Intra and interspecific competition among invasive and native species during early stages of plant growth

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1 Plnt Eol (2011) 212: DOI /s z #687 Intr nd interspeifi ompetition mong invsive nd ntive speies during erly stges of plnt growth Seem Mngl Roger L. Sheley Jeremy J. Jmes Steven R. Rdosevih Reeived: 2 April 2010 / Aepted: 23 June 2010 / Pulished online: 4 Mrh 2011 Ó Springer Siene+Business Medi B.V Astrt Plnt ompetition is primry eologil proess limiting grsslnd restortion suess. Approprite restortion tehniques require n understnding of the degree to whih intr nd interspeifi ompetition ontrol invsive nd ntive plnt growth. The ojetive of this study ws to determine how the intensity of intr nd interspeifi ompetition hnges during erly stges of plnt growth. Two invsive (Bromus tetorum nd Tenitherum put-meduse) nd two ntive (Pseudoroegneri spit nd Po seund) speies were grown in dillel ompetition experiment, either lone or in 1:1 inry omintions nd exposed to two levels of N (no N or S. Mngl (&) Environmentl Siene, Oregon Stte University, Corvllis, OR 97331, USA e-mil: seem.mngl@oregonstte.edu R. L. Sheley J. J. Jmes USDA-Agriulturl Reserh Servie, Estern Oregon Agriulturl Reserh Center, A Hwy 205, Burns, OR 97720, USA S. R. Rdosevih Forest Eosystems nd Soiety, Oregon Stte University, Corvllis, OR 97331, USA Present Address: S. Mngl Deprtment of Environmentl Siene, Poliy & Mngement, University of Cliforni Berkeley, 137 Mulford Hll #3114, Berkeley, CA 94720, USA e-mil: mngls@erkeley.edu 400 mg N kg -1 soil dded) in greenhouse. Totl iomss for eh speies ws quntified over four hrvests nd ompetitive effets were lulted. Our results show tht the reltive mgnitude of intr nd interspeifi ompetition hnges through time. Intrspeifi ompetition ws intense for ntive speies t the initil hrvests nd therefore importnt in ontriuting to the outome of finl size of ntive speies seedlings. Interestingly, lueunh whetgrss imposed interspeifi ompetition on nnul grsses t the first two hrvests nd ppered to e etter ompetitor thn Snderg s luegrss. We found tht fst growing invsive speies eme more ompetitive ompred to slow growing ntive speies with inresing N nd pper to estlish positive feedk mehnism etween size nd resoure uptke. Opportunities to improve restortion suess exist from determining the optimum omintion of density, speies proportion, nd their sptil rrngement in vrious eosystems nd environments. Keywords Intrspeifi ompetition Interspeifi ompetition Hrvest time Nitrogen Restortion Dillel design Introdution Exoti plnt invsion poses serious eologil nd onservtion thret to ntive plnt ommunities (Vitousek et l. 1996; Simerloff 2005). Conversion

2 532 Plnt Eol (2011) 212: of perennil grsslnd to nnul grsslnd hs een identified s the gretest eologil thret to the ntive iot of the semi-rid steppe of the North Ameri (D Antonio nd Vitousek 1992; Germino et l. 2004). Annul grsses, prtiulrly hetgrss (Bromus tetorum L.) nd medushed (Tenitherum put-meduse L. Nevski), re outstnding exmples of this invsion. Sine their introdution, these invsive grsses now dominte millions of hetres of rid lnd in the western United Sttes, leding to loss of speies diversity, livestok nd wildlife hitt, nd inreses in fire frequeny (D Antonio nd Vitousek 1992; Vitousek et l. 1996; Pimentel et l. 2005). Improved mngement strtegies re ritilly needed to seletively ontrol invsive plnts, while promoting the estlishment of ntive speies (Brown et l. 2008). It is eoming inresingly ler tht mngers must ddress the underlying eologil proesses nd mehnisms of suession if desired vegettion dynmis re to endure (Luken 1997; Sheley nd Mngold 2003; Mngold et l. 2006). Competition mong plnts is primry eologil proess limiting grsslnd restortion suess (Allen 1995; Brown et l. 2008). The term ompetition is used in the sense of negtive interferene (i.e., ny diret or indiret negtive impt) of one plnt over nother (Fowler 1986; Csper nd Jkson 1997) or the ility to void or tolerte suppression (Golderg nd Brton 1992). Approprite restortion tehniques require n understnding of the degree to whih intr nd