a vegetation-based method for ecological diagnosis

ENVIRONMENTAL AUDITINGA Vegetation-Based Method for Ecological Diagnosisof Riverine WetlandsCLAUDE AMOROS *GUDRUN BORNETTECHRISTOPHE PASCAL HENRYLaboratoire de dynamique des ecosystemes aquatiques

peri-fluviaux et genie ecologique; E.S.A.C.N.R.S. 5023, Ecologie des Eaux Douces et des Grands

FleuvesUniversite Claude Bernard Lyon I69622 Villeurbanne Cedex, France

ABSTRACT / The management of riverine wetlands, recog-nized as a major component of biodiversity in fluvial hydro-systems, is problematic. Preservation or restoration of suchecosystems requires a method to assess the major ecologi-cal processes operating in the wetlands, the sustainability ofthe aquatic stage, and the restoration potential of each river-

ine wetland. We propose a method of diagnosis based onaquatic macrophytes and helophytes. Plant communities areused because they are easy to survey and provide informa-tion about (1) the origin of a water supply (i.e., groundwater,seepage, or surface river water) and its nutrient content, (2)effects of flood disturbances, and (3) terrestrialization pro-cesses. The novelty of the method is that, in contrast to avail-able typologies, it is based on the interference of gradientsresulting from several processes, which makes it possible topredict wetland sustainability and restoration potential.These predictions result from knowledge of the processesinvolved in terrestrialization, i.e., the influence of flood distur-bances, occurrence of groundwater supplies, trophic de-gree, and water permanency of the habitat during a yearlycycle. The method is demonstrated on five different riversystems.

Riverine wetlands, as ecotones between a main riverchannel and the terrestrial ecosystems (Holland andothers 1990, Pinay and others 1990), are recognized as amajor component of biodiversity in fluvial hydrosystems(Baker and others 1991, Usseglio-Polatera 1994, Obrd-lik and others 1995, Bornette and others 1998a,b). Insome cutoff channels, the life duration of the aquaticstage may be only a few decades, the succession ratedepending on the influences of allogenic processes(water dynamics, erosion, and deposition) and auto-genic processes (population dynamics, eutrophicationand terrestrialization) (Bravard and others 1986, Millerand others 1995). In pristine river systems, fluvialdynamics create new wetlands, offsetting the terrestrial-ization of the older ones (Amoros and others 1987, Saloand Kalliola 1989, Muller 1995). Nowadays, in manyrivers, anthropic impacts such as embankments impedethe fluvial processes that were able to create newwetlands (e.g., flooding, channel migration); therefore,preservation and restoration of the relict wetlands arethe only remaining options for environmental manage-ment (Henry and Amoros 1995, Petts and Amoros1996). Whether for preservation or restoration pur-

poses, a method is needed to assess the ecologicalprocesses, the duration of the aquatic stage, and thepotential for restoration of each riverine wetland.

A method that provides diagnosis of riverine wet-lands is necessary for several reasons. Firstly, measuresoften proposed for the preservation of rare species orrare communities do not guarantee the maintenance ofcommunities of interest (Wittig 1982, Runhaar andothers 1996). Because wetland ecosystems may changerapidly, their dynamics must be considered to makegood forecasts on community survival. Secondly, ecologi-cal processes may have been altered through modifica-tions in river dynamics or surrounding land use. Thepresent stage reflects only indirectly the previous ecologi-cal processes, and therefore the method should be ableto infer the previous stage (restoration target) from thepresent one. The method should also permit identifica-tion of potential vegetation communities. At the land-scape level, the method should be used for mappingecosystems, each characterized by its major processes.This map could then be used as a management tool tohelp the decision-makers in their choice of preservationand restoration objectives (for example, to sustain thediversity of types of processes at floodplain section level).

Classification methods, based on environmental forc-ing factors such as hydrological processes, are used forinventories (Cowardin and others 1979, Mitsch andGosselink, 1993). However, since the focus was on abroad scale, such methods may not permit separation of

KEY WORDS: Aquatic macrophytes; Ecological processes; Ecosys-tem dynamics; Disturbance; Environmental manage-ment; Sustainability; Restoration

*Author to whom correspondence should be addressed.

DOI: 10.1007/s002679910017

Environmental Management Vol. 25, No. 2, pp. 211–227 r 2000 Springer-Verlag New York Inc.

riverine wetlands on the basis of the ecological pro-cesses that drive their dynamics. Phytosociological ap-proaches also were proposed for inventory and assess-ment of wetlands. These methods are frequently basedon the relative frequency of rare species or communi-ties (Hughes 1995, Novitzki 1995) and appear irrelevantwhen the structuring role of the habitat interferes withother processes, such as disturbances or the history ofthe ecosystems (Zobel 1992, Mucina 1997). Some au-thors have proposed not using a stable syntaxonomicsystem, as this cannot include genetic processes, distur-bance history, and past management, but rather the useof stable classification structures based on available data(Feoli and Lausi 1981, Mucina 1997). Such systems havebeen proposed for classifying all wetland types (Se-meniuk and Semeniuk 1995) that use broadly appli-cable classification criteria, but they presently fail toencompass the very high variability of processes thatoccur in most types of wetlands (Salo and others 1986,Broyer and others 1997, Bornette and others 1998a,b).A more precise classification classification method isstill needed for riverine wetlands, where communitiesresult from the interactions among geomorphologicalprocesses, water quality, successional processes, distur-bances, and the contingency of diaspore migration infavorable, short-lived ecosystems (Klein and others 1995,Muller 1995, Bornette and others 1998a,b). The hydro-geomorphic classification proposed by Brinson andothers (1995) uses first principles of geomorphology,hydrology, and hydrodynamics to separate wetlands intofunctional classes at a gross level. Since this classifica-tion requires hydrologic data, water-quality data, vegeta-tion cover, soil type, as well as the knowledge of watersources and hydrodynamics, its use for inventoriesremains rather time-consuming and expensive. Wepropose a method based on vegetation data and verysimple geomorphic variables, aiming: (1) to deducehydrodynamics and water quality, and (2) to predictwetland sustainability and restoration potential. Ourgoal is to propose a method to diagnose the processesoperating in riverine wetlands. The efficiency of themethod implies consideration of the main processes(eutrophication, terrestrialization, fluvial dynamics,groundwater supplies) that interact and govern struc-tures and dynamics of riverine wetlands (Foeckler andothers 1991, Muller 1995, Bornette and others 1998a,b).Aquatic macrophytes and helophytes were used, asthese are mostly perennial sessile organisms, usuallyidentifiable in the field. Furthermore, plant communi-ties are highly organized by the major processes pre-sented below (Tremolieres and others 1991, Bornetteand others 1994a, Klein and others 1995, Haury and

others 1996), and, as primary producers, should governthe other communities at least in part.

