Biogeomorphologic succession dynamics in a Mediterranean river system Dov Corenblit, Johannes Steiger and Eric Tabacchi D. Corenblit (corenbli@cict.fr), King's College London, Dept of Geography, Strand London, WC2R 2LS, UK, and CNRS, UMR 6042, GEOLAB Á Laboratoire de ge´ographie physique et environnementale, FR-63057 Clermont-Ferrand, France. Á J. Steiger, Clermont Univ., Univ. Blaise Pascal, Maison des Sciences de l'Homme, 4 rue Ledru, FR-63057 Clermont-Ferrand Cedex 1, France, and CNRS, UMR 6042, GEOLAB Á Laboratoire de ge´ographie physique et environnementale, FR-63057 Clermont-Ferrand, France. Á E. Tabacchi, Univ. de Toulouse III, ECOLAB Á Laboratoire d'e´cologie fonctionnelle, UMR 5245 CNRS/UPS/INPT, 29 rue Jeanne Marvig, FR-31055 Toulouse Cedex 04, France.

Biogeomorphologic succession (i.e. reciprocal adjustments between vegetation and geomorphologic dynamics) of the Mediterranean River Tech, France, was analysed using aerial photographs over a period of sixty years between 1942 and 2000. A spatial analysis of the biogeomorphologic succession was undertaken considering effects of flood regime. Interactions between vegetation dynamics and flood events largely controlled the spontaneous replacement of the dense riparian forest removed in October 1940 during an exceptional high magnitude flood event with a recurrence time !100 yr. In response to this major disturbance event, the fluvial landscape demonstrated a very high resilience emphasizing the existence of a positive feedback driven by pioneer riparian vegetation. The observed feedback corresponded to landform accretion, vegetation succession and to an increase of biogeomorphologic stability under current hydrogeomorphologic and bioclimatic conditions. The evolution of the biogeomorphologic system toward stabilisation appeared to be non-linear with a threshold occurring thirty years after the exceptional destructive flood event. This threshold materialized a reinforcement of biogeomorphologic cohesive forces driven by vegetation dynamics. This study showed the control of riparian vegetation on the dynamics of Mediterranean fluvial landscapes and pointed to the need to improve our knowledge about biogeomorphologic succession cycles and threshold dynamics within different biogeomorphologic settings.

Riparian vegetation dynamics depend on hydrogeomorphologic parameters [START_REF] Stromberg | Flood flows and dynamics of Sonoran riparian forests[END_REF][START_REF] Florsheim | Bank erosion as a desirable attribute of rivers[END_REF], and in return living and dead riparian plants modulate sediment, diaspore, nutrient and water flows within fluvial corridors [START_REF] Gurnell | Effects of deposited wood on biocomplexity of river corridors[END_REF][START_REF] Opperman | Livewood'': geomorphic and ecological functions of living trees in river channels[END_REF]. Fluvial corridors are constituted of river channels, their margins and the zone of expansion of frequent floods occupied by riparian vegetation. Under intermediate and highly dynamic settings, negative and positive feedbacks between water, sediment and vegetation dynamics produce a complex and shifting mosaic of habitats, landforms and associated vegetation communities [START_REF] Ward | Riverine landscape diversity[END_REF][START_REF] Steiger | Hydrogeomorphologic processes affecting riparian habitat within alluvial channel-floodplain river systems: a review for the temperate zone[END_REF]. Biogeomorphologic positive feedbacks occur where plant growth or reproduction is controlled by hydrogeomorphologic processes; and where these hydrogeomorphologic processes are to some extent regulated by the characteristics of vegetation [START_REF] Francis | Perspectives on biogeomorphology, ecosystem engineering and self-organisation in island-braided fluvial ecosystems[END_REF]). The ability of some riparian plants to grow on deposited sediment implies that sediment deposition favours plant biomass production. In return, biomass production then increases surface roughness and sediment deposition [START_REF] Francis | Allogenic and autogenic influences upon riparian vegetation dynamics[END_REF], Corenblit et al. 2009a). An increasing body of research demonstrated over the last thirty years the control of dead or living vegetation on hydrodynamics [START_REF] Green | Modelling flow resistance in vegetated streams: review and development of new theory[END_REF], fluvial morphogenesis [START_REF] Hupp | Riparian vegetation and fluvial geomorphologic processes[END_REF] and fluvial landscape dynamics (Ward et al. 2002, Pettit and[START_REF] Naiman | Origins, patterns, and importance of heterogeneity in riparian systems[END_REF]. These studies suggested that vegetation plays key roles in determining fluvial ecosystem structure, function and channel geometry. However, only recent investigations focused explicitly on feedbacks between hydrogeomorphologic and riparian vegetation dynamics [START_REF] Bendix | Hydrological and geomorphological impacts on riparian plant communities[END_REF][START_REF] Naiman | Biophysical interactions and the structure and dynamics of riverine ecosystems: the importance of biotic feedbacks[END_REF][START_REF] Dollar | A framework for interdisciplinary understanding of rivers as ecosystems[END_REF]. In particular, the fundamental role of pioneer vegetation and dead wood in creating, maintaining and constructing fluvial landscapes in early stages of succession has been illustrated only recently [START_REF] Johnson | Woodland expansion in the Platte River, Nebraska, patterns and causes[END_REF][START_REF] Gurnell | Riparian vegetation and island formation along the gravel-bed Fiume Talgliamento, Italy. Á Earth[END_REF], 2005, Naiman et al. 2005). The resulting effects on fluvial ecosystem structure and function have been demonstrated at local scales from single plants to community patches, and pioneer riparian vegetation including herbs, shrubs and young trees have now explicitly been identified in fluvial systems as ''ecosystem engineers'' sensu [START_REF] Jones | Organisms as ecosystem engineers[END_REF] [START_REF] Edwards | A conceptual model of vegetation dynamics on gravel bars of a large Alpine river[END_REF][START_REF] Gurnell | Trees as riparian engineers: the Tagliamento river, Italy. Á Earth Surf[END_REF][START_REF] Corenblit | Reciprocal adjustments between landforms and living organisms: extended geomorphologic evolutionary insights[END_REF][START_REF] Francis | Perspectives on biogeomorphology, ecosystem engineering and self-organisation in island-braided fluvial ecosystems[END_REF].