interspeifi ompetition influene invsive nd ntive plnt growth. Clssil ompetition theory predits intrspeifi ompetition should e greter thn interspeifi ompetition euse individuls of the sme speies shre similr resoure requirements (Tilmn 1982; Arssen 1983; Spitters 1983; Fowler 1986; Golderg nd Brton 1992). Reviews of the reltive strength of intr nd interspeifi ompetition in experimentl plnt studies tend to find little evidene for onsistently stronger intrspeifi ompetition mong interting speies (Golderg nd Brton 1992; Gurevith et l. 1992). However, results from more reent studies not inluded in these reviews ffirm these vrile results, with some studies showing intense intrspeifi ompetition (Sheley nd Lrson 1994; Velgl et l. 1997; Wssmuth et l. 2009), while others inditing intense interspeifi ompetition (Young nd Mngold 2008; Vsquez et l. 2008; Blnk 2010). One possile explntion for these ontrditory findings my e euse the interprettion of ompetitive intertions hs een lrgely sed on ssessing individul plnt iomss over one lifehistory stges (Connolly et l. 1990; Golderg nd Brton 1992; Frrer et l. 2010). Therefore, these studies my not fully pture the vrition in strength nd diretion of plnt plnt intertion whih my vry onsiderly mong life-history stges (Golderg et l. 2001; Howrd nd Golderg 2001). Plnts pss through different physiologil stges s their development progresses nd ompetition ours not only within speies, ut lso within nd etween stges of different speies (Connell 1983; Shoener 1983; Cmeron et l. 2007). Germintion, emergene nd initil root nd shoot development my e prtiulrly sensitive to ompetition (Foster nd Gross 1997, 1998; Foster 1999). It hs een shown tht smll seedlings my e muh more sensitive to ompetition thn dult plnts (Foster 1999; Suding nd Golderg 1999). Other reserh, however, hs indited tht ompetition my e reltively rre erly fter germintion, ut my e ritil in determining the finl plnt iomss (Golderg et l. 2001). Confliting results onerning the differenes in ompetitive effet mong studies rise euse studies hve lrgely overlooked the vlue of understnding ompetition over sequentil hrvests during the erly stges of plnt growth. This highlights the need to expliitly investigte when during the erly stges of estlishment plnts re most sensitive to ompetition. Competitive intertions re strongly ffeted y resoure grdients (Grime 1977; Tilmn 1988). Grime s model predits ompetition will hve less influene on plnt ommunity struture in resoure poor environments ompred to resoure rih environments (Grime 1977), while Tilmn s model predits tht ompetition will e similr in resoure poor nd resoure rih environments (Tilmn 1988). However, even fter dedes of experimentl nd oneptul work, empiril dt hve not onsistently supported either model (Golderg et l. 1999), nd the dete onerning how ompetitive intertions vries with produtivity remins unresolved. In semi-rid nd -rid grsslnds, nitrogen (N) is often the most limiting resoure fter wter (Dehler 2003; Vsquez et l. 2008). Reserh hs indited

3 Plnt Eol (2011) 212: tht inresed N fvors invsive speies over ntive speies (Chmers et l. 2007; Jmes et l. 2008) nd deresed N vilility fvors ntive speies (MLendon nd Redente 1992; Lowe et l. 2003). However, dominne y nnul grsses hs een oserved even in low N environments (Vsquez et l. 2008; Young nd Mngold 2008). Plnts pture more N during periods of high growth rte or during high plnt demnd for N (Collier et l. 2003). Thus, speies ility to respond to N depends on plnt growth ptterns. If oexisting speies differ in phenology nd timing of mximum growth rte, durtion of N vilility my differentilly impt speies N quisition (Bilrough nd Cldwell 1997). For exmple, pility of fst growth y invsive speies, espeilly during the erly growth phse, llows them to pture more resoures (e.g., N) thn slow growing ntive speies. This provides invsive speies ompetitive dvntge in lter stges of growth ompred to ntive speies (Lmers nd Poorter 1992; Rdosevih et l. 2007). Knowledge of the plnt growth stge tht responds to N vilility tht influenes ompetition is ritil to understnding the mehnisms regulting dominne of invsive speies, espeilly