The paper is organized in four parts: the firstpresents environmental gradients induced by the majorprocesses that control the dynamics of riverine wet-lands. In the second step, using an extensive survey ofbibliographic data, relationships between each environ-mental gradient and plant communities are established:this section provides, for each degree of environmentalconstraint, the potential floristic communities thatshould occur in the riverine wetlands. In the thirdsection, a conceptual model, based on knowledge of theprocesses involved in wetland dynamics, predicts the lifeduration and restoration potential for each type ofriverine wetland. In the last section, the use of themethod is examplified on a Rhone River wetland andon four cutoff channels from other European rivers,and the limits of the method are discussed.

Identification of Main Processes

Previous studies have demonstrated that within agiven climatic area, the main processes governing floris-tic communities in riverine wetlands are: (1) the originof water supply (i.e., groundwater, see page, or surfaceriver water) and its nutrient content, (2) the frequencyand intensity of flood disturbances, and (3) terrestrial-ization processes (processes by which, aquatic ecosys-tems become terrestrial) (Bornette and Amoros 1991,Tremolieres and others 1991, Bornette and others1994a, Klein and others 1995).

Trophic Degree and Groundwater Supplies

In some river floodplains, nutrient-poor groundwa-ter originating from hillslope aquifers, often flows intocutoff channels, sustaining their oligotrophic status(Kohler and others 1974, Carbiener and Kapp 1981).On the other hand, river seepage and surficial riverwater are often richer in nutrients, favoring eutrophica-tion of the wetlands (Carbiener and others 1990,Wassen and others 1990). Ecological succession alsocontributes to eutrophication: succession results in theaccumulation of organic matter and the clogging of thesubstrate, which thereby reduces groundwater supplies(Rostan and others 1987). Plant species are stronglylinked to the nutrient content of the water. Therefore,the composition of plant communities will be governedby the trophic degree of the riverine wetlands (Kohlerand others 1974, Haslam 1982, Carbiener and others1990). Previous studies have also demonstrated that insome cases, groundwater supplies can limit plant produc-tion because of the low temperature and low nutrientcontent of the water, and slow down succession by partly

C. Amoros and others212

washing away dissolved and fine particulate organicmatter (Bornette and others 1994a).

Flood Disturbances

Floods also strongly influence the structure anddynamics of plant communities in riverine wetlands. Ithas been previously demonstrated that for an intermedi-ate frequency and intensity of flood disturbances, spe-cies-rich plant communities occur, these being patchilyorganized as a shifting mosaic, remaining stable throughdynamic equilibrium (Connell 1978, Bornette and oth-ers 1994a, Barrat-Segretain and Amoros 1996, Bornetteand Amoros 1996). When disturbances are less frequentor have low intensity, they are insufficient to impede orslow down succession and terrestrialization proceeds(Foeckler and others 1991, Bornette and others 1994b,Muller 1995). Conversely, when disturbances are toofrequent or intense, they can impede vegetation growth(Bilby 1977, Bornette and Amoros 1991). The scouringability of water during floods depends on its velocity andthus mainly on the drainage capacity of the formerchannel (mean depth and width) and its relative slope,which in turn depends on its sinuosity. Depending ongeomorphological and morphological features that may,in some cases, reduce water velocity, floods can alsobring fine sediment to the wetlands instead of scouringthem, promoting terrestrialization and eutrophication(Bhowmik and Adams 1989, Jongman 1992, Martinetand others 1993, Bornette and others 1998a,b). Further-more, fine sediment can contribute to clogging of thesubstrate, impeding groundwater supplies that mayhave decreased the nutrient content of the water orwashed away part of the fine sediment deposits.

Terrestrialization

During terrestrialization, hydrophyte communitiesare progressively replaced by helophytes, and finally byterrestrial species. Terrestrialization is mainly due toorganic matter production and deposition and silt andsand deposition during floods. Substrate granulometrywill thus afford information on the net flood effects.Coarse substrate (pebbles, gravel) usually indicates highflood scouring, whereas thick layers of organic matterindicate that terrestrialization is due to autogenic pro-cesses and that the wetland is not scoured by floods(Rostan and others 1987, Schwarz and others 1996).Coarse substrate as well as thick layers of fluid organicmatter are usually unfavorable for plant recruitment(Wetzel 1979, Carpenter 1981, Barrat-Segretain 1996),whereas a cohesive substrate (e.g., silt) is favorable formost plant growth and recruitment (Barko and Smart1986, Bornette and others 1998a,b). Thus, substrate will

also influence the floristic composition and speciesrichness of plant communities.

Elaboration of the Method

Plants Ranked on Trophic Gradient

Aquatic macrophytes have long been recognized asbeing strongly dependent on the nutrient content ofthe water, namely [N-NH4

1] and [P-PO432] in calcare-

ous rivers (Kohler and others 1974, Carbiener andothers 1990). Based on an extensive bibliography sur-vey, the aquatic plant species have been classified in thethree trophic degrees: oligotrophic, mesotrophic andeutrophic (Tables 1 and 2). For some species, such asLudwigia palustris, we did not find any information;these were therefore not documented. Oligotraphentspecies are usually correlated to [N-NH4

1] and [P-PO4

32] content that remains lower than 40 µg/liter onaverage; and mesotraphent species are usually corre-lated with [N-NH4

1] and [P-PO432] content that re-

mains lower than 80 µg/liter (Carbiener and others1990, Schnitzler and others 1996). Because nutrientsare only potentially available in the sediment of dystro-phic ecosystems, the oligotrophic degree used in plantstudies includes both oligotrophic hardwater ecosys-tems and dystrophic ecosystems, as defined in limnologi-cal studies (Hutchinson 1957).

The species found in groundwater-supplied ecosys-tems, or that are indicative of such supplies, have alsobeen documented. The indication of groundwater sup-plies was based on both bibliographic data and informa-tion collected along the Rhone River floodplain (highconductivity, alkalinity, and nitrate content of the wa-ter).

