Community Structure and Abiotic Determinants of Salt Marsh Plant Zonation Vary Across Topographic Gradients

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1 Esturies nd Costs (2011) 34: DOI /s Community Struture nd Aioti Determinnts of Slt Mrsh Plnt Zontion Vry Aross Topogrphi Grdients Bo-Shn CUI & Qing HE & Yun AN Reeived: 16 Deemer 2009 / Revised: 13 Septemer 2010 / Aepted: 10 Deemer 2010 / Pulished online: 21 Jnury 2011 # Costl nd Esturine Reserh Federtion 2011 Astrt We investigted ompositions of plnt ommunities nd their ioti determinnts in the Yellow River estury, Chin. Along topogrphi grdient, we quntified plnt ompositions nd ioti ftors in different vegettion zones, nd exmined the reltionships etween plnt ommunities nd ioti ftors using nonil orrespondene nlysis nd the effets of vegettion shding using removl experiment. The reltionships etween plnt ommunities nd ioti ftors differed etween high elevtions nd low elevtions. Slinity nd flooding oppositely relted with distriutions of plnt ommunities t low elevtions, ut they ppered to operte synergilly t high elevtions. The effets of vegettion shding were found to vry ross the topogrphi grdient, inditing sptil vritions in potentil positive intertions mong plnts. These results suggest tht sptil vritions in determinnts of ommunity struture should e ddressed in future studies in esturine nd ostl systems, s well s in other nturl hitts. Keywords Estury. Flooding. Slinity. Slt mrsh. Yellow River Delt. Wetlnds B.-S. CUI (*) : Q. HE Stte Key Joint Lortory of Environmentl Simultion nd Pollution Control, Shool of Environment, Beijing Norml University, Beijing , Chin e-mil: uis@nu.edu.n Q. HE : Y. AN (*) Deprtment of Plnt Siene, Shool of Agriulture nd Biology, Shnghi Jio Tong University, Shnghi , Chin e-mil: nyun@sjtu.edu.n Introdution Costl hitts worldwide re distinguished for vegettion zones ross steep environmentl grdients (Chpmn 1974). These onspiuous vegettion zones re generlly onsidered to e produts of medition of oth physil stress nd plnt ompetition (reviewed y Bertness nd Pennings 2000). Trde-offs in plnts etween stress tolerne nd ompetitive ility led to ompetitively superior speies monopolizing enign hitts nd displing ompetitively inferior speies to stressful hitts. Despite of ppliility in severl ostl sites, this rule of slt mrsh plnt zontion hs een ritiized for generlity ross geogrphil regions (Bertness nd Pennings 2000; Pennings et l. 2005; Friñ et l. 2009). Estlished rules of slt mrsh ommunity orgniztion my not e diretly pplile to unstudied systems (Kunz nd Pennings 2008; Friñ et l. 2009). An importnt hypothesis eing tested urrently is tht rules of slt mrsh ommunity orgniztions vry mong geogrphil sites on osts (Bertness nd Pennings 2000; Pennings et l. 2003; Pennings nd Sillimn 2005; Pennings et l. 2009). Geogrphi vritions in limte my indue different slinity ptterns ross mrsh elevtions (Bertness nd Pennings 2000; Wng et l. 2007). Tking ltitude grdients in limte (whih re urrently repetedly used to exmine vritions in ptterns nd proesses of slt mrshes) for exmple, in low-ltitude slt mrshes, the hot limte leds to elevted slinities t upper/middle elevtions of the mrshes ompred with high-ltitude slt mrshes where the limte is muh ooler nd slinity is reltively low (Bertness nd Pennings 2000). This differene in slinities etween low- nd high-ltitude slt mrshes ws suggested to generte vritions in mehnisms of slt mrsh plnt orgniztions (Pennings

2 460 Esturies nd Costs (2011) 34: nd Bertness 2000; Pennings et l. 2003). Sintiln (2009) lso ttriuted ltitudinl ptterns of slt mrsh plnt diversity to limte-indued differenes in slinity ptterns etween low nd high ltitudes in Austrli. Besides differenes in slinity ptterns, Kunz nd Pennings (2008) suggested differenes in flooding regime to e the most plusile hypothesis ounting for differenes in plnt diversity etween Georgi nd Texs slt mrshes. At finer sles, mehnisms of slt mrsh plnt orgniztion re suggested to vry with mrsh elevtion (Pennings nd Cllwy 1992; Lenssen et l. 1999; Friñ et l. 2009). In Clifornin slt mrsh, Pennings nd Cllwy (1992) indited tht lthough flooding, soil slinity, nd ompetition ll interted to determine plnt zontion ptterns, the reltive importne of these ftors differed t different elevtions. Moreover, the preise roles of the two min physil