The synergic development of fluvial landforms and vegetation communities (i.e. biogeomorphologic succession) driven by pioneer engineering riparian vegetation corresponds to a positive feedback materialized by the shift from a strictly hydrogeomorphologically driven system dominated by bare sediment to an autogenic ecologically driven system dominated by stabilized, forested, and only periodically flooded riparian islands and floodplains [START_REF] Gurnell | Riparian vegetation and island formation along the gravel-bed Fiume Talgliamento, Italy. Á Earth[END_REF][START_REF] Geerling | Succession and rejuvenation in floodplains along the river Allier (France)[END_REF][START_REF] Corenblit | Reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors: a review of complementary approaches[END_REF][START_REF] Francis | Perspectives on biogeomorphology, ecosystem engineering and self-organisation in island-braided fluvial ecosystems[END_REF]). The biogeomorphologic succession is composed of four main phases during which particular sets of biophysical processes take place (for a full description see [START_REF] Corenblit | Reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors: a review of complementary approaches[END_REF]): 1) geomorphologic, i.e. bare sediment, during alluvial bar accretion or after vegetation destruction by floods; 2) pioneer, i.e. seedling and saplings on the bare or rejuvenated alluvial bars; 3) biogeomorphologic, i.e. pioneer herbs, shrubs or trees on the alluvial bars within the exposed zones of the active channel; and 4) ecological, i.e. riparian forests on stabilized and disconnected islands and floodplains. Pioneer riparian vegetation plays its crucial engineering role (sensu [START_REF] Jones | Organisms as ecosystem engineers[END_REF]) by trapping sediment and preventing erosion mainly during the biogeomorphologic phase which is characterised by a high degree of hydrogeomorphologic connectivity.

In the current European and North American bioclimatic settings, the cycle of biogeomorphologic succession (i.e. from rejuvenated stages materialised by the domination of bare substrates to mature stages characterized by the domination of dense post-pioneer riparian forests which may in turn be rejuvenated, starting the cycle over again) alternates between periods of biogeomorphologic organisation toward mature and stabilized stages, and sudden and brief periods of succession rejuvenation which coincide with exceptional floods [START_REF] De ´camps | Historical influence of man on the riparian dynamics of a fluvial landscape[END_REF][START_REF] Hughes | Floodplain biogeomorphology[END_REF][START_REF] Gurnell | Riparian vegetation and island formation along the gravel-bed Fiume Talgliamento, Italy. Á Earth[END_REF]. The balance between resistive forces largely controlled by vegetation and destructive or regenerative forces determined by the hydrogeomorphologic disturbance regime is critical in defining the frequency and the spatial extent of biogeomorphologic regression or progression in succession [START_REF] Baptist | Assessment of the effects of cyclic floodplain rejuvenation on flood levels and biodiversity along the Rhine River[END_REF].