in N-limited systems. Moreover, ompetitive intertions my vry temporlly, s individuls modify the resoure pool, nd sequentil destrutive hrvests re needed to investigte speies intertions over time. Suessful restortion of invsive nnul grss dominted grsslnds requires full understnding of the reltive strength nd mgnitude of plnt intertions during the erly stges of plnt growth nd how this is impted y N vilility. The rod ojetive of this study ws to determine how the ompetition type nd intensity of intr nd interspeifi ompetition hnges during erly stges of plnt growth. Sine the performne of n individul plnt my e modified y soil nutrient vilility, espeilly soil N, we exmined these responses under two levels of N vilility. We onduted ompetition experiments in the greenhouse euse of the need to mnipulte the mount of N reeived y plnts. Although experiments under ontrolled environmentl onditions n rrely e diretly extrpolted to field onditions, they re useful for exploring the potentil for prtiulr ftors to hve n impt (Novoplnsky nd Golderg 2001). We hypothesized tht the type of ompetition nd its intensity tht lrgely dominte etween invsive nd ntive speies would vry mong hrvest time during erly stges of plnt growth. Bsed on the expettion tht fst growth y invsive speies will llow them to quire more N nd suppress N pture y ompeting speies during periods of rpid growth, we predited tht the ompetitive intensity for invsive speies will e higher thn ntive speies with higher N vilility. Mterils nd methods Seletion of plnt speies nd soil We seleted two invsive nd two ntive speies tht o-our in the Intermountin West. Chetgrss nd medushed re ommon nd widespred oolseson invsive nnul grsses tht hve invded muh of the Intermountin West (Turner et l. 1963; Mk 1989; Dunn et l. 2004). Ntive perennil speies, lueunh whetgrss (Pseudoroegneri spit (Pursh) A) nd Snderg s luegrss (Po seund J. Presl) re mjor hereous speies in the region nd widely used for restortion (Zltnik 1999). Seeds for this study were olleted from lol popultions during 2006 nd stored in moistureproof ontiner t ool, dry lotion t outside ir temperture. Seeds of hetgrss nd medushed were de-wned prior to seeding. Soil ws olleted from the Northern Gret Bsin Experimentl Rnge, 16 km southest of Riley, Oregon ( E, N). Soil t the site ws fine, montmorilloniti, mesi Xeri Hplrgid. The top m of the soil ws olleted in spring The soil ws then dried nd sieved through 6 mm mesh sreen to rete uniform texture nd thoroughly mixed. Soil nlysis for nitrte N, mmonium nd inuted N reveled only 9 lg minerlizle N g -1 soil. Experimentl design, tretments, nd smpling This greenhouse study ws onduted during June through Novemer 2008 t Oregon Stte University, Corvllis, OR, USA. A ompletely rndomized design with ll the possile pirs mong individuls of eh speies (hetgrss, medushed, lueunh whetgrss, nd Snderg s luegrss) were omined using dillel design. The dillel design uses

4 534 Plnt Eol (2011) 212: only one or two individuls of eh speies, nd therefore llows study of oth intr nd interspeifi ompetition within frmework of sustitutive experiment (Hrper 1977; Rdosevih et l. 2007). Fourteen different omintions for the four speies were otined. For eh omintion there were two N tretments; either no N (N-) or 400 mg N kg -1 soil (N?) dded. There were 10 replites per tretment per hrvest dte with totl of four hrvests t 15, 30, 60, nd 90 dys fter plnting (DAP). Hene, there were 14 speies omintions 9 2 N tretments 9 10 replites 9 4 hrvests = 1,120 experimentl units. The performne of n individul plnt n e ffeted y the size of its neighor, therefore the per pit effet of ompetition must e ontrolled (Golderg 1987). This hs een onsidered s the ommon prolem of studies ompring intr nd interspeifi ompetition (Golderg nd Brton 1992). In order to hieve uniform plnt size, seeds of eh speies were plnted seprtely in liner trys nd similr sized seedlings were seleted nd trnsplnted into the pots for the dillel experiments. Pots for the 15 nd were 10 m in dimeter y 7.5 m in depth, while the pots for the 60 nd were 15 m dimeter y 10 m in depth. Smller pots nd lrger pots were filled with 500 nd 1,500 