stressors, slinity nd flooding, in mediting plnt zontion were lso suggested to vry with elevtion in Chinese slt mrsh (He et l. 2009). Compred with slt mrshes on the Atlnti ost of North Ameri where most influentil slt mrsh plnt zontion studies were onduted nd in severl other geogrphil regions, suh s Austrli (reviewed in Sintiln 2009), Western Europe (e.g., Bokelmnn nd Neuhus 1999; vn Wijnen nd Bkker 1999; Hukle et l. 2002) nd South Ameri (e.g., Cost et l. 2003; Alerti et l. 2008), slt mrshes on the western Pifi ost of Asi hve een muh less investigted. Chpmn (1977) hs lssified slt mrshes on the western Pifi ost of Asi s Sino-Jpnese group sed on floristi nd vegettion riteri. However, when Adm (1990) ws doing glol review of slt mrsh vegettion, he ould found little informtion on the flor nd vegettion of slt mrshes in this geogrphi region (exept for slt mrshes in Jpn). A few studies on Chinese slt mrshes hve risen in reent yers (e.g., Wng et l. 2006; Wu et l. 2008; Ji et l. 2009). However, most of these studies foused on Sprtin invsions rther thn strutures nd orgniztions of ntive nd hrteristi speies in Chinese slt mrshes. Nevertheless, studies investigting strutures nd orgniztions of ntive nd hrteristi speies in Chinese slt mrshes re impertive fing the rpid invsion of Sprtin from southern to northern slt mrshes on the Chinese ost. In this rtile, we investigte ompositions nd ioti determinnts of plnt ommunities long topogrphi grdient in the Yellow River estury on the western Pifi ost of northern Chin, where most plnt ommunities re still ntive ut fing the invsion of Sprtin lterniflor (herefter, Sprtin; He et l. 2009). The mrshes in the estury re reently formed due to high sediment deposit nd rpid lnd retion (Li et l. 2009). In suh rpidly developing mrshes, vegettion zontion hs een suggested to e minly ontrolled y ioti ftors either diretly or indiretly vi the ltertion of ompetitive reltionships (Wng et l. 2010). Here, we first desrie the ompositions of plnt ommunities nd ioti onditions in different vegettion zones long the topogrphi grdient, nd then test the following hypotheses speilly: long the topogrphi grdient, (1) the reltionships etween plnt ommunities nd ioti ftors differ etween low elevtions nd high elevtions; (2) the effets of vegettion shding (nd hene potentil positive intertions) vry with elevtion. Our study from previously little-studied ostl geogrphi region provides new informtion on the hypothesis tht mehnisms of ostl plnt ommunity orgniztion vry geogrphilly nd within site. Mterils nd Methods Study Site Field work ws onduted t site (Fig. 1; N, E) in the Yellow River estury, northern Chin. Slt mrshes nd mudflts in the estury re 1 6 km in width nd re still ruing rpidly. Tides in the estury re irregulrly semidiurnl. The study site hs typil temperte monsoon limte, eing dry nd ool ll the yer round, exept hot, riny summers. Refer to Li et l. (2009) for detiled desription of the limte ondition. The flor of the estury is reltively simple. Dominnt vsulr plnt speies inlude Sued sls (dominting low nd high mrshes; herefter, Sued), Tmrix hinensis (dominting terrestril orders of the slt mrshes; herefter, Tmrix), nd Phrgmites ustrlis (dominting uplnds ner the river ourse, see Fig. 1; herefter, Phrgmites). In reent yers, smooth ordgrss ntive to North Ameri, Sprtin, hs invded the mudflts in the E d e E Yellow River estury Pifi Oen N37 48 N37 46 N 0 2km Fig. 1 Mp of the study site nd the topogrphi grdient. The rrowheded line indites the topogrphi grdient. Lines e, divisions etween mudflt nd Sprtin low mrsh, Sprtin low mrsh nd Sued low mrsh, Sued low mrsh nd high mrsh, high mrsh nd terrestril order, nd terrestril order nd uplnd, respetively. Refer to He et l. (2009) for the lotion of the study site on the western Pifi ost