Species Tolerance to Flood Scouring andSubstrate Requirements

Floods periodically disturb aquatic plant communi-ties by uprooting or breaking plants. In contrast withthe trophic degree, little research has been carried outto classify plant species according to their tolerance toscouring floods. The tolerance of species to distur-bances has been documented from data collected fromRhone and Ain (France) riverine wetlands, (Bornetteand Amoros 1991, Bornette and others 1994a, 1998a,b).It remains difficult to assess whether a species is reallytolerant to floods (as it is found in flood disturbedecosystems and nowhere else) or if it requires environ-mental conditions that occur only in flood-disturbedecosystems. For example, Phalaris arundinacea is stronglyindicative of flood disturbances in several river systems(Meriaux and Wattez 1980, Castella and Amoros 1986),

Method for Ecological Diagnosis of Riverine Wetlands 213

Table 1. Relative frequency of occurrence of hydrophyte species in three trophic levels (in calcareouswaters)—oligotrophic, mesotrophic, and eutrophic—and usual occurrence in groundwater-supplied ecosystemsaccording to the bibliographya

Hydrophyte species Oligotrophic Mesotrophic EutrophicGroundwater

supplies References

Azolla filiculoides * * Carbiener and others (1990)Berula erecta * * (.) * Carbiener and others (1990)Ceratophyllum demersum * Kohler (1975)Callitriche platycarpa (.) * * * Wiegleb (1978)Chara globularis * * Bornette and others (1996)C. hispida * * Krause (1981)C. major * ** * Stewart and Church (1992)C. vulgaris * ** * Wattenhoffer (1984)Eleocharis acicularis * ** (.) * Haslam (1978)Elodea canadensis (.) * * * Haslam and others (1975)Fontinalis antipyretica (.) * * * Klein and others (1995)Groenlandia densa ** * Carbiener and others (1990)Hippuris vulgaris * ** * * Schmider and Ottow (1985)Hottonia palustris * * * Carbiener and others (1990)Hydrocharis morsus-ranae * ? Schmider and Ottow (1985)Juncus articulatus * * * Kohler and others (1974)Juncus subnodulosus * * Kohler and Schiele (1985)Lemna gibba * Rodwell and others (1995)L. minor * ** Cernohous and Husak (1986)L. trisulca * * * Carbiener and others (1990)Ludwigia palustrisLuronium natans * * ? Haury and others (1996)Mentha aquatica * * * * Kohler and others (1974)Menyanthes trifoliata * * Rodwell and others (1995)Myriophyllum spicatum (.) * ** Kohler and Schiele (1985)M. verticillatum * * * Klein and Carbiener (1989)Najas marina (.) * Cernohous and Husak (1986)Najas minorNuphar lutea * * * Klein and Carbiener (1989)Nymphea alba * * * Pott (1980)Nymphoides peltata * ? Haury and others (1996)Oenanthe fluviatilis * * Klein and others (1995)Polygonum amphibium * Cernohous and Husak (1986)P. coloratus * * Carbiener and Kapp (1981)P. compressus * * Haslam and others (1975)P. crispus * Wiegleb (1978)P. lucens * * Cernohous and Husak (1986)P. natans * * (.) * Kohler and others (1974)P. nodosus * Meriaux and Wattez (1980)P. pectinatus (.) * ** Westlake (1973)P. perfoliatus * * Klein and others (1995)P. pusillus * * Kohler and Schiele (1985)Ranunculus circinatus * * * Carbiener and others (1990)R. fluitans * * Monschau-Dudenhausen (1982)R. trichophyllus * * * Haury and others (1996)Riccia fluitans * ? Klein and others (1995)Sagittaria sagittifolia * * Rodwell and others (1995)Sparganium emersum * ** ** Schnitzler and others (1996)Spirodela polyrhiza * Wiegleb (1978)Stratiotes aloides * Brammer (1979)Thelipteris palustris * * Grootjans and others (1991)Trapa natans * Haury and others (1996)U. minor * Rodwell and others (1995)U. vulgaris * Rodwell and others (1995)Vallisneria spiralis * Haslam and others (1975)Wolffia arrhiza * Haury and others (1996)Zannichellia palustris * * * Kohler and Schiele (1985)

a(.) indicates very sporadic occurrence; if the species occurs in several trophic levels, its preference is indicated, when possible, by two stars insteadof one. ‘‘?’’ indicates that the species is probably linked to groundwater supplies, but that not enough data were available. Some informativereferences are given.


as it requires a sandy substrate (Petit and Schumacker1985) that is found only in disturbed ecosystems alongthe Rhone River (Castella and Amoros 1986). Such asubstrate occurs frequently along the Saone River (atributary of the Rhone River) and favors the occurrenceof this species, despite the absence of flood scouring.We therefore considered it necessary to classify speciesaccording to both their potential to withstand floodscouring, according to results obtained by investigations

along the Rhone and Ain rivers, and the substrate onwhich they are able to grow, according to previousstudies (Tables 3 and 4). Furthermore, we provideinformation on habitat and vegetation characteristicsfor each category of flood scouring in Table 5.

To determine the tolerance of species according tothe scouring effect of a flood, we first assessed the floodeffect on a set of 15 riverine wetlands. The river’soverflow frequency into 12 Rhone River wetlands was

Table 2. Relative frequency of occurrence of helophyte species in three trophic levels (in calcareouswaters)—oligotrophic, mesotrophic, and eutrophic—and usual occurrence in groundwater-supplied ecosystemsaccording to the bibliography

Helophyte species Oligotrophic Mesotrophic EutrophicGroundwater

supplies References

Acorus calamus * * Haslam and others (1975)Agrostis stolonifera *a * * Haslam and others (1975)Alisma plantago-aquatica * * ** Haslam and others (1975)Bidens tripartita * Weber-Oldecop (1970)Butomus umbellatus * Schnitzler and others (1996)Caltha palustris * * * Schnitzler and others (1996)Carex acutiformis * * Rodwell and others (1995)Carex elata * * * ? Haury and others (1996)Carex pseudo-cyperus * ** Rodwell and others (1995)Catabrosa aquatica * * Haslam and others (1975)Cladium mariscus ** * * Rodwell and others (1995)Eleocharis palustris * * (.) Haslam and others (1975)Galium palustre * * Haury and others (1996)Glyceria maxima * Meriaux and Wattez (1980)Hydrocotyle vulgaris * * Grootjans and others (1991)Iris pseudacorus * * * Haslam and others (1975)Leersia orhyzoides * Haury and others (1996)Lycopus europaeus * Haury and others (1996)Lysimachia nummularia * Haury and others (1996)Lysimachia vulgaris * * Rodwell and others (1995)Lythrum salicaria * * * Haury and others (1996)Mentha suaveolensMyosotis scorpioides * * * ? Schnitzler and others (1996)Nasturtium officinale * * * Carbiener and others (1990)Phalaris arundinacea * * * Schnitzler and others (1996)Phragmites australis * * * Haury and others (1996)Polygonum hydropiper * * Haury and others (1996)Ranunculus linguaRorippa amphibia * ** Haury and others (1996)Rumex hydrolapathum * Haury and others (1996)Samolus valerandi * *Scirpus lacustris * * * Haslam and others (1975)Scrophularia auriculata * * * Rodwell and others (1995)Scutellaria galericulataSolanum dulcamaraSolidago giganteaSparganium erectum * Haslam and others (1975)Stachys palustrisTypha latifolia * * Haslam and others (1975)Typha angustifolia * * Haslam and others (1975)Urtica dioica * ? Rodwell and others (1995)Veronica a.-aquatica * Klein and others (1995)Veronica beccabunga * * Schnitzler and others (1996)

aSymbols are as defined in Table 1.