g of soil, respetively. A preliminry study demonstrted tht smller pot sizes did not lter seedling growth ompred to lrger pots during the first 30 dys of growth nd therefore, the performne of the individul plnt ws not ffeted y the mount of spe tht surrounded it. After trnsplnting the similr sized plnts to their respetive pots, one-time ddition of N tretments ws pplied (N- nd N?). Pots without dded N (N-) were irrigted with wter, while the pots with N ddition (N?) were irrigted with wter nd liquid ure (40% N) to hieve onentrtion of 400 mg N kg -1 soil. Throughout the experiment, the soil ws irrigted dily with wter using hnd-held sprinklers to ring the soil to pproximte field pity. The ottoms of the pots were overed to void N lehing. Mid-dy nd night ir temperture in the greenhouse rnged from 21 to 25 C nd 18 to 21 C, respetively. Averge solr rdition in the greenhouse ws 1,250 lmol m -2 s -1. At eh hrvest seedlings were removed from the pots nd seprted. Roots were wshed over fine msh sreen. Eh seedling ws dried t 65 C for 48 h, fter whih totl iomss ws determined for eh seedling. Growth nlysis nd sttistil proedures We nlyzed our dt with the fous on how the reltive strength nd mgnitude of ompetition hnges mong speies over time of hrvests nd in response to N vilility. We lulted the ln response rtio () of eh trget individul on the sis of totl seedling iomss (Hedges et l. 1999; Golderg et l. 1999): ln RR ¼ ln ðbm mixture =BM lone Þ; where BM mixture is the totl iomss of trget speies i with neighor i (intrspeifi ompetition) or j (interspeifi ompetition) nd BM lone is the men totl iomss of trget speies i grown s single individul (no ompetition). The response rtio is mesure of ompetition intensity, the degree to whih neighor plnt influenes trget plnt growth (Golderg et l. 1999; Weigelt nd Jolliffe 2003). The sttistil properties of hve een exmined in detil y Hedges et l. (1999) nd hve een shown to hve mjor dvntges over other potentil indies of ompetition intensity. We nlyzed the ompetition intensity of trget individuls on neighors using n ANOVA tht tested the effets of trget speies, time of hrvest, nd ll seond-order intertions. Comprisons etween test groups were mde using Tukey HSD tests t the 5 level of signifine. We were lso interested to know if ompetition intensity mong speies differed for N tretments hene N ws lso inluded in the model. Therefore, Tukey HSD test ws lso performed to sttistilly ompre the rtios of in no dded N tretments to tht in the dded N tretments for vrious ompetition senrios. We lso presented the men iomss of eh seedling growing in different N tretments nd ompetition senrios. All sttistil nlysis ws performed using S plus 7. for Mirosoft windows (S plus 2005). Results Overll, DAP nd N hd miniml effet on ompetition intensity, ut neighor nd trget speies identity hd signifint impts on ompetition

5 Plnt Eol (2011) 212: Tle 1 Results of ANOVA (df, F, nd P) testing for effets of DAP, nitrogen, trget speies, nd neighor speies on ompetitive effet () Soure df F P DAP Nitrogen Trget Neighor DAP 9 nitrogen DAP 9 trget DAP 9 neighor Nitrogen 9 trget Nitrogen 9 neighor Trget 9 neighor DAP 9 nitrogen 9 trget DAP 9 nitrogen 9 neighor DAP 9 trget 9 neighor DAP 9 nitrogen 9 trget 9 neighor Residuls 64 intensity (Tle 1). Neighor speies differed signifintly in their ility to suppress the growth of trget speies experiening either intr or interspeifi ompetition. However, neighor speies did not differ in their response to N nd DAP (neighor 9 N nd neighor 9 DAP, respetively; Tle 1). Effet of hrvest time on intr nd interspeifi ompetitive intertions In oth N tretments, Snderg s luegrss hd lowest intrspeifi ompetition intensity nd highest interspeifi ompetition intensity t every hrvest. The only exeption ws tht intrspeifi ompetition ws the dominnt type for Snderg s luegrss t 15 DAP growing in N- (Fig. 11). Similrly, intrspeifi ompetition ws the min type of ompetition deteted t in oth N tretments for lueunh whetgrss (Fig. 11, 2). At, interspeifi ompetition with invsive speies ws the dominnt type nd no ompetition ws deteted t 60 nd in N- (Fig. 11). In N?, interspeifi ompetition