3 Esturies nd Costs (2011) 34: estury, eoming dominnt speies in these hitts nd is moving up rpidly. Other plnt speies inluding Sliorni europe, Sripus spp., nd Junus spp. our only sprsely or in smll pthes in the mrsh hitts. Plnt speies in the uplnds re muh more diverse, inluding Clmgrostis pseudophrgmites, Trirrhen shriflor, Impert ylindril, et. Prllel Trnset Smpling To quntify ompositions of plnt ommunities nd their reltionships with ioti ftors ross topogrphi grdients from the lower limits of vsulr plnts to uplnds, five zones (hitts) long the topogrphi grdient were delimitted sed on elevtion nd vegettion: (1) the uplnd dominted y Phrgmites; (2) the terrestril order dominted y Sued nd Tmrix; (3) the high mrsh dominted y Sued; (4) the low mrsh dominted y Sued (herefter, Sued low mrsh) with few other speies, suh s Phrgmites nd S. europe; nd (5) the low mrsh newly formed y Sprtin invsions (herefter, Sprtin low mrsh) (Fig. 1). In August 2009, we loted two to five prllel trnsets of 80 m in length in eh zone. The numer of trnsets in eh zone ws pproximtely proportionl to the width of tht zone. We set 2 2 m qudrt every 20 m long eh trnset (five qudrts in totl for eh trnset) nd reorded ll hereous speies nd their over vlues (visully estimted) in eh qudrt. Cover vlues of shruy nd rorel speies were estimted in 5 5 m qudrt onentril with the orresponding 2 2 m qudrt. Most plnt speies inluding rre speies were reorded. We used over vlues (rther thn other speies vriles) for nlyses of vegettion ompositions, sine (1) speies ompositions in our study site were reltively simple nd over vlues ould well desrie strutures of the plnt ommunities; (2) over vlues were one of the vriles used frequently y previous studies in similr hitts (e.g. Vine nd Snow 1984; Bertness nd Ellison 1987; Trut 2005; Kunz nd Pennings 2008). A soil ore of topsoil (5.05 m in dimeter 5 m in depth) ws olleted t the enter of eh qudrt to determine soil pore wter slinity, moisture ontent, ulk density nd ph. Soil orgni mtter nd totl nitrogen were not determined, sine our previous studies in the estury suggested these ftors hd only minor reltionships with vritions in vegettion (He et l. 2009). Permnent Plot Smpling nd Quntifition of Aioti Ftors To quntify mong-zone nd sesonl vritions in ioti onditions ross the topogrphi grdient, we rndomly estlished m plots in eh zone delimitted ove nd soil ore of topsoil (5.05 m in dimeter 5 m in depth) ws olleted in eh plot monthly from June through August 2009 to determine soil pore wter slinity, moisture ontent, ulk density nd ph. Soil ores were weighed, then dried t 60 C for 48 h in n oven nd reweighed to determine moisture ontent nd ulk density. Dried soil ws mixed with known volume of deionized wter (5:1 queous suspension, 1-min virtion). The ph of the superntnt ws mesured fter 30 min. The slinity of the superntnt ws mesured fter 24 h using n eletroni meter, nd the originl soil pore wter slinity ws lulted sed on the initil moisture ontent of the ore (Pennings et l. 2005). Due to the hrsh working onditions in the newly formed esturine mrsh (the mrsh ws very extensive nd hd mny tidl reeks), dily oservtions of flooding t permnent lotions were logistilly diffiult. Thus, we estimted the flooding frequeny in eh zone ording to our field experiene working in the estury (>3 yers). The estimted flooding frequenies were lso verified y experiened lol fishers. Removl Experiment As vegettion feedks my ply n importnt role in mediting edphi onditions, whih ment oth ioti nd ioti ftors were importnt in determining ptterns in these mrshes, we further onduted removl experiment to quntify the potentil feedks of vegettion shding on edphi onditions (other prole feedk effets, suh s sustrte stiliztion nd wve uffering s reviewed in Pennings nd Bertness 2000, were not onsidered in the present pper). In erly June 2009, we estlished m plots (>20 m distnes etween plots) in eh zone of the uplnd, the terrestril order nd the high mrsh where vegettion shding ws suggested to ply role in generting positive intertions in plnts (Bertness nd Pennings 2000). We lipped ll the oveground vegettion in hlf of these plots (rndomly seleted). The left plots (ontrol) were mintined without tretments. Clipping tretments were repeted iweekly s needed. We did not estlish shded neighor-removl tretments to mimi vegettion shding, sine (1) suh tretments were found to hve similr effets on soil slinity s ontrol tretments (lthough not for redox potentil; see Bertness nd Hker 1994; Bertness nd Yeh 1994) nd (2) previous studies suggested tht omprisons etween ontrol nd neighor-removl tretments should e n effetive wy to investigte the effets of vegettion shding on edphi ftors (Bertness nd Hker 1994; Bertness nd Yeh 1994; Hker nd Bertness 1999;Pennings et l. 2003). In middle Septemer 2009, soil ores were olleted in ll plots nd six dditionl soil ores were olleted in re pthes (typil in the terrestril order ut rre in other zones) whih were nturlly generted y

