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INVASION OF ALIEN PLANTS IN FLOODPLAINS - A COMPARISON OF EUROPE AND JAPAN

Norbert Miillerl and Shigetoshi 0kuda2 'University of Applied Sciences Erfurt, Faculty of Landscape Architecture, Leipziger Str. 77, 99085 Erfurt, Germany

Yokohama National University, Institute of Environmental Science & Technology, 7907 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan

Abstract

The vegetation of floodplains is compared in central Europe and northern- and central Japan focusing on the questions: - which alien plants have successfully naturalised and where is their native country, - which vegetation types are preferred by invasive plants, - what are the reasons for the fast expansion of aliens in floodplains?

In central Europe, 130 alien plants are naturalised in the vegetation ofriparian landscapes. In floodplains of northern and central Japan, 124 aliens have been found.

In both areas, most alien species come from North America. Whereas Europe has only 6 species from Japan, 40 species which are native to central Europe are naturalised in Japan.

Alien plants prefer the annual and perennial herbaceous vegetation in both Europe and Japan (about 80 % of all species). Fewer invaders can be found in the vegetation of floodplain shrubs and -forests and only a few in aquatic vegetation.

The success of aliens is related to human impacts in river ecosystems in both areas. While there are few or no aliens in the more natural upper courses of rivers, they become more abundant in the lower courses, where river dynamics are weakened, due to the influence of dams and where there are settlements, which are sources for the dispersal of aliens.

Introduction

Riparian landscapes are important corridors for plant dispersal (e.g. Johansson et al. 1996, van der Pijl 1982). Riparian habitats play a central role in the process of invasion and naturalisation of alien plants (PySek and Prach 1993). Many successful invaders in the natural vegetation were first observed in floodplains. The greatest number of alien plant species which have naturalised in the natural vegetation of central Europe (agriophytes, sensu Kamysev 1959), are found in floodplain vegetation. Twelve of the 13 most frequent invaders of central Europe can be found in floodplains (Lohmeyer and Sukopp 1992).

In this paper we compare alien plants in floodplains in the temperate to submeriodonal zone of Europe and Japan in order to answer the following questions: - which aliens have successfully naturalised and where is their native country, - which habitats in floodplains are most vulnerable to plant invasions, - what are the reasons for the fast expansion of aliens in floodplains?

Plant Invasions: Ecological Mechanisms and Human Responses, pp. 32 1-332 edited by U. Starfinger, K. Edwards, I. Kowarik and M. Williainson c', 1998 Backhzlys Publishers, Leiden, The Netherlands

N. Miiller and S. Oh,..

Basic data and compared rivers 3

Data about naturalised invasive plants in central Europe were collected using the 1 : - of agriophytes in central Europe (Lohmeyer and Sukopp 1992) and for alpine rii c - - (Miiller 1995a). These lists cover all of Austria, Belgium, Denmark, German:. Liechtenstein, Luxembourg, the Netherlands, and Switzerland, as well as parts of Franc. Italy, Poland, the Czech Republic and Slovakia. Information about naturalised plan: - in Japan was gathered from the list of agriophytes for the rivers of central and non]:- em Japan (Okuda n.p.), which includes Hokkaido, Hokuriku, Tohoku and Kanto. A5 ..

result, the information distiilguishes between different river types (e.g. small and b i ~ braided and bent rivers). The floodplains studied ranged in elevation fronz sea level r ca. 1500 m.

Japanese rivers originate in the alpine region, tend to be short and with steep slopc'. Times of peak run-off are in early summer, due to snow melt, and in autumn durin; the typhoon season. In contrast, European rivers can be separated into two types, base; on their time of peak run-off. Alpine rivers originate in the Alps, with most of thei- ' Y

,7

catchment area in the alpine zone; peak run-off occurs in early summer. Lowland ri\ - ers originate in the Low Mountains; peak run-off is during early spring. Rivers, suci- as the Rhine and the Danube, represent both types because of their great length.

Number and origins of naturalised alien plants

Currently, 130 alien plant species have become naturalised in central European and 124 in Japanese floodplains (Table 1). In Japan, as well as in Europe, most aliens arc' originally from North America (Fig. 1). Regarding the species from warmer areas, i n

America

Sou,th &Trop. America

Europe L

Asia h Japan

Central Europe Others I

0 5 10 15 20 25 30 35 40 45

species number

Fig. I. Origins of alien plants in floodplains of central & northern Japan (124 species) and central Europe (1 30 species)

Invasion of alien plants in floodplains -3 2 -7

Japan most came from South and Tropical America (18). whereas in Europe mol-i. species are from the Mediterranean. The reason may be that the study areas ill .Inpan include larger regions of the submeridional zone than in Europe (Mcusel et (11. 1965). An interesting fact is that Japan has received 40 species from Europe whereas, ill ccil-

tral Europe, only 6 naturalised species are native to Japan.

Table 1. Alien plants in Central European and Japanese floodplains (Source Lohmeyer & Sukopp 1992, Muller 1995a, Okuda n.p.)

In roman: species in central Europe In italic: species in Japan In bold: species in both areas Underlined species with origin in ccntral Europe or Japan Main habitat (vegetation type) in floodplains: a=annual vegetation; p=perennial veg.; s=woody (shrubs & forests) veg.; w=aquatic veg.

Species Main habitat Native country

Acer negundo L. S

Acorus calamus L. W

Aesculus hippocastanum L. S

Agrostis gigan tea Roth. P Agrostis scabra Willd. P Agrostis stolon[fera L. P Ailanthus altissima (Mill.) Swingle S

Allium paradoxum (M.B.) G.Don S

Althernanthera nodzflora R.Br. P Amaranthus albus L. a Amaranthus blitoides S.Watson a Amaranthus hybridus agg. a Amaranthus lividus L. a Amaranthus patulus Bertol. U

Amaranthus retroflexus L. a Amaranthus viridis a Ambrosia artemisia<folia L var. elat. Desc. ( I

Ambrosia trzfida L. LI

Amorpha fruticosa L. S

Anchusa officinalis L. P Andropogon virginicus L. P Anthoxanthum odoratum L. P Apera spica-venti (L.) P.B. a Arctium tomensosum Mill. P Arrnoracia rusticana Gaertn P Arrhenatherum elatius (L.) P Artemisia absinthium L. P Artemisia annua L. a Artemisia verlotiorum Lamotte a Artistolochia clematitis L. S

Aster lanceolatus Willd. P Aster novae-angliae L. P Aster novi-belgii L. P .Aster subulatus Michx. a Aster tradescantii L. P Aster X salignus Willd. P Atriplex acuminata W. et K. P .4vena fatua L. P Azolla filiculoides Lam. W

Barbarea vulgaris R. Br. a Bidens connata Muhlenb. a

North America East Asia South-East Europc Tet~zptlr-~r to Zo rlo North America SL N-E Asia E I I I - L Z S ~ ~ I North China &: N. Korea Asia Tvopicctl A4.~itr North Amcrici~ North America North Amcr~ca Mediterranean So~lt11 ,I rrl(>~.i[~~l Amcrica T 1 ~ 0 / ~ i ~ ~ ~ i l z4t?z~1~icti No/.th .I rilclr.ic.~r Nor.11~ .4 trler.ic,cr North America East Europe Nor.tlr & Cciultr-~rl :l t?lc-'~.icu Ellvtrsitr Eurasia Eurasia South-East E~lrope E L I ~ O P C Eurasia Asia South-East Asia S o ~ ~ t l l E~1ropt' North America North ,4lner1ca North Alncrica S o ~ ~ r l l :l nlcr.icu North America North Amci-ica Eurasia Elr/.(r.\ itr & !\or-tli A frictr Tropical America E111.0/)(~ North .America

324 N. Muller and S. Okuda

Table l . Cont.


Bidens frondosa L. Bidens pilosa L. (incl. var. minor Sherfj Brassica juncea Czern. Brassica napus L. (B. oleracea X rapa) Brassica nigra (L.) Koch Brassica rapa L. Briza maxima L. Briza minor L. Bromus catharticus Vahl Bromus sterilis L. Bromus tectorum L. Bryonia alba L. Buddleja davidii Franch. Bunias orientalis L. Capsella bursa-pastoris (L.) Med. Carduus acanthoides L. Carduus crispus L. Carduus nutans L. Cerastium glomeratum Thuill. Chaenorhinum minus (L.) Lange Chenopodiurn album L. Chenopodium ambrosioides L. Chenopodium ficifolium Sm. Chrysanthemum vulgare (L.) Bernh. Cichorium intybus L. Coix lacryma-jobi L. Conium maculatum L. Conyza bonariensis Cronq. Conyza canadensis L. Conyza sumatrensis Walker Coreopsis lanceolata L. Crassocephalum crepidioides L. Cuscuta campestris Yuncker Cuscuta gronovii Willd. Cuscuta lupuliforrnis Krocker Cuscuta pentagona Engelm. Cyperus eragrostis Lam. Dac<vlis glomerata L. Datura stramonium L. Delphinium anthriscvolium L. Digitaria ischaemum (Schreber) Miihlenbg. Digitaria sanguinalis (L.) Scop. Diodia teres Walt. Diodia virginiana L. Echinochloa crus-galli (L.) P.B. Echinochloa muricata (Beauv.) Fern. Echinocystis lobata (Michx.) Torr. et Gray Echinops sphaerocephalus L. Egeria densa PI. Eichhornia crassipes Solms-Laub. Elodea canadensis Michx. Elodea nutalli St. Johann Epilobium ciliatum Raf. Eragrostis albensis Scholz Eragrostis curvula (Schrad.) Nees

North America Tropical Zone Asia Cultivated plant Europe Mediterranean Mediterranean Mediterranean South America South Europe Europe & East Med. South-East Europe China Eurasia Mediterranean Europe & Asia Minor Eurasia Eurasia Europe Submed. Europe Eurasia South America Eurasia Eurasia Eurasia Tropical Asia Eurasia South America North America South America North America Africa North America East-North America Eurasia North America Tropical America Eurasia Mexico to E . North America China Eurasia Mediterranean North America North America Eurasia 3

East-North America South Europe to West Asia Argentina Tropical America North America North America North America Sibiria ? South Africa

Invasion of alien plants in jloodplains

Table l . Cont.

Species

P

Main habitat

P - -

Native country

Eragrostis minor Host. Eranthis hyemalis (L.) Salisb. Erechtites hieracifolia Rafin. Erigeron annuus (L.) Pers. Erigeron philadelphiczis L Euphorbia maculata L. Fagopyrum cymosum Meisn. Festuca arundinacea Schreb. Festuca pratensis Huds. Fraxinus pennsylvanica Marshal1 Galinsoga ciliata (Rafin.) Blake Galinsoga parviflora Cav. Geranium carolinianum L. Helianthus strumosus L. Helianthus tuberosus L.s.1. Heracleum mantegazzianum Somm. et Lev. Hesperis matronalis L. Holcus lanatus L. H?ipericum perforatum v. anazist. (DC:) Hvuochoeris radicata L. Impatiens glandulifera Royle lmpatiens parviflora DC. Iris pseudacorus L. Lactuca serriola L. Lactuca serriola L. Lamium puyuureum L. Lepiditlm virginicum L. Linaria canadensis Dzim. Lindernia dubia (L.) Pennell Lolium multifIorz~m Lam. Lolitim perenne L. Ludwigia decurrens Walt. Matricaria chamomilla L. Matricaria perforata Merat Melilotus alba Med. Melilotus officinalis (L.) Pall. Mentha spicata L. em. Huds. Mimulus guttatus DC. Mvosotis scorpioides L. Myriophyllum brasiliense Cambess. Nasturtizim officinale R.Br. Oenothera biennis L. Oenothera erythrosepala Borbas Oenothera laciniata Hill Oenothera rosea LJH4r. Oenothera stricta Ledeb. ex Link Orychophragmus violacezis (L.) O.E. Schulz Oxalis corymbosa DC. Oxalis fontana Bunge Panicum capillare L. Panicum dichotomifIorum Michx. Papaver dubium L. Papaver rhoeas L. Parthenocissus inserta (Kerner) Fritsch Paspalum dilatatum Poir.

Eurasia South-East Europe North America North America North America North America India & China Eurasia Europe North America South America Mexico North America North America North America Caucasus Europe Europe Europe Europe lndia Central Asia Eurasia Eurasia Europe Europe North America North America North America Europe Eurasia Tropical America West Asia Europe & West Siberia Eurasia Eurasia Cultivated Plant West-North America Ezirope Brazil & Sozith America Ezirope North America North America North America America South America China South America North America North America North America South Europe Mediterranean & Eurasia North America South America

N. Miiller and S. Okuda

Table I . Cont.


Paspalum distichum L. Pastinaca sativa L. Persicaria pilosa Kitag. Phleum pratense L. Physocarpus opulifolius (L.) Maxim. Picris hieracioides L. subsp. hieracioides Plantago lanceolata L. Plantago virginica L. Poa pratensis L. Poa trivialis L. Populus X canadensis Moench Portulaca oleracea L. Reseda lutea L. Reseda luteola L. Reynoutria japonica Houtt. Revnoutria sachalinensis Nakai Reynoutria X bohemica Robinia pseudacacia L. Rudbeckia hirta L. Rudbeckia laciniata L. Rumex acetosella L. Rumex confertus Willd. Rumex conglomeratus Murra-v Rumex crispus L. Rumex longifolius DC. Rumex obtus(folius L. Rumex triangulivalvis (Danser) Rech,fil. Saponaria officinalis L. Scilla sibirica Andr. Sedum sarmentosum Bunge Senecio inaequidens DC. Senecio vulgaris L. Setaria pumila (Poiret) R. et Sch. Setaria viridis (L.) P.B. Sicyos angulatus L. Silene alba (Mill.) E.H.L. Krause Silene armeria L. Sinapis alba L. Sinapis arvensis L. Sisymbrium chrysanthum Jord. Sisymbrium officinale Scop. Sisyrinchium atlanticum Bickn. Solanum americanum Mill. Solanum nigrum L. Solidago altissima L. Solidago canadensis L. Solidago gigantea Ait. Solidago graminifolia (L.) Salisb. Sonchus asper Hill. Sonchus oleraceus L. Sorghum halepense Pers. Spiraea alba Du Roi Stellaria media (L.) Vill. Symphoricarpos albus (L.) Blake Taraxactim officinale Weber

Tropical Zone Eurasia India & Malaysia & China Eurasia North America Eurasia Europe North America Eurasia Eurasia East-North America Mediterranean? South Europe South Europe & West Asia East Asia Japan & Sachalin Neotyp North America North America East-North America Europe Eurasia Eurasia Europe North Europe Eurasia Canada & North America Europe East Europe Korea & North China South Africa Europe Southern Eurasia Mediterranean & Eurasia North America Eurasia Europe Mediterranean Mediterranean South-west Europe Eurasia North America North America Tropical Zone North America North America IVorth America IVorth America Eurasia Eurasia Mediterranean East-North America neogen East-North America Europe

lnvasion of alien plants in jloodplains 327

Table I . Cont.


Telekia speciosa Baumg. Thlaspi arvense L. Trijolium dubium Sibth. Trzfolium prntense L. Trifolium repens L. Tulipa sylvestris L. Valevianella l o c ~ ~ s t a (L . ) Latervnde Verbascum densiflorum Bertol. Verbascum thapsus L. Verbena bonaviensis L. Verbena officinalis L. Veronica arveizsis L. C'evonicn hedevifolia L. Veronica peregrina L. Veronica persica Poir. Vzllpia myzlros (L.) C. C. Gm el. Xanthium albinum (Widder) H. Scholz Xanthizlm italicum Moretti Xanthiurrl occidentnle Bertol. Xanthium orientale L. Xanthium saccharatum Wallr. Em. Widder

South-east Europe West Asia Eurasia Europe Europe East Mediterranean Europe South Europe Europe South Anlevica Eurasia Eurasia Ajkica Europe America Wesr Asia Eurasia North America America North Americu West India ? North America

Aliens in different types of the floodplain vegetation

In order to get an overview of which vegetation types are preferred by plant invasions, the floodplain vegetation was separated into annual and perennial (herbs and grasses), woody (shrubs and trees) and aquatic vegetation. Each alien species was ordered to the vegetation type, in which it occurs with the highest frequency (Table 1 , Fig. 2).

Central Europe

Most naturalised species here grow in annual and perennial vegetation types. These are under the strongest influence of river dynamics, such as periodic inundation and erosion and accumulation of bedload. Fifty-one species grow in the annual vegetation (Bidentetea-communities), which is well developed along the lowland rivers, such as the Rhine and the Elbe. Especially rich is the Polygonum lapathifolium-community, which is dominated by aliens such as Xanthium saccharaturn, Chenopodium jicifolium and diverse Amaranthus species (Lohmeyer and Sukopp 1992).

Perennial vegetation types (excluding woody vegetation) contain 54 naturalised species. In the inundation area of big lowland rivers, the Convolvulus sepium-community is most frequent and especially rich in aliens, such as Helianthus tuberosus, Solidugo gigantea, S. canadensis, Aster salignus, A. lanceolatus and the annuals Brassica nigra and Irnyatiens glandul$era. Along rivers with frequent, but short inundation, Rudbeckia laciniata, Impatiens glandulifera and Reynoutria japonica are often common in the Petasites hybridzrs-community. Reynoutria japonica often forms species poor communities due to its strong competitive nature (Lohineyer and Sukopp 1992). Along alpine rivers, Arrhenatherum elatius. Con-yza canadensis, Solidago gigantea, S. canadensis

328 N. Muller and S. Okudu

Annual T l l

Perennial I I

vegetation I Aquatic

vegetation Y Japan

Central Europe

0 10 20 30 40 50 60 species number

Fig. 2. Species number of alien plants in different types of the floodplain vegetation in central Europe ( 1 30 species) & central & northern Japan ( 124 species)

and lmpatiens gland~rliferu are most frequent and can be found in the Bavburea vulgaris- and Festuca arundinacea-community. Impatiens gland~rlifera often forms species poor communities on new created gravel bars after highwater events.

Twenty species occur in the woody vegetation. In warmer parts of central Europe. for example the alpine Rhine, the lower Danube and at some southern alpine rivers (Brenta and Tagliamento), Buddleja davidii is a frequent alien in willow communities on young gravel bars. In willow floodplain forests of lowland rivers, Acer negundo is the most common tree (Lohmeyer and Sukopp 1992). lmpatiens parviflora and I. glandulifera are frequent in the herb layer of many floodplain forests (Alno-Ulmion communities) of lowland and alpine rivers. Robinia pseudoacacia can spread from plan- tations into natural vegetation, forming pioneer forests. Along the southern alpine rivers (Lippert et al. 1995) and the Upper Rhine (Bocker in lit.), it colonises old gravel bars which are episodically inundated.

Only four species naturalised in aquatic habitats. Most common is Elodea canadensis. Acortrs calamus is the most important agriophyte in old channels.

Central - and northern Japan

Similar to Europe, most introduced species are naturalised in Japan in the annual and perennial vegetation (Fig. 2). Fifty-eight species were found in annual vegetation types. In the middle and lower courses of Japanese rivers, the Polygonum thunbergii-community is typical near the main river course. In these sites, which are frequen~ly disturbed by flooding, important aliens are Aster subulatus, Bidens frondosa and Echinochloa crus-galli. On higher sites, frequent aliens in the Bidens pilosa-community are diverse Amaranthus species, Bidens pilosa and Chenopodium ambrosioides. A community dominated by Erigeron annuus and the native Erigeron sumatr-ensis can be found on drier sites (Okuda 1996).

The perennial vegetation has 58 aliens; most occur in communities of the Artemisietea. They colonise sites on gravel bars higher to the water level than the annual vegetation. This comn~unity, which is decreasing due to civil engineering mea- sures, the endemic Aster kantoensis is typical, although aliens, such as Bidens pilosa, Lepidium virginicum and Oenothera biennis, have invaded in recent years. Today, newly created gravel bars are often colonised by aliens that become dominant, such as

Invasion of alien plants in jloodplains 329

Helianthus strumosus, H. tuberosus, Solidago altissima and S. gigantea (Okuda et al. 1995). Rumex crispus, R. obtus~olius and R. conglomeratus are characteristic species of the Rumex crispus-community (Miyawaki and Okuda 1972; Okuda 1996). Nas- turtium officinale from Europe has naturalised in old channels (tables in Okuda et al. 1995). In contrast to the gappy herbaceous vegetation, grass dominated communities (Phragmitetea and Miscanthetea) are poor in aliens. These communities are only influenced by inundation, therefore the vegetation cover is dense and only competitive species, such as Festuca arundinacea,are able to invade.

Only four aliens are important in the woody vegetation. On dry gravel bars, Robinia pseudoacacia has expanded greatly in the last 20 years, which is documented in detail for the Tama river by Tokyo (Fig. 3). The expansion is connected with decreased river dynamics, due to the construction of dams in its upper course (compare next chap- ter). Robinia pseudoacacia can form its own floodplain forests, replacing the natural Pinus denszjlora forests on these sites. The herb layer is often dominated by Festuca arundinacea. In younger successional stages of the floodplain forests, Amorpha fruticosa and Buddleja davidii are typical for central Japan (Okuda 1996).

Four species are naturalised in the aquatic vegetation. Most frequent is Egeria densa, which colonises the lower courses of Japanese rivers.

The floodplain vegetation of northern Japan, especially Hokkaido, is more influenced by European species than those of central Japan. The perennial vegetation types show strong similarities to the central European floodplain vegetation of alpine rivers, probably due to the similar climate in both areas.

The role of natural and man-made disturbances

In general, disturbed habitats (natural or human disturbances) are regarded to be more vulnerable to invasion of aliens than climax habitats (Drake and Mooney 1989, Kowarik 1995, Lohmeyer and Sukopp 1992). The vulnerability of riparian landscapes to the invasion of aliens is often explained by the occurrence of natural disturbances (flooding), as well as to the greater human impact in this ecosystem (Beerling 1995, Ferreira and Moreira, Muller 1 995b).

Our survey study showed that most aliens in floodplains were found in annual and perennial vegetation; these habitats are found where river dynamics are the strongest. This is in contrast to woody vegetation (shrubs and floodplain forests), where there are fewer invaders. While the sites of the woody vegetation are normally influenced only by water dynamics, annual and perennial vegetation can be additionally influenced by erosion and accumulation of bedload (gravel and sand). This results in the occurrence of regularly spaced open patches, making it possible for fast invaders to settle. This may be one general explanation for the vulnerability of floodplains to the invasion of aliens.

There is a strong connection between human impact and the fast expansion of aliens. In European alpine rivers, floodplain vegetation changed rapidly after the construction of upstream dams. Due to the decrease in river dynamics, species of typical floodplain vegetation (scree communities, willow shrubs and dry grassland communities) are re- placed by species of ruderal and wetland communities (e.g. Muller et al. 1992, Muller 1995a, b). At the Isar, the number of aliens increased conspicuously 30 years after the construction of an upstream power plant dam. Mainly, Erigeron annuus, Impatiens

330 N. Muller and S. Okuda

H Water . B are gravel

Annual communities Willow shrubs

Herbs dom. perennial com. Robinia pseudoacacia forest

Grass dom. perennial com. Quercus acutissima forest

Artificial bare grounds, trails and gardens

Fig. 3. Alterations of floodplain vegetation and spreading of Robinia pseudoacacia L. at the Tama river (by Tokyo, 100 m a.s.1.) due to the decreasing river dynamics between 1976 and 1995 (after Okuda et al. 1976; 1995)

glandulfera, I. parvzflora, Oenothera biennis, and Solidago gigantea established successfully. On the other hand, typical species of gravel bar vegetation (e.g. Myricaria germanica, Chondrilla chondrilloides) decreased or became extinct (Miiller 1995~). The same process has been documented for braided rivers in central Japan. In the Kanton district, many typical floodplain species (e. g. Aster kantoensis, Ixeris tamagawaensis) are decreasing, and aliens are increasing, as a consequence of strong flood regulation by dams (Washitani 1997).

The success of aliens is mainly due to man-made changes of the disturbance re-

Invasion of alien plants in ftoodplains 33 1

gime. Under natural conditions, the gravel bars of braided rivers are poor in nutri- ents, periodically dry, and strongly disturbed by the erosion and accumulation of bedload and flooding. Most species in natural alpine floodplains are spread by wind dispersal and form no permanent seedbank (Muller and Scharm 1997). Typical is a vegetation full of gaps, where there is no competition between individual plants. Nutrient avail- ability increases in riparian gravel bars that are impacted by dams (Muller 1995b). Due to fewer large and frequent disturbances, plants can colonise which have the capacity to build up a permanent seedbank (Miiller and Scharrn 1997). In contrast to species of natural floodplains, a lot of successful aliens have a permanent seedbank e.g. Ambro- sia trifida, Barbarea vulgaris (Muller and Scharm 1997, Tachibana and ltoh 1997, Takenaka et al. 1996, Washitani 1997). Reduced stress and disturbance levels mean that competition becomes the most important factor affecting the vegetation on gravel bars. In this way, competitive alien plants can drive out less competitive native species.

Increased human impact, but not the invasion of aliens, is responsible for the decrease of native species in river ecosystems. The example of floodplains shows the importance of natural disturbances in ecosystems for the survival of plant populations.

Acknowledgements

We are grateful to Peter Bracken (Sydney) for improving the English manuscript, the Ph.D, student Hur Mi Sun (Yokohama National University) for preparing some figures and two anonymous reviewers for comments to the manuscript.

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