Relations Between River Plant Richness in the Portuguese Floodplains and the Widespread Water Knotgrass (Paspalum Paspalodes)

Relations between river plant richness in the Portuguese floodplains

and the widespread water knotgrass (Paspalum paspalodes)

Ivan Bernez1,2,*, Maria Teresa Ferreira2, Antonio Albuquerque2 & Francisca Aguiar21Laboratoire d �Ecologie & Sciences Phytosanitaires, UMR EQHC INRA-AgroCampus-Rennes,65, rue de Saint-Brieuc, 35042, Rennes cedex, France2Forestry Department, Instituto Superior de Agronomia, Universidade Tecnica de Lisboa,Tapada da Ajuda 1349-17, Lisboa codex, Portugal(*Author for correspondence: E-mail: [email protected])

Key words: aquatic plants, amphibious plants, river management, invasion, widespread vegetation, Paspalum

paspalodes (Michx) Scribner

Abstract

The distribution and invasive status of the amphibious plant Paspalum paspalodes (water knotgrass) areobserved. The P. paspalodes distribution clearly shows that it is an invasive plant established in Portugal,which has never before been recorded as invasive in Portugal. The purpose of our study is to determine thevarious stages of invasion of this plant on a large biogeographical scale, in five basins or groups offloodplain river basins belonging to five Portuguese regions covering the principal floodplains of the southand centre of the country. The composition of the river plant community in terms of structure and speciesrichness was obtained from a data set of 401 sites. In some cases, water knotgrass could be a threat forparticular river flora and consequently for the integrity of the river system. In other cases, it occurs in highlydegraded places, where other taxa considered as invasive plants and/or indicators of degraded ecosystemsare present. The pattern of invasion of P. paspalodes is discussed in relation to risk assessment for the otherriver plants and compared to the North European invasion processes of P. paspalodes. We conclude thatthe evolution of the distribution of this predominant Portuguese water plant has to be carefully followed insome of the studied floodplains, as well as its North European distribution limits. In-depth survey should becarried out to relate the current biogeographical changes to the hypothesis of indication in climate changes.

Introduction

Over the past years, most research on invasion andmanagement of aquatic alien species has beenparticularly directed towards distribution andcontrol, rather than the prediction of the invasionprocess (Wade, 1997). This has to be linked to thedifficulties and the lack of results in predictinginvasion (Roy, 1990). Because individual featuresalone are insufficient to describe processes (Mack,1996), invasive species performances in other re-gions or habitats were also sometimes compared(Williamson & Fitter, 1996; Muller & Okuda,

1998) as well as the climatic profiles of their nativeranges (Lonsdale, 1999). For every case of plantinvasion, it is important to determine the stage ofinvasion (Brundu et al., 2001) and its impact onthe ecosystem integrity and the biodiversity. Manycommunications at the 46th Congress of theFrench Association of Limnology, Metz, Decem-ber 2003, discussed cases of invasive taxa causingtrouble to the aquatic communities with conse-quences on biodiversity. More has to be done todetermine if one particular taxa causes problemsor not at a global (regional, national, interna-tional) level, but also at a local scale, where

Hydrobiologia (2005) 551:121–130 � Springer 2005J.N. Beisel, L. Hoffmann, L. Triest & P. Usseglio-Polatera (eds), Ecology and Disturbances of Aquatic SystemsDOI 10.1007/s10750-005-4454-1

management action could be carried out. Such anapproach has not yet been applied to Paspalumpaspalodes (Gramineae family, taxonomy fromTutin et al., 1964–1993), an invasive species inPortuguese rivers, although it has been mentionedmany times in river surveys (Ferreira & Moreira,1995, 2000; Ferreira et al., 1998; Aguiar et al., inpress, 2001; Bernez et al., 2002a). At the earlierphenological stage, when ecological surveys aredone at the beginning of spring, it can sometimesbe difficult to differentiate it from Paspalum vag-inatum, a species that occurs more often in coastalhabitats and particular sites such as rice fields andirrigation crops (Franco & Rocha-Afonso, 1971–1994). In the Mediterranean basin, knotgrasseswere initially reported in France, where it wasdeliberately introduced from South America as acrop near the Garonne River in 1802; in Portugal,it was first reported in 1887 along the Tagus bank,and described in 1909 in one of the first Portugueseflora, Flora Portuguesa by G. Sampaio (Aguiaret al., in press). Until the 1970s its distribution wasconfined to areas of coastal and alluvial flood-

