Belg. Journ. Bot. 137 (2) : 140-154 (2004)

VEGETATION COMPOSITION AND ZONATION OF A MEDITERRANEANBRAIDED RIVER FLOODPLAIN

Jose Antonio MOLINN· *, Concepci6n PERTlNEZ1, Alberto DIEZl & Miguel Angel CASERMEIR02

1 Departamento de Biologfa Vegetal n, Universidad Complutense de Madrid, 28040-Madrid, Spain2 Departamento de Edafologfa, Universidad Complutense de Madrid, 28040-Madrid, Spain

(* Author for con'espondence)

Received 3 March 2003 ,. accepted 7 June 2004.

ABSTRACT. - This paper examines the spatial distribution of riparian plant communities ina pre-Pyrenean braided river floodplain from three perspectives: floristical, environmental andseral. Fourteen plant-community types and 4 soil texture classes were recognised. Shrublandsof Salix purpurea, Salix elaeagnos, Coriaria myrtifolia, Populus nigra, Tamarix gallica andTamarix canariensis are widespread in the banks and floodplain. These shrublands are the cli­max vegetation in areas with more intense river disturbance or, in areas with less intense riverdisturbance they represent an intermediate step towards arboreal climax vegetation. Forests ofblack poplar (Populus nigra) and white willow (Salix neotricha) are the climax vegetation onsilty clay and sandy loam soils. Saltcedar (Tamarix gallica , Tamarix canariensis) copsesappear to constitute the climax vegetation on sandy loam that has less moisture effectiveness.

KEy WORDS. - Riparian vegetation, pre-Pyrenean stream, northern Spain, classification,ordination, numerical analysis, plant-soil relationship.

INTRODUCTION

As more is learned about how riparianecosystems function, their role in the landscape isseen to be increasingly important. Riparian areasare in fact an interface between terrestrial andaquatic environments supporting important eco­logical functions and a diversity of plants and ani­mals. Riparian corridors have been related to lon­gitudinal and lateral patterns of species distribu­tion as well as to species flows and exchangesacross ecotonal and ecoclinal boundaries (HuGHES1988, BAKER 1989, TABACCHI 1994, Mouw &ALABACK 2003).

The river environment is subjected to theaction of intense dynamic processes such as rising

and falling water levels, erosive effects, and theformation of deposits (ABERNETHY & WILLBY1999), which produce a great spatial and temporalheterogeneity in the habitats (LARGE et al. 1993).These disturbances are fundamental to thedynamics of riparian plant communities, as thespecies have to adjust their distribution to the newconditions (VAN Looy et al. 2003). These commu­nities are distributed in a transversal dimension(LYON & SAGERS 1998). The main abiotic factorsaffecting the pattern of riparian vegetation distrib­ution are changes in soil texture and bioclimate,and to a lesser degree, height and distance fromrunning water (ANmOLINI & DE DOMINICIS 2001).In Spain, several studies have focused on the phy­tocoenological diversity and zonation of riparian

VEGETATION OF A BRAIDED RIVER FLOODPLAIN 141

TABLE 1

Bioclimatic indices and diagnoses according to Rivas-Mart{nez (1997) and climatic data from Elias & Ruiz (1977)

Altitude P T It I, Bioclimatic belt Ombric type

Tamarite 233 475 14.2 248 19.7 Mesomediterranean DryMonz6n 279 461 14.5 250 19.0 Mesomediterranean DrySosa 324 448 14.7 233 20.6 Mesomedherranean DryBarbastro 341 536 14.7 261 18.7 Mesomediterranean DryBinefar 380 414 13.0 221 17.5 Mesomediterranean DryEl Grado 467 768 13.5 225 19.6 Mesomediterranean SubhumidGraus 580 694 11.9 173 19.3 SuprameditelTanean SubhumidPuebla de Castro 649 489 13.3 202 19.8 Supramediterranean Subhumid

P : mean annual preciphation in mm ; T: mean annual temperatures in QC ; L : thermicity index = T + T,"ldmin +T,"ld=" whh T,"ldmin = minimum monthly mean temperature of the coldest month, T,oldmn, = maximum monthly meantemperature of the coldest month; I, : continentality index = Twonn - T,"ld, with TwMm = mean temperature of thewarmest month, T,"ld = mean temperature of the coldest month.

ecosystems (DiAZ et at. 1987, MOLINA & ARNAlZ1987, LARA et at. 1996, VALLE & GARciA-BAQUERO1996, ALCARAz et at. 1997, BIURRUN 1999, MOLINA& MORENo 1999, SALAZAR et at. 2001) but noneparticularly in a braided river system. This type ofecosystem, of which few examples exist in Spain(PEDRAZA 1996), usually consists of a stream witha shallow channel in coarse alluvium comprisingseveral deeper threads of faster flowing cunents,which are continually breaking up and reconven­ing in constant fluctuation.

The goals of this research project are toinvestigate in phytosocio1ogica1 terms the floristi­cal composition of riparian plant communitiesand their transversal distribution in aMediterranean braided river section. Moreover,we also aim to study vegetation complexes from asuccessional viewpoint.

MATERIAL & METHODS

STUDY AREA

The Cinca river is the main Spanish Pyreneanriver. The middle Cinca river is open and very unstableas it undergoes important fluctuations in water volume,accumulating considerable gravel deposits that causethe flow to run along numerous channels. The width ofthe channel and the bank is very variable, although thelatter normally occupies a large part of the flood plain.The vegetation on the banks is mainly composed ofshrublands dominated by Salix elaeagnos and Salixpurpurea (ALCAZAR & FERRAN 1998). Water from the

middle Cinca river is used for irrigation and hydroelec­tricity (GIL et al. 2001).

The study area is defined whh the climatic dataprovided in Table 1. It has a Mediterranean macrobio­climate. The climatic indices reveal a moderately tem­perate and oceanic-semicontinental region. There is achange of bioclimatic belt from Mesomediterranean toSupramediterranean towards 500 meters of altitude.Ombric types range from dry to subhumid. Climax veg­etation consists of continental and basophilous ever­green woodlands of Quercus rotundifolia in theMesomediterranean belt. Continental and basophilousmarcescent forests are the potential vegetation in theSupramediterranean belt. The potential riparian vegeta­tion of the upper terraces corresponds to basophilousMediterranean riparian forests dominated by Ulmusminor (RIVAS-MART1NEZ 1987).

VEGETATION DESCRIPTION AND SOIL ANALYSIS

Using a phytosociological approach (BRAUN­BLANQUET 1964, MUELLER-DoMBols & ELLENBERG 1974,WESTHOFF & VAN DER MAAREL 1978), 53 releves at11 localities were made in river environments, record­ing all the identified habitats (Fig. 1). The range of plotsizes varied according to the physiognomy of the plantcommunities: 1-50 m' in herbaceous communities,4-10 m' in chamaephytic communities, 20-100 m' inshrublands and 50-400 rn' in forests. The followingparameters were recorded in each releve : UTM coordi­nates, altitude (m a.s.l.), distance from running water(m), and relative elevation above the river margin (cm)as a surrogate for the duration of flooding. Also, sub­strate grain-size was recorded visually in each sampling

142 BELGIAN JOURNAL OF BOTANY 137

FIG. 1. - Study area (rectangle) and its geographical posi­tion.

plot during low water levels in 2003. The relative coverof each of the seven substrate classes (blocks 250 ­4000 mm; pebbles = 64 - 250 mm; gravel = 2­64 mm ; sand = 0.2 - 2 mm; sandy silt = 0.01­0.2 mm; silty clay = < 0.01 mm; organic mud) wasestimated using five classes (1 : cover:S 5% ; 2 : cover:S 25% ; 3: cover :S 50% ; 4: cover :S 75% and 5 :cover> 75%).

The releves were ascribed to plant-communitytypes and these were placed in a syntaxonomical frame­work (RIVAS-MARTfNEZ et al. 2001). In each locality atransversal section of the plant-community distributionwas also made in order to establish a final schematictransect. The dynamic-zonal interpretation of the land­scape is according to GEHU & RNAS-MARTfNEZ (1982)and RNAS-MARTfNEZ (1994). The nomenclature of theplants follows CASTROVlEJO et al. (1986-2001) andTUTIN et al. (1964-1980) with the following exceptions:

Galiwn lucidum All. subsp. fruticescens (Cav.) A. &O. Bolos and Salix neotricha Goerz.

Samples from the top soil (1-10 cm) were collect­ed in most of the river plant-community types studied.Soils where it was difficult to obtain samples, such asthe flooded soils of certain river-margin vegetation orvery stony soils with shrubs, were not collected. A totalof 13 samples were analysed for pH and electrical con­ductivity in water suspension, organic carbon content,soil texture classified into three types (sand, clay andsilt) and soluble content of Ca'+, Mg'+, Na+, K+ mea­sured by AAS (ISRIC 1993). Field moisture wasanalysed in a core sample according to SMITH &MULLINS (1991).

For numerical analysis, all the releves were com­bined in one table. In this table, sporadic taxa with onlyone presence were excluded. In order to focus the seralstudy on a river segment with homogeneous mesocli­mate, a stretch of about 50 km from the middle Cincariver with a dry Mesomediterranean bioclimate waschosen. In the final table the releves located outside thissection were not considered. The Braun-Blanquet scalevalues were transformed to the ordinal scale of VAN DERMAAREL (1979). The matrix obtained (36 releves x61 species) was processed by multivariate analysis toobtain plant-communities and to seek relationshipsbetween the distribution of the vegetation, the seralcomplexes and the succession phenomena.Classification analysis used Similarity Ratio to com­pute the distance between every pair of samples, andComplete Linkage as distance-optimising clustering.These algorithms have been demonstrated to be usefulin syntaxonomical studies (MUCINA & VAN DER MAAREL1989). The computer program used was SYNTAX (PODANI1993). The clusters obtained by classification wereanalysed by Detrended Correspondence Analysis(DCA) using CANOCO (TER BRAAK & SMILAUER 1998).

RESULTS

VEGETATION COMPOSITION

Fourteen plant-community types were recognisedin the Middle Cinca Basin. Their floristical com­position is shown in Tables 2-5 and Appendix 1.Seven phytosociological classes were identified:Phragmito-Magnocaricetea, Molinio-Arrhena­theretea, Bidentetea tripartitae, Thlaspietearotundifolii, Salici purpureae-Populetea nigrae,Rhamno-Prunet'ea and Nerio-Tamaricetea(Appendix 2).

VEGETATION OF A BRAIDED RIVER FLOODPLAlN

TABLE 2

Helophytic and water margin vegetation

Releve number 1 2 3 8 9 4 5 14

Altitude (m) 330 330 255 330 330 255 255 255

Plot surface (m2) 4 4 20 1 2 40 10 4

Localities B B E B B E E E

Ai Ai Ai Az Az B B B

Typha domingensis

I

3 1 2 + 1

Typha latifolia 1 4 4

funGus subnodulosus + 2 4

Phragmites australis + 4 5I

1

Paspalum paspalodes ~Phalaris arundinacea 1

Species of Phragmito-Magnocaricetea

Lythrum salicaria + + 2

Lycopus europaeus +

Companions

funcus articulatus 3 1

Salix pUlpurea (pI.) + +

Sporadic species 0 0 4 2 1 2 2 2

143

The helophytic commumtles (Phragmito­Magnocaricetea) occur in the channel of the riverwhere true aquatic vegetation is absent. Swampscharacterised by Typha latifolia and Typha domin­gensis (Table 2, reI. 1-3) are found within thechannel growing with their basal parts submerged.Phragmites australis reed-swamps are found onemerged silty-sandy banks and dykes (Table 2,reI. 4, 5). Water margin vegetation includesswamps characterised by funGus subnodulosus(Table 2, reI. 8, 9) on shallow silty banks andPhalaris arundinacea sedges on sandy-pebblybanks covered with a thin layer of mud (notreleved).

Wet pastures of Molinio-Arrhenathereteaclass occur dispersed along the banks. Dense tus­sock grasslands dominated by Schoenus nigricans(reI. 11) are found on sandy banks, and forbsdominated by Mentha longifolia on sandy-pebblebanks (reI. 12). Mediterranean rushes of Scirpusholoschoenus (not releved) and perennial rhi­zomatous grasslands of Paspalum paspalodes

have been identified on siltY-sandy deposits(Table 2, reI. 14). Other marginal forbs identifiedare those with autumnal growth of Polygonumlapathifolium which develop on nutrient-richdeposits of the channel (Bidentetea tripartitae,reI. 13).

The vegetation of sandy-pebbly areas, some­what raised with respect to summer water levels,includes open chamaephyte communities charac­terised by Andryala ragusina (Thlaspietea rotun­difolii, Table 3, reI. 15-19). It has a thermo­xerophilous significance in the territory (BRAUN­

BLANQUET & BOLOS 1958).The braided shores and alluvial plains are

settled by a dynamic series of shrublands foundon a diversity of substrates, sometimes clearlypioneering, which precede riparian forest in thesuccession. The riparian shrublands (Salicetaliapurpureae, Salici purpureae-Populetea nigrae)are floristically characterised in the territory bySalix elaeagnos, Salix purpurea, Coriaria myrti­folia, Populus nigra (shrub), Tamarix canariensis

144 BELGIAN JOURNAL OF BOTANY 137

TABLE 3

Chamaephytic vegetation

Releve number 15 16 17 18 19

Altitude (m) 330 350 350 240 240

Plot surface (m') 10 10 4 10 10

Localities B A A F F

C C C C C

Andryala ragusina I 3 3 2 2 2

Companions

Sanguisorba minor subsp. muricata + 1 2 +

Helichrysum stoechas 2 1 2

Galium lucidum subsp. fruticescens + +

Ononis natrix subsp. natrix 1 +

Paronychia capitata 3 +

Plantago sempervirens + 2

Piptatherum miliaceum + +

Sporadic species 4 0 0 3 3

and Tamarix gallica (Table 4, except releves 36­40, 48). The occurrence of Salix purpurea andSalix elaeagnos is common in pre-PyreneanSpanish rivers (RIVAS-MARTINEZ et al. 1991,BWRRUN 1999). Nevertheless, the presence ofCoriaria myrtifolia, a thermophilous element(FONT QUER 1962), gives a singular originality tothis floristical combination.

Shrublands and copses dominat((d byTamarix canariensis and Tamarix gallica whereSalix is normally absent (Nerio-Tamaricetea,Table 4: reI. 36-38) as well as the communities ofArundo donax, physiognomically correspondingto a tall graminetum (Table 6 : reI. 39-40) wereoccasionally found in the floodplain on fine sandysubstrates.

Riverine forests (Populetalia albae, Salicipurpureae-Populetea nigrae) were found onbanks on fine substrates. These are high, denseforests with abundant shrubs, floristically domi­nated by Populus nigra or Salix neotricha (Table4, reI. 48; Table 5, reI. 49, 46, 45). Among theaccompanying shrubs are some willows such asSalix purpurea, Salix elaeagnos, hawthorns suchas Rubus caesius, Rosa micrantha and Rosa cani-

na and other shrubs such as Coriaria myrtifolia.Within these forests some floristic variabilityalong the altitudinal gradient can be observed:Salix neotricha, a Mediterranean element, is moreabundant at lower altitudes where Salix elaeagnosand Fraxinus angustifolia tend to be absent.Populus nigra forest edges dominated byCrataegus monogyna have been found locally inthe territory (Rhamno-Prunetea, Table 5, reI. 53).

CLUSTER ANALYSIS

Classification analysis on the selectedreleves provided a dendrogram in which sevenmain clusters can be distinguished and relatedwith plant-communities (Fig. 2). Cluster Aincludes helophytic vegetation (Phragmito­Magnocaricetea) characterised by Typha domin­gensis. Two subgroups can be distinguished: onewith the swamps characterised by Typha latifolia(A), samples 1-3) and the other with low water­margin vegetation characterised by funcus sub­nodulosus (A2, samples 8, 9). Cluster B is floristi­cally characterised by Phragmites australis. Thiscluster groups tall water-margin vegetation of

TABLE 4

Shrubby vegetation

Releve number 20 21 25 26 36 37 38 39 40 28 29 30 31 32 33 48 34 35Altitude (m) 350 350 330 310 270 270 270 270 255 330 330 330 330 310 350 330 350 240Height (m) 3 1,5 1,5 3 1,5 5 2 2,5 2,5 3 2,5 2,5 5 4 2,5 20 1,5 2Plot surface (sq. m) 50 50 100 50 100 100 50 50 20 50 40 40 50 50 50 100 40 50Localities A A B C D D D D E B B B B C A B A F

D, D, D, D, DJ DJ DJ E E F F F F F F G, F FSchoenus nigricans 1Salix purpurea 2 + 3

~I3 2 1 +

Salix elaeagnos 2 3 1 2 1 1 3 2 5 3Tamarix canariensis + 3 3 3 3 4 3 5 + 2 1 1 +

Tamarix gdllicaArundo donax + 5 5 2Coriaria myrtifolia 1 + 1 4 5 5 4 4 2 3 4 3Populus nigra 3 2 + 2 + 3 1 2 3 2 3 3 1 5 3

Species of Salici purpureae-Populetea nigrae

Rubus ulmifolius 1 1 1 1 2 2Rubus caesius 4Fraxinus angustifolia 3

CompanionsDittrichia viscosa + + 2 2 3 + +Dorycnium pentaphyllum + 1 2 + + 1Piptatherum miliaceum + 1 2 + + +Phragmites australis + + + + 1Melica ciliata + + + 1 +Plantago lanceolata 1 1 1 1Saccharum ravennae + + + 1Conyza albida 1 3 1 +Sanguisorba minor subsp. 1 + + +

muricatafunGus articulatus 1 1 +Scirpus holoschoenus + + 2Phalaris arundinacea 1 + +Euphorbia segetalis + 1 +Retama sphaerocarpa + + +Helichrysum stoechas 1 + 1Galium lucidum subsp.

!ruticescens 1 1 +Andryala ragusina + + 1Plantago coronopus subsp. 2 1 2

coronopusMedicago sativa 1 + 1DauClls carota subsp. carota + 1 +Foeniculum vulgare subsp. + 1 +

piperitumMelilotus alba 1 1 .Brachypodium phoenicoides 1 +Agrostis stolonifera 1 3Sedum album + +Aster squamatus + +Centaurea aspera + +Artemisia campestris subsp. + 1

glutinosaRubia peregrina + +Scrophularia canina + +Thymus vulgaris 1 1Fumana ericoides + 1Ononis natrix subsp. natrix + 1

Sporadic species 5 0 1 3 3 3 0 1 2 0 0 0 2 3 1 0 2 3

146 BELGIAN JOURNAL OF BOTANY 137

TABLE 5

Forest vegetation

Releve number 53 49 46 45

Altitude (m) 330 330 255 240

Height (m) - 30 20 15

Plot surface (m2) 10 200 400 100

Localities B B E F

G, G, G2 G2

Populus nigra 1 5l 1 3

Salix neotricha + I 4 4

Crataegus monogyna 0= 3

Species of Salicetalia purpureae and

Salicion discolori-neotrichae

Salix purpurea 1 1 1 3

Species of Rhamno-Prunetea and

Pruno-Rubion ulmifolii

Cornus sanguinea 1 2

Rubus caesius 4 3

Coriaria myrtifolia 1 1

Rubus ulmifolius 1

Fraxinus angustifolia 3

Companions

Phragmites australis + 2

Phalaris arundinacea 2 1

Equisetum ramosissimum 1 +Sporadic species 0 1 8 7

reeds (Phragmito-Magnocaricetea, samples 4, 5)and rrnzomatous grasslands of Paspalum pas­palodes (Molinio-Arrhenatheretea, sample 14).Cluster C is composed of chamaephytic commu­nities of Andryala ragusina (Thlaspietea rotundi­foW, C, samples 15-19).

Cluster D is floristically characterised byTamarix spp. On the one hand it includes theSchoenus nigricans community (Molinio­Arrhenatheretea, Dj, sample 11) as well as willowshrublands characterised by Salix purpurea (D2,

Salici pUlpureae-Populetea nigrae samples 20,21, 25, 26). On the other hand, it comprisesTamarix-dorninated shrublands and copses (D],Nerio-Tamaricetea, samples 36-38). Group E is

characterised by Arundo donax formations(Nerio-Tamaricetea, samples 39, 40). Cluster F ischaracterised by the high abundance of theCoriaria myrtifolia. It mainly contains riparianshmblands where Salix elaeagnos is frequentlypresent (Salici purpureae-Populetea nigrae, sam­ples 28-35) in conjunction with a Populus nigraforest (sample 48). Cluster G included a well­stmctured Populus nigra forest (Salici purpureae­Populetea nigrae sample 49) and its spiny shmbedges of Crataegus monogyna Rhamno­Prunetea, sample 53), as well as forests dominat­ed by Salix neotricha (G2, samples 45, 46).

The classes with the greatest number of ele­ments were distributed in two or three clusters:

VEGETATION OF A BRAIDED RIVER FLOODPLAIN 147

J.

E

000000011111112222333342233334335444123894545678910156678908901238453965

theretea, Salici purpureae-Populetea nigrae andRhamno-Prunetea communities, and finishingwith Thlaspietea rotundifolii communities ondrier soils. The ordination also revealed thetendency of riparian shrubs of Salici purpureae­Populetea nigrae to separate into two groups, onecentred on Salix spp. and Coriaria myrtifolia, andthe other on Tamarix spp. Axis 2 is not relatedwith any gradient.

The ordination analysis qualifies the conclu­sions drawn from the classification. For example,the Tamarix-dominated shrublands and copses,and the tall Arundo donax grarninetum segregatedat the lower end of axis 2 of the ordination dia­gram, which emphasises the separation. of theclasses Nerio-Tamaricetea and Salici-Populeteain an area of contact between both units such asthe one under study. The ordination also relatesthe forests of Salix neotricha more closely to wil­low shrublands than to other riparian forests suchas those dominated by Populus nigra.

SPATIAL DISTRIBUTION

FIG. 2. - Clustering using Similarity Ratio and CompleteLinkage.

Phragmiti-Magnocaricetea (in clusters A and B),Nerio-Tamaricetea (in clusters D and E) andSalici pU1pureae-Populetea nigrae (in clusters D,F and G), whereas the samples of classes with lownumerical representation in the territory, such asMolinio-Arrhenatheretea (rei. 11 in cluster D, rei.14 in cluster B) and Rhamno-Prunetea (reI. 41 incluster G), were integrated within these firstclasses.

ORDINATION ANALYSIS

The ordination diagram obtained using DCAillustrates two main sources of variation (Fig. 3).Axis 1 describes an altitudinal sequence startingwith the communities growing on submergedsoils (Phragmito-Magnocaricetea) , followed byhygrophilous communities occurring on floodingsoils, which are in turn distributed from bottom totop in Nerio- Tamaricetea, Molinio-Arrhena-

The helophytic and water-margin communi­ties are found inside or in the bank of the channel(A, B), while the rest of the communities arelocated above the water level (Fig. 4, Table 6).The complex of riparian forests and shrublands isdistributed along an elevational gradient on whichcan be found: groves of Salix spp. and Tamarixspp. (Dz) and also groves of Populus nigra (G1) atlower elevations, and finally shrublands ofCoriaria myrtifolia (F) and groves of arborealSalix neotricha (Gz) at higher elevations. Thecommunities of Arundo donax (E) and thechamaephytic vegetation of Andryala ragusina(C) occupy the higher relative elevations on thefloodplain.

The communities which are closer to theriver are herbaceous river-margin communities(e.g. Ab B, Az), although communities of this typemay also be situated at a considerable distancefrom the channel, in superficial water poolsremaining from former channels (e.g. DJ). Thewoody river-plant communities occupy a largepart of the floodplain at distances ranging fromnear to far from the channel (e.g. Gz, GJ).

(13

30340 16 017

3~ :18 [J]15DeIhlaspietea

19

148

5

4

3

2

1

BELGIAN JOURNAL OF BOTANY 137

Rhamno-Prunetea53e

n DRCHARD

49

4b2~~1

alici-Populet;&.:J

1 9 %~O,. 26 ~ rt21P ct§ °0·. d4~ ¥0}~ 6 <:26l5

B 5\!JL:h ~Q,:.

i9/6l~

Phragmito-Magnocaricetea G q +114

o

CHANNEL 37. '+74036 V v·~8 Nerio-Tamaricetea

FLDDDPLAIN

o 2 4

Axis 1

6 8 10

FIG. 3. - DCA ordination of releves. Circles: swamps of Typha latifolia and T. domingensis ; dotted circles: swamps offunGus subnodulosus ; +-marked circles: reeds of Phragmites australis ; +-marked triangles up : grasslands of Paspalum pas­palodes and grasslands of Schoenus nigricans; triangle down: shrublands of Tamarix spp. ; +-marked triangles down: sedges ofArundo donax; squares: scrubs of Andryala ragusina; diamonds: shrublands of Salix pupurea and Salix elaeagnos ; hexagons:Salix neotricha and Populus nigra-dominated forests; +- marked hexagons: spiny edge of Crataegus monogyna. Summary infor­mation of DCA: eigen value of axis 1 =0.8542, eigen value of axis 2 =0.5413.

FIG. 4. - Schematic transversal vegetation distribution in the middle Cinca river floodplain

TABLE 6

Environmental data (mean values) for the jloodplain plant community groups

Ar Az B C Dr Do D3 E F Gr Gz

Altitude -0.15 0.36 0.54 1.89 0 0.89 1.09 2.67 1.58 0.69 1.43Distance from the river (m) -2 14 2 43.7 143 41.6 127.5 126.6 45.3 204 17.75Substrate grain-size

(dominant classes) Silty clay/ Gravel! Gravel! Pebbles/ Sand Pebbles/ Sandy silt Sandy silt Gravel! Silty clay Sandy siltOrganic Pebbles Pebbles Gravel Sand Pebbles

mud

TABLE 7

Soil data (mean values) for seven plant community groups

C Dr Do D3 E Gr Go Mean± S.E.

pH 8.2 8.1 7.9 8.1 8 7.9 7.95 8.02 ± O.llElectrical Conductivity (MS) 291 305 309.6 314.5 331 365 321.5 319.66 ± 23.64Organic Carbon (%) 0.30 0.12 0.43 0.83 0.57 1.46 1.77 0.78 ± 0.61Field moist (%) 6.98 40.73 5.24 2.10 2.63 26.98 7.53 13.17 ± 14.81Particle size distribution (%) :Sand 79.96 98.37 83.80 75.33 76,56 2.28 63.34 67.18 ± 33.78Clay 6.47 0.30 6.46 9.80 6.52 42.52 13.54 12.23 ± 13.94Silt 13.57 1.33 9.73 14.87 16.91 55.20 23.12 19.25 ± 17.20Textural class Loamy sand Sand Loamy Sand Sandy loam Sandy loam Silty clay Sandy loamSoluble cations (cmo1 kg-r)Ca2+ 4.07 2.54 7.75 4.73 6.51 7.02 4.48 5.30 ± 1.85Mgz+ 0.21 0.18 0.25 0.25 0.25 0.22 0.21 0.22 ± 0.03Na+ 0.47 0.39 0.48 0.65 0.23 1.10 0.64 0.57 ± 0.28K+ 0.52 0.05 0.37 0.30 0.36 1.08 0.41 0.44 ± 0.32

150 BELGIAN JOURNAL OF BOTANY 137

The dominant substrate grain-size is veryheterogeneous in the soils of the floodplain. Thecommunities which are flooded for the longestperiod (AI) grow on silty clay and organic mud. Arange of communities can be found growing onsand/gravel/pebbles, including herbaceous com­munities and shrublands. Riparian shrublandscharacterised by Tamarix, Salix, Populus orCoriaria (Dz, F) as well as the chamaephyticscrubs of Andryala (C) are found on coarse sub­strates. Finer substrates are dominant in Tamarixcopses (D3), Arundo donax community (E) andSalix neotricha forests (G2). Silty clay is dominantin the Populus nigra forest (GI).

SOIL-VEGETATION RELATIONSHIPS

The soils of the seven vegetation groups hada low organic carbon content, basic pH (> 7.9),and low electrical conductivity (291-365 IlS)(Table 7). Thus, the soils can be characterised aspoor in nutrients, basic and mineral. The loworganic carbon content denotes their fluvial ori­gin. The moistest of the soils analysed was thesandy soil supporting the Schoenus nigricanscommunity, which marked the water level at themoment the releve was taken. A field moisturegradient can be observed in riparian forests andcopses, which decreasingly goes from copsesdominated by Tamarix, through white willow(Salix neotricha) forests, to the black poplar(Populus nigra) forest.

Sand was the predominant soil fraction withproportions between 63% and 98%, with theexception of the group comprising the Populusnigra forest and its spiny edge in which silt is thedominant soil fraction. Mean proportions of claywere mainly below 10% except under forest veg­etation. The textural classes of the soils underwoody vegetation were: loamy sand under lowwillow shrubs, sandy loam under Tamarix copsesand Salix neotricha forests, and silty clay underthe Populus nigra forest.

The soluble salt content of the soils wasvery low, which can be attributed to the the highmobility of the salts and the sandy texture ofthese soils. The dominant cation was Ca2+ withvalues between 2.5 and 7.7 cmol kg,l whereas the

other cations were present at concentrationsbelow 1 cmol kg,l.

DISCUSSION AND CONCLUSIONS

Shrublands with Salix purpurea, Salixelaeagnos, Coriaria myrtifolia, Populus nigra,Tamarix gallica and Tamarix canariensis are veryabundant in the floodplain. All of these shrub­lands are rich in Populus nigra, but not in Salixneotricha; however, these two species are themost common liparian trees in the stretch. Somedifferences in the shrub vegetation compositionmay be related to environmental conditions.Shrubs with Coriaria myrtifolia and Salix(!laeag­nos seem to be more abundant at higher relativeelevations than shrubs with Tamarix and Salixpurpurea whiCh are more often located at lowerrelative elevations. Tamarix shrublands withoutSalix appear on sandy loam soils rather than onthe loamy sand soils of the above-mentionedshrublands. Furthermore, open areas of the flood­plain on elevated sandy-pebbly alluvial depositsare colonised by chamaephytic communitiestolerant to xeric conditions, which are here inter­preted as a type of permanent vegetation.

Riparian shrublands play a key role either aspermanent vegetation or as an intermediate stagetowards forests (RlvAs-MARTlNEZ 1982). Withinthe shrublands investigated, those characterisedby different species of willow (Salix purpurea,Salix elaeagnos) and salt-cedar (Tamarix gallica,Tamarix africana) growing on coarser substratesappear to constitute the potential vegetation inlower relative elevations and unstable conditions.

As a consequence of the succession phenom­ena, an increase in the height and structural com­plexity of the plant communities can be seen.Upland shrubs are characterised by Coriaria myr­tifolia, Populus nigra and Salix elaeagnos butSalix is absent in soils with less effective fieldmoisture. Our results suggest that several types ofarboreal vegetation that act as climax riparianvegetation may be distinguished. These includeforests as well as copses occurring on finer sub­strates and higher relative elevations. Forestsdominated by black poplar (Populus nigra) andwhite willow (Salix neotricha) are found on silty

VEGETATION OF A BRAIDED RIVER FLOODPLAIN 151

clay and sandy loam soils, respectively, whereas

copses of salt-cedar (Tamarix spp.) are found on

sandy loam with coarser particle size distribution

in relation to the aforementioned forests.

As a result of the analysis of environmental

factors and classification of the floodplain vegeta­

tion in a semicontinental Mediterranean braided

river system we draw the following conclusions:

1) Floodplain plant communities are distrib­

uted in a transversal section along an edaphic gra­dient of moisture and soil texture (ANorOLINI & DE

DOMINICIS 2001). At the floodplain scale, relative

elevation is a better predictor of the composition

and structure of the riparian plant-communities

than distance from the river.

2) Riparian forests and copses are associated

with fine substrates and soil textures of the flood­

plain. Our data revealed that soil features such as

texture and field moisture effectiveness may con­

dition the succession routes and thus the potential

arboreal vegetation.

3) Floodplain vegetation is constituted of

groups of inter-related communities which can be

interpreted as series of vegetation complexes

(RIVAS-MARTlNEZ 1994) where several succession­

al and dynamics stages coexist (LETEINTURIER etal. 2000, SUZUKI et al. 2002). Within the geoseries

studied here, several types of riparian series can

be identified: the willow series (Salix purpureaand Salix elaeagnos), the salt cedar series

(Tamarix gallica and Tamarix canariensis) and

the black poplar and white willow series (Populusnigra and Salix neotricha).

ACKNOWLEDGEMENTS

This research was supported by Instituto deEstudios Altoaragoneses (C.E.H.LM.O. and C.E.S.B.).

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VEGETATION OF A BRAIDED RIVER FLOODPLAIN 153

APPENDIX 1

LOCALITIES OF RELEvEs

Locality A : Cinca river, El Grado, near electricalpower station, 31TBG7167, 350 rn.

Locality B : Cinca river, El Grado, Puente de lasPilas, 3lTBG6961, 330 rn.

Locality C: Cinca river, Barbastro, FincaFigueruela, 3lTBG6658, 310 rn.

Locality D: Cinca river, Castej6n del Puente,Torre de Paisanto, 31TBG6548, 270 rn.

Locality E: Cinca river, Monz6n, in loco dicto"El Merendero", 31TBG6544, 255 rn.

Locality F: Cinca river, Monz6n, ConcheI,3lTBG644I, 240 rn.

Locality G: Cinca river, Santa Lecina, SanMiguel de Lecina, 3lTBG6129, 210 ill.

Locality H: Cinca river, Albalate de Cinca,3ITBG6223, 170 rn.

Locality I: Vero river, Lecina, 3lTBG07, 620 rn.Locality J : Vero river, Huerta de Vero, 31TBG06,

430rn.Locality K : Alcanadre river, Peralta de Alcofea,

30TYM36, 310 rn.

SPORADIC SPECIES OF TABLES 2-5

Table 2. ReI. 3, Scirpus lacustris subsp. tabernae­montani (2), Scirpus maritimus (1), Lysimachia vul­garis (+), Calystegia sepium (+) ; ReI. 8, Berula erecta(1), Mentha longifolia (+) ; ReI. 9, Veronica beccabun­ga (+); ReI. 4, Epilobium hirsutum (2), Pulicariadysenterica (1) ; ReI. 5, Polygonum lapathifolium (1),Paspalum paspalodes (1) ; ReI. 14, Aster squamatum(+), Polygonum lapathifolium (+).

Table 3. ReI. 15, Thymus vulgaris (2), Sedumalbum (1), Avenula bromoides subsp. bromoides (1),Fumana ericoides (1) ; ReI. 18, Populus nigra (pI.) (1),Chondrilla juncea (+), Sedum sediforme (+) ; ReI. 19,Dittrichia viscosa (1), Sonchus tenerrimus (+),Euphorbia segetalis (+).

Table 4. ReI. 20, Polypogon viridis (2), Limllnbienne (1), Carex viridula (1), Apium nodiflorum (+),Carex distans (+) ; ReI. 25, Equisetum ramosissimum(1); ReI. 26, Equisetum arvense (+), Salix neotricha xSalix triandra (+), Lythrum salicaria (3); ReI. 36,Pulicaria dysenterica (+), Dipsacus fullonum (+),Xanthium italicum (+) ; ReI. 37, Picris hieracioides (1),Trifolium pratense (+), Hedera helix (+); ReI. 39,Cistus clusii (+); ReI. 40, Marrubium vulgare (1),

Conyza bonariensis (+); ReI. 31, Rosa micrantha (+),Ephedra fragilis subsp. fragilis (+) ; ReI. 32, Asparagusacutifolius (2), Bromus erectus (1), Plantago semper­virens subsp. sempervirens (1) ; ReI. 33, Pistacia tere­binthus (+); ReI. 34, Rosmarinus officinalis (2),Onobrychis supina subsp. supina (+); ReI. 35,Gypsophila struthium subsp. hispanica (+), Dichantiumischaemum (+), Sedum sediforme (+).

Table 5. ReI. 49, Rosa micrantha (1) ; ReI. 46,Lycopus europaeus (1), Solanum dulcamara (+);Agrostis stolonifera (3), Epilobium hirsutum (2),Lysimachia vulgaris (1), Scirpus holoschoenus (1),Lythrum salicaria (1), Aster squamatus (+); ReI. 45,Dittrichia viscosa (1), Piptatherum miliaceum (+),Sanguisorba minor subsp. muricata (+), Foeniculumvulgare subsp. piperitum (1), Artemisia verlotiorum(+), Brachypodium phoenicoides (+), Plantago semper­virens (+).

RELEvES NOT INCLUDED IN THE TABLES

ReI. 11, locality A, area 50 rn', Schoenus nigri­cans 5, Salix purpurea 1, Populus nigra 1, Phragmitesaustralis +, Scirpus holoschoenus 2, Salix elaeagnos 1,Coriaria myrtifolia 1, Tamarix canariensis +.

ReI. 12, locality J, area 10 rn', Mentha longifolia3, Agrostis stolonifera 1, Salix purpurea 2', Populusnigra 1'.

ReI. 13, locality G, area 20 rn' : Polygonum lap­athifolium 3, Echinochloa crus-galli 2, Amaranthusretroflexus +, Chamaesyce prostrata +, Xanthiumitalicum +, Amaranthus blitoides +, Cyperus fuscus 1,Atriplex prostrata +, Digitaria sanguinalis +, Rumexpalustris 1, Lythrum salicaria 1, Apium nodiflorum 1,Plantago lanceolata +, Verbena officinalis +,Calystegia sepium 1, Sonchus tenerrimus 1, Populusnigra (pI.) 1, Veronica beccabunga +, Equisetum ramo­sissimum +, Plantago major +, Lycopus europaeus +,Salix neotricha (pI.) +.

APPENDIX 2

SYNTAXONOMICAL SCHEME

PHRAGMITO-MAGNOCARICETEA Klika in Klika &Novac 1941

PHRAGMITETALIA W. Koch 1926Phragmition communis W. Koch 1926(Table 2, reI. 1-3, 4-5)

154 BELGIAN JOURNAL OF BOTANY 137

NASTURTIO-GLYCERETALIA Pignatti 1953Glycerio-Sparganion Br.-El. & Sissingh inBoer 1942 (Table 2, reI. 8-9)

THIASPIETEA ROTUND/FOLl! Br.-El- 1948ANDRYALETALIA RAGUSINAE Rivas Goday in RivasGoday & Esteve 1972

Glaucionflavi Br.-El. ex Tchou 1948 (Table 3,reI. 15-19)

SALICI PURPUREAE-POPULETEA NIGRAE (Rivas­Martfnez & Canto ex Rivas-Martfnez, Bascones, T.E.Dfaz, Fernandez-Gonzalez & Loidi) Rivas-Martfnez,Fernandez-Gonza1ez, Loidi, Lousa & Penas 2001

POPULETALIA ALBAE Br.-El. ex Tchou 1948Populion albae Br.-El. ex Tchou 1948 (Table5, reI. 45-46, 49 ; Table 4, reI. 48)

SALICETAUA PURPUREAE Moor 1958Salicion triandro-neotrichae Br.-Bl. &O. Bolos 1958 (Table 4 except reI. 36-38,48)

RHAMNO-PRUNETEA Rivas Goday & Borja ex Tlixen1962

PRUNETALIA SPINOSAE Tlixen 1952Pruno-Rubion ulmifolii O. Bolos 1954(Table 5, reI. 53)

NERIO-TAMARICETEA Br.-El. & O. Bolos 1958TAMARICETALIA Br.-El. & O. Bolos 1958 em. Izco,Fernandez Gonza1ez & A. Molina 1984

Tamaricion africanae Br.-Bl. & O. Bolos 1958(Table 4, reI. 36-38)Imperato cylindricae-Erianthion ravemzaeBr.-El. & O. Bolos 1958 (Table 4, reI. 39-40)

MOLINIO-ARRHENATHERETEA Tlixen 1937HOLOSCHOENETAUA VULGARiS Br.-El. ex Tchou 1948

Molinio-Holoschoenion vulgaris Br.-Bl. exTchou 1948 (reI. 11)

PLANTAGINETALIA MAJORIS Tlixen & Preising inTlixen 1950

Mentho-Juncion inflexi De Foucault 1984(reI. 12)

PASPALO-HELEOCHLOETALIA Br.-Bl. in Br.-Bl.,Roussine & Negre 1952

Paspalo-Agrostion verticillati Br.-El. in Br.­Bl., Roussine & Negre 1952 (Tap1e 2, reI. 14)

BIDENTETEA TRIPARTlTAE Tlixen, Lohemeyer &Preising ex von Rochow 1951

BWENTETALIA TRIPARTlTAE Br.-El. & Tlixen ex Klika& Hadac 1944

Bidention tripartitae Nordhagen 1940 em.Tlixen in Poli & J. Tlixen 1960 (reI. 13)