Table 3. Distribution of hydrophytes according to disturbance levels and substraterequirements of the speciesa

Hydrophytespecies

Flood disturbance Substrate granulometry

ReferencesNoneLow

scouringIntermediate

scouringHigh

scouringSilting

in Peat MudSilt/clay Sand Coarse

Azolla filiculoidesBerula erecta (.) ** ** ** * * * * * * Haslam and others (1975)Callitriche platycarpa (.) ** ** * * * * Haslam (1987)Ceratophyllum demersum ** ** [**] ** * * * * * Haslam and others (1975)Chara globularis * (.)C. hispida * * Carbiener and others (1990)C. major ** ** ** (.) * * Corillion (1975)C. vulgaris ** ** *Eleocharis acicularis * ** *Elodea canadensis (.) ** ** * * * * * (.) Meriaux (1982)Fontinalis antipyretica * * * Westlake (1975)Glyceria fluitans * * * Felzines (1977)Groenlandia densa * ** ** * * * * * Guinochet and de Vilmorin (1973)Hippuris vulgaris * * ** * * * * * Mooney and O’Connell (1990)Hottonia palustris * * ** * * * * * Brock and others (1989)Hydrocharis morsus-ranae **Juncus articulatus ** ** *Juncus subnodulosus * * * * Mooney and O’Connell (1990)Lemna gibba **L. minor ** ** [**] **L. trisulca * ** * * * * * Klein and Carbiener (1989)Ludwigia palustris * * *Luronium natans ** **Mentha aquatica ** ** ** ** * * * * * * Haslam and others (1975)Menyanthes trifoliata * * * Mooney and O’Connell (1990)Myriophyllum spicatum * * * ** *M. verticillatum ** ** * * * * * Haslam and others (1975)Najas marina * [*] ** *Najas minor *Nuphar lutea ** * (.) * * * * (.) Haslam and others (1975)Nymphea alba ** * * * * Mooney and O’Connell (1990)Polygonum amphibium * Wolff (1987)Potamogeton coloratus * * * * * * Guinochet and de Vilmorin (1973)P. compressus * ** * * Haslam and others (1975)P. crispus * * [*] * * * * * Cernohous and Husak (1986)P. lucens ** ** *P. natans ** ** ** * * * * * Haslam (1987)P. nodosus * [*] ** * * Guinochet and de Vilmorin (1973)P. pectinatus * [*] * * * * * Guinochet and de Vilmorin (1973)P. perfoliatus (.) * * * * Guinochet and de Vilmorin (1973)P. pusillus ** * * * * * * Bornette and Amoros (1991)Ranunculus circinatus * ** ** *R. fluitans **R. trichophyllus * ** *Riccia fluitans *Sagittaria sagittifolia ** * * * Rodwell and others (1995)Sparganium emersum ** ** * * * * * Haslam and others (1975)Sparganium minimum (.) * Guinochet and de Vilmorin (1973)Spirodela polyrhiza ** [**] [**] **Stratiotes aloides * * *Thelipteris palustris **U. minor * * Guinochet and de Vilmorin (1973)U. vulgaris **Vallisneria spiralis **Zannichellia palustris (.)

aDistribution of species according to disturbance levels are only based on data collected along the Rhone and Ain cutoff channels. Disturbancelevels where abundant populations occur are marked by **; levels where the species occurs more or less frequently, but is never abundant, aremarked by *. Sparse occurrences are marked by (.). [*] and [**] indicate that the species should present the same tolerance as in the preceding andfollowing classes. Stars are underlined when bibliographic information is available. Species abundant in cutoff channels submitted to alluvialdeposition during floods or backflows are marked by an asterisk. Substrate requirements of the species were documented from bibliography, andsome informative references are given.


derived from knowledge of the stage–discharge relation-ships of the river at the upstream end of each cutoffchannel, the discharge frequency distribution of theriver, and the stage at which water flows into the cutoff

channel (Table 6). S. Quignard, Compagnie Nationaledu Rhone, unpublished data). With regard to the threeriverine wetlands of the Ain River, overflow dischargeswere quantified by direct observations at several dis-

Table 4. Distribution of helophytes according to disturbance levels and substrate requirements of the speciesa

Helophytespecies

Effect of flood disturbance Substrate granulometry

ReferencesNoneLow


scouringHigh

scouringSilting

in Peat MudSilt/clay Sand Coarse

Alisma plantago-aquatica

* ** * * * Haslam (1987)

Bidens tripartita * * [*] *Butomus umbellatusCarex acutiformis ** ** (.) * * * Rodwell and others

(1995)Carex elata ** ** (.) * * * * Mooney and

O’Connell (1990)Carex pseudo-cyperus ** * * * * Mooney and

O’Connell (1990)Cladium mariscus ** * * Rodwell and others

(1995)Eleocharis palustris (.) * * * Rodwell and others

(1995)Galium palustre * * * * * * * * Rodwell and others

(1995)Glyceria maxima * Felzines (1977)Impatiens glandulifera (.)Iris pseudacorus ** ** *Leersia orhyzoidesLycopus europaeus * * [*] *Lysimachia nummularia * [*] *Lysimachia vulgaris ** * (.) * * Rodwell and others

(1995)Lythrum salicaria ** * * * * * * Rodwell and others

(1995)Mentha suaveolens (.) (.)Myosotis scorpioides (.) * ** ** *Nasturtium officinale * ** **Phalaris arundinacea (.) ** ** ** * * * Petit and Schumacker

(1985)Phragmites australis ** ** * * * * * * * Pautou and Girel

(1981)Polygonum hydropiper (.) ** [**] ** *Ranunculus lingua (.)Rorippa amphibia * ** ** * * * Guinochet and de

Vilmorin (1973)Rumex hydrolapathumScirpus lacustris * * * * * * * * Mooney and

O’Connell (1990)Scrophularia auriculata * [*] [*] *Scutellaria galericulata * [*] [*] *Solanum dulcamara *Solidago gigantea

Sparganium erectum ** ** * Mooney andO’Connell (1990)

Stachys palustris (.)Typha latifolia * * * * * Mooney and

O’Connell (1990)Veronica

beccabunga 1 anagallis* [*] *

aSee footnote to Table 3 for definitions and explanations of symbols.


