. o f water potential and ga'> exchange trai t'>. (A) O ' crall mult ivari ate analy '> i\ of var iance (M A:'>lOVA) re<,u lh for

,~a ter poten tial ('f' ~ ). photO'>) nthctic rate (A). and "atcr-u'>C efficicnc)

rWl ' l:. ). Significant fac tor'> arc in bo ld . (B) Standardi;cd canon1ca l

coeffi cient '> arc pre<,cnted to indicate the amoum o f vari ati on de-.cribcd b) each re<,pon-,c vari able in the model prc<,entcd. ( ) Roy\ greatest

root for -.peci lied l inear contra'ot '> indicate-. difference-. among nati ve and

inva'>i' c -.pcc ic-. at \ pcc ific rc<,ource availabilities. Degree-. of freedom for both the numerator (dfn ) and denominator (dfd ) are prc-.cntcd .

(AJ Overall M A OVA: Roy\ grcatc-. t root

Source

)'<

w Functional group Block

1 x W N x functional group W x functional group

x W x functional group

Value

0.12() 1.83() 0.442 0 .075 0.11i7 0.04 0 .058 0 .0 19

f

5.07 74.07 17.9()

1. 85 6.72 1.6 1 2.35 0.78

dfn

3 3 3 5 3 3 3 3

(B ) Overall M Al OVA: Standardi ; ed canonical coefficien t-.

Source '·.P .... A W E

N 0.486 0.5 13 I. I I 0 w 0.8 11 0.630 0.77 1 1-unctional group 0.98 1 0.228 0.84 I Block 0.3 79 0. 7 10 1.033 1 X W 0.593 - 0.627 0.968

x functional group 0.430 1.224 0.059 W x functional group 0.9 18 0.534 0.809

x W x functional group 0. 142 0 .603 I . 183

(C) Linear contra'ol 'o: Roy\ grcatc-.t root

Contra'> !: t ative ' '· inva<, i, ·e '>pec ic'>

lf1gh . high water lligh . low water Low . high water Lo\\ . low water

Value

0.025 0.025 0.068 0.047

r 1.0 1 1.0 1 2.74 1.88

dfn

3 3 3 3

dfd

12 1 121 122 123 121 122 122 122

d fd

12 1 12 1 12 1 12 1

p

0.002 <0.000 1 <0.000 1

0.1 09 0.0003 0. 190 0.076 0.507

p

0.390 0 .390 0.046 0.1 36

native species, though RGR was similar between the species groups. A I high , low water availability, natives maintained simi lar toia l biomass but a slightly higher RGR than in va. ivc species. Thus, although biomass and RGR declined in respon. e to decreased water availab ility in both nati ve and invas ive spe­cies, the species group achieving higher bioma . and/or RGR under a given treatment combination depended on oi l water availability. In contrast, invasive . pecies had higher total biomass bu t slightly lower RGR than nati ve . pecics under both the low treatments, regardless of water avail ability. Under all treatment combinati ons, in vas i ve spec ie. had higher SLA than nati ve species. Constructing cheaper ti . ue. may prov ide inva ive spe­cies a growth advantage under both low and high CLambers and Poor1er, 1992) as well as under well -watered conditions (Grotkopp and Rejmanek, 2007; James and Drenov. ky, 2007; James, 2008). However, under low water availability, high SLA may be disadvantageous. as it provides greater surface area for transpiration (Lambcr ct al., 2008). Additionally, maintaining a higher RMR than nati ves under both high . high water and low

, high water may have provided inva ivcs with greater acce .. to so il nutrients (Aer1s and Chapin, 2000). At lower water availabil ­ity, nati ve spc ie invested more total biomass into root. than did invasive species. Increased allocati on to roots under drought conditions is a key adaptation to maintaining plant water status

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Fig . 2. Gas exchange and water po ten ti al traits of nat ive and invasive

perennial species. inc luding {A ) photosynthetic rate, ( B) instantaneous

water-usc effi c iency ( W E). and ( ) plant water potential. Data arc means

±SE (N = S- 9). Note: Sec Fi g. I legend for definit ions of abbreviat ions.

(Lamber et al. , 2008), enabling nati ve specie to maintain greater biomas than inva ive . pecies under decreased water availability.

In contrast to our initi al hypothe e , nati ve and in vas ive spe­cies were similar w ith re pect to in tantaneous phys iologica l measurement , including midday water potential, photosynthetic

pri1 2012 1 D RI ' 0 \ 'SK Y I f AL.-T R. \IT PI AS IICIT Y '" D 1'\V·\ SIVI· . PECILS 635

T\llll 3. Stati\ti cal analy\i ' of nitrogen allocation and c.:on-,e f\a ti on U«il'>. (A )

()' erallmultimriatc analy' i' o f variance (M A NO ) re'>u lt-, for nitrogen

(N) allocation and consen ation trail\. including green leaf t • -,ene-,ccd

leaf t • photo,ynthetic 1 w.e effic iency (PN ). and nitrogen produc­

ti \ ity ( P). Significant factors arc in bold. ( 8 ) . tandardi~:ed canonica l

coe ffi cients arc prc<,cntcd to indicate the amount o f , ·ariation d<:scrihcd by

each rc!>po m.c vari able in the model prc!>cnted . ( ) Ro) \greatest root for

-,pcc ified linear contra. ts indicates diffe rences among nati' c and im·a-.i' c

specie' at <,pccific resource a\·ailabilitie'> . Degree!> o f freedom for both the numerator (dfn ) and denominator (dfd ) arc presented.