ws dominnt ompetition type for lueunh whetgrss t lter hrvests; however, interspeifi ompetition with hetgrss ws deteted only t (Fig. 12). In N-, oth intr nd interspeifi ompetition ws deteted t initil hrvests for hetgrss, ut miniml effet of ompetition ws deteted for lter hrvests (Fig. 11). In N?, no ompetition ws deteted t (Fig. 12), while t every other hrvest (30, 60, nd ) ddition of N resulted in intrspeifi ompetition within this nnul grsses. In N?, t, hetgrss lso experiened interspeifi ompetition from ntive speies, ut not t 60 nd. Medushed experiened intrspeifi ompetition t every hrvest for oth N tretments (Fig. 1d1, d2). Exept t, in N-, medushed lso experiened interspeifi ompetition with ssoited speies. However, in N?, miniml effet of interspeifi ompetition ws deteted t every hrvest for medushed. Effet of hrvest time on ompetition intensity For Snderg s luegrss, in N-, mximum ompetition intensity ws deteted t 15 nd nd in N?, it ws deteted t (Fig. 11, 2). Intense ompetition t in N? resulted in 40% derese in Snderg s luegrss iomss ompred to its iomss grown lone (Fig. 22). For lueunh whetgrss in N-, the ompetition intensity ws highest t initil hrvest () resulting in 30% redution in its iomss ompred to where grown lone (Figs. 11, 21). Similrly, in N?, the ompetition intensity ws highest during initil hrvests (15 nd ) for lueunh whetgrss; however, more intense ompetition ws deteted t thn t with 72% derese of iomss elow tht grown lone t (Figs. 12, 22). The ompetition intensity ws higher during initil hrvests in N- for hetgrss; however, more intense ompetition ws deteted t ompred to 15 DAP with 50% derese of iomss elow tht grown lone t (Figs. 11, 21). In N?, hetgrss iomss ws nerly similr etween different ompetition senrios t nd onsequently, ompetition intensity ws nerly 0 t, while ompetition intensity ws mximum t nd for this invsive grss (Figs. 12, 22). In N?, hetgrss iomss deresed 47 nd 17% t 30 nd, respetively, elow tht grown in isoltion (Fig. 22). Similrly, for medushed, in N-, the ompetition intensity ws highest during initil hrvests (Fig. 1d1). However, t 60 nd 90

6 536 Plnt Eol (2011) 212: Snderg's luegrss trget Snderg's luegrss Blueunh whetgrss Blueunh whetgrss trget Chetgrss trget d Chetgrss N- N+ 2 Snderg's luegrss trget Blueunh whetgrss Chetgrss Medushed Snderg's luegrss Snderg's luegrss Chetgrss Blueunh whetgrss d Medushed Medushed Snderg's luegrss Blueunh whetgrss Blueunh whetgrss trget Blueunh whetgrss Chetgrss trget d Chetgrss Snderg's luegrss Snderg's luegrss Chetgrss Chetgrss Blueunh whetgrss Medushed Medushed Medushed - - d1 Medushed trget - - d2 Medushed trget Medushed Snderg's luegrss Blueunh whetgrss Neighor speies Chetgrss Medushed Snderg's luegrss Blueunh whetgrss Neighor speies Chetgrss

7 Plnt Eol (2011) 212: Fig. 1 Effet of time of hrvest (DAP) on intr nd interspeifi ompetitive intertions () of different neighor speies on Snderg s luegrss, lueunh whetgrss, hetgrss, nd d medushed trget plnts under N- nd N?. Brs represent men ± SE (n = 10). Negtive vlues indite ompetition nd positive vlues indite filittion. More negtive vlues indite greter ompetitive effets of neighors. The dshed line indites = 0 (i.e., no ompetitive effet). Brs with different letters indite signifint differenes in in response to DAP within neighor speies using Tukey HSD tests (P \ 5). Asterisks indite signifint differenes etween N tretments determined y Tukey HSD tests (P \ 5), with omprisons mde only etween sme trget speies nd sme neighor speies t prtiulr DAP DAP, intense ompetition ws lso deteted nd resulted in derese of iomss y 38 nd 30%, respetively, ompred to medushed grown lone (Figs. 1d1, 2d1). In N?, the ompetition intensity ws highest t for medushed. N ddition resulted in filittion for medushed when ompeting with ssoited speies t 60 nd (Figs. 1d2, 2d2). Comprison of ompetition intensity etween N tretments Competitive effet of neighor on trget speies ws not influened y N supply (P = 0.11, Tle 1), lthough identity of trget speies were influened differentilly y N, s indited y signifint N 9 trget speies intertion (Tle 1). For ntive speies, N ddition resulted in less intrspeifi