Biodiversity assessment following a naturality gradient of riverbank protection structures in French prealps rivers

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To link to this article : doi:10.1016/j.ecoleng.2012.12.105

URL : http://dx.doi.org/10.1016/j.ecoleng.2012.12.105

To cite this version : Cavaillé, Paul and Dommanget, Fanny and

Daumergue, Nathan and Loucougaray, Gregory and Spiegelberger,

Thomas and Tabacchi, Eric and Evette, André Biodiversity assessment

following a naturality gradient of riverbank protection structures in

French prealps rivers. (2013) Ecological Engineering, vol. 53 . pp.

23-30. ISSN 0925-8574

Any correspondance concerning this service should be sent to the repository

administrator: [email protected]

Biodiversity

assessment

following

a

naturality

gradient

of

riverbank

protection

structures

in

French

prealps

rivers

Paul

Cavaillé

,

Fanny

Dommanget

,

Nathan

Daumergue

,

Gregory

Loucougaray

,

Thomas

Spiegelberger

_,

_Eric

_Tabacchi

_,

_André

_Evette

a_{Irstea,UREMGR,2ruedelapapeterieBP76,38402Saint-Martin-d’Hères,France}

b_{CNRS,UniversityofToulouse,InstitutNationalPolytechnique,EcoLab,Bât.4R1,118routedeNarbonne,31062Toulousecedex9,France}

Keywords: Beetlediversity Bioengineering Plantdiversity Riverbank Ripariancorridors

a

b

s

t

r

a

c

t

Erosioncontrolofriverbankisfrequentlynecessarytoprotecthumaninvestmentssituatedalongrivers.

Thetechniquechosenforsucherosioncontrolconstructionmayhavemajorimpactsonbiodiversityand

onthefunctioningofrivercorridors.Evenifthereisagreement,thatbiodiversityshouldbeonecriterion

forchoosingembankmentstechniqueslittleisknownaboutwhethersuchtechniquescanaccommodate

biodiversity.

Weaimedtodeterminecoleopteranandplanttaxonomicdiversitiesalonganaturalitygradientof

riverbankprotectionsystems,rankingfromentirelycivilengineeringstructures,throughcombined

con-structions(mixingcivilengineeringandbioengineering),topurelybioengineeringstructures.

Fifteensites(fivesitesofeachtechnique)weresampledintheRhône-Alpesregion(S.E.France).On

eachsite,vegetationwassampledalongthreetransectsfromthebottomtothetopoftheriverbankand

flyingbeetlesbytrapping.

Intotal,werecorded148plantspeciesand78beetlegenera.Wefoundsignificantlyloweranimaland

plantdiversitieswithincivilengineeringconstructionsthanintheothertwotechniques.Diversitiesof

bothtechniquestendedtobehigher,althoughnotsignificantly,incombinedtechniquesthaninpurely

bioengineeringones.Furthermore,civilengineeringstructuresweremoresubjecttoinvasionbyexotic

plantspeciesthanthetwoothertechniques.Theseresultsquantifyandhighlighttheinterestof

bioengi-neeringtechniquescomparedtocivilengineeringinenhancingbiodiversityandlimitinginvasivespecies

techniques.

1. Introduction

In addition to its high level of biodiversity, riparian corri-dorssupplymultipleecosystemservicessuchasnutrientcontrol, floodcontrol,shadingeffectandsocialaspects.Ripariancorridors havethereforebeenwidelyrecognizedaskeycomponentin land-scapes(Décamps,2011).Thereisastrongcausallinkagebetween biodiversityandecosystemfunctioning(Maestreetal.,2012). Bio-diversity loss may induce a modification of natural ecosystem functioningandecosystemservices(McNeely,2010).

However,WesternEuropeanripariancorridorsandtheir veg-etation have been widely affected by centuries of human use. Rivercorrectionsandassociatedfloodcontrolhavebeen primar-ilyperceivedhasagreat humanachievementin environmental

∗Correspondingauthor.