The biogeomorphologic succession dynamics were studied in this article at the reach scale specifically in a Mediterranean context within the fluvial corridor of the River Tech, over a period of sixty years from 1942 to 2000. Two main questions were addressed using the River Tech as a case study. 1) Within a Mediterranean context, contrasted in respect to flow regime, does pioneer riparian vegetation generate positive feedbacks leading to vegetation succession combined with floodplain construction and stabilisation? If the response is positive, over which time spans act these positive feedback mechanisms? 2) Is the biogeomorphologic succession in the Mediterranean context linear in time and homogenous in space? A biogeomorphologic succession would be considered being homogenous in space when the quality (vegetation physiognomy) and the time required in reaching post-pioneer stages are identical in each location. In order to answer these questions, the spatial and temporal dynamics of biogeomorphologic succession of the River Tech were quantified and simulated using respectively geographical information systems (GIS) and the Markov chain.

Material and methods

Study reach

The River Tech, western Pyrenees, France, is a 85 km long gravel bed river which drains a Mediterranean mountainous catchment of 750 km 2 (Fig. 1). The contrasted pluvio-nival hydrological regime reflects the Mediterranean influence, with high flows occurring from April to May and from November to January (10Á13 m 3 s (1 ), and low flows occurring between July and September (4Á5 m 3 s (1 ). High magnitude flash floods generally occur in autumn when riparian vegetation is fully developed. This river was chosen because it remains a relatively natural system compared to most French and European rivers. Furthermore, a very exceptional and destructive flood with a peak discharge of 2500 m 3 s (1 recorded in the piedmont zone occurred in October 1940. Before this destructive flood, the River Tech was occupied by dense riparian forests [START_REF] Jacob | La Basse valle ´e du Tech sous les eaux en octobre 1940[END_REF]. Aerial photographs document that erosion during the 1940 flood completely removed riparian vegetation within the entire fluvial corridor. The present study uses this context of a completely rejuvenated river system with only bare sediment within the entire fluvial corridor as its reference situation for the analysis of biogeomorphologic succession dynamics.

Hydrogeomorphologic dynamics

The flood regime, i.e. the magnitude, frequency, duration, and timing of floods, is a main control of fluvial landscape dynamics [START_REF] Poff | The natural flow regime[END_REF][START_REF] Naiman | Flow variability and the biophysical vitality of river systems[END_REF]. A hydrological time series established from peak discharges ]300 m 3 s (1 (return period 01 yr) recorded since 1940 at a gauging station 2 km upstream from the study reach, was used to characterize the flood regime of the River Tech during the five different periods determined according to the dates of the aerial photographs (1942Á1953, 1953Á 1962, 1962Á1974, 1974Á1988, and 1988Á2000). Change analyses of river planform geometry were performed in order to quantify geomorphologic changes associated with the flood disturbance regime for each of the five periods. Measurements using the GIS MapInfo TM included four variables: 1) active channel area (m 2 ). The active channel on aerial photographs corresponded to water channel plus bare sediment. It is the most exposed and unstable zone within the fluvial corridor [START_REF] Gurnell | Effects of deposited wood on biocomplexity of river corridors[END_REF]; 2) mean active channel width (m) measured with MapInfo TM every 100 m along the main channel, with a total of 52 transects; 3) lateral mobility of the main channels between two successive dates. The distance (m) measured between the main active channels at time t (1 and time t on the 52 transects was used to represent mobility; 4) sinuosity index [START_REF] Schumm | Sinuosity of alluvial rivers on the Great Plains[END_REF] or braiding index [START_REF] Brice | Index for description of channel braiding[END_REF] according to whether the river pattern was meandering or braiding. The sinuosity presents a measure of the intensity of channel meandering and the braiding index a measure of the overall braiding intensity of the river. The statistical significance of differences between dates was tested using one-way analyses of variance (ANOVA). ANOVAs were performed with repetition because spatial measurements were repeated in the same location at different dates and thus the data did not follow the criterion of independence.

Biogeomorphologic succession dynamics

The diachronic study of the biogeomorphologic succession dynamics of the River Tech was carried out using GISbased analysis (MapInfo TM ). The spatial analyses were undertaken within a 5 km long river reach selected in the piedmont zone of the River Tech (Fig. 1). They were based on six sets of black and white aerial photographs with scales ranging from 1:17 000 to 1:30 000 and covering a period of sixty years : 1942, 1953, 1962, 1974, 1988 and 2000. The choice of biogeomorphologic succession analyses at ten year periods was validated by testing shorter, three year periods, of two complementary black and white aerial photograph sets taken in between each of the ten year period. This qualitative control of landscape structural change based on stereoscopic analyses attested the continuity in changes between dates used for measurements at the step of ten years. All aerial photographs were taken during the vegetation growth period in July and August. Daily discharges during these different low water periods were equivalent between all dates (Â5 m 3 s (1 ). Aerial photographs were georeferenced with MapInfo TM . Quadratic error was estimated as B5 m. The spatial extent of the study area and its lateral limits, corresponding to the topographic break line of the Holocene terraces, were defined from the 1942 aerial photographs and during field investigations.