plains in the main basins of Portugal. Nowadaysthis taxon has a tendency to extend to NorthernEurope (A. Dutartre, E. Tabacchi, personal com-munication) and the phenomena seems to be new:it is confirmed by the fact that the water knotgrasswas not mentioned in 1997 in a synthesis onaquatic and riparian invasive plants covering theFrench territory (Peltre et al., 1997), justifying thepresent synthesis of its presence in our area ofstudy.

Our purposes are to synthesise several worksfrom the 1990s to 2002 and study the distributionat a large biogeographical scale and to establishthe invasive status of the amphibious plantP. paspalodes, which is the most important interms of occurrence (Table 1, adapted from Ber-nez, 2002b) and the most widely distributedthrough the five main floodplains of SouthernPortugal. It includes, (i) a first overview of itsextension in Portugal and (ii) a state of the dis-tribution and optimal habitats of that plant forsurvey of its Northern European limit. As regardsour study, our objectives are to see, (i) to which

Table 1. For each taxa considered as invasive for Portuguese floodplains, cover frequency over 4 (>50% of cover in the Braun-

Blanquet scale), over 3 (>25%) and the frequency (�freq.�) of occurrence on 401 sites from 5 floodplains rivers of South Portugal

(100 m. reach studied per site), see Figure 1 for the sampling scheme and the distribution of Paspalum paspalodes

Taxa %>4 %>3 Freq.

Paspalum paspalodes 15.5 40.6 69.6

Arundo donax 6.7 11.9 26.4

Oenanthe crocata 3.5 13.2 58.9

Ranunculus peltatus 3.0 7.5 22.9

Apium nodiflorum 2.5 14.9 53.4

Azolla filiculoides 2.0 2.7 5.7

Panicum repens 1.5 7.0 9.2

Phragmites australis 1.5 4.7 12.5

Lotus uliginosus 1.2 2.5 11.0

Myriophyllum aquaticum 0.7 3.7 5.5

Myriophyllum spicatum 0.7 1.5 11.0

Cyperus eragrostis 0.5 4.2 33.7

Eichornia crassipes 0.5 1.5 2.2

Ceratophyllum demersum 0.2 3.2 11.2

Xanthium strumarium 0.2 2.0 17.2

Alisma lanceolatum 0.0 2.2 21.4

Aster squamatus 0.0 0.0 13.7

Bidens frondosa 0.0 1.2 30.4

Lemna gibba 0.0 1.5 9.7

Potamogeton pectinatus 0.0 0.5 1.7

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extent this is an invasive plant (e.g., what are thedifferent structures of knotgrass colonies in Cen-tral and Southern Portugal), (ii) is the distributionhomogenous within each river basin and in thevarious basins, (iii) whether it could be a threat forother plants or communities and for biodiversity?

Materials and methods

Sampling and study area

Five main floodplains of Central and SouthernPortugal (Fig. 1) were studied and a total of 401sites were sampled along the river course over alength of 100 m.; the lateral limits (Fig. 2) corre-sponded to the maximal annual flows, which areusually visible thanks to erosion lines on the rivermargins or signs and lines on riparian woody trees.All species found in water and on both margins

and inner banks were identified and allocateda relative cover class (six abundance classes:0: absent; 1: <5%; 2: >5–30%; 3: >30-50%; 4:>50–75%; 5: >75%), similar to the Braun-Blan-quet scale (1932). The Portuguese part of thecatchment area of the Tagus River has an area ofabout 25000 km2. Yearly average rainfall is810 mm and air average annual temperature is15 �C, however, local differences within the basinare very large, especially between the Northernand Southern parts. The river basin of the Oeste,formed by a group of Western rivers flowingtowards the Atlantic Ocean, is the smallest of ourstudy (2000 km2). The Sado Basin is a typicalMediterranean lowland basin located in SouthwestPortugal (8000 km2), where the annual averageprecipitation is 600–700 mm. The Portuguese partof the Guadiana basin has an area of 12,000 km2,the average annual temperature is 12–18 �C andrainfall is 600 mm. The Algarve basin (3000 km2),