E-mailaddress:[email protected](P.Cavaillé).

control.However,shortlyafterinitialsuccesssuchartificial land-scapes spotlight a major conflict between riparian ecosystem conservationandhumanpatrimonyprotection(Pahl-Wostl,2006), assuchriverstraighteninghasleadtoadrasticlossofbiodiversity in riparian systems. In urbanized areas as well as in agricul-tural landscapes,spaceavailableforriparianvegetationisoften reducedtotheriverbankonly.Thus,organismscanhardly circu-latealongthereremnantsofthepastripariancorridorsespecially whencivilengineeringerosioncontrolstructuresareused(Nilsson etal.,2005).Suchinterruptionsinthecorridormosaicmaylower biologicalcontinuitybyreducingcirculation,refugeandfeeding opportunities,andbyincreasingthermalcontrastsduringsummer. Alteredlandscapesanddisruptedcorridorresultinaglobal ripar-ianecosystemloss(Poffetal.,1997;TocknerandStanford,2002). Consequently,biodiversityconservationshouldbeastrategyfor practitionerstoenhanceecosystemfunctioning(Isbelletal.,2011). In addition, direct exposure to flood and high flood fre-quencyleadtohighpropagulefluxesonbaresoilsandpromote

colonizationbyexoticspeciesandsubsequentlyinvasion(Décamps et al., 1995). Therefore, exotic species richness is significantly greaterinriparianzonescomparedtouplandsites(Stohlgrenetal., 1998).

Habitat restoration hasbeen clearly identified as a key ele-menttoreversenegativeanthropic impactsonbiodiversityand ecosystemsfunctioning(Helfieldetal.,2012).Publicpolicysuch astheEuropeanWaterFrameworkDirective(WFD;2000/60/EC) encouragesaglobal“goodecologicalstatus”ofaquatic environ-mentincludingmorphologicalconditionssuchasstructureofthe riparianzone.Astudyorderedin2005bytheFrenchMinistryof EcologyandSustainableDevelopmenthighlightsthat51%ofthe surfacewatermasswouldnotreachtheobjectiveof“good ecologi-calstatus”by2015(IFEN,2006).Improvingbiodiversityonartificial riverbanksmayactinfavourofthisobjectiveandinparticularif alternativestocivilengineeringareusedsuchasbioengineering techniques(LiandEddleman,2002).

Bioengineeringtechniquesforerosioncontrolconsistsinthe uses of living plantsin order tomimic natural vegetationable to resist intense scouring (Gray and Sotir, 1996; Norris et al., 2008;EvetteandFrossard,2009).Bioengineeringappliedto river-banksaimsatcombiningbothartificialandnaturalcomponents toprovideprotectionattheentirebanklevel(Evetteetal.,2012). Inadditiontotheirerosioncontrolcapacity,bioengineering struc-turesaresupposedpromotingtherecoveryofindigenousspecies andtoensurebetterplantcoveragethanartificiallyreconstructed embankmentsusingcivilengineeringtechniques(Pezeshkietal., 2007).One oftheargumentsgiven forchoosingbioengineering structuresinsteadofcivilengineeringonesisthatthefirst pro-mote the re-establishment of a functional and self-sustainable ecosystem;however,toourknowledge,noquantitativescientific evidenceshave been produced to supportthis idea. Untilnow socio-economicalissuesarethemainreasonstowhetherprotect ornot.Riverandlandmanagers’finaldecisiononwhichriverbank protectiontechniquewillbeusedismainlybasedonseveral cru-cialpointssuchashydro-engineeringaspects,bedmaterialand watercoursebankstability(SchiechtlandStem,1996).However,no quantitativeecologicalaspectisforthemomenttakenintoaccount inthedecisionline.

Todate,many scientificstudiesonbioengineeringstructures alongriversfocusonmethodologicalaspectssuchasstructures, construction methods, mechanical resistanceand thechoice of species(Bretonetal.,underreview),comparedtothefewworks whicharededicatedtobiodiversitysubsequentlyrelatedto ero-sioncontrolstructuresonwatercoursebanks.Since2002andthe earlyworkontheenvironmentalandecologicaladvantagesof bio-engineeringstructures(LiandEddleman,2002),fewstudieshave usedquantitative methodstoinvestigaterelation between bio-diversityanderosioncontrolstructures.Someworksaredealing withplantspeciesand habitatdiversity assessmentbut remain studycasesonveryfewsites(Lietal.,2006;SudduthandMeyer, 2006).Ontheotherhand,studieshavebeendoneonriverbank restorationbyremovingbankfixationorprotectionand compar-ativebiological surveyonrestoredandnon-restored siteswere made(Helfieldetal.,2007;Januschkeetal.,2011).Thoughthereis abigknowledgegapconcerningquantitativebiological compari-sonofriverbanktechniquesforerosioncontrolandtheirimpact onecosystemproperties(Pahl-Wostl,2006).Toourknowledge, nostudyhasyetdemonstratedtherelationshipbetweenmaterials andtechniquesusedforerosioncontrolandassociatedchangesin biodiversityontheconstructions.