Four phases based on the biogeomorphologic succession classification established by [START_REF] Corenblit | Reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors: a review of complementary approaches[END_REF] were defined on aerial photographs using MapInfo TM : 1) geomorphologic/pioneer dominated by bare sediment; 2) biogeomorphologic dominated by dense herbs; 3) biogeomorphologic dominated by sparse or dense shrubs; 4) ecological dominated by dense trees. Areas dominated by human activities were also digitized. A regular grid of 10 )10 m, constituting 19 000 georeferenced squares of 100 m 2 , represented the basic framework for the spatial analysis based on the distance measured between the main active river channels at all different dates.

The biogeomorphologic succession dynamics were analyzed for each period by calculating surface cover of each of the four biogeomorphologic succession phases. Three different diversity indexes were calculated to define related landscape properties: Shannon Diversity (H?), dominance (D) (for explanations and formulas see [START_REF] Hammer | PAST: palaeontological statistics package for education and data analysis[END_REF], and fragmentation which is the number of disjointed objects of a same thematic category [START_REF] Bogaert | Landscape fragmentation assessment using a single measure[END_REF]. In order to quantify the transition dynamics between biogeomorphologic succession phases, and to estimate the absorbing or attractive power of the mature ecological phase, the transitions were calculated for each period of time between successive aerial photographs. This work was performed through transition analyses using transition matrix established from the GIS-approach. The matrix describes the dynamics of the system between two dates where each element of the matrix represents the individual transition probability calculated for one biogeomorphologic phase shifting to another biogeomorphologic phase after one time increment. Transition analysis was performed 1) at the reach scale in order to define the general tendencies of biogeomorphologic succession between 1942 and 2000; and 2) along the transverse gradient of hydrogeomorphologic disturbance from the main channels to the floodplain, in order to perform a spatial analysis of the biogeomorphologic succession. The distance (m) to the main active channels from their position at time t (1 was used in order to analyze the combined effect of the initial state within each 10)10 m grid cell and their location along the transverse disturbance gradient.

A Markov chain [START_REF] Markov | Extension of the limit theorems of probability theory to a sum of variables connected in a chain[END_REF][START_REF] Usher | Modelling ecological succession, with particular reference to Markovian models[END_REF][START_REF] Turner | Spatial simulation of landscape changes in Georgia: a comparison of 3 transition models[END_REF][START_REF] Balzter | Markov chain models for vegetation dynamics[END_REF][START_REF] Coppedge | Markov models of land cover dynamics in a southern Great Plains grassland region[END_REF]) was applied at the reach scale in order to simulate and quantify the frequency and the mean return and maintenance times of each phase of the biogeomorphologic succession and for each couple of dates. The Markov chain was parameterised by the transition probabilities between the discrete states of the biogeomorphologic succession. The probability distributions were defined using the GIS transition matrix for each couple of dates (Supplementary material Table S1).

Biogeomorphologic succession maturation (progression in succession, i.e. from phase 1 to 4) versus rejuvenation (regression in succession, i.e. from phase 4 to 1) dynamics were studied on the transverse gradient within the fluvial corridor. In order to perform this analysis four different transition classes of maturation and five transition classes of rejuvenation were defined (Fig. 2). All statistical analyses were performed with SYSTAT v. 11 and PAST v. 1.27 (Hammer et al. 2001).