Figure 1. Sampling effort (401 sites) & distribution in the 5 river basins (A: Algarve rivers, G: Guadiana rivers, O: Oeste rivers, S: Sado

rivers, T: Tagus rivers) and Cover of Paspalum paspalodes: small points: cover = 0; medium circles: cover >0 & <3; large circles: >3.

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formed by a group of rivers flowing southward ischaracterised by its Mediterranean–Atlantic cli-mate (500 mm of average rainfall and 17 �C): thedry season begins in mid-spring and inter-annualflow variations are very high.

In this study, we followed the terminology forplant invasions developed by Brundu et al. (2001).The invasive/non-invasive status was attributedaccording to a bibliographic reviewing and expertjudgement (Bernez et al., 2002b).

Data analysis

Classical statistical analysis and a polynomialmodel are used to present the tendency to invasionat local and regional scales. Canonical Analysis(CA) was carried out to detect and illustratefloristic affinities and spatial preferences at thedifferent stages of development of Paspalumpaslodes colonies and the comparison between the

five basins using CANOCO software (Ter Braak &Smilauer, 1998).

Results

On a global scale, Figures 1 (according to thelegend) and 3 confirm the dominance of the colo-nies and the high frequency of water knotgrassgiven in Table 1 in comparison with other invasivespecies of Portugal. In this survey including 401sites, about two sites out of three are colonised byP. paspalodes; in 5% of the cases it shows a coverof over 75% of the site and in 40% of the cases itshows a cover of 30% of the site.

Figure 4 shows that the biggest river basins,Guadiana and Tagus rivers, have the highest cover(scale5, >75% of the studied stretch of land),when the three other basins only reach a lowerscale cover (4, between 50 and 75%). Figure 4

Sampling area

Figure 2. Schematic sampling area with the lateral limits of the studied river course.

0

5

10

15

20

25

30

35

Pasp0 Pasp1 Pasp2 Pasp3 Pasp4 Pasp5

Figure 3. Cover of Paspalum paspalodes in frequency (%) of the six Braun-Blanquet scale values pooled for the five river basins.

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shows the relation between water knotgrass coverand the species mean richness and mean variationsof species richness of the plant communities of the401 studied sites. Considering the global situation

(Southern and Central Portugal together: bottomright-hand side of Fig. 4), it is obvious that thehighest species richness reaches a cover of 50–75%of P. paspalodes. On the river basin scale a similar

0

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60

Pasp0 Pasp1 Pasp2 Pasp3 Pasp4 Pasp50

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Pasp0 Pasp1 Pasp2 Pasp3 Pasp4 Pasp5

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Pasp0 Pasp1 Pasp2 Pas3 Pasp4 Pasp50

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Pasp0 Pasp1 Pasp2 Pas3 Pasp4 Pasp5

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Pasp0 Pasp1 Pasp2 Pasp3 Pasp4 Pasp50

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Pasp0 Pasp1 Pasp2 Pasp3 Pasp4 Pasp5

Total South & Center of PortugalTAGUS

SADOOESTE

GUADIANAALGARVE

Figure 4. Mean cover of Paspalum paspalodes in the six Braun-Blanquet scale values in relation to the Richness for each of the five

basins and for the complete data set of South and Center of Portugal floodplains. Polynomial trends are given.