Theaimsofthisstudywere(i)toevaluateandcompareanimal andvegetalbiodiversityonthreeriverbanktechniquesforerosion controland(ii)toassessthecapacityofdifferentriverbank pro-tectiontechniquestolimitinvasionofexoticspecies.Theapplied

aimofthisstudywastointegrateecologicalaspectsintothe deci-sionlinebyprovidingecologicaldata.Weprovideguidelinesto practitionersonwhatecologicalconditionstheycanexpectonthe riverbanktechniquechoseninordertodevelopstrategieslinkedto riverbankmanagementandallowrestorationofecosystemgoods andservices.Wehypothesizethatcivilengineeringtechniquesdo notpromotebiodiversityasgoodasbioengineeringtechniques. 2. Materialsandmethods

2.1. Sites

Weassessedbiologicaldiversityonfivereplicatesofthree river-banktechniques(Fig.1):

-Civilengineeringembankments:riprapprotection(rocksusedto stabilizeshorelines,againstscouranderosion)notedas “Min-eral”.

-Mixed embankments combining civil engineering techniques (riprapatthelowerpartofthebank)andbioengineering tech-niques(cuttingsandwoodyplantationattheupperpart)noted as“Mixed”.

-Completely bioengineeringembankments: (willow fascines at thelowerpartofthebankwithcuttingsandwoodyplantation attheupperpart)notedas“Vegetal”.

Thesethreetechniqueswereconsideredasthreelevelofa nat-uralitygradient.Naturalitycanbedefinedbytheamountofliving vegetationusedintheconstructionasclassificationkeycriterion (Brunel,2009;Werdin-Pfistereretal.,2009).Mineralstructures wereconsideredasman-madeenvironmentswithlittleplant cov-erage,differentfromtheenvironmentfoundonnaturalmountain riverbank.Mixed methods showintermediate plant cover with revegetationon halfof thebank.Bioengineering structuresare completelyvegetalandthereforeareexpectedtoshowthe high-estvegetationcoverandthusthehighestlevelofnaturality.These threeengineeringtechniquestherefore constitutethreedistinct levelsonanaturalitygradient.

WeselectedfifteenriverbanksalongsixdistinctFrenchprealps riversbelongingtotheRhonecatchmentarea.Alargeandintense prospectionatterritorialunitsforrivermanagementorganizations wasperformedtolistenoughcomparablebanks.Table1listssites characteristics.Asaltitudeprofoundlyaffectsecological commu-nitycomposition,studysiteswereselectedatanarrowrangeof 270m(between200and470masl).Onlysitesbetween20and 30mlongandwithanestablishedvegetationwerechosenfor anal-ysis.AllthestudysiteswerelocatedinthefoothillsoftheFrench Alpscharacterizedbyhighlyfragmentedhabitats.

2.2. Vegetation

VegetationwasassessedusingthecontactpointmethodinJune andJuly2009.Thismethodencompassesthevertical organiza-tionofthevegetation(tree,shrubandherbaceouslayers).Plant speciesdiversityandfrequencieswereestimatedusinga2-m-long stickwitha diameterof1cm. Measurementsweretakenevery metrealongthree20mtransectsplacedparalleltotheshoreone locatedatthewaterline(Transect1),oneinthemiddle(Transect 2)andoneatthetop(Transect3)oftheriverbankembankment (sixtymeasurementsper site).Vegetationwasidentified tothe specieslevelusingvariousidentificationkeysandflora(Rameau, 1994;LauberandWagner,1998;Aeschimannetal.,2004).Exotic speciesweredefinedonthebasisofestablishedlistsrelevantto thearea(Mulleretal.,2006),alistofexoticinvadersinSwitzerland

Fig.1.Schematicrepresentationofriverbanktechniquesstudied.(a)Civilengineeringembankments:riprapprotection,notedas“Mineral”.(b)Mixedembankments combiningcivilengineering(riprapatthelowerpartofthebank)andbioengineeringtechniques,notedas“Mixed”.(c)Completelybioengineeringembankments:willow fascinesatthelowerpartofthebankwithcuttings,notedas“Vegetal”.

(LandoltandBäumler,2010)andalistestablishedforthe admin-istrativedistrictofIsère(Gentiana,2006).

2.3. Fauna

Coleopteranswereidentifiedtogenus(Insecta,Coleoptera).This order, and more specifically theCarabids, is used as proxy for habitatdiversity,hydromorphologicalprocessesandhabitathealth status(Boscainietal.,2000;Kleinwaïchteretal.,2003).Moreover, morphologicalattributesofColeopteranscanprovideinformation ontheirfunctionalandtrophicstatus(VanLooyetal.,2005).Flying coleopteransweretrappedusingtwoFlora®_{“YellowWell”traps,}

locatedatthetwoextremitiesin themiddle ofthebank (tran-sect2)andplacedatthecanopylevel.TrapsweresetinJuneand

July2009andremainedinthefieldforsevendays.Insectswere identified using a binocular microscope and appropriate refer-enceworks(Picard,1929;Paulian,1941;Hoffmann,1945;Guignot, 1947;Balachowsky,1949).

2.4. Statisticalanalysis

Differencesinplantspeciesandcoleopterangenerarichness, andinfrequencyandnumberofexoticspeciesamongthethree typesofdevelopmentwasassessedusingKruskal–Wallistests.In caseofasignificantmaineffect,pairedMann–Whitneytestswere carriedouttodetectdifferencesbetweeneach technique. Over-allstatisticalriskwasassessedusingtheHolm’scorrection(Holm, 1979).Aninter-classprincipalcomponentanalysis(PCA)(Doledec

Table1

Characteristicsandlocalizationsofembankments.

Sitecode Constructiontype Locality River Latitude Longitude Constructionworkyear Altitude(masl) 6V Vegetal LesEchelles Guiersvif 45◦_{26’19.15”N 5}◦_{45’36.75”E 2005 393}

35V Vegetal VillardBonnot Vorz 45◦_{15’01.29”N 5}◦_{54’01.62”E 2006 242}

7V Vegetal StGeoireenValdaine Ainan 45◦_{28’02.24”N 5}◦_{39’40.90”E 2004 381}

38V Vegetal LeGrandSerre Galaure 45◦_{15’45.62”N 5}◦_{06’29.65”E 2007 370}

39V Vegetal Cluse Arve 46◦_{04’13.98”N 6}◦_{33’18.72”E 2005 470}

6Mix Mixed LesEchelles Guiersvif 45◦_{26’19.15”N 5}◦_{45’36.75”E 2005 393}

30Mix Mixed Grenoble Isère 45◦_{11’55.65”N 5}◦_{44’02.36”E 2002 210}

32Mix Mixed Gière Isère 45◦_{11’20.94”N 5}◦_{47’20.62”E 2005 210}

39Mix Mixed Cluse Arve 46◦_{04’13.98”N 6}◦_{33’18.72”E 2005 470}

40Mix Mixed Bonneville Arve 46◦_{04’33.18”N 6}◦_{24’26.63”E 2004 444}

7Min Mineral StGeoireenValdaine Ainan 45◦_{28’02.24”N 5}◦_{39’40.90”E 2004 381}

35Min Mineral VillardBonnot Vorz 45◦_{15’01.29”N 5}◦_{54’01.62”E 2006 242}

37Min Mineral ChateauneufdeGalaure Galaure 45◦_{14’09.59”N 4}◦_{58’30.83”E 2004 257}

38Min Mineral LeGrandSerre Galaure 45◦_{15’45.62”N 5}◦_{06’29.65”E 2007 370}

Fig.2.Meanplantspeciesnumbersforeachriverbankprotectiontechnique accord-ingtotheirpositionsonthebank.Differentlettersindicateastatisticaldifference betweensamples.Errorbarsindicatestandarderrors.The“Alltransect”bar repre-sentthemeanofthetotalnumberofplantspeciesfoundforeachtechnique.

and Chessel,1987)wascarriedouttoanalyzetheoverallplant pattern. AMonte-Carlo test wasprocessedtodetectsignificant differencesinfloristic compositiononstructuresusingdifferent engineeringtechniques(MetropolisandUlam,1949).Alltestswere performedusingRstatisticallanguage(R,2005)withade4package (Thioulouseetal.,1997)formultivariateanalysis.

3. Results 3.1. Vegetation

Weidentified126plantspeciesfromthe15sampledsites. Plant species richness differedalong thenaturality gradient (Kruskal–Wallistest:p-value=0.006,Table2).“Vegetal”or“Mixed” techniques having higher species plant diversity compared to “Mineral”technique(Mann–WhitneyUtestwithHolmcorrection: p-value=0.024and0.036,Table2).Therewasnosignificant dif-ferencebetweenmeanspeciesrichnesson“Vegetal”and“Mixed” techniquesalthoughdiversitytendedtobehigheronthelatter, especiallyinthemiddleandatthebottomofthebank(Fig.2).

Floristiccompositiondifferedsignificantlyalongthenaturality gradient(Monte-Carlotest:p-value=0.003).Thedistributionand spacingofthethreesphericalclusters(mn:Mineral;mx:Mixed; v:Vegetal)onthePCAfactorialmapprovidesinformationonthe variabilityoffloristiccompositions(Fig.3).However,somespecies contributemorethanotherstothesegregationofthethreesets,e.g. multipleSalicaceaespecies(SalixviminalisL.,SalixpurpureaL.,Salix triandraL.,SalixfragilisL.,SalixmyrsinifoliaSalisb.andSalix pen-tandraL.)characterized“Vegetal”technique.“Mineral”technique wascharacterizedbyspecieslikeBuddlejadavidiiFranch.,Humulus lupulusL.,RobiniapseudoacaciaL.,UrticadioicaL.and Parthenocis-susquinquefoliaL.“Mixed”techniquewasmainlycharacterizedby thepresenceofCornussanguineaL.,CalamagrostisepigeiosL.,Salix incanaL.,ViburnumopulusL.andLoniceraxylosteumL.(Fig.4).The inertiaoftheinter-classanalysisrepresents18.19% ofthetotal varianceamongthesites.Inaddition,66.16%oftheinformation explainedbythisinter-classPCAwasexplainedbyaxis1.Salicaceae speciesaswellasshrubslikeC.sanguineaandV. opulusmainly explainedaxis1componentonthepositivesideoftheaxis,andto alesserextentbyspecieslikeB.davidiiandR.pseudoacaciaonthe negativeside.Axis2expressed33.84%oftheinformation.Inthe upperpartofthisaxis,thediscriminatoryspeciesweremainly:S. viminalis,S.purpureaandS.triandra.Inthelowerpart,C.sanguinea,

Fig.3. Inter-classanalysisofvegetationonthreeriverbanksprotectiontechnique. mn:Mineral;mx:Mixed;v:Vegetal.Thedistributionandspacingbetweenthethree sphericalclustersprovidesinformationonthevariabilityoffloristiccompositions betweenthedifferenttechniques.Thesizeoftheellipseprovidesinformationabout intra-techniqueplantspeciesvariability.

C.epigeios,S.incanaandV.opuluswerethediscriminatoryspecies (Fig.4).

The area of the ellipse provided information about intra-techniquesplantspeciesvariability(Fig.3).“Mixed”and“Vegetal” techniquesshowcomparablevariabilityofplantspecies composi-tion,higherthanthatof“Mineral”technique.

Fig.4. Spatialdistributionofallspeciesencounteredonthethreeriverbanks pro-tectiontechnique.Speciescodesrefertothefirstthreelettersofthegeniusand thespeciesnames.Percentagesofvarianceobservedaregivenoneachaxis.A correspondencetableisgivenintothesupplementaryfiles.

Table2

ProbabilityvaluesoftheKruskal-WallistestandMann-Whitneytestforvegetationsurvey,flyingcoleopteraandinvasivespecies.

Statisticaltests Pairwisecomparison Vegetationsurvey Aeriancoleoptera Invasivespecies

Probabilityvalues Probabilityvalues Number Frequency

Kruskal–Wallis 0.006** _0.006** _{0.308 0.027}*

Vegetal/Mixed 0.247 0.083 0.817 1

Mann–Whitney Mixed/Mineral 0.024* _0.03* _{0.363 0.063}

Vegetal/Mineral 0.036* _0.038* _{0.812 0.045}*

*_{Indicatestatisticalsignificantdifferences(p-value<0.05).} **_{Indicatehighlysignificantdifferences(p-value<0.01).}

3.2. Fauna

Weidentified42distinctgeneraofflyingColeopterafromthe 14sampledsites(everytrapshavebeenlostforonesitefromthe “Vegetal”technique).Fig.5indicatesthemeannumbersofflying ColeopterageneracollectedinFlora®_{yellowtrapsforeachthree}

techniques.“Vegetal”supportedameanof12.2differentgenera ofcoleopterans,“Mixed”onessupported7.2and“Mineral” tech-niquessupported3.0genera.Ageniusnameslistwasincludedin

supplementaryfiles.

Riverbank techniques significantly influenced the number of coleopterangenera (Kruskal–Wallisp-value=0.006,Table 2). “Mixed”and“Vegetal”techniquesweresignificantlyricherthan “Mineral”technique(Mann–WhitneyUtestwithHolmcorrection: p-value=0.03and0.038;Table2).