A. O' crall M A OVA: Ro) \ greate't root

Source

\\' Functiona l group Bloc k N x \V N x functional group W x functional group N x W x functional group

Value

0.722 0.549 0.250 0.075 0.165 0.098 0 .037 0 .045

I·

24.38 18.54 8.51 2.08 5.56 3.33 1.25 1.53

B. Standardi ;cd canonical coe f fic ienh

Green Senc'>ced ourcc leaf 1 leaf

w Functional group Block Nx W

x functional group W x functional group

x W x fun cti onal group

C. Roy''> greatc" root Cont ra~l: mi ve vs.

inva; ivc '>pcc ie>

High . high water High . IO\\ water Low . high water Lo\\ N , low water

0.11 5 1.0 10

-0.456 -0.1 32 -0.37 1

0 .579 - 0 .098

Value

00-+4 0. 13 1 0 .0 16 0.049

0.329 -0. 186

0 .48 1 0.934 0.032 0.304

- 0 .398 0.660

F

1.-!7 4.41 0.56 1.65

dfn

4 4 4 5 4 4 4 4

-0.432 0.9 2

-0.367 0.474 0 .188 0.655 0 .378 0 .355

dfn

4 4 4 4

dfd

135 135 136 138 135 136 136 136

1.503 0.5 10 1.03

- 0 298 1.102

- 1.304 1.1 9 1

- 0 .941

dfd

135 135 135 135

p

<0.0001 <0.0001 <0.0001

0.072 ().0004 0.01 2 0 .294 0. 198

p

0.2 15 0 .002 0.695 0. 166

rates, and W E. Midday water potential became increas ingly negati ve, and photosynthetic rates decl ined in the low-water trea tments. In contrast, WU effic iency increa ed under low water avai lability. driven in large part by trong declines in sto­matal conductance (data not shown). Midday water potentials indicate plant water tatus during the most tressful portion of the clay. when plant are balancing radiative heat loads with transpirational water loss . Under drought cond ition . plants close or partia lly c lo e their stomata to limit water losses, and as a result, photosynthetic rates decline (Ca per et al.. 2006). Over the long term. decreased photo. yntheti c rate limit carbon gain and thus the bu ilding blocks ava il ab le for new biomass pr ducti on. ln contrast to the effects of water on midday water potential and gas exchange, the impact of decreased N avail ­ab ility were more muted and were dri ven in large part by im­pacts on W UE, with W E increas ing w ith greater availability . Greater W E can come at the cost of greater requirements because of increased investment in photo yntheti c machinery (Wright ct al. , 200 I . 2003) and thus lower PN E (Martin et al. . 20 I 0). In support we observed hi gher green leaf . lower P UE, and higher WUE in plant grown at high . low water than at high , high water.

Likewise, allocation and conservation traits were imilar among our suite of nati ve and in vas ive spec ies. itrogen pro-

ducti vity and P UE were the mo'>t important driver~ of trait relation ship~ under changing resource availabilities. itrogen productivity was highest at high . high water and ~ignificantly reduced under all oth r treatments. Fast -growing plants wilh hi gh allocation to photosynthetic tissues typically have higher NP. s more biomass is invested in nonphotosynth tic ti'>sucs (e.g., greater RMR at low resource availabilities ), P declines (Larnbers ct al. , 2008) . s described previously, P E is in­versely correlated with W E and thus declined with decreased soil wat r availabilit y. Green leaf was low in well -watered plants, most likely because of biomass dilution. In contra:-t, droughtcd plants tended to have higher scnesccd leaf concen­tration ~ . indicati e of poorer resorption proficiency . The pro­cess of resorpti On requires translocation of nutrients 10 storage tissues .. which can be negatively affected by low so il water avatlabtltty (Wnght and Wcstoby, 2003; Renteria and Jaramillo. 20 I I ). Overall. five of the six species achic cd comp lete rc­:orption (<7 g ·kg 1 N; sensu Killingbeck , 1996) under at least one treatment combination . with two species ( . munroww and C. stoebe ) being hi ghly 1 roficicnt under all trcatmenls. Prc\'i ­ous greenhouse work with . stoebe indicated its hi gh -usc efficiency is linked to a long mean retenti on time, \~hi c h de­pends in part on proficient resorption (D 'I mperi o. 2005 ). All species showed . imilar allocation patterns, with root pool' becoming increasingly dominant in terms of whole-plant budgets a. resource a ailability declined. The. c changes were more strongly dri en by changes in biomas. allocation patterns (i.e., increased RMR with decreased resource avai labi lity ) than by changes in ti ssue nutrient concentrations. These data st ress the importance of nonphotosyn thcti c ti ssues to whole -plant nutrient budgets and the need to look beyond leaf Iraits \\hen studying conservati on mechanisms.