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Figure 5. Plan of the first 2 axis of Correspondence Analysis (CA) for the five river basins (A: Algarve rivers, G: Guadiana rivers,

O: Oeste rivers, S: Sado rivers, T: Tagus rivers); Cover of Paspalum paspalodes is given using a six-classes Braun-Blanquet scale; the

signification of some plants acronyms are given in the text or refered to in a synthesis of previous publications (Ferreira & Moreira,

1995, 2000; Aguiar et al., in press, 2001; Ferreira et al., 2001, 2002; Bernez et al., 2002a).

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trend can be observed in very different situationsof community species richness. The Sado River haslow richness level, irrespective of the variation ofcover of P. paspalodes but this species shows veryfew cases of high covers in that river. In theAlgarve rivers, the species richness is higher whenPaspalum had a cover >0% and <5%. The Tagusriver basin showed an increase of species richnesswhen Paspalum reached higher covers. Guadianaand Oeste river basins show similar trends to theglobal approach. Figures 1 and 4 together showthat small coastal urbanised river basins of theOeste and Algarve have a high cover of waterknotgrass, just like the big basins of Guadiana andTagus. The more isolated Sado Basin shows alesser development, with some high cover areas onthe main course of the Sado River and no Pasp-alum in many tributaries.

For all analyses of Figure 5, the eigenvalues ofthe first two axes of the five CorrespondanceAnalysis (CA) are generally low, considering thosecommonly obtained in ecology studies (Ter Braak& Smilauer, 1998) but correspond to thoseobtained in hydrobiology for other type of riversand regions (Bernez et al., 2004). Overall, thestream hierarchy mainly determines river plantdistribution: the five CA presented per basin showthe particularity of each basin in term of commu-nity structure. For more details on the communitiesdescriptions see previous publications (Ferreira &Moreira, 1995, 2000; Aguiar et al., in press, 2001;Ferreira et al., 2001, 2002); Bernez et al., 2002a).Considering the sites where various covers of waterknotgrass occurred, where the P. paspalodes is notpresent (PASP0 on the graphs), we usually havecommunities representative of the upper courses(river basins A: Algarve, O: Oeste, S: Sado) ordriest stretches (G: Guadiana and T: Tagus). Acover of P. paspalodes over 25% (>3) usuallycorresponds to communities that are verydegraded, e.g., dominated by Phragmites orTamarix. Concerning the Guadiana River, we cansee (axis 2) that communities of conservationinterest (e.g., with Gratiola ssp., Gladiolus sp.,Narcissus sp.) are related to medium coverof P. paspalodes (scale 2 or 3). When the informa-tion shared by the first axis reaches 15%, e.g. forAlgarve and Oeste basins, it correspondsto important communities without Paspalumopposed to others where it occurs at different levels

of abundance. In fact, the proportion of commu-nities without Paspalum is lower on the other threebasins, and these balance is not observed in theirrespective CA. The Tagus basin, the first axis cor-responds to a gradient of abundance, and thecentral position of the plan of the CA (corre-sponding to the average situation of the basin) iscovered with Paspalum between 30 and 50%. Thisaverage situation occurs between >0 and 5% ofcover in the Guadiana and Oeste basins. No aver-age trend exists in the Sado and Algarve basins. Incoastal urbanised smaller river basins of the Al-garve and Oeste rivers, linked with bigger basins, itresults in a high extension of P. paspalodes; there isless development in the more isolated Sado Basin,although some high covers do exist on the maincourse of the Sado River (Fig.1).

Discussion

Paspalum paspalodes is usually considered anaturalised species in the Iberian Peninsula (Tutinet al., 1964–1993; Carvalho e Vasconcellos, 1970;Franco & Rocha-Afonso, 1971–1994; Castroviejo,1986–1993; Valdes et al., 1987; Stace, 1997),without a very clear invader status. In the presentstudy, its frequent occurrence and some importantcovers could be demonstrated as well as its wide-spread distribution in a large part of Portugal.There are no floodplains in the South that do notprovide habitat for P. paspalodes. At the largestscale of our study very high covers of waterknotgrass correspond to a decrease in speciesrichness, however higher values of species richnessare reached with important P. paspalodes cover.These trends have been demonstrated in Australia(Fox & Fox, 1986) and should be discussed in termof levels of disturbances: when invasion occurs innatural communities, the level of disturbance canhave an impact on species richness. In theGuadiana basin, it could be interesting to monitorthe sites of conservation interests; in such habitats,the medium cover of water knotgrass leads us tothink that a propagation of the plant could dam-age the existing communities of conservationinterests (e.g., with Gratiola ssp., Gladiolus sp.,Narcissus sp.).

In the five basins, the high development ofP. paspalodes (>4) is linked to the occurrence of

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very common communities of Phragmites orPhalaris, e.g., still degraded habitats. High speciesrichness levels are not always related to goodecosystem quality, but sometimes to the impactof disturbance. This is the way our results shouldbe interpreted, concerning the main cases ofhigh species richness coupled to high cover ofP. paspalodes. An opportunistic status can be gi-ven to that taxon, colonising opened habitats thatfrequently show high species richness in the earlierstage of colonisation.

No average trend in the CA of the Sado andAlgarve basins where observed (Fig. 5) thus sug-gesting that the water knotgrass is not well estab-lished or established in a diversity of cases, or it isat an earlier stage of colonisation. Sites where theP. paspalodes is absent correspond to uppercourses where (i) it was not possible to facilitate itscolonisation (shaded habitats, isolation of thesites) or (ii) the driest stretches of the intermittentrivers. Drought may be the factor that relativelycontrols the extension of P. paspalodes in Portugal.This amphibious species needs wet habitats. Therecent change in its northern distribution as wellas the presence of some high cover patches inEurope (e.g., in the river Loire and Garonne,France – A. Dutartre, personal communication.)make its accurate study necessary. The possibilityof linking these changes with recent temperatureincreases and climate changes could be a hypoth-esis to be to consider.

This overview of the P. paspalodes distributionshows that the plant is sometimes still a threat forwater plant communities (the biggest river basinsof Portugal are impacted), that some relativelydisconnected basin (e.g., the Sado Basin) are in away still preserved but might be threatened in thefuture, such as a few plots in the Guadiana basin,where rare species are growing near the waterknotgrass. Each river seems to have its ownresponse, or its own stage of colonisation that hasto be related to the respective river basin history(Bernez et al., 2002c, 2004) and the date of arrivalof the invasive plant. There is real difficulty inmaking a direct link between the occurrences of aplant and the species richness levels of its com-munities without considering the level of distur-bance of the habitats involved.

The particular characteristics of P. paspalodes(amphibious plant, colonising water courses and

margins, high vegetative reproduction capacity,easy drift of propagules), might be some of thecharacters of success of invasion (Williamson &Fitter, 1996), and when compared with otherinvasive taxa (Bernez et al., 2002b) it proves thatusing biological traits to respond to predictionneeds in invasion management is justified: simplebiological characters could be useful to describe thespreading potentials (Rejmanek & Richardson,1996; Chicouene, 2002). Risk assessment for aqua-tic (Willby et al., 2000) and riparian plants (Pollocket al., 1998), has been focused on the biologicaltraits of successful invaders (Ashton & Mitchell,1990), including their physiological genetic anddemographic features, dispersal strategies (Kean &Barlow, 2000) and life-history traits related tocompetition and reproduction. It should beenlarged to include ecological traits, particularrelationships with environmental conditions,species richness (Bornette et al., 1994) and space-temporal community variation (Starfinger, 1998). Itshould also be related to the community charac-teristics and the river habitat features and theprevailing physico-chemical environment (Ferreiraet al., 2002).

Acknowledgements

This study was funded by the PortugueseFoundation for Science and Technology (FCT)project SAPIENS PCOTI/MGS/42584/2001.The FCT supported the PhD grant of FransiscaAguiar (PRAXISXXI/BD/11173/97) and the post-doctoral grant of Ivan Bernez (SFRH/BPD/1513/2000) and it occurred in a Lavoisier program ofthe French ministry of foreign affairs. The authorsare thankful to Miss Bouillon for suggestingEnglish improvements.

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