Results

Hydrogeomorphologic dynamics

The flood regime of the River Tech showed highly irregular and contrasted dynamics (Table 1). The associated geomorphologic responses were also contrasted with a highly dynamic period in the 1960s and 1970s and a loss of dynamics after this period (Table 2). The simple linear regression models showed that the variables of control for geomorphologic changes were mainly 1) the flood peak discharge (n 06 for each model): channel mobility: F 0 41.07, p B0.01, R 2 00.91; active channel surface area: F 0 26.27, pB0.01, R 2 00.87; active channel mean width: F 028.90, p B0.01, R 2 00.88 and 2) the average flood peak discharges (n 06 for each linear regression models): channel mobility: F 057.77, p B0.01, R 2 00.93; active channel surface area: F 021.09, p00.01, R 2 00.84; mean active channel width, F 019.90, p 00.011, R 2 00.83. One way ANOVAs showed the statistical significance of the adjustments between the different periods for the mean Figure 2. Model showing the different possibilities of maturation and rejuvenation processes observed on the River Tech, defined here as transition classes between the four phases of biogeomorphologic succession (geomorphologic/pioneer phase; biogeomorphologic phase (dominance by herbs); biogeomorphologic phase (dominance by shrubs); ecological phase). The processes of maturation (plain arrows) present four transition classes: 1) progressive changes from one biogeomorphologic succession phase to the next (e.g. from geomorphologic/pioneer to biogeomorphologic dominated by herbs); 2) changes from one biogeomorphologic succession phase to the second next one (e.g. from the geomorphologic/pioneer phase directly to biogeomorphologic dominated by shrubs without the establishment of the biogeomorphologic phase dominated by herbs); 3) changes from the first to the last biogeomorphologic succession phase (from the geomorphologic/pioneer phase directly to the ecological phase without the establishment of the biogeomorphologic phase dominated by herbs nor the biogeomorphologic phase dominated by shrubs); 4) no changes. The processes of rejuvenation (dotted arrows) present five transition classes: 1) no changes; 2) regressive changes from one successive phase to the next (e.g. from ecological to geomorphologic/pioneer); 3) regressive changes from one successive phase to the second next one (e.g. from ecological directly to biogeomorphologic dominated by herbs without the establishment of the biogeomorphologic phase dominated by shrubs); 4) regressive changes from the last to the first biogeomorphologic succession phase, i.e. from the ecological phase directly to the geomorphologic phase without the establishment of the biogeomorphologic phase dominated by shrubs or by herbs; 5) no changes; i.e. persistence in the geomorphologic/pioneer phase.

active channel width: F 4,255 025.6, pB0.0001; and for channel mobility: F 5,306 0145, p B0.0001.

Biogeomorphologic maturation versus rejuvenation along the transverse gradient of disturbance

Spatial analysis of biogeomorphologic maturation versus rejuvenation along the transverse gradient of disturbance revealed a tendency to maturation toward biogeomorphologic stability materialised by the dominance of the ecological phase (phase 4 of the biogeomorphic succession) and the decrease in landscape diversity (Table 3). Biogeomorphologic maturation corresponded to the progressive colonization of the floodplain by dense riparian forests which started initially between 1942 and 1953 within the immediate margin of the active channels.

The zones adjacent to the initial (1942) position of the active channels evolved between the first ten year period between 1942 and 1953 without floods (Table 1) mainly toward the ecological phase, whilst the more distant zones developed toward the biogeomorphologic phase with sparse pioneer herbaceous and shrubby covers (Fig. 3 and4; Table 3). Figure 5 illustrates the clear establishment of these two contrasted succession patterns.

The biogeomorphologic succession was completely rejuvenated according to the resumption of floods between 1953 and 1974 mainly in the first 30 m of the active channels (Fig. 3 and4; Table 3). Conjointly, a large proportion (around 50%) of the sparse herbaceous and shrubby communities observed in 1953 beyond the first 30 m from the active channels were not removed by water flow and evolved toward the ecological phase with dense riparian forests. The transition analysis (Supplementary material Table S1) clearly revealed that the probability of rejuvenation decreased systematically as a function of the initial maturation phase of the biogeomorphologic succession (i.e. low probability of rejuvenation from previous biogeomorphologic and ecological phases) and the distance to main active channels (Fig. 6).

After a period of thirty years a so far unequivocal maturation in succession from the outer limits of the fluvial corridor to the margins of the active channels occurred. The maturation transition class number 4 (see Fig. 2 for explanation) described in Fig. 3 and4 occurred systematically beyond 170 m from the active channels after 1974. The occurrence of the maturation transition class number 3 implicating a direct evolution from bare substrate (geomorphologic/pioneer phase) to dense riparian trees (ecological phase) (Fig. 2) was particularly stimulated according to the proximity of active channels (Fig. 3 and4). Complete rejuvenation, i.e. the creation or maintenance of bare sediment, tended to concentrate during the last twenty years only within the first ten meters at the proximity of active channels (Fig. 3 and4) despite the occurrence of several intermediate to high magnitude floods (Table 1).