4 462 Esturies nd Costs (2011) 34: disturnes nd ould often e permnent for yers, nd then edphi ftors were determined s desried ove. Sttistil Anlysis One-wy nlyses of vrine (ANOVAs; followed y post ho multiple omprisons, LSD test) were generlly used to test differenes in ioti ftors mong zones long the topogrphi grdient, mong months nd mong tretments in the removl experiment. Dt were log 10 (x+1) trnsformed if neessry to inrese normlity nd homogeneity of vrine. Those dt tht did not meet the ssumptions of ANOVA fter usul trnsformtions were nlyzed using nonprmetri multiple omprisons (Steel test) or nonprmetri Kruskl Wllis tests. ANOVAs nd nonprmetri Kruskl Wllis tests were performed using SAS Version 9.1 (SAS Institute, Cry, North Crolin, USA). Nonprmetri multiple omprisons were performed using R Version 2.7 (R Development Core Tem 2008). The reltionships etween plnt ommunities nd ioti ftors were nlyzed using nonil orrespondene nlysis (CCA). CCA is one of the most widely used grdient nlyses y whih speies omposition n e diretly relted to mesured environmentl vriles (Plmer 1993). In order to investigte differenes in the plnt ommunity-ioti ftor reltionships etween low-elevtion hitts (Spritn low mrsh, Sued low mrsh nd high mrsh) nd highelevtion hitts (high mrsh, terrestril order nd uplnd), two CCAs were performed for low-elevtion hitts nd high-elevtion hitts, respetively. High mrsh ws onsidered s trnsitionl zone etween the low-elevtion nd the high-elevtion hitts (Trut 2005), nd it ws inluded in oth low-elevtion hitts nd high-elevtion hitts for the CCAs. An dditionl CCA ws performed to detet the reltionship etween plnt ommunities nd ioti ftors ross the entire topogrphi grdient. Speies perentge over vlues in ll qudrts were log 10 (x+1) trnsformed nd used for CCA speies mtries, nd ioti dt inluding flooding frequeny (medium vlues of the estimted rnges for eh zone, see Results ) werelsolog 10 (x+1) trnsformed (Plmer 1993) nd used for CCA environment mtries. CCA ws performed using Cnoo for Windows 4.5 (ter Brk nd Šmiluer 2002). Results Plnt Communities Aross Topogrphi Grdients Aross the topogrphi grdient in the Yellow River estury, ompositions of plnt ommunities differed mong different zones (Tle 1).In the low mrsh,there were minly two distinguishing vegettion zones: the Sprtin zone sewrd nd the Sued zone lndwrd. No other vsulr plnt speies were found in the Sprtin zone while severl other speies, suh s Phrgmites nd S. europe, ourred in reltively low overge in the Sued zone (Tle 1). On the lower order of the Sued zone, Sprtin lso formed some pthes. In the Tle 1 Compositions of plnt ommunities in different zones long the topogrphi grdient Speies Uplnd (n=15) Terrestril order (n=20) High mrsh (n=25) Sued low mrsh (n=25) Sprtin low mrsh (n=10) Typh spp. 2± Slix mtsudn 24± Cynnhum hinense 9± Sonhus rvensis 18± Tripolium vulgre 2± Cirsium setosum 2± Apoynum venetum 13± Trirrhen shriflor 22± Impert ylindri 7± Equisetum rvense 2± Glyine soj 3± Clmgrostis pseudophrgmites 7± Phrgmites ustrlis 55±25 3±5 1±4 12±24 0 Tmrix hinensis 5±9 19± Sued sls 0 71±29 64±24 68±25 0 Sliorni europe ±2 0 Sprtin lterniflor ±8 85±14 Dt re perentge over vlues (shown s mens±se)

5 Esturies nd Costs (2011) 34: high mrsh, Sued ws the only dominnt plnt speies. Although there were some smll pthes of Phrgmites, the men overge of Phrgmites in the high mrsh ws muh lowerthninthesued low mrsh (Tle 1). In the terrestril order t higher elevtions, shruy speies, Tmrix, formed onspiuous zone of shrus, despite tht Sued ws still dense. In the uplnd, Phrgmites repled Sued nd Tmrix nd eme the dominnt speies in this zone. Other min plnt speies tht ourred in the uplnd inluded T. shriflor, C. pseudophrgmites, nd n rorel speies, Slix mtsudn (Tle 1). Aioti Ftors Aross Topogrphi Grdients Among-zone vritions Flooding frequeny generlly deresed from the lowest Sprtin zone to the uplnd. The Sprtin nd the Sued low mrshes were flooded frequently while the terrestril order nd the uplnd were little flooded (Fig. 2). The high mrsh, s trnsitionl zone etween low nd high elevtions, ws flooded with medium frequeny of 5 20%. All the four edphi ftors determined differed signifintly mong different zones long the topogrphi grdient. Soil slinity rehed onspiuous pek in the high mrsh, nd it ws muh lower in the other zones oth lndwrd nd sewrd; in ontrst, soil moisture ontent nd ph were lowest in the high mrsh ut were highest in the uplnd nd the Sprtin low mrsh (Fig. 2). Soil ulk density ws reltively low in the uplnd nd high in other zones (Fig. 2e). Sesonl vritions Soil slinity vried sesonlly in eh of the five zones while soil ulk density showed strong sesonl vrition only in the Spritn low mrsh (Fig. 2). Signifint sesonl vritions in soil moisture ontent were deteted only in the terrestril order, the high mrsh nd the Spritn low mrsh. Soil ph differed mong months in eh of the zones exept the Sued low mrsh. Despite of these sesonl vritions, the ptterns of mong-zone vritions long the topogrphi grdient for eh ftor were generlly similr when determined in different months. For exmple, soil slinity ws onstntly highest in the high mrsh zone nd lowest in the uplnd nd the Spritn low mrsh (Fig. 2). Reltionships Between Plnt Communities nd Aioti Ftors For the three zones t low elevtions, i.e., the Sprtin low mrsh, the Sued low mrsh, nd the high mrsh: The first xis of the CCA ordintion explined most of the totl vrine of the speies-environment reltion (Tle 2). All the five ioti ftors signifintly orrelted with the first xis ut showed no reltionship with the seond xis (Tle 2). Flooding, moisture ontent, nd ph positively Floodling frequeny (%) Slinity (PSU) Moisture ontent (%) ph Bulk density (g m -3 ) d e ** d d d ** ** * ** Uplnd Mx Medium Min 0 1 * ** * de e Terrestril order ** * High mrsh Sued low mrsh June August orrelted while slinity nd ulk density negtively orrelted with the first xis (Tle 2). Plnt ommunities in the Spritn low mrsh, the Sued low mrsh nd the high mrsh were minly distriuted on the right, the middle nd the left long the first xis, respetively (Fig. 3). For the three zones t high elevtions, i.e., the high mrsh, the terrestril order, nd the uplnd: the first xis of the CCA ordintion lso explined most of the totl vrine of the speies-environment reltion (Tle 2). All the five ioti ftors signifintly orrelted with the first xis, nd only flooding nd moisture ontent showed signifint 12.5 ** d 55 July ** ** ** Sprtin low mrsh Fig. 2 Sesonl vritions in ioti ftors from June through August Flooding frequeny; soil slinity; soil moisture ontent; d soil ph; nd e soil ulk density. Dt re shown s mens±se exept flooding frequeny dt whih were shown s estimted mximl, minimum nd medium vlues. For edphi ftors, the smple sizes re eight exept those in the Sprtin low mrsh determined in June whose smple size re six, due to two smples lost during smple nlysis. Signifint differenes mong months re indited y str (P<0.05) or two strs (P<0.01) ove eh of the r groups. For differenes mong zones, rs of eh month in eh suplot shring letter re not signifintly different from one nother. Sttistil omprisons for the flooding frequeny dt whih were estimted were infesile 95

6 464 Esturies nd Costs (2011) 34: Tle 2 Summry of the CCA ordintions Low-elevtion hitts High-elevtion hitts All hitts Axis 1 Axis 2 Axis 1 Axis 2 Axis 1 Axis 2 Sttistis Eigenvlue Speies-environment orreltions % vrine Correltions Slinity 0.693** ** ** 0.262* Flooding 0.681** ** 0.404** 0.805** 0.386** Moisture ontent 0.642** ** 0.275* 0.256* 0.510** ph 0.567** ** ** 0.340** Bulk density 0.275* ** ** Correltions indite intr-set orreltions of ioti ftors with the first two ordintion xes *P<0.05; **P<0.01 Perentge vrine of speies-environment reltions explined y the first two ordintion xes reltionships with the seond xis (Tle 2). Slinity, flooding nd ulk density positively orrelted while the others negtively orrelted with the first xis. Plnt ommunities in the uplnd were distriuted on the left of the CCA iplot (Fig. 3). In ontrst, plnt ommunities in the high mrsh were minly distriuted t the lower right of the iplot. Plnt ommunities in the terrestril order were distriuted on the upper right of the iplot (Fig. 3). For ll the zones long the topogrphi grdient The first xis nd the seond xis of the CCA ordintion explined 58.2% nd 29.3% of the totl vrine of the speies-environment reltion, respetively. All the ioti ftors signifintly orrelted with the first nd the seond xis, exept ulk density whih orrelted only with the first xis (Tle 2). Slinity, flooding nd ulk density negtively orrelted while ph nd moisture ontent positively orrelted with the first xis (Tle 2). Flooding, moisture ontent, nd ph positively orrelted while slinity negtively orrelted with the seond xis (Tle 2). Plnt ommunities in the uplnd were distriuted on the right of the CCA iplot (Fig. 3). Plnt ommunities in the terrestril order nd the high mrsh were minly distriuted on the lower left of the iplot (Fig. 3). The Sued nd the Sprtin low mrshes were distriuted on the upper left of the iplot (Fig. 3). Vegettion Feedks y Shding Results from the removl experiment showed tht vegettion removl signifintly elevted soil slinities in the uplnd nd in the terrestril order rther thn in the high mrsh (Fig. 4). Signifint effets of shding on soil moisture ontent nd ulk density were only deteted in the uplnd (Fig. 4, d). Signifint effets of shding on soil ph were not deteted in ny zone (Fig. 4). Compred with vegettion-removl plots in the terrestril order, nturl re pthes hd signifintly higher level of soil slinity nd ulk density (Fig. 4, d). Differenes in soil moisture ontent nd ph etween vegettion-removl plots nd nturl re pthes were not deteted (Fig. 4, ). Disussion Zontion Pttern in the Yellow River Estury In the Yellow River estury, the most onspiuous vegettion zones re Sued-dominted low mrshes nd high mrshes. Sued hs wide tolerne to slt nd flooding stress (Song et l. 2008; He et l. 2009), eing one of the min slt mrsh plnt speies in slt mrshes ross the temperte ost of Chin (Yng nd Chen 1995; Editoril Committee of Wetlnd Vegettion in Chin 1999). Sued domintes extensive mrshes in the Yellow River estury, extending from the low mrsh frequently flooded to the high mrsh nd the inlnd terrestril order where re little flooded nd slinity re extremely high. In the terrestril order t upper elevtions, the shruy speies Tmrix grows reltively vigorous ove Sued plnts, forming striking zone of shruy plnts long the estury. Tmrix is lso one of the min slt mrsh plnt speies ross the temperte ost of Chin (Editoril Committee of Wetlnd Vegettion in Chin 1999). Despite of the wide stress tolerne, Sued nd Tmrix re repled y dense

7 Esturies nd Costs (2011) 34: Axis 2 (eig = 0.038) - Axis 2 (eig = 0.168) - - Axis 2 (eig = 0.391) Sprtin low mrsh Sued low mrsh - Slinity - - Moisture ontent ph Flooding Bulk density Slinity Bulk density High mrsh Terrestril order Axis 1 (eig = 0.390) Axis 1 (eig = 0.771) Moisture ontent Axis 1 (eig = 0.775) Moisture ontent Bulk density Flooding ph Uplnd ph Slinity Flooding Fig. 3 CCA iplots showing reltionships etween plnt ommunities nd ioti ftors t low elevtions (), t high elevtions (), nd ross ll elevtions () Phrgmites plnts nd pthes of severl other plnt speies in the uplnd. Besides the ntive vegettion zones, we would like to ddress the emerging zone formed y Sprtin invsions in the Yellow River estury, due to the potentil effets of Sprtin invsions on ptterns nd proesses of the mrsh eosystem in the estury. The effets of Sprtin invsions on the Yngtze esturine mrshes in southern Chin hve lredy proved to e signifint (Wng et l. 2006; Wuet l. 2008; Li et l. 2009). In the Yellow River estury in northern Chin, Sprtin ws only distriuted in few pthes of ommonly less thn 10 m 2 in size in the mudflt efore 2007; Amzingly, in 2009, Sprtin mrsh zone of severl hundred meters in width ws formed in the mudflt, nd Sprtin now is tending to spred rpidly sewrd to newly formed mudflts in the estury nd lndwrd to the ntive Sued low mrshes (Qing He, personl oservtion). Determinnts of Vegettion Zontion Aross Elevtions In generl, our study shows tht ioti ftors orrelte with the zontion of vegettion ross the topogrphi grdient in the Yellow River estury. Moreover, our results suggest tht the reltionships etween plnt ommunities nd ioti ftors differ etween low- nd high-elevtion hitts long the topogrphi grdient. Below, we would rgue tht flooding nd slinity, the two most pronouned ioti ftors (Bertness nd Ellison 1987; Bertness 1991, ; Bertness et l. 1992; Pennings et l. 2005), ply different roles in mediting vegettion zontion etween low- nd high-elevtion hitts long topogrphi grdients in the estury. In the estury, lthough flooding frequeny generlly deresed with inresing elevtion long the topogrphi grdient, slinity rehed pek in the high mrsh (Tle 1 nd Fig. 2). At low elevtions, flooding frequeny deresed while slinity inresed from the Spritn low mrsh through the high mrsh. As reveled y the CCA ordintion, flooding nd slinity oppositely relted with distriutions of plnt ommunities t the low elevtions (Fig. 3). The lower limits of these plnt speies, suh s the low limit of Sued, t low elevtions re most proly medited y flooding rther thn slinity (see experimentl studies in He et l. 2009), while their upper limits re most proly medited y slinity. Besides, s the upper oundry of the urrent Sprtin zone is still elow the lower limit of Sued, the role of ompetition y Sprtin in mediting the lower limit of Sued should e limited. However, with Sprtin rpidly moving up towrds the Sued low mrsh, ompetition will e predited to ply role in mediting the low limit of Sued few yers lter.

8 466 Esturies nd Costs (2011) 34: Fig. 4 Summry of the removl experiment investigting vegettion feedks y shding. Dt re shown s mens±se (n=6). Differenes etween vegettionpresent nd vegettion-removl plots re indited y the P vlues ove eh r group. For rs of terrestril order in eh suplot, rs shring letter re not signifintly different from one nother Slinity (PSU) Vegettion-present Vegettion-removl Nturl re pth P < P = 0.62 P = Moisture ontent (%) P = P = 0.56 P = 0.51 ph P = 0.28 P = 0.25 P = 0.60 Bulk density (g m -3 ) d P = P = 0.81 P = Uplnd Terrestril order High mrsh 0.8 Uplnd Terrestril order High mrsh At high elevtions, however, oth flooding frequeny nd slinity deresed with inresing elevtion (Tle 1 nd Fig. 2). Flooding nd slinity ppered to synergilly medite distriutions of plnt ommunities t high elevtions (Fig. 3). Even though, the role of slinity my e more importnt in diretly generting ptterns of plnt ommunities t high elevtions. At lest, slinity lone ould well explin the segregtion of plnt ommunities etween in the terrestril order nd in the uplnd (Fig. 3). The extreme sline onditions of the terrestril order (see Tle 1 nd Fig. 2) re pprently eyond the slt tolerne of Phrgmites who is only pseudohlophyte (Nikolevskii nd Smirnov 1968) nd who hs limited slt tolerne (Adms nd Bte 1999). However, omintion of slinity nd flooding rther thn slinity lone would etter explin the segregtion of plnt ommunities etween in the high mrsh nd in the terrestril order (Fig. 3). Tmrix hs wider slt tolerne thn Phrgmites (He et l. 2008), yet we d rgue tht the vigorous growth of Tmrix in the terrestril order is proly medited y positive intertions with Sued who often forms dense nopies nd n signifintly redue soil slinities in the terrestril order (see Fig. 4). Thin dwrfish Sued plnts in the high mrsh hd little effet on soil slinity (Fig. 4), resulting in higher soil slinity levels in the high mrsh thn in the terrestril order, despite of none sttistil signifine (see Tle 1 nd Fig. 3). In ontrst to the low limits of plnts, the upper limit of Sued t high elevtions ws suggested to e ontrolled y ompetition (He et l. 2009). Whether ompetition medites the upper limits of Tmrix remins uninvestigted. In ddition to the pronouned roles of flooding nd slinity in mediting the vegettion zontion, our study lso indites tht soil moisture ontent, ph, nd ulk density my ply some role in mediting the vegettion zontion in the Yellow River estury. It hs een suggested tht soil moisture ontent ould ffet seed germintion of slt mrsh plnts (Noe nd Zedler 2000) nd tht soil ph n ffet performnes of mrsh plnts y ltering the form of inorgni ions ville to the plnts nd the plntmyorrhizl ssoitions (Montemyor et l. 2008; Gormlly nd Donovn 2010). However, in our study, whether the sttistilly signifint ut limited vritions in these ioti ftors do mtter to the plnts hs to e further experimentlly exmined. Tken together, our study supports the plnt zontion prdigm whih suggests tht ioti ftors ply role in mediting ostl vegettion zontion (Bertness nd Ellison 1987; Bertness 1991, ; Bertness et l. 1992; Bertness nd Pennings 2000; Pennings et l. 2005). Moreover, our study supports the urrent hypothesis tht the preise ontrolling ftors mediting vegettion zontion vry ross elevtions on osts (Pennings nd Cllwy 1992; Lenssen et l. 1999; Friñ et l. 2009). As determinnts of vegettion pttern n vry with elevtion nd differ due to the sptil sle used (see Mtthews et l. 2009), sptil vritions in the determinnts of vegettion pttern should e more expliitly ddressed in future studies.

9 Esturies nd Costs (2011) 34: The Role of Vegettion Feedks Our study shows tht the most signifint effet of vegettion shding on edphi onditions in the three zones t upper elevtions is the effet of shding on soil slinity. Shding signifintly redued soil slinity in the uplnd nd the terrestril order ut hd little effet on soil slinity in the high mrsh. The effet of shding ws most signifint in the terrestril order where ws flooded only during very few exeptionl high tides in lte utumn nd storm tides. In the terrestril order, Sued often forms dense nopies wheres disturne-generted re pthes where soil slinity is extremely high (see Fig. 4) would e permnent without vegettion over (Qing He, personl oservtion). Although in the uplnd soils without vegettion shding still mintined low level of slinities, vegettion shding lso redued soil slinity. The elevted soil slinity in the terrestril order nd the uplnd without vegettion shding my relte with hnges in evportive onentrtion of groundwter slts t the soil surfe (Slm et l. 1999; Joágy nd Jkson 2004; Werner nd Lokington 2006). In ontrst to the terrestril order nd the uplnd, vegettion removl hd little effet on soil slinity in the high mrsh where flooding frequeny ws out 5~20%. This is proly result from reltively thin dwrfish Sued plnts whih hve limited effets of shding in the high mrsh. These results onfirm previous studies whih suggested tht the removl of vegettion would strongly elevte soil slinity (Bertness nd Yeh 1994; Hker nd Bertness 1999; Brewer et l. 1997; Pennings et l. 2003). Moreover, our study indites tht vegettion shding n signifintly influene edphi onditions t further upper elevtions, suh s the uplnd nd the terrestril order esides the high mrsh long ostl topogrphi grdients, nd tht vegettion shding my not ply role in high mrshes where slt stress is extremely severe restriting plnts from forming enough nopies to hve sustntil feedk. Sine vegettion feedks n meliorte edphi onditions even in terrestril orders nd uplnds where re little flooded ut not in high mrshes t middle tidl heights, positive intertions mong plnts re preditle to ply role in mediting orgniztions of plnts t the lndwrd end of nd vry ross ostl topogrphi grdients. In onlusion, long the topogrphi grdient in the Yellow River estury on the Pifi ost of northern Chin, different plnt ommunities dominted different zones of vrying ioti onditions. The reltionships etween plnt ommunities nd ioti ftors differed etween low- nd high-elevtion hitts long the topogrphi grdient. Slinity nd flooding oppositely relted with distriutions of plnt ommunities t the low elevtions, ut they ppered to operte synergilly t the high elevtions. The role of vegettion feedks n vry ross the topogrphi grdients; t upper elevtions eyond the high mrsh, vegettion feedks (nd hene potentil positive intertions) n still ply role in mediting orgniztion of plnt ommunities. Sptil vritions in determinnts of ommunity struture should e therefore ddressed in future studies in esturine nd ostl systems, s well s in other nturl hitts. Aknowledgments We thnk Xinxin Liu, Ynyn Hu, Ynzi Ci, nd Jifeng Deng for invlule field nd/or lortory ssistne, Chenghun Wng, Xio Sun, two nonymous reviewers for ritil omments, nd Yueling Liu nd Junzhng Lv for ess to the study site. This study ws funded y Ntionl Nturl Siene Foundtion of Chin (U ), Ntionl Key Bsi Reserh Progrm of Chin (2006CB403303), nd the Fundmentl Reserh Funds for the Centrl Universities (2009SD-24). 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