ompetition t ompred to N-. However, N ddition resulted in intense interspeifi ompetition for Snderg s luegrss growing in ompetition with lueunh whetgrss t every hrvest (exept for, P = 7). On the other hnd, ompetition with invsive speies in N? resulted in greter ompetition intensity of Snderg s luegrss nd lueunh whetgrss thn in N- t 15 nd, respetively (P \ 5). Exept t 15 nd, N? resulted in signifintly higher negtive for hetgrss grown with intrspeifi ompetition (P \ 5), i.e., intrspeifi ompetition redued the iomss of hetgrss when N? ws ompred with N- t lter stges (60 nd ). However, N ddition resulted in the redution of intrspeifi ompetition intensity of medushed t (P = 1). At 15 nd 30 DAP, hetgrss experiened highly negtive y Snderg s luegrss euse of N ddition s indited y ompring N? with N-. For medushed, N? resulted in greter interspeifi ompetition y hetgrss t (P = 04). However, medushed redued hetgrss growth when N? ws ompred to N- s indited y highly negtive ompetition intensity t 60 nd (P \ 01). Disussion Suessful restortion involves identifying nd modifying ritil eologil proesses within omplex we of interonneted proesses nd mny ollterl intertions our to influene vegettion dynmis (Christensen et l. 1996). Plnt ompetition is often ssumed to e primry eologil proess determining the outome of restortion efforts (Allen 1995; Brown et l. 2008). Competition during erly stges of growth n ritilly influene individul plnt growth nd determine future development ptterns (Foster nd Gross 1997, 1998; Suding nd Golderg 1999). Smll differenes in initil size nd growth rtes etween individuls nd speies ould potentilly determine long-term developmentl ptterns. An in-depth understnding of ompetitive effets mong invsive nd ntive speies during erly stges of plnt growth my provide informtion helpful in ltering this proess to fvor vegettion dynmis towrd ntive speies during restortion. Our overll results suggest tht invsive nnul grsses nd ntive perennil grsses re sujet to oth intr nd interspeifi ompetition during erly growth stges; however, the type differed mong hrvests, lending support for the hypothesis tht ompetition type vries mong hrvests during the erly stges of plnt growth. This emphsizes the need to investigte ompetition t severl points over time (Foster nd Gross 1997, 1998; Foster 1999; Gison et l. 1999), prtiulrly when life yles differ (Gison et l. 1999). Unfortuntely, most previous studies on the effet of invsive speies on ssoited ntive speies re sed on single hrvest, usully t the end of the growing seson (Vsquez et l. 2008; Young nd Mngold 2008; Blnk 2010). These studies hve therefore lrgely overlooked hnges plnt my experiene when it

8 538 Plnt Eol (2011) 212: Snderg's luegrss trget Snderg's luegrss trget N Biomss (g) Biomss (g) Snderg's luegrss Blueunh whetgrss Chetgrss Medushed Snderg's luegrss Blueunh whetgrss Chetgrss Medushed Blueunh whetgrss trget Blueunh whetgrss trget Biomss (g) Biomss (g) Blueunh Snderg's whetgrss luegrss Chetgrss Medushed Blueunh Snderg's whetgrss luegrss Chetgrss Medushed Chetgrss trget 90DAP Chetgrss trget Biomss (g) 4 Biomss (g) Chetgrss Snderg's luegrss Blueunh whetgrss Medushed 0 Chetgrss Snderg's luegrss Blueunh whetgrss Medushed 5 4 d1 Medushed trget 5 4 d2 Medushed trget Biomss (g) 3 2 Biomss (g) Medushed Snderg's luegrss Blueunh whetgrss Chetgrss 0 Medushed Snderg's luegrss Blueunh whetgrss Chetgrss Neighor speies Neighor speies

9 Plnt Eol (2011) 212: Fig. 2 Effet of time of hrvest (DAP) on iomss (g) of different neighor speies on Snderg s luegrss, lueunh whetgrss, hetgrss, nd d medushed trget plnts under N- nd N?. Brs represent men ± SE (n = 10). Biomss mens were used to lulte ompetitive effets () presented in Fig. 1 psses through different growth stges (Connell 1983; Shoener 1983; Cmeron et l. 2007). A previling notion is tht ompetition from invsive grsses is the primry ostle for ntive grsses estlishment (Allen 1995; Brown et l. 2008). While we did find interspeifi ompetition to e the predominnt type of ompetition for the ntive speies, whih is in greement with previous ompetition studies (Lowe et l. 2003; Vsquez et l. 2008; Young nd