Markov chain

The Markovian analysis with an iteration step of ten years (1000 iterations) based on the previous calculated transition matrix for each couple of dates showed the low frequency of occurrence of the biogeomorphologic phase for each period between 1942 and 2000 (Fig. 7; Table 4). The biogeomorphologic phase appeared to be unstable and transitory. Conversely, the frequency of occurrence of the ecological phase was high. From a given date to the next, the biogeomorphologic phase showed systematically a tendency to progress toward the ecological phase (Fig. 6). The Markovian simulation, based on the transition matrix calculated for the period without floods (1942Á1953), revealed a clear tendency toward stabilization of the landscape structure; the biogeomorphologic succession was achieved over a thirty year period simulated at the scale of the corridor with 100% of domination by the ecological phase (Fig. 7; Table 4). This Markov chain can be 1953Á1962, 1962Á1974, 1974Á1988, and 1988Á 2000) oscillated mainly between the geomorphologic/ pioneer phase and the ecological phase with a rapid transition of less than ten years through the biogeomorphologic phase (Fig. 7; Table 4). The Markovian analysis also demonstrated the general tendency of the fluvial corridor 1) to a progressive decrease of the occurrence frequency of the geomorphologic/pioneer phase and 2) to a net increase in frequency of the ecological phase which appeared to be more and more absorbent in time at the scale of the corridor (Table 4). Considering the biogeomorphologic succession, the absorbance efficiency of the ecological phase reached its maximum at the scale of the fluvial corridor after thirty years. The colonization by dense riparian forests of the areas distant from the active channels ultimately stimulated vegetation growth and succession close to the active channels. In accordance with these landscape dynamics, the transitory biogeomorphologic phase at a short timescale (ten year resolution) was difficult to observe by the simulations.

Discussion

Feedback dynamics

This study illustrated the role of riparian vegetation dynamics in structuring fluvial landscapes in space and time at the reach scale. In the Mediterranean context of the River Tech the interactions between riparian vegetation and hydrogeomorphologic disturbances clearly generated a positive feedback that rippled through the entire fluvial corridor in thirty years following the flood of October 1940 which entirely destructed and cleared the riparian forest. This time period corresponds to the relaxation time of the River Tech fluvial landscape structure. Such resilience dynamics of construction and stabilization of islands and floodplains with riparian forests within a few decades were observed in the temperate context on the River Tagliamento, Italy [START_REF] Edwards | A conceptual model of vegetation dynamics on gravel bars of a large Alpine river[END_REF][START_REF] Gurnell | Riparian vegetation and island formation along the gravel-bed Fiume Talgliamento, Italy. Á Earth[END_REF][START_REF] Gurnell | Trees as riparian engineers: the Tagliamento river, Italy. Á Earth Surf[END_REF], on many other French rivers [START_REF] Pautou | Space-time units in floodplains: the example of the Drac River upstream of Grenoble (French Alps)[END_REF], Pie ´gay and Salvador 1997, Lie ´bault and Pie ´gay 2002), and in North America [START_REF] Miller | Changes in the landscape structure of a southeastern Wyoming Riparian zone following shifts in stream dynamics[END_REF][START_REF] Friedman | Channel narrowing and vegetation development following a great plains flood[END_REF]. Similar positive feedbacks between hydrogeomorphologic processes and vegetation dynamics were also described in tropical rivers where riparian vegetation contributes to concentrate matter and energy flows within active channels constrained by dense riparian forests [START_REF] Junk | A general review of tropical South American foodplains[END_REF]Furch 1993, Wende and[START_REF] Wende | Anabranching rivers: ridgeform alluvial channels in tropical northern Australia[END_REF]. It is suggested that these positive feedback dynamics occur in temperate, and also in tropical contexts mainly because 1) vegetation growth and resilience are regularly stimulated by the climatic conditions within these regions; and because 2) flood regimes encompass high frequency and low to intermediate magnitude events which contribute to support the continuity of the process of landform construction by delivering fine sediment between destructive floods [START_REF] Steiger | Sediment deposition along the channel margins of a reach of the middle River Severn, UK[END_REF][START_REF] Corenblit | Reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors: a review of complementary approaches[END_REF]). However, the spatial pattern of stabilizing effects of vegetation, its lateral and longitudinal extension within the fluvial corridor and its duration, may vary according to contrasting bioclimatic, hydrogeomorphologic and anthropogenic contexts. The question if the results obtained on the River Tech could be transposed to other rivers remains at least partially unanswered, even though the biogeomorphologic succession may evolve in different settings according to a similar pattern or may be truncated. [START_REF] Wolman | Relative scales of time and effectiveness of climate 111 in watershed geomorphology[END_REF] suggested that within arid climatic river systems large floods can produce irreversible effects in the landscape structure because vegetation development is limited. [START_REF] Osterkamp | Changes accompanying an extraordinary flood on a sandbed stream[END_REF] demonstrated the aptitude of semi-arid rivers to rejuvenate regularly the biogeomorphologic succession at large scales within the fluvial corridors because vegetation concentrates only near the main channel. According to this landscape structure, [START_REF] Burkham | Channel changes of the Gila River in Safford valley, Arizona 1846Á1970[END_REF] and [START_REF] Schumm | Channel widening and floodplain construction along Cimarron River in southwestern Kansas[END_REF] observed respectively on the semi-arid River Gila, Arizona, USA and on the River Cimarron, New Mexico, USA, repetitive alternations between periods of concentration and important widening of the active channel. An equivalent landscape structure led on the River Tech to significant rejuvenations in the areas covered only by sparse vegetation during the 1960s and 1970s.