charge levels. Sinuosity, determining the slope withinthe channel, has a hydraulic effect and was thus pro-vided for the set of channels. Furthermore, high sinuos-ity is usually correlated with a high drainage capacity ofthe channels (mean depth and width within formermeanders), which also decreases flow velocity (Table 6).We documented the occurrence of macrophyte speciesfor different levels of flood scouring (Tables 3 and 4).Four degrees of flood scouring were distinguished: (1)undisturbed or very rarely disturbed ecosystems, (2)rarely or only slightly disturbed ecosystems, (3) moder-ately disturbed ecosystems; and (4) ecosystems dis-turbed with high frequency and/or intensity. The num-ber of cutoff channels used as a database for delineationof species flood tolerance is indicated for each trophicdegree (Table 6). The method used for classifyingspecies according to their tolerance to flood scouring isillustrated in Table 7. For each species and eachdisturbance level, the relative frequency of occurrenceis calculated by dividing the number of occurrences ateach disturbance level by the total number of occur-

rences of the species (indicated between arrows afterthe species name). The highest abundance observed ineach class is also documented, as this reflects thepotential success of the species. The classification isthen based on the relative frequency of occurrence aswell as on the highest abundance. If the species occursat several disturbance levels, the levels where abundantpopulations occur are indicated by two asterisks, thelevels where the species occurs at various degrees offrequency, but is never abundant, are marked by asingle asterisk. Abundance is always considered ofprime importance compared to the occurrence fre-quency, as the latter parameter depends on the numberof potential habitats at each disturbance level. Sparseoccurrences are indicated by a period in parentheses.Single and double asterisks in brackets indicate that thespecies occurs with such an index at both lower andhigher disturbance levels and that it should present thesame tolerance between the two. Such ranking ofspecies according to their disturbance tolerance isbiased by their nutrient tolerance (and conversely, the

Table 5. Channel habitat and vegetation characteristics for each degree of flood scouring

Flood disturbance

None Low scouringIntermediate

scouring High scouring

Morphometry high sinuosity (usually.2)

intermediate sinuosity(usually lower than1.5)

intermediate to lowsinuosity (usuallyfrom 1 to 1.5)

low sinuosity (usuallylower than 1.25)

Substrate granulometry fine, muddy or peaty fine, silt, clay mixedwith mud or peat

silt, sand, coarse in themost scoured parts

mostly coarse silty sandin some refugia

Species richness low intermediate high intermediate to lowPlant cover intermediate to high high intermediate to high intermediate to low

Table 6. Ranges of overflowing frequency and sinuosity for each degree of flood scouringa

Flood disturbance

None Low scouringIntermediate

scouring High scouring

Overflow frequency (days/y)Rhone River 0–2 2–3.5 10–37 12–37Ain River 0 62 5–11

SinuosityRhone River 1.81–12.86 1.08–1.56 1.05–1.26 1–1.41

average: 6.2 average: 1.27 average: 1.13 average: 1.21Ain River 5 1.26 and 1.52 1.05

average: 1.39Channel number

Oligotrophic 3 3Mesotrophic 2 6 4 4Eutrophic 5 16 9

aThe number of cutoff channels (or floristic zones) belonging to each trophic and disturbance degree and used in the method elaboration isindicated below.


ranking of species according to their nutrient require-ments is biased by their disturbance tolerance), whenthe two phenomena interfere. For example, Potamogetoncoloratus, requiring low nutrient levels, is associated withonly slightly disturbed ecosystems. Indeed, increasingdisturbance frequency is correlated with increasingnutrient levels, as river water is usually nutrient-rich.However, this species is tolerant of physical disturbances(Kohler and others 1974, and personal observation). Insuch cases therefore, the flood tolerance of speciesappears narrower than it is in reality.

Species occurrence according to substrate types wasalso documented. Grain-size classes were peat, mud, siltand clay, sand, and coarse substrate (gravel, pebbles).

When aquatic plants are too scarce, or when speciesdo not have a clear indicating value, plant communitiessometimes become insufficient for assessment of thedegree of flood disturbance. In these cases, the vegeta-tion-based diagnosis must be supported by additionalinformation concerning geomorphology, morphom-etry, substrate granulometry of the channel, and plantcommunity organization (Table 5). For confirmation ofthe diagnosis, field observations of the assessed wetlandshould be compared to the following description ofeach disturbance degree.

Undisturbed or very rarely disturbed ecosystems. Suchecosystems are usually very sinuous cutoff channels(very low slope; Table 5) and/or of high depth andwidth, i.e., high drainage capacity (Bravard and Gilvear1993). For example, the sinuosity of the undisturbedcutoff channels of the Rhone River ranges from 1.81 to12.86 (Table 6). These wetlands are either infrequentlyinundated or are inundated at velocities that neverdisturb the substrate or the vegetation. In this case,floods are unable to export the fine sediment andorganic matter that progressively fill in the wetlands.The substrate of riverine wetlands is peaty or muddy,depending on the trophic degree. However, in somecases, coarse substrate can be abundant, due to ground-water supplies, which export fine sediment deposits. Insuch cases, wetlands function as groundwater streams[described by Kohler and others (1974) and Carbienerand others (1990)] and should thus remain stable aslong as such groundwater supplies persist (Bornetteand others 1994a). Such ecosystems are poor in hydro-phyte species at the spatial scale of a few quadrat meters,because competition eliminates the less competitivespecies (Bornette and others 1998a,b). Vegetation coveris usually high, and species are organized as largemonospecific patches or stripes along the banks.

Rarely or only slightly disturbed ecosystems. Such formerchannels have intermediate to low sinuosity (1.08–1.56in Rhone and Ain River wetlands; Table 6). Floods only

slightly disturb vegetation, and partly export some ofthe fine sediment and organic matter, but this scouringeffect remains insufficient to stop succession. Further-more, floods also bring fine sediment (silt, clay) promot-ing terrestrialization. The substrate, resulting from bothfluvial deposits and organic matter accumulation is amixture of river silt or clay and organic matter (Rostanand others 1987). Aquatic plant richness is higher thanin the absence of disturbances because of the limitationof competitive exclusion by sporadic flood distur-bances; and the recruitment of new species brought byfloods (Bornette and others 1998). Plant cover is highin all but the most deeply scoured parts of the channels.

Sediment accumulation due to floods can be effec-tive to various degrees in such wetlands, depending on

Table 7. Illustration of the method usedfor classifying species according to theirdisturbance tolerancea

Flood disturbance

NullLow


scouringHigh

scouring

A: Relative frequencyAlisma plantago-

aquatica (12) 16% 52% 16% 16%Berula erecta (17) 5% 30% 24% 41%Carex elata (20) 50% 45% 5% 0%Lemna minor (17) 35% 35% 0% 30%Nasturtium

officinale (11) 0% 27% 9% 64%B: Highest abundance

Alisma plantago-aquatica 0.4 0.5 0.33 0.22

Berula erecta 0.4 3.21 2.9 3.38Carex elata 3.85 3.83 3 0Lemna minor 2.4 3.55 0 3.7Nasturtium

officinale 0 0.33 1.6 0.9C: Classification

Alisma plantago-aquatica * ** * *

Berula erecta (.) ** ** **Carex elata ** ** (.)Lemna minor ** ** [**] **Nasturtium

officinale * ** **

aA: relative frequency calculated for each disturbance degree bydividing the number of occurrences in the disturbance degree by thetotal number of occurrences of the species (indicated in parenthesesafter the species name). B: highest mean abundance of the speciesobserved in each disturbance degree. C: resulting classification. If thespecies occurs in several disturbance degrees, the degrees whereabundant populations occurs are marked by **, the degrees where thespecies occurs more or less frequently, but is never abundant, aremarked by *. Sparse occurrences are marked by (.). [*] and [**]indicate that the species is absent but is assumed to have the sametolerance as for the upper and lower levels. For more details, see text.


their morphometry: if the former channels are veryshort and widely open to the river, backflows of riverwater are slowed down when entering the channel,instigating fine sediment deposition and acceleratingterrestrialization. In the same way, as a consequence ofvery low flow velocity, sediment accumulation occurs inthe more sinuous and frequently overflowed channels.In these cases, species richness is intermediate to low,depending on water turbidity, induced by frequentbackflows or overflows of eutrophic and turbid riverwater, and substrate stability: frequent substrate inputsmay bury plant and impede their regrowth.

Moderately disturbed ecosystems. Such former channelshave low sinuosity (1.05–1.26 in cutoff channels of theRhone and Ain Rivers, Table 6). They are characterizedby fine substrate along the banks and coarser substratein the deepest, more scoured areas of the channels.

Flood disturbances affect these ecosystems with inter-mediate frequency (overflow frequency from 10 to 37days per year along the Rhone River) and disturbvegetation. The scouring effect of floods slows down oreven stops succession. The major part of organic matterproduced in the cutoff channels is exported, whereasfloods bring coarser sediment (sand, silt, and evengravel) and propagules. Substrate is therefore fre-quently disturbed, and granulometry is very heteroge-neous at the patch level (Bornette and Amoros 1996).Substrate accumulation results from fluvial deposits andis restricted to protected areas. Richness is theoreticallythe highest, because of the dynamic equilibrium estab-lished by the intermediate frequency and/or intensityof flood disturbances (Connell 1978, Pickett and others1989). Competition remains low and floods frequentlybring diaspores, allowing the recruitment of new spe-cies in patches cleared by the disturbance. Species areorganized as small patches, providing high speciesrichness at the sample level. Vegetation cover should bethe most extensive in zones of fine sediment accumula-tion.

Ecosystems disturbed with high frequency and/or intensity.These former channels have low sinuosity and flood

disturbances are very frequent or intense. Sinuosityranges from 1 to 1.41 and overflow frequency from 12 to37 days per year in the Rhone riverine wetlands.Sinuosity and overflow frequencies are close to thoseobserved in the previous case, but cutoff channels differby their lower hydraulic capacity, favoring scouring byfloods. The substrate is predominantly coarse, mixed invery small, protected areas (along the banks, for ex-ample) with silt or mud. These wetlands are mostlyeutrophic, as the river (usually nutrient-rich) frequentlyoverflows and inputs nutrients. However, oligotrophicwetlands may occur along oligotrophic rivers.

The scouring effects of floods stop succession andimpede sediment accumulation as well as vegetationgrowth in most parts of the wetlands. Competitionremains very low and floods frequently bring diaspores,but recruitment is limited to a few dead zones. Richnessis intermediate to low, depending on the refuges avail-able and on the intensity and frequency of floodscouring. Vegetation cover remains very low.

Predictions on Ecosystem Sustainability andRestoration Potential

Wetland Life Duration

Forecasts on wetland life duration (Table 8) arebased on the major processes involved in terrestrializa-tion, i.e., the influence of flood disturbances, theoccurrence of groundwater supplies, the trophic de-gree, and water permanency during the yearly cycle.Any water-table lowering, resulting, for instance, fromwater abstraction or riverbed deepening, will also in-crease the terrestrialization, but this process is notconsidered in the predictions afforded by the method,since aquatic vegetation cannot predict any trend inwater-table lowering.

The increase in trophic degree favors terrestrializa-tion through the increase in macrophyte productionand the subsequent accumulation of organic matter.Groundwater supplies may slow down successional pro-cesses and terrestrialization by limiting plant produc-tion and washing away part of the fine sediment. Flooddisturbances can scour the sediment and thus slowdown or impede terrestrialization. The scouring of finesediment favors the maintenance of pioneer communi-ties, limits organic matter accumulation, and sustainsgroundwater supplies by conserving coarse substrate.Conversely, floods that inundate ecosystems withoutscouring them (too low velocity) contribute to theirterrestrialization and eutrophication by carrying anddepositing fine sediment (usually nutrient rich) thataccelerates the filling in of the wetlands and clogs thesubstrate, reducing groundwater supplies. Further-more, in cutoff channels that are only slightly or notdisturbed, great depth affords the potential of a longerlife duration of the aquatic stage. Conversely, the waterlevel in the shallowest wetlands can drop during thesummer and favor helophyte colonization and thussediment accumulation and terrestrialization.

Frequently disturbed ecosystems have a high poten-tial life duration, regardless of their trophic degree,their depth, and the occurrence of groundwater sup-plies, because disturbances impede succession. Pioneercommunities persist in such flood-scoured sites (Table


8). Moderately disturbed ecosystems also have a highpotential life duration (Bornette and Amoros 1996),which could be reduced in the more eutrophic cases, ifgroundwater supplies or disturbances remain insuffi-cient to compensate for terrestrialization resulting fromorganic matter accumulation. Life duration will prob-ably be reduced in ecosystems that are periodicallydewatered if groundwater supplies do not contributewith disturbances to slow down complete terrestrializa-tion.

The life duration of the aquatic stage in infrequentlyand never disturbed ecosystems depends mostly ontheir depth, their trophic degree, and the occurrence ofgroundwater supplies (Bornette and others 1994a). Inpermanently aquatic ecosystems, life duration is relatedto depth and occurrence of groundwater supplies thatreduce plant productivity (Bornette and others 1998a,b).The life duration of the aquatic stage is shorter intemporary habitats because water turnover from ground-water supplies is low. In wetlands that are never or onlyslightly supplied by groundwater, duration of the aquaticstage is related to the trophic status combined withdepth. Among eutrophic channels with high productiv-ity, the deepest habitats have an intermediate lifeduration, while the shallowest have a short one (Bornetteand Amoros 1996).

In wetlands frequently inundated but not scoured,terrestrialization proceeds rapidly because of siltation(Martinet and others 1993). The increase in trophicdegree induced by these nutrient-rich deposits in-creases the macrophyte production and terrestrializa-tion rate. Consequently, these channels should have anintermediate to low life duration, depending on theirdepth and the frequency of sediment inputs.

Reversibility of Terrestrialization

Reversibility is the feasibility of returning an ecosys-tem to a previous successional stage (Amoros andothers 1987) with a moderate effort (moderate energyinput or financial cost). Consequently, reversibility con-cerns potential for restoration. Succession leads to theprogressive filling in of the wetlands with organic matterand/or fluvial silt and clay, to their progressive discon-nection from groundwater, eutrophication, terrestrial-ization, and finally disappearance of the aquatic stage.Reversibility means the potential of an ecosystem tobecome less eutrophic, be deeper, and have a longer lifeduration. High reversibility supposes that an ecosystemis changing towards the terrestrial stage due to a processthat could be reversed by restoration (Henry andAmoros 1995). Intermediate reversibility supposes highsuccess of restoration in terms of ecological effects, but

Table 8. Prediction of life duration and reversibility of terrestrialization of aquatic ecosystemsa

Flood disturbance Water permanency

Potential life duration

Oligotrophic ecosystemgroundwater supplies

Mesotrophic ecosystemgroundwater supplies

Eutrophic ecosystemgroundwater supplies

1 0 1 0 1 0

None high 3 2 3 2 2 1low (periodic dewatering) 2 1 2 1 1 1

Low scouring high 3 2 3 2 2 1low (periodic dewatering) 2 1 2 1 1 1

Intermediate scouring high 3 3 3 3 3 2low (periodic dewatering) 3 2 3 2 2 2

High scouring high 3 3 3 3 3 3low (periodic dewatering) 3 3 3 3 3 3

Potential of restorationNone high n.c.b 2 n.c. 2 1–2 1

low (periodic dewatering) 2–3 2 2–3 2 1–2 1Low scouring high n.c. 2 n.c. 2 1–2 1

low (periodic dewatering) 2–3 2 2–3 2 1–2 1Intermediate scouring high n.c. n.c. n.c. n.c. n.c. 1

low (periodic dewatering) 3 2 3 2 2 1High scouring high n.c. n.c. n.c. n.c. n.c. n.c.

low (periodic dewatering) n.c. n.c. n.c. n.c. n.c. n.c.

aFor life duration, each functional type has been classified into: (1) low life duration (likely from 10 to 30 years), (2) intermediate life duration(likely from 30 to 60 years), and (3) high life-duration (likely from 60 years to more than 100 years). Potential of restoration was estimated forecosystems that have intermediate to low life duration; each functional type is classified into: (1) low reversibility; (2) intermediate reversibility, and(3) high reversibility. For the significance of the classes of reversibility, see text.bn.c.: not concerned: high life-span, see text.


a short life-span for this restored stage, or intermediatesuccess of restoration in terms both of ecological effectsand sustainability of the restored stage. Low reversibilityconcerns habitats where restoration of a previous succes-sional stage would need excessively high energy inputor those where the sustainability of the restored stagewould probably be low (Amoros and others 1987).

Channels that are frequently and moderately dis-turbed are not candidates for restoration if they aresufficiently deep, because floods should impede terres-trialization (Table 8). In the same way, the deepestcutoff channels that are infrequently disturbed have ahigh life-span and thus do not need any restoration.

The efficiency of restoration in the shallowest wet-lands increases if groundwater supplies occur (Henryand Amoros 1995, Henry and others 1995). Waterturnover and washing of fine sediment will be moreefficient if the slope within the channel is high andhydraulic capacity low. Eutrophic ecosystems have lowreversibility, as high nutrient content favors macrophyteproduction and terrestrialization. The occurrence ofgroundwater supplies should, however, increase thisreversibility, as they can contribute to limit macrophytedevelopment and siltation. In silted wetlands whereplants indicate the occurrence of floods, restorationprocesses that increase the frequency or the scouringefficiency of overflows should be considered.

Application of the Method andConcluding Remarks

Floristic data must be collected throughout riverinewetlands, in order to see the whole spectrum of thecommunities. We usually take samples from 2-m stripscrossing the channel from bank to bank, separated by25 or 50 m depending on channel length (Bornette andothers 1994b, 1998a,b). The abundance of each speciesin each sample [estimated using the Braun-Blanquet(1964) abundance indices] allows determination ofdominant species. Some species occur very sporadically(only a few individuals) due to particular environmentalconditions at the sampling date, or because they havebeen brought by floods and remain alive, but will notprosper. Such species have a lower indicative value thanthe dominant ones. In each sample, the cover percent-age of each grain-size class (peat, mud, silt/clay, sand,and coarse substrate) must be estimated. Depth, width,water flow, occurrence of any scoured patch, shade,total plant cover, and spatial plant patterns (such assmall intermingled patches or large patches or stripsalong the banks) also should be documented. Channelsinuosity is obtained by dividing the whole channel

length (distance from the upstream to the downstreamconfluence with the river at the date of cutoff) by thelinear distance between these two points.

When the wetland exhibits clear upstream-to-down-stream vegetation zonation, it is necessary to discrimi-nate the floristic zones using a multivariate analysis(principal component analysis for example) and toclassify all zones according to their trophic degrees andthe effect of floods. Indeed, floods usually affect thedifferent parts of the riverine wetlands differently, thusmaking it necessary to diagnose each part of thewetland. For example, braided cutoff channels oftenpresent a large and deep downstream section that canbe less disturbed (higher capacity decreasing flowvelocity) than the shallowest and narrowest upstreamareas.

Floristic data must be compared with Tables 1 and 2to classify the wetland into oligotrophic (i.e., oligotro-phic and dystrophic), mesotrophic, or eutrophic and toassess any groundwater supply. For eutrophic ecosys-tems, it is often difficult to detect the occurrence ofgroundwater supply, because of the lack of indicativeeutraphent species. However, permanent water flowassociated with the absence of permanent upstreamconnections to the river suggests the occurrence ofgroundwater supply. For example, a former channel ofthe Rhone River dominated by Nuphar lutea, Nympheaalba, Thelipteris palustris, Menyanthes trifoliata, and Potamo-geton coloratus for hydrophytes and by Phragmites austra-lis, Cladium mariscus, Carex elata, and Juncus subnodulosusfor helophytes has been classified under oligotrophicwetlands. The occurrence of Potamogeton coloratus, Thelip-teris palustris, and Cladium mariscus suggests that theformer channel is supplied by groundwater. This diagno-sis is confirmed by the water turn over observed in theshallowest parts of the channel (permanent low flowdespite the fact that it is not supplied by river waterduring low water level or by any other stream).

As a second step, floristic data must be comparedwith Tables 3 and 4 to assess the effects of floods. In theprevious example, Nuphar lutea, Nymphea alba, Phrag-mites australis, Cladium mariscus, and Carex elata growpreferentially in less disturbed channels. Cladium maris-cus and Menyanthes trifoliata grow preferentially on peatysubstrate that is usually linked to a low disturbance level.The observation of dominant peaty substrate, aquaticplants organized in large patches, helophytes as stripsalong the banks, high plant cover, and high sinuosity(12.13) are all indications, confirming through Table 5,that disturbances must be infrequent. The wetlandpresents deep water zones (more than 3 m) indicating,together with its low trophic degree and groundwater


supplies, that its potential life-span is high. The shallow-est parts are probably preserved from terrestrializationby permanent groundwater fluxes. Thus, such a wetlandshould not need any restoration.

The method was applied on four riverine wetlandsbelonging to four different river systems (Table 9). Themethod requires a map to measure the sinuosity of thewetland and some simple data on habitat conditions

Table 9. Test of the method on four cutoff channels from different river systems

River system Ain River, France Saone River, France Danube River, Hungary Rhine River, Netherlands

Cutoff channel Creux de Fouchoux La Bouillie Szilos-Fanos Kil near HurwenenDate of sampling 1992 1994 1984–85 1955Reference Bornette and

others (1998)Bornette and others

(unpublishedreport)

Rath (1987) van Donselaar (1961)

Sampling plots (N) 13 11 28 18% of each grain-size

class on thewhole channel

Coarse substrate 75Sand 10Silt 15Mud 100 ca 100 ca 100

Channel habitat andvegetationcharacteristics

Mean width (m) 10 35 50–300 125–200Mean depth (m) 0.7 0.6 1 0.4Sinuosity 1.05 1.07 2.47 2.41Hydrophyte

species richness21 13 9 12

Helophyte speciesrichness

10 12 7 25

Plant cover low high intermediate tohigh (60%)

intermediate(50%)

More abundant species and averageabundance (max. 5)

Phalaris arundinaceaSparganium emersumBerula erectaPotamogeton pusillusCallitriche platycarpaCarex elataPotamogeton perfoliatusMentha aquaticaElodea canadensisMyositis scorpioides

1.31.110.80.70.70.60.60.40.4

Phragmites australisNuphar luteaCarex acuta?Ceratophyllum demersumHydrocharis morsus-ranaeLemna minorSpirodela polyrhizaCarex pseudocyperusStratiotes aloidesLemna trisulcaLysimachia vulgarisChara globularis

3.52.510.90.50.40.40.40.40.40.40.3

Scirpus lacustrisGlyceria maximaPhragmites australisPperfoliatusNymphea albaNuphar luteaPolygonum amphibiumPotamogeton crispusRorippa amphibiaCeratophyllum demersumLemna minor

1110.70.60.60.50.30.30.10.1

Glyceria maximaPhragmites australisEquisetum fluviatileLemna minorMentha aquaticaScirpus lacustrisTypha angustifoliaRumex hydrolapathumCarex acutaNuphar luteaSparganium erectumHydrocharis morsus-ranaeRorippa amphibiaIris pseudacorusLemna trisculca

1.31.20.80.80.80.70.70.60.60.60.60.50.50.50.4

Ecological diagnosisTrophic level mesotrophic eutrophic eutrophic eutrophicDisturbance level high scouring low scouring null nullGroundwater

suppliesyes yes, but low absent low

Potential lifeduration

high low-intermediate low low-intermediate

Restorationpotential

n.c. low-intermediate low low-intermediate

n.c.: not concerned, high life span.


and vegetation (depth, width, transparency, flow veloc-ity, substrate, relative plant cover). Our diagnosis agreedwith the informations provided by the authors.

We propose a method of ecological diagnosis ofriverine wetlands using relatively simple and rapidfieldwork. The method efficiency results partly from theextensive bibliographic survey concerning species toler-ances and requirements. In the future, the methodshould also integrate bibliographic information concern-ing other plant communities that occur in some otherriver systems in the same habitat conditions. The effi-ciency of the method would also be considerably im-proved if plant traits could replace plant species (Mu-cina 1997). This should be possible for disturbanceeffects, because they select for plant growth form andreproductive strategies (Bornette and others 1994c),but trophic degrees should select physiological traitsthat are less identifiable at the species level (Dendeneand others 1993, Rolland and Tremolieres 1995).

As predictions are based on aquatic plant communi-ties, the method is limited by the usual restraints ofaquatic plant growth. The deepest parts of the waterbod-ies, the areas shaded by riparian trees, or those whereflow velocity is too high, cannot be considered for theecological diagnosis, as vegetation growth is impeded.In this case, as in the previous cases, geomorphologicaland substrate data combined with field observationsmust be used. However, these predictions suppose thatsubstrate accumulation (where possible), and develop-ment of plant communities have proceeded. Wetlandsrecently cutoff, or those where substrate has beenrecently removed, are difficult to classify, because theperiod required for plant colonization has not beenapplied. For example, a recently cutoff meander (in thepast 10 years) which is no longer inundated may notpresent typical community organization and substrate,as granulometry may remain coarse and pioneer plantcommunities may still occur. In such a case, it would bedifficult to provide a reliable diagnosis. However, thedivergence between geomorphological data (in thisexample, a very sinuous channel) and substrate (in thisexample, coarse substrate), matched with field observa-tions (chances of river overflow, overflow indications)should reveal the bias and make it possible to identifythe processes operating in such wetlands.

Acknowledgments

The present study benefited from the financialsupport from the Compagnie Nationale du Rhone andthe French Water Agency, Rhone–Mediterranee–Corse.We are very grateful to M. H. Barrat-Segretain andJean-Claude Rostan for comments on a first draft of the

paper, to J. M. Friedman and M. C. McKinstry forremarks to improve a previous version of the manu-script, and to T. Partrick for linguistic assistance.

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a vegetation-based method for ecological diagnosis

Documents

pusillus ranunculus circinatus

upper rhine plain

theoretical habitat templets

bring ne sediment

aquatic vascular plants

high drainage capacity

moderately disturbed ecosystems

organic matter accumulation