Mngold 2008), intrspeifi ompetition ws lso dominnt within ntive speies t the initil hrvest () nd therefore, importnt in ontriuting to the outome of finl size of ntive speies seedlings. Bsed on these results, there ppers to e sustntil need for restortion efforts to refully lne the seeding rte used during restortion with the numer of sfe site nd potentil for intrspeifi ompetition y ntive speies. We found intrspeifi ompetition to e dominnt for invsive speies s suggested y previous studies (Vsquez et l. 2008; Young nd Mngold 2008; Blnk 2010). However, invsive speies lso experiened interspeifi ompetition y ssoited plnt speies, thus, interspeifi ompetition likely ontriutes to the finl size of seedling invsive speies. Blueunh whetgrss negtively ffeted nnul iomss t the first two hrvests s indited y negtive ompetition intensity. There ppers to e window of opportunity for lueunh whetgrss to suppress nnul grsses in their seedling stge, nd one estlished, lueunh whetgrss my e le to mintin itself through perennil resoure llotion s suggested y Jos et l. (1996). Blueunh whetgrss lso resulted in greter interspeifi ompetition for Snderg s luegrss t every hrvest, while it ould potentilly tolerte the presene of Snderg s luegrss when grown in the sme pot. These results suggest lueunh whetgrss is etter ompetitor thn Snderg s luegrss nd ould e more suessful restortion speies. Overll, intr nd interspeifi ompetition intensity for ntive speies ws highest during initil hrvests ompred to lter hrvests for oth N tretments, lending support for hypothesis tht ompetition intensity would vry mong hrvests. In ddition to intense ompetition t erly stges of growth for invsive speies, we lso deteted intense ompetition t lter stges of growth. Medushed nd hetgrss lso experiened filittion from ssoited ntive speies t lter hrvests in N dded tretments. One mehnism to explin this pttern ould e the role of ntive grss seedlings s nurse rops y providing greter nutrient vilility to the invsive speies, t lest temporrily s suggested y Blnk (2010). Grime s C S R theory predits ompetition will hve greter influene on plnt ommunity struture in resoure rih environments ompred to resoure poor environments (Grime 1977, 1979). Although we did oserve ompetition in oth low nd high N environments, we lso oserved n inrese in ompetition intensity s N vilility inresed, supporting the preditions of Grime (1979). Plnts n hve preferentil uptke of, or differentil needs for, prtiulr forms of nutrients depending on their physiologil effiienies (Mkne et l. 2002). We found tht nnul grsses eme more ompetitive ompred to ntive speies with inresing N, lending support for our hypothesis tht inresing N would inrese the ompetitive effets of invsives on ntives. The fst growing invsive plnt speies gin more ess to resoures thn the slower growing ntive plnt speies nd pper to estlish positive feedk mehnism etween size nd resoure uptke (Csper nd Jkson 1997; Blnk 2010). Reserhers hve theorized tht ntive perennil grsses perform etter nd re more ompetitive thn invsive nnul grsses under low N vilility (Wedin nd Tilmn 1990; Wilson nd Gerry 1995; Herron et l. 2001). For exmple, the CSR theory predits tht ntive speies would perform etter in low N environments (Grime 1979). However, we did not detet suh ptterns. Sine some minerl N (9 lg minerlized N g -1 soil) ws still present in our ontrol N tretment, this mount of N my hve een suffiient for the nnuls to outperform the perennils. Therefore, low N ontent lredy presents in the soil might hve een enough for greter growth response y invsive plnts. However, mintining resoure levels nd minimizing the loss of

10 540 Plnt Eol (2011) 212: previously ptured resoures is ritil under low N vilility (Berendse nd Aerts 1987). These ttriutes my improve the ompetitive ility of ntive plnts over invsive plnts espeilly under low N over time. It is ler tht ompetition etween invsive nd ntive speies influene struture, pttern, nd dynmis of plnt distriutions (Crwley 1997), ut the reltive role of ompetition my vry drmtilly mong different eosystems. While we did find ompetition type nd ompetition intensity to vry t different hrvests, ution in interpreting our findings is needed euse this study ws performed in greenhouse in environmentlly moderte onditions. Furthermore, our neighorhoods onsisted of single neighor, very unlikely sitution in semi-rid grsslnds. Future work with these speies nd other speies in the field is needed to exmine the generlity of these findings. Lnd mngers hve reognized the need to ontrol ompetition during the initil phse of restortion (Sheley nd Mngold 2003). Overoming the rriers to desired speies estlishment is entrl to restoring nnul grss infested eosystems. Muh literture suggests tht seeding t very high rte shifts the ompetitive lne in fvor of desired speies nd mximizes the hnes of seed rehing sfe site during restortion (Jos et l. 1996; Velgl et l. 1997; Seloom et l. 2003). Our study suggests tht limits to seeding rte exist euse of intrspeifi ompetition mong desired speies t initil stges of growth. Sine density, speies proportion, nd their sptil rrngement determine the outome of ompetition (Rdosevih 1987, 2007), these three ftors ould e designed to minimize intrspeifi ompetition mong desired ntive speies nd mximize interspeifi ompetition ginst nnul invsive speies, espeilly during the first few weeks fter germintion. Opportunities to improve restortion suess exist from determining the optimum omintion of these three ftors in vrious eosystems nd environments. Aknowledgments The uthors wish to thnk Jne Mngold for tking time to review the erlier drft of this mnusript, Brett Binghm nd field tehniins for ssistne in projet implementtion nd dt olletion. This projet ws funded y the USDA-Agriulturl Reserh Servie s portion of the re-wide EBIPM of nnul grsses projet. Referenes Arssen LW (1983) Eologil omining ility nd ompetitive omining ility in plnts: towrd generl evolutionry theory of oexistene in systems of ompetition. Am Nt 122: Allen EB (1995) Restortion eology: limits nd possiilities in rid nd semirid lnds. In: Roundy BA, MArthur ED, Durnt E, Hley JS, Mnn DK (Compilers) Proeedings wildlnd shru nd rid lnd restortion symposium. USDA generl tehnil report INT-GTR-315, Ogden, pp 7 15 Berendse F, Aerts R (1987) Nitrogen-use-effiieny: iologil meningful definition? Funt Eol 1: Bilrough CJ, Cldwell MM (1997) Exploittion of springtime ephemerl N pulses y six gret sin plnt speies. Eology 78: Blnk RR (2010) Intrspeifi nd interspeifi pir-wise seedling ompetition etween exoti nnul grsses nd ntive perennils: plnt soil reltionship. Plnt Soil 326: Brown CS, Anderson VJ, Clssen VP et l (2008) Restortion eology nd invsive plnts in the semirid west. Invsive Plnt Si Mng 1: Cmeron T, Wering H, Rohni P, Sit S (2007) Two-speies symmetri ompetition: effets of ge struture on intrnd interspeifi intertions. J Anim Eol 76:83 93 Csper B, Jkson R (1997) Plnt ompetition underground. Annu Rev Eol Syst 28: Chmers JC, Meyer SE, Whittker A, Roundy BA, Blnk RR (2007) Wht mkes Gret Bsin sgerush eosystems invsile y Bromus tetorum? Eol Monogr 77: Christensen DL, Herwig BR, Shindler DE, Crpenter SR (1996) Impts of lkeshore residentil development on orse woody deris in north temperte lkes. Eol Appl 6: Collier MD, Fotelli MN, Nhm M et l (2003) Regultion of nitrogen uptke y Fgus sylvti on whole plnt levelintertions etween ytokinins nd solule N ompounds. Plnt Cell Environ 26: Connell JH (1983) On the prevlene nd reltive importne of interspeifi ompetition evidene from field experiments. Am Nt 122: Connolly J, Wyne P, Murry R (1990) Time ourse of plnt plnt intertions in experimentl mixtures of nnuls density, frequeny, nd nutrient effets. Oeologi 82: Crwley MJ (1997) Prefe to the first edition. In: Crwley MJ (ed) Plnt eology. Blkwell Siene Ltd, London D Antonio CM, Vitousek PM (1992) Biologil invsions y exoti grsses, the grss/fire yles, nd glol hnge. Annu Rev Eol Syst 23:63 87 Dehler CC (2003) Performne omprisons of o-ourring ntive nd lien invsive plnts: implitions for onservtion nd restortion. Annu Rev Eol Syst 34: Dunn CA, Jhett JJ, Brown ML et l (2004) Assessing the eonomi, environmentl, nd soietl losses from invsive plnts on rngelnd nd wildlnds. Weed Tehnol 18:

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