On the River Gila, Arizona, USA, the impacts of an exceptional flood still persisted in semi-arid landscapes after a period of fifty years [START_REF] Burkham | Channel changes of the Gila River in Safford valley, Arizona 1846Á1970[END_REF][START_REF] Tooth | Process, form and change in dryland rivers, a review of recent research[END_REF]. This slow evolution is in contrast with the River Tech for which a relaxation period following the destructive 1940 flood was estimated to be about thirty years. A general tendency to the decrease of the number of active channels was also observed within human impacted, regulated and channelized rivers [START_REF] Eschner | Hydrologic and morphologic changes in channels of the Platte River Basin in Colorado, Wyoming, and Nebraska: a historical perspective[END_REF][START_REF] Bravard | La me ´tamorphose des rivie `res des Alpes franc ¸aises a `la fin du Moyen A ˆge et a `l'e ´poque moderne[END_REF][START_REF] Johnson | Woodland expansion in the Platte River, Nebraska, patterns and causes[END_REF][START_REF] Marston | Channel metamorphosis, floodplain disturbance, and vegetation development: Ain River, France[END_REF][START_REF] Bravard | River incision in south-east France, morphological phenomena and ecological effects[END_REF][START_REF] Shafroth | Riparian vegetation response to altered disturbance and stress regimes[END_REF]. These dynamics were generally accompanied by the progression of riparian forests which contributed to reinforce the tendency toward channel incision and lateral biogeomorphologic stabilization. However, irreversible riparian forest dieback often occurred after few decades on regulated and channelized rivers due to important vertical channel incisions and to the absence of rejuvenation by lateral migration in the Figure 3. Probability of occurrence of the transition classes of maturation and rejuvenation at time t'1 (y axes) as a function of the distance to the active channels at time t (x axes). Blue colours correspond to transition classes of maturation and green colours correspond to transition classes of rejuvenation (transition classes are defined in Fig. 2).

interval between very exceptional floods [START_REF] Steiger | Channelization and consequences on floodplain system functioning on the Garonne River, SW France[END_REF].

The case of the River Tech which remains a relatively dynamic system in comparison to many other European rivers demonstrated that in the current Mediterranean bioclimatic context natural pioneer vegetation dynamics can be sufficient to produce a positive feedback leading to the expansion of riparian forests in the entire fluvial corridor and to floodplain construction. Thus, it is reasonable to argue that even moderate anthropogenic attenuation of flow variability and sediment transit strongly enhances this evolutionary trajectory in current Mediterranean bioclimatic conditions.

The mechanisms leading to the construction of islands and floodplains and the correlative expansion of postpioneer riparian forests in thirty years can be explained on the River Tech according to local interactions between pioneer vegetation and hydrogeomorphology. The very good aptitudes of pioneer ligneous vegetation to regenerate quickly or to resist destruction by floods and to stabilize landforms [START_REF] Francis | Perspectives on biogeomorphology, ecosystem engineering and self-organisation in island-braided fluvial ecosystems[END_REF] were attested by the empirical transition analysis (Supplementary material Table S1) and the Markov chain simulations (Fig. 7; Table 4). The biogeomorphologic phase, in particular when dominated by pioneer shrubs which intercept frequently water, sediment and diaspores [START_REF] Gurnell | Trees as riparian engineers: the Tagliamento river, Italy. Á Earth Surf[END_REF] was transitory and evolved rapidly and mainly toward the ecological phase (Fig. 3, 4 and6; Supplementary material Table S1). The aptitude of pioneer ligneous vegetation to resist mechanical destruction during floods, to regenerate and to establish efficiently in disturbed areas was demonstrated by field studies for example on the Tagliamento River, Italy [START_REF] Francis | Survival and growth responses of Populus nigra, Salix elaeagnos and Alnus incana cuttings to varying levels of hydric stress[END_REF]. Populus nigra and Salix spp. in particular have a very good fitness especially in response to intermediate hydrogeomorphologic disturbances (Karrenberg et al. 2002, Corenblit et 2009a). Their life-history includes traits such as efficient dispersal, rapid germination, rapid root and aerial structure growth rates to withstand flooding and drought. Their ability to reproduce by vegetative processes was experimentally demonstrated on the Mediterranean River Dro ˆme, France [START_REF] Barsoum | Relative contributions of sexual and asexual regeneration strategies in Populus nigra and Salix alba during the first years of establishment on a braided gravel bed river[END_REF]) and on the River Tagliamento (Gurnell et al. 2005, Francis andGurnell 2006). These different aptitudes control and largely increase the potential of stabilized and disconnected riparian forested islands and floodplains [START_REF] Gurnell | Riparian vegetation and island formation along the gravel-bed Fiume Talgliamento, Italy. Á Earth[END_REF].