WeobservedmanygeneraassociatedwithSalicaceaeincluding varioussaproxylicCetoniaspecieswithlarvaegrowingin decom-posingwood(Cetoniaaurata,Potosiacupreabourginii).Therewere alsoCerambycidaespecieslikeLamia textor,ofwhich thelarvae developinwillowstumpsandroots,Necydalismajorinthehigh cavitiesoftrees,andAromiamoschata,ofwhichthelarvaedevelop in rottingstumps and branches.Many herbivorouscoleopteran specieswereidentified,especiallyVariimordassp.(oftenseenon Apiaceae),Hopliafarinosa(whichfeedsonpollen),Oedemerassp. (the adultof which isherbivorous and thelarva xylophagous). SomewereassociatedwithSalicaceaelikeC.aurata,Chaetocnema aridulaandAlticaaenescens.Predatoryspecieswerealsoidentified, includingthecarnivorousAdrastuslimbatusandOrchesiamicans. 3.3. Exoticplantspecies

Althoughexoticplantspeciesweresometimesfoundonsites thathavebeenconstructedusingbioengineering,theywerenot amongthetendominantspeciesobservedinthestudy.In con-trast, two exotic species, B.davidii and R. pseudoacacia, belong

Fig.5.Coleopterameangenusnumbersinyellowtrapsonthethreeriverbanks pro-tectiontechnique.Differentlettersindicateastatisticaldifferencebetweensamples. Errorbarsindicatestandarderrors.

Fig.6.Exoticmeanspeciesnumbersonthethreeriverbanksprotectiontechnique. Differentlettersindicateastatisticaldifferencebetweensamples.Errorbarsindicate standarderrors.

tothetenmostabundantspecieson“Mineral”structure (100% of observed exotic species). Fallopia sp. was the main genius representedon“Mixed”structure(83%ofobservedexoticspecies) andon“Vegetal”structure(100%ofobservedexoticspecies). Over-all exoticspeciesrichness variedfrom 1.75to 2.2betweenthe techniques (Fig.6)anddidnot differsignificantly betweenthe three techniques(Kruskal–Wallis test p-value=0.308; Table 2). Invasivespeciesfrequencywassignificantlydifferentaccordingto thetechnique(Kruskal–Wallistestp-value=0.027;Fig.7),reaching amaximumonmineraldevelopments(42.5)comparedtovegetal (10)and mixed(13).Frequencyofinvasivespecies was signifi-cantly differentbetween“Mixed” and“Mineral” techniquesbut not between“Vegetal”and “Mixed”techniquesbecauseofhigh variability on“Vegetal”developments(fromf=0.34 tof=22.71; Mann–WhitneyUtestwithHolmcorrection:p-value=0.063and 0.045;Table2).Exoticinvasivespeciesweredifferentaccordingto thetechnique.

Fig.7.Exoticmeanspeciesfrequenciesonthethreeriverbanksprotection tech-nique.Differentlettersindicateastatisticaldifferencebetweensamples.Errorbars indicatestandarderrors.

4. Discussion

4.1. Vegetationdiversity

Thissurvey providesa firstassessment ofplant speciesand Coleoptera genera richness associated with three categories of riverbankprotectiontechniques.Basedonfieldobservations,we highlightsignificantdifferencesinplantdiversitybetween con-ventionalcivilengineeringandbioengineering.Inaccordancewith ourhypothesis,significantlyfewerplantspecieswerefound con-structionusingon“Mineral”structuresthanoneitherofthetwo othersprobablyduetothesubstratein“Mineral”structuresthat physicallylimitscolonizationbyplants. Incontrast,weobserve nosignificantdifferencesinplantspeciesrichnessbetween “Vege-tal”and“Mixed”techniques.Thehighernumberofplantspecies werefoundon“Mixed”structurescomparedto“Vegetal”onesmay beexplainedbyalargeproportionofwillowsfromthefascines installedatthewaterlinewhichshowanimportantgrowthrate; willowsarepartoffast-growing speciesusuallyusedriverbank bioengineering (Adamet al.,2008).Thiswillowcanopy rapidly developsadensecoveragethatcanpartiallyinhibitcolonizationof theunderstorybyherbaceousplants.Forthe“Mixed”technique, thepresenceofvegetationfreeriprapatthebottomofthebank restricts colonizationand coverageby Salicaceae species tothe upperpart.Riprapattheinterfacebetweentheaquatic environ-mentandthefirstwillowcuttingsprovideopenspaceavailablefor colonizationbyhalophyticspecies(Chenetal.,2010).Thislower partconstitutesadifferenthabitat,distinctfromthatofferedby the“Vegetal”techniquewherewoodyspecies,mostlywillows, pre-dominate.Speciestypicallyfoundonthis“Mixed”structuresare(i) typicalwetlandspecieslikeTetragonolobusmaritimus,Juncus acu-tiflorus,CarexpseudocyperusandPhragmitesaustralis,(ii)meadow specieslikeHolcusmollis,(iii)edgespeciesandspeciesofwoodland fringeslikeLigustrumvulgare,L.xylosteumandCrataegus monog-ynaand(iv)treesfromalluvialzoneslikePopulusnigraandPopulus alba.Mostofthosespecieswerenotfoundon“vegetal”structures becauseofthepredominanceofwillowsduetoalackoflightand space.Finally,periodicsedimentationfillsinthegapsbetweenthe blocksandcreatesaloam-richsubstratefavourablefor coloniza-tionbyhelophyticspecies.Ourresultsarethereforeconsistentwith otherstudieswherethepresenceofsomelitterandloamincreases both thedensity and diversity of pioneerplants (Langladeand Décamps,1996).

4.2. Faunadiversity

FlyingColeopteraweremorediverseon“Vegetal”(12.2genera onaverage)and“Mixed”(7.2generaonaverage)structureswhich havemorecomplexvegetationandgreaterfloristicdiversitythan on“Mineral”(3.0generaonaverage)structures.Ourresultsare consistentwithotherstudieswhichshowthattheabundanceof Coleopteranspeciescanberelatedtothepresenceofdifferent veg-etationstrata,fromgrassesthroughbushestotrees(Burel,1989). Ingeneral,insectpopulations––especiallyCarabidae––dependon vegetation structure and heterogeneous habitats (Verdonschot etal.,2007).Alongrivercorridors,hotspotsofbiodiversityare prin-cipallysituatedinregionswherehabitatdiversityishigh(Tews etal.,2004).Suchhabitatheterogeneityprimarilyreflectsgradients ofdisturbance,sedimentgrainsize,moistureandfertilitythatare tightlylinkedtoriverdynamicsthatrecurrentlyrenewsthe habi-tatmosaicanddrivessuccession.Inparticularsmallandmedium floodscreatemorespatialvariabilityandthereforemore hetero-geneity(Pollocketal.,1998;Helfieldetal.,2007).

Although “Mixed”structures harbour greaterbiodiversity in terms of plant species richness (30 species on average), more

generaof flying Coleopterawere foundon “Vegetal”structures (12.2generaonaverage).Thissuggeststhattheplantspecies iden-tityhasamajorimpactoninsectdiversity.Salixspeciesprobably attractsmany nectar-feedinginsectsbyproviding animportant nectarsourceavailableearlyintheyear(inJanuaryandFebruary) when other nectar sources are scarce(Newsholme, 1992).The highnumberofcoleopteraassociatedwithSalicaceaefoundinthis studyconvergeswithNewsholme’s observations.Vegetal cover and biomass werehigher on “Vegetal”structures compared to thetwootherswhatcouldexplainthehighercoleopteran diver-sityon“Vegetal”structures.Moreoverthat shorezonescontain largenumberofpredatorsandscavengersincludingcarabid bee-tlesthatfeedonwrackorcarrionthathasbeendeposedonthe shore(Kleinwächteretal.,2003).Wecanassumethat“Vegetal”and “Mixed”structuressupportmorevegetationcomparedto “Min-eral” structuresand canthus producemore wrack and induce higheranimaldrowningmortality.Ingeneral,shorezonesarean importantfeedingzoneformanyspeciesincludingcarabidspecies (StrayerandFindlay,2010).

4.3. Invasivespecies

Thisstudydidnotrevealanysignificantdifferenceinthe num-berof exotic plants between “Mineral”, “Mixed”and “Vegetal” techniques (respectively 1.75; 2.2 and 1.8). The occurrence of exotic plantspecies hasbeen often related tothe intensity of humanactivity(Pyˇseketal.,2010).Alltechniquesstudiedinvolve heavyhumaninterventionlikeearthmovingordiggingactivities. Itcan beassumedthat thesoilswereallsubject toan equiva-lentdegreeofperturbationand comparablepropagulepressure explainingwhythedifferenttechniquesharbourasimilarnumber ofexoticplantspecies.Incontrast,thisstudyshowsthatthe fre-quencyofexoticspecieswashigheron“Mineral”structuresthan ontheothertechniques.Exotic plantsspecies identifiedin this studyarecharacterizedbyhighgrowthrateswhichgavethema competitiveadvantageinpioneerhabitats(CallandNilsen,2003; Tallent-HalsellandWatt,2009).Moreovertherelativeabundance ofinvasiveplantscouldbeexplainedbybioticinteractions,notably relatedtocompetition(Levineetal.,2004).Thepresenceof com-petitorson“Mixed” and “Vegetal”structures couldrestrict the vigourandinhibitthepropagationofexoticspecieswhatisin par-ticularseenonbioengineeringstructures;whereitisverylikely thatthehighdensityofwillowrestrictedtheperformanceofexotic species. Interestingly,B. davidii wasthe only species on “Min-eral”techniqueandFallopiasp.wastheonlyinvasivespecieson “Vegetal”technique.Asmineralstructurescouldexperiencehigher temperaturesduringsunnyweatherthanvegetalizedbanks,high temperatureonmineraltechniquescouldbeherehypothesized asanenvironmentalfilterforspeciessensitiveforheatstress.B. davidiiisaMediterraneanandtropicalspecies,canbesupposed tostandhightemperatures(Tallent-HalsellandWatt,2009;Watt etal.,2010).Colonisablesubstrataavailablecanalsobean expla-nation;Fallopiasp.rhizomesneedmorespacetodevelopandend upinaviableorganism.

4.4. Perspectivesandapplications

Theapplicativepurposeofthisstudyistoprovideuseful infor-mationtodecision-makersforplanningsustainablemanagement strategiesofmountainriverbanks.Alongsmallrivers,riverbank protectionsshouldbeplantinsteadofself-recoveryofnatural veg-etationin order topromote a rapid vegetal cover and to limit exoticspeciesinvasion.Thebioengineeringtechniquesusing will-ows controls erosion, limit colonization by exotic heliophilous speciesandprovidesfoodresourcesandhabitattoentomofauna.

Salicaceae are particularly adapted to riverbank restoration techniques. The morphological (deep roots, dense coverage), mechanical(flexiblewood)andphysiological(rapidgrowth, pro-duction of large amounts of nectar and pollen) properties of thisgenus limiterosionandensurea goodreturn tonaturality (SchiechtlandStem,1996;FrossardandEvette,2009).However, suchrestorationtechniquesshouldalwaysincludealargediversity ofspeciesandgrowthforms.Thus,creatingdifferentplantstrata (grasses,bushes,andtrees)ensureshabitatdiversity.River restora-tionprogrammecouldalsoaimatincreasinghabitatdiversity.For instance,itispossibletopromotetherecruitmentofhelophytic species by creatinginterface areasbetween thewater and the bank,freeofbushesandtreesandwheresedimentcanbedeposed tofacilitatetheaccumulationofacolonisablesubstrate.“Mixed” techniquewithriprapatthebaseofthebankandbioengineering techniquesontheupperpartrepresentatrade-offbetweenvegetal coveringandhabitatdiversity.

Finally yet importantly, bioengineering structures represent agood alternativeintheFrenchEnvironmentalLaw, as admin-istrative procedures are simpler (Decree n◦_{93-743 of the law}

92-3article10).Forcivil-engineeringstructures,an environmen-talimpactassessmentisrequiredwhatbringstheadministrative delayto6–9monthstoobtainaconstructionpermit.

5. Conclusion

Theintensificationofhumandisturbancealongmountainriver lead to a dramatic decrease in riparianbiodiversity. Maintain-ingbiologicalcontinuitybetweensmallremnantrefugezonesis apriorityfortheconservationofbiodiversitythreatenedspecies (Miller,2006).Thepresentstudyshowsthecapacitiesof restora-tionecology and,more specifically,bioengineering, topromote the recovery of coleopteran and plants of riparian forests. In addition,“Mixed”and“Vegetal”embankmenttechniquesenhance connectivitybetweendifferenthabitatsandthuscontributetothe restorationofecologicalcorridors.Increasedconnectivitybetween fragmentedlandscapepatchesislikelytoincreasegeneticdiversity withinmodifiedlandscapes(Neavesetal.,2009).

Inafurtherstep,thisstudyshouldbeextendedtotheaquatic compartment.Benthic invertebratesseemstobeagood indica-torofnaturalitywiththeirdiversitycorrelatedtotheamountof organicsubstrate(woodand roots)onthebanks(Sudduth and Meyer,2006).Thequestioningcouldalsoberaisedtoafunctional analysisoftherestoredecosystems(LavorelandGarnier,2002; Hooperetal.,2005).

Acknowledgements

Thisworkwouldnothavebeenpossiblewithoutthehelpof theEco-ErosionUnitof theMountainEcosystemResearch Unit ofIrstea,GrenobleandtheUniversityofToulouse,CNRSEcoLab, Toulouse.TheFrench-SwissInterregIVProjectGeni’Alpandthe Agencedel’EauRhone-Alpesarethankedforprovidingfunding. AppendixA. Supplementarydata

Supplementary data associated with this article can be found,intheonlineversion,athttp://dx.doi.org/10.1016/j.ecoleng. 2012.12.105.

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