From our data. it is ev ident thai limited water availability con­stra ined responses to avai labi lity, and overall. soil' ater avail ­abi lity was the major driver of plant traits. as evidenced by the PCA. For example, drought limited total biomass production, even in the high treatment. Drought also had strong impacts on traits related to allocation and conservation. limiting resorp­tion, decreasi ng instantaneous P E, and reducing P. Drought li mits plant access to soi l , by affectin g so il biolon ica l ;nd

~ 0

physica l processc. that innuencc so il supply and plan t physi -ologica l processes that innucncc plant uptake. In dry soi ls. soi l microbial acti vity is reduced. limiti ng decomposition and miner­alization (Burke. 1989) . Of greater importance in this gn;enhouse study, drought l imits nutrien t . upply to roots by decreasing movem ntthrough soils via di ffusion or mass now (I unham and

yc. 1973 ). A s soils dry. diffusion rates decrease because of re­duced nutrient mobility in the soil. and reduced transpiration rates limit nutrient mass now rates through so il. both of which reduce plant nutrient uptake (Lambcrs ct al. . 2008).

M any au thors have argued that high avail abilit y fa ors in-vas ive species and that low N ava ilab ility favors native spec i e~.

However. ev idence i: mounting that in vasive spec ies arc suc­cessful under resource - limiting co nd i ti ons (e.g .. Fu nk and Vitousek. 2007; James ct al., 20 II ). Most work in this area has focused on the role of resource uptake and usc (e.g., Drcnovsky et al. , 2008). but many of the traits associated w ith success in low-re ource envi ronment are those related to resource con­servation and storage (Berend. e. 1994: Aerts. 1999). A lthough many authors have mea ured soft trai ts. like LA. that corre­. pond to leaf longevi ty. they are only a proxy for resource con­servation potential. In thi . study. inva ive pecies had higher SLA. but native and invas ive species had very similar allocation

634

T\HII 2. Stati-.t ical analy-.i-. o f water potential and ga'> exchange trai t'>. (A) O ' crall mult ivari ate analy '> i\ of var iance (M A:'>lOVA) re<,u lh for

,~a ter poten tial ('f' ~ ). photO'>) nthctic rate (A). and "atcr-u'>C efficicnc)

rWl ' l:. ). Significant fac tor'> arc in bo ld . (B) Standardi;cd canon1ca l

coeffi cient '> arc pre<,cnted to indicate the amoum o f vari ati on de-.cribcd b) each re<,pon-,c vari able in the model prc<,entcd. ( ) Roy\ greatest

root for -.peci lied l inear contra'ot '> indicate-. difference-. among nati ve and

inva'>i' c -.pcc ic-. at \ pcc ific rc<,ource availabilities. Degree-. of freedom for both the numerator (dfn ) and denominator (dfd ) are prc-.cntcd .

(AJ Overall M A OVA: Roy\ grcatc-. t root

Source

)'<

w Functional group Block

1 x W N x functional group W x functional group

x W x functional group

Value

0.12() 1.83() 0.442 0 .075 0.11i7 0.04 0 .058 0 .0 19

f

5.07 74.07 17.9()

1. 85 6.72 1.6 1 2.35 0.78

dfn

3 3 3 5 3 3 3 3

(B ) Overall M Al OVA: Standardi ; ed canonical coefficien t-.

Source '·.P .... A W E

N 0.486 0.5 13 I. I I 0 w 0.8 11 0.630 0.77 1 1-unctional group 0.98 1 0.228 0.84 I Block 0.3 79 0. 7 10 1.033 1 X W 0.593 - 0.627 0.968

x functional group 0.430 1.224 0.059 W x functional group 0.9 18 0.534 0.809

x W x functional group 0. 142 0 .603 I . 183

(C) Linear contra'ol 'o: Roy\ grcatc-.t root

Contra'> !: t ative ' '· inva<, i, ·e '>pec ic'>

lf1gh . high water lligh . low water Low . high water Lo\\ . low water

Value

0.025 0.025 0.068 0.047

r 1.0 1 1.0 1 2.74 1.88

dfn

3 3 3 3

dfd

12 1 121 122 123 121 122 122 122

d fd

12 1 12 1 12 1 12 1

p

0.002 <0.000 1 <0.000 1

0.1 09 0.0003 0. 190 0.076 0.507

p

0.390 0 .390 0.046 0.1 36

native species, though RGR was similar between the species groups. A I high , low water availability, natives maintained simi lar toia l biomass but a slightly higher RGR than in va. ivc species. Thus, although biomass and RGR declined in respon. e to decreased water availab ility in both nati ve and invas ive spe­cies, the species group achieving higher bioma . and/or RGR under a given treatment combination depended on oi l water availability. In contrast, invasive . pecies had higher total biomass bu t slightly lower RGR than nati ve . pecics under both the low treatments, regardless of water avail ability. Under all treatment combinati ons, in vas i ve spec ie. had higher SLA than nati ve species. Constructing cheaper ti . ue. may prov ide inva ive spe­cies a growth advantage under both low and high CLambers and Poor1er, 1992) as well as under well -watered conditions (Grotkopp and Rejmanek, 2007; James and Drenov. ky, 2007; James, 2008). However, under low water availability, high SLA may be disadvantageous. as it provides greater surface area for transpiration (Lambcr ct al., 2008). Additionally, maintaining a higher RMR than nati ves under both high . high water and low

, high water may have provided inva ivcs with greater acce .. to so il nutrients (Aer1s and Chapin, 2000). At lower water availabil ­ity, nati ve spc ie invested more total biomass into root. than did invasive species. Increased allocati on to roots under drought conditions is a key adaptation to maintaining plant water status

20 -'7 1/)

~ 15 E 0 E :::1. -

10

<( 5

-'7 90 0 E 0 60 E :::1. -w

::l 30 3:

-1 -ca Q. ::E -2 -~ ;,..

-3

A

B

c

l T

l

..J w ..J w

~

T

~

T ,.,_.

II

I '

Q. en en Q.

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I I

~

.

T

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,"

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lli

::E 0 <(

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~ ~

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.... en w 0 ...

I

I

I Vol. 99

I]

<( c ..J ...

I

~

High N, high water

High N, low water

Low N, high water

Low N low water

Fig . 2. Gas exchange and water po ten ti al traits of nat ive and invasive

perennial species. inc luding {A ) photosynthetic rate, ( B) instantaneous

water-usc effi c iency ( W E). and ( ) plant water potential. Data arc means

±SE (N = S- 9). Note: Sec Fi g. I legend for definit ions of abbreviat ions.

(Lamber et al. , 2008), enabling nati ve specie to maintain greater biomas than inva ive . pecies under decreased water availability.

In contrast to our initi al hypothe e , nati ve and in vas ive spe­cies were similar w ith re pect to in tantaneous phys iologica l measurement , including midday water potential, photosynthetic

pri1 2012 1 D RI ' 0 \ 'SK Y I f AL.-T R. \IT PI AS IICIT Y '" D 1'\V·\ SIVI· . PECILS 635

T\llll 3. Stati\ti cal analy\i ' of nitrogen allocation and c.:on-,e f\a ti on U«il'>. (A )

()' erallmultimriatc analy' i' o f variance (M A NO ) re'>u lt-, for nitrogen

(N) allocation and consen ation trail\. including green leaf t • -,ene-,ccd

leaf t • photo,ynthetic 1 w.e effic iency (PN ). and nitrogen produc­

ti \ ity ( P). Significant factors arc in bold. ( 8 ) . tandardi~:ed canonica l

coe ffi cients arc prc<,cntcd to indicate the amount o f , ·ariation d<:scrihcd by

each rc!>po m.c vari able in the model prc!>cnted . ( ) Ro) \greatest root for

-,pcc ified linear contra. ts indicates diffe rences among nati' c and im·a-.i' c

specie' at <,pccific resource a\·ailabilitie'> . Degree!> o f freedom for both the numerator (dfn ) and denominator (dfd ) arc presented.

A. O' crall M A OVA: Ro) \ greate't root

Source

\\' Functiona l group Bloc k N x \V N x functional group W x functional group N x W x functional group

Value

0.722 0.549 0.250 0.075 0.165 0.098 0 .037 0 .045

I·

24.38 18.54 8.51 2.08 5.56 3.33 1.25 1.53

B. Standardi ;cd canonical coe f fic ienh

Green Senc'>ced ourcc leaf 1 leaf

w Functional group Block Nx W

x functional group W x functional group

x W x fun cti onal group

C. Roy''> greatc" root Cont ra~l: mi ve vs.

inva; ivc '>pcc ie>

High . high water High . IO\\ water Low . high water Lo\\ N , low water

0.11 5 1.0 10

-0.456 -0.1 32 -0.37 1

0 .579 - 0 .098

Value

00-+4 0. 13 1 0 .0 16 0.049

0.329 -0. 186

0 .48 1 0.934 0.032 0.304

- 0 .398 0.660

F

1.-!7 4.41 0.56 1.65

dfn

4 4 4 5 4 4 4 4

-0.432 0.9 2

-0.367 0.474 0 .188 0.655 0 .378 0 .355

dfn

4 4 4 4

dfd

135 135 136 138 135 136 136 136

1.503 0.5 10 1.03

- 0 298 1.102

- 1.304 1.1 9 1

- 0 .941

dfd

135 135 135 135

p

<0.0001 <0.0001 <0.0001

0.072 ().0004 0.01 2 0 .294 0. 198

p

0.2 15 0 .002 0.695 0. 166

rates, and W E. Midday water potential became increas ingly negati ve, and photosynthetic rates decl ined in the low-water trea tments. In contrast, WU effic iency increa ed under low water avai lability. driven in large part by trong declines in sto­matal conductance (data not shown). Midday water potentials indicate plant water tatus during the most tressful portion of the clay. when plant are balancing radiative heat loads with transpirational water loss . Under drought cond ition . plants close or partia lly c lo e their stomata to limit water losses, and as a result, photosynthetic rates decline (Ca per et al.. 2006). Over the long term. decreased photo. yntheti c rate limit carbon gain and thus the bu ilding blocks ava il ab le for new biomass pr ducti on. ln contrast to the effects of water on midday water potential and gas exchange, the impact of decreased N avail ­ab ility were more muted and were dri ven in large part by im­pacts on W UE, with W E increas ing w ith greater availability . Greater W E can come at the cost of greater requirements because of increased investment in photo yntheti c machinery (Wright ct al. , 200 I . 2003) and thus lower PN E (Martin et al. . 20 I 0). In support we observed hi gher green leaf . lower P UE, and higher WUE in plant grown at high . low water than at high , high water.

Likewise, allocation and conservation traits were imilar among our suite of nati ve and in vas ive spec ies. itrogen pro-

ducti vity and P UE were the mo'>t important driver~ of trait relation ship~ under changing resource availabilities. itrogen productivity was highest at high . high water and ~ignificantly reduced under all oth r treatments. Fast -growing plants wilh hi gh allocation to photosynthetic tissues typically have higher NP. s more biomass is invested in nonphotosynth tic ti'>sucs (e.g., greater RMR at low resource availabilities ), P declines (Larnbers ct al. , 2008) . s described previously, P E is in­versely correlated with W E and thus declined with decreased soil wat r availabilit y. Green leaf was low in well -watered plants, most likely because of biomass dilution. In contra:-t, droughtcd plants tended to have higher scnesccd leaf concen­tration ~ . indicati e of poorer resorption proficiency . The pro­cess of resorpti On requires translocation of nutrients 10 storage tissues .. which can be negatively affected by low so il water avatlabtltty (Wnght and Wcstoby, 2003; Renteria and Jaramillo. 20 I I ). Overall. five of the six species achic cd comp lete rc­:orption (<7 g ·kg 1 N; sensu Killingbeck , 1996) under at least one treatment combination . with two species ( . munroww and C. stoebe ) being hi ghly 1 roficicnt under all trcatmenls. Prc\'i ­ous greenhouse work with . stoebe indicated its hi gh -usc efficiency is linked to a long mean retenti on time, \~hi c h de­pends in part on proficient resorption (D 'I mperi o. 2005 ). All species showed . imilar allocation patterns, with root pool' becoming increasingly dominant in terms of whole-plant budgets a. resource a ailability declined. The. c changes were more strongly dri en by changes in biomas. allocation patterns (i.e., increased RMR with decreased resource avai labi lity ) than by changes in ti ssue nutrient concentrations. These data st ress the importance of nonphotosyn thcti c ti ssues to whole -plant nutrient budgets and the need to look beyond leaf Iraits \\hen studying conservati on mechanisms.

From our data. it is ev ident thai limited water availability con­stra ined responses to avai labi lity, and overall. soil' ater avail ­abi lity was the major driver of plant traits. as evidenced by the PCA. For example, drought limited total biomass production, even in the high treatment. Drought also had strong impacts on traits related to allocation and conservation. limiting resorp­tion, decreasi ng instantaneous P E, and reducing P. Drought li mits plant access to soi l , by affectin g so il biolon ica l ;nd

~ 0

physica l processc. that innuencc so il supply and plan t physi -ologica l processes that innucncc plant uptake. In dry soi ls. soi l microbial acti vity is reduced. limiti ng decomposition and miner­alization (Burke. 1989) . Of greater importance in this gn;enhouse study, drought l imits nutrien t . upply to roots by decreasing movem ntthrough soils via di ffusion or mass now (I unham and

yc. 1973 ). A s soils dry. diffusion rates decrease because of re­duced nutrient mobility in the soil. and reduced transpiration rates limit nutrient mass now rates through so il. both of which reduce plant nutrient uptake (Lambcrs ct al. . 2008).

M any au thors have argued that high avail abilit y fa ors in-vas ive species and that low N ava ilab ility favors native spec i e~.

However. ev idence i: mounting that in vasive spec ies arc suc­cessful under resource - limiting co nd i ti ons (e.g .. Fu nk and Vitousek. 2007; James ct al., 20 II ). Most work in this area has focused on the role of resource uptake and usc (e.g., Drcnovsky et al. , 2008). but many of the traits associated w ith success in low-re ource envi ronment are those related to resource con­servation and storage (Berend. e. 1994: Aerts. 1999). A lthough many authors have mea ured soft trai ts. like LA. that corre­. pond to leaf longevi ty. they are only a proxy for resource con­servation potential. In thi . study. inva ive pecies had higher SLA. but native and invas ive species had very similar allocation

63o A\II ·R IC .,, JOl R' ·\1 or BmA'-' !Vol. 99

..... 0

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75

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-Ql

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28

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-..... 0

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0 E 0 100 E

-w ::J 50 z 0.

-..... 'en 28 .:1!

C) -z -nl <1>

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tn <1> c: <1>

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D - High N o high water [===:::J High N 0 low water [===:::J Low N o high water

Low N 0 low water

T·

rr

I I I

Fi g. 3. it rogen ( ) allocati on and con'ocrva ti ontraits of native and invasive perennial species. including (A) 1 product ivity ( P). (B) photmoynthctic

usc efficiency (PN E), (C) green leaf , and (D) scnesccd leaf . Data arc means ±SE (N = 2- 9). Note : Sec Fig. I legend for defi nitions of abbrev iati ons.

and conservati on patterns. Plants w ith hi gher P UE and lower scncsccd lea f (and thus greater resorpti on proficiency, sensu Killingbeck. 1996) had enhanced plant performance. as as­~esscd by total plant bi omass. at the end of the experiment (P :5 0.005 for both variab les: data not shown). These correlations stress the i mportancc of resource conservati on traits for the suc­cess of in vas i ve spec ies in resource-poor sy tems, and further rc. ca rch and empha. is should be placed on these traits in in va­sive spec ies. Without data on traits such as nutrient-usc effi­c iency, mean retention time, resorpti on, and storage, we w ill fail to recognize key mechani sms supporting the role of inva­sive spec ies in resource-poor environments.

ontrary to expectations, native and invasi ve specie. were sim ilarl y plastic for mos t meas ured traits. The grcatc. t dif­ferences in pl as ti c ity between native and invas i ve . pecies were observed for in stantaneous measurements (A, WU E, and plant water potential ), with in vas i ve spec ies being more plastic in response to resource ava il ab ility for all three traits. Although there was a marg inall y signifi ca nt difference in pl as ti city for N all oca ti on and con. er va tion trait s, no differ­ence in p las ti city ex isted betwe n nati ve and invasive spe-

c ies for growth and allocation traits. These results are simil ar to th ose of a study co mparing related spec ies of in va. ive and nati ve woody vines, in which nati ve and in vas i ve spec ies had simil ar plasticity for 14 out of 17 physio log ica l and growth traits measured. thoug h overall pl as ti c ity was grea ter in in vas i ve th an in nati ve species (0 unk oya et al. , 20 I 0) . Likewi e. in two phylogenetically paired studies of nati ve and invas i ve spec ies aero. sa range of li fe forms , nati ve and invas ive spec ies did not differ in their plasti c ity for a suite o f growth and phys io log ica l traits (Funk , 2008; Godoy et al. , 20 I I ). These studi es suggest pia. ticity alone may not predict the succes. of invasive species. Finall y, plasti city varied greatl y depending on the trait measured and was not consisten t among the trait groupings. The most plasti c traits generally were those that require only small change in allocati on of resources or function ing and/or arc fairly rever. ible, such as photosynthetic rate, P E, to tal biomass, and P. In contras t. those trai ts that require (or are strong ly influenced by) more long- term change in ti ssue constructi on were less pl asti c, such as RGR , SLA, green leaf , and senescedleaf 1, which were the least plastic trait .

April 20121 D RI '0' \f.' 1 r \I .- TR,\1 1 PI i\S 1 ICI 1 Y ·"-D I,V.\SI\ r Sl'l Cll s 6J7

90

"' .§.. 0

g_ 60

z

"' 0 ..... 30

- Root N pool c::J Stem N pool c::J Green leaf N pool [ii!i Senesced leaf N pool

I ELEL PSSP

~'"F

! ~ I ACMI SPMU "CEST "LID A

Fig. 4. Total nitrogen ( ) poob of nati ve and invas ive perennial 'PC­cics grown at two levels and two water leve ls. Tota l 1 poo ls arc com­posed of root. ~ tcm . green leaf. and scncsccd leaf pools. Treatment codes arc a<, follows: H HW. high . high wa ter: H LW, high . low water; L II W, IO\ , high wa ter: L LW. low . low water. Data arc means ( 7- 9). Note: Sec Fig. I legend for definitions of other abbrev iati ons.

Multiple resource limitati ons constrain plant growth and plast ic it y and may be particu larly strong driver. o f plant growth and function in arid , nutrient -poor systems (Bloom et al., 1985: Gleeson and Tilman, 1992; James ct al. , 2005; Valladares ct al.. 2007). The inOuence o f multiple resource limi tations on pattern s of trait convergence or divergence as well as trait pl asti c ity among nati ve and in vas ive spec ies has direct implications for advancing theories of in vas ion and in vasion resistance. In thi s study, inva. ivcs tended to have higher SLA. support ing the no­ti on that inva~ ive specie. tend to be positioned further along the leaf econom ics spectrum toward resource capture (Wright et al. , 2004 ). W e also found , however, strong ev idence for functional similarity and plasticity between nati ve and in vas i ve species.

N en

~

0

-1 5

SPMU • SPMU •

ELEL

··CEST 0

"LIOA ELEL SPMU 0 0 SPMU

0 0

·cesr 0

ELEL • "UOA • 'CEST

ACMI • "LIOA ELEL 0

·c•sr Senesced 1£ t N 0 • Npe ' ·uoA e •c"'' Water potential

ACMI Total biomass e , RMR. PSSP

0 PSSP WUE

PSSP

• Axis 1 (80.0%)

1.0

Fig. 5. Principal components analysis of species and plant functional traits. Colors represent different treatment combinations (black. H 1HW: white. H LW; gray, L HW; red= L LW). Note: cc legends of Figs. I and 4 for definitions of abbrevia ti ons.

T \ II II 4. Pla'>ticit) indicc' for mea'>urcd ph) ,iological and morphological \·ariables. Pla\ticit) \\a' mc<Ntred according to tht: rl!lati\C di-,tanco.: pla'>ticity indt:x. "' dc,crihcd in alladan.:'o ct al. (2006) . Data arc means

I SE ( = 6). 1ote: Sec Tahlc' 1-3 for definitions of abbrc' iations .

(A ) Gnm th anti allocation Jraih

TotallmHna" RMR RGR SLA /;"/\111111 l'iwnoide 1 P.\l'tllltlroegllcria

\f1iCOlll

Achillea millejf>lium Spluwmlcea

llllllll"(){lllll

Centuureu .\toehe Luwria dalnuuica

0.24 ± 0.06 0.26 ± 0.07

O.JJ ± 0.07 0.:19 0.07

0.28 ± 0.07 0.34 ± ().()7

(13) Water potential. A. \ E

E l'irmoide.\ P 1picaw A . mille.folium S. mu11ma11a C. 1toehe L. dalmatica

Water potent1al

0.16 ± 0.04 0.07 ± 0.02 0.19 (l.0-1 0.23 ± O.Cl:'\ 0.2-1 ± 0.05 0.2-1 ± 0.07

( ) 1 al location anti con,cnation

/:". elw11oides P .1pica1a A. mille(olium S. mu1mla 11a

. . 1toehe L. dalmcuica

Grt:cn lear

0.11 ± 0.03 0.09 ± 0.02 0 .()7 ± 0 .()2

0.08 ± 0.02 0.07 ± 0.02 0.13 ± 0.03

0.07 0.02 0.1 ± 0.02 O.OJ ± 0.0 I 0.06 0.0 I 0. 14 ± 0.03 0.02 ± 0.00

0.(>4 0.0 I 0.15 0.0:1 0.0 I + O.<Xl 0. 14 + 0.(>4 0.17 ± 0_0-l (J.02 ± 0.0 I

0.06 :1: O.<l2 0.15 0.0-1 0.0 I O.!Xl 0.06 ± 0.0 I 0. 17 ± O.Cl4 0.03 0.0 I

A \\o 1:

0.56 :t- 0. 12 0.20 ± 0.06 0.61 ± 0. I 0 0. I I ± 0.0.1 o.55 ± o. 1 1 o.:n ± o.os 0.-19 0.1 I 0.-11 ± 0. I I 0.61 0.09 0.18 ± 0.05 0.58 ± 0.10 (l..\9 ± 0.08

0.29 ± 0.06 0.20 ± 0.06 0.23 ± 0.05 0. 1 I ± 0.0.1 0.3-1 ± 0.09 0 .. 13 0.08 0.24 ± 0.05 0.-11 ± 0.10 0 28 ± 0.07 0. I K 0.05 0.2-1 ± 0.06 0.39 ().08

particularly with respect to nutrien t allocation and conservation traits. supporting community assembly hypothese~ based on habi tat filtering (Tecco ct al.. 20 I 0). Importantly , oUJ· data shO\ that multiple resource limitations influence the degree of trait convergence or divergence between in vasive and native spe­cies. The limited number or spec ies used in thi s study and the lack of phylogenctica ll y controlled comparisons cons train our abi lity to generalize beyond our parti cular system. cvcrth '­lcss, these data make a strong case for improved understanding or how multiple resource and environmental strcssors inlluence dif'fcrcnces in resource conscr ati on and resou rce capture traits between nati ve and in vas i ve species if we are to further ad­vance theori es of invasion and in vas ion res istance.

LITE R AT RECITED

AI:R Is. R. 1996. utricnt rc;orp ti on from ;cnc;c ing leave' or pt:n.:nnial' : Arc there general paucrn, ·J Journal of l:"cologr 84: 597 608 .

A~:R·Js. R. 1999 . Interspecific competiti on in natural plan t com muni ­ti cs: Mechanisms. tradc-offs and plant- soil fCt:dbad,, . ./ounwl of Erperimental Bo ta11y 50: 29 37.

AI·RJS. R .. AND F. S. CII J\1'1 . 2000. The mino.:ral nutrition or wild plant\ revis ited: A re-eva luat ion of processes and paucrn,. Adi 'IIIIC£'.1 in Ecological Research 30: I 67.

BAKER. H. 1965. Characteristics and modes of origin of weeds. In II. G. Baker and G. L. Stebbins lt.!ds.l. Genetic; of co loni7ing species. 147-168. Academ ic Press. ew York. cw York. SA .

BtoRENDSE, F. 1994. Compet ition between pl ant populations at low and high nutrient supplies. Oikos 71: 253- 260.

BLOOM . A. J .. F. S. CJJAPJ 0 • AND H. A. M OONEY. 1985. Resource limita­tion in pl ants-An economic analogy. An11ua/ Re l'ie ,. of Ecology and Systematics 16: 363- 392.

63o A\II ·R IC .,, JOl R' ·\1 or BmA'-' !Vol. 99

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Fi g. 3. it rogen ( ) allocati on and con'ocrva ti ontraits of native and invasive perennial species. including (A) 1 product ivity ( P). (B) photmoynthctic

usc efficiency (PN E), (C) green leaf , and (D) scnesccd leaf . Data arc means ±SE (N = 2- 9). Note : Sec Fig. I legend for defi nitions of abbrev iati ons.

and conservati on patterns. Plants w ith hi gher P UE and lower scncsccd lea f (and thus greater resorpti on proficiency, sensu Killingbeck. 1996) had enhanced plant performance. as as­~esscd by total plant bi omass. at the end of the experiment (P :5 0.005 for both variab les: data not shown). These correlations stress the i mportancc of resource conservati on traits for the suc­cess of in vas i ve spec ies in resource-poor sy tems, and further rc. ca rch and empha. is should be placed on these traits in in va­sive spec ies. Without data on traits such as nutrient-usc effi­c iency, mean retention time, resorpti on, and storage, we w ill fail to recognize key mechani sms supporting the role of inva­sive spec ies in resource-poor environments.

ontrary to expectations, native and invasi ve specie. were sim ilarl y plastic for mos t meas ured traits. The grcatc. t dif­ferences in pl as ti c ity between native and invas i ve . pecies were observed for in stantaneous measurements (A, WU E, and plant water potential ), with in vas i ve spec ies being more plastic in response to resource ava il ab ility for all three traits. Although there was a marg inall y signifi ca nt difference in pl as ti city for N all oca ti on and con. er va tion trait s, no differ­ence in p las ti city ex isted betwe n nati ve and invasive spe-

c ies for growth and allocation traits. These results are simil ar to th ose of a study co mparing related spec ies of in va. ive and nati ve woody vines, in which nati ve and in vas i ve spec ies had simil ar plasticity for 14 out of 17 physio log ica l and growth traits measured. thoug h overall pl as ti c ity was grea ter in in vas i ve th an in nati ve species (0 unk oya et al. , 20 I 0) . Likewi e. in two phylogenetically paired studies of nati ve and invas i ve spec ies aero. sa range of li fe forms , nati ve and invas ive spec ies did not differ in their plasti c ity for a suite o f growth and phys io log ica l traits (Funk , 2008; Godoy et al. , 20 I I ). These studi es suggest pia. ticity alone may not predict the succes. of invasive species. Finall y, plasti city varied greatl y depending on the trait measured and was not consisten t among the trait groupings. The most plasti c traits generally were those that require only small change in allocati on of resources or function ing and/or arc fairly rever. ible, such as photosynthetic rate, P E, to tal biomass, and P. In contras t. those trai ts that require (or are strong ly influenced by) more long- term change in ti ssue constructi on were less pl asti c, such as RGR , SLA, green leaf , and senescedleaf 1, which were the least plastic trait .

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Multiple resource limitati ons constrain plant growth and plast ic it y and may be particu larly strong driver. o f plant growth and function in arid , nutrient -poor systems (Bloom et al., 1985: Gleeson and Tilman, 1992; James ct al. , 2005; Valladares ct al.. 2007). The inOuence o f multiple resource limi tations on pattern s of trait convergence or divergence as well as trait pl asti c ity among nati ve and in vas ive spec ies has direct implications for advancing theories of in vas ion and in vasion resistance. In thi s study, inva. ivcs tended to have higher SLA. support ing the no­ti on that inva~ ive specie. tend to be positioned further along the leaf econom ics spectrum toward resource capture (Wright et al. , 2004 ). W e also found , however, strong ev idence for functional similarity and plasticity between nati ve and in vas i ve species.

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T \ II II 4. Pla'>ticit) indicc' for mea'>urcd ph) ,iological and morphological \·ariables. Pla\ticit) \\a' mc<Ntred according to tht: rl!lati\C di-,tanco.: pla'>ticity indt:x. "' dc,crihcd in alladan.:'o ct al. (2006) . Data arc means

I SE ( = 6). 1ote: Sec Tahlc' 1-3 for definitions of abbrc' iations .

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A \\o 1:

0.56 :t- 0. 12 0.20 ± 0.06 0.61 ± 0. I 0 0. I I ± 0.0.1 o.55 ± o. 1 1 o.:n ± o.os 0.-19 0.1 I 0.-11 ± 0. I I 0.61 0.09 0.18 ± 0.05 0.58 ± 0.10 (l..\9 ± 0.08

0.29 ± 0.06 0.20 ± 0.06 0.23 ± 0.05 0. 1 I ± 0.0.1 0.3-1 ± 0.09 0 .. 13 0.08 0.24 ± 0.05 0.-11 ± 0.10 0 28 ± 0.07 0. I K 0.05 0.2-1 ± 0.06 0.39 ().08

particularly with respect to nutrien t allocation and conservation traits. supporting community assembly hypothese~ based on habi tat filtering (Tecco ct al.. 20 I 0). Importantly , oUJ· data shO\ that multiple resource limitations influence the degree of trait convergence or divergence between in vasive and native spe­cies. The limited number or spec ies used in thi s study and the lack of phylogenctica ll y controlled comparisons cons train our abi lity to generalize beyond our parti cular system. cvcrth '­lcss, these data make a strong case for improved understanding or how multiple resource and environmental strcssors inlluence dif'fcrcnces in resource conscr ati on and resou rce capture traits between nati ve and in vas i ve species if we are to further ad­vance theori es of invasion and in vas ion res istance.

LITE R AT RECITED

AI:R Is. R. 1996. utricnt rc;orp ti on from ;cnc;c ing leave' or pt:n.:nnial' : Arc there general paucrn, ·J Journal of l:"cologr 84: 597 608 .

A~:R·Js. R. 1999 . Interspecific competiti on in natural plan t com muni ­ti cs: Mechanisms. tradc-offs and plant- soil fCt:dbad,, . ./ounwl of Erperimental Bo ta11y 50: 29 37.

AI·RJS. R .. AND F. S. CII J\1'1 . 2000. The mino.:ral nutrition or wild plant\ revis ited: A re-eva luat ion of processes and paucrn,. Adi 'IIIIC£'.1 in Ecological Research 30: I 67.

BAKER. H. 1965. Characteristics and modes of origin of weeds. In II. G. Baker and G. L. Stebbins lt.!ds.l. Genetic; of co loni7ing species. 147-168. Academ ic Press. ew York. cw York. SA .

BtoRENDSE, F. 1994. Compet ition between pl ant populations at low and high nutrient supplies. Oikos 71: 253- 260.

BLOOM . A. J .. F. S. CJJAPJ 0 • AND H. A. M OONEY. 1985. Resource limita­tion in pl ants-An economic analogy. An11ua/ Re l'ie ,. of Ecology and Systematics 16: 363- 392.

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