The implications of pioneer ligneous vegetation, in particular Populus nigra, Salix spp. and large woody debris able to resprout, in forming growing sediment accretion points are fundamental in temperate contexts in generating biogeomorphologic succession gradients [START_REF] Pautou | Space-time units in floodplains: the example of the Drac River upstream of Grenoble (French Alps)[END_REF][START_REF] Gurnell | Effects of deposited wood on biocomplexity of river corridors[END_REF][START_REF] Francis | Perspectives on biogeomorphology, ecosystem engineering and self-organisation in island-braided fluvial ecosystems[END_REF]. Along the transverse gradient of hydrogeomorphologic disturbance, from the main active channels to the alluvial plain, pioneer meso-hydrophilous herbaceous vegetation and Salix spp. and Populus nigra developing in the exposed zones on alluvial bars demonstrated very strong resilience and physical resistance to mean-annual floods (T 2Á3 yr) during a complementary field study undertaken by Corenblit et al. (2009b) on the River Tech between 2002 and 2004. Discharges corresponding to the mean annual flood are assumed to be dominant for bedload transport in river channels, representing the theoretical optimum level for geomorphologic adjustment within channels and their immediate margins [START_REF] Wolman | Magnitude and frequency of forces in geomorphic processes[END_REF]). Despite the erosion potential of the floods recorded between 2002 and 2004, pioneer vegetation stabilised the substrate, trapped large amounts of fine sediment and diaspores and progressed in succession near the active channels in a period of only three years (Corenblit et al. 2009b). The present study demonstrated in complement to the field observations that in such a biogeomorphologic context, and if no major destructive flood event occurs, fluvial landscape structure tends to be predisposed to evolve toward the domination by forested islands and floodplains at the scale of the fluvial corridor over thirty years. The main point to be made here is that such geomorphologic predisposition to riparian forest and floodplain resilience has a biological origin linked to specific adaptations of pioneer riparian vegetation life-history traits, including morphological and biomechanical traits, in response to river disturbance regimes [START_REF] Lytle | Adaptation to natural flow regimes[END_REF][START_REF] Bornette | A model of plant strategies in fluvial hydrosystems[END_REF], Corenblit et al. 2009a).

Equilibrium conditions of the biogeomorphologic succession

The analysis of biogeomorphologic succession rejuvenation versus maturation dynamics suggested that the evolution of dynamic equilibrium (i.e. landscape structure displaying relatively stable characteristics with minor feedback mechanisms taking place, and to which it will return after disturbance) within the fluvial corridor is non-linear in time and non-homogenous in space at the scale of the reach scale as shown in Fig. 3 and Supplementary material Table S1. This observation can be associated mainly to 1) contrasting rates and patterns of vegetation establishment on the immediate margins and at long distances from active channels, and to 2) the consecutive reinforcement of the biogeomorphologic stability at the scale of the entire fluvial corridor over a thirty year period. The reinforcement of biogeomorphologic stability occurred when a threshold was reached. This threshold corresponded to the reinforcement of biogeomorphologic resistive forces after an initial state during which geomorphological remobilisation forces dominated. The occurrence of this threshold in time was largely controlled by vegetation dynamics, occurring when riparian forests became denser on the floodplain while pioneer vegetation establishment continued to be stimulated at the proximity of water channels on alluvial bars. Thus, a dynamic equilibrium for landscape structure resulting concurrently from maturation and rejuvenation processes was reached on the River Tech after thirty years following the catastrophic flood of October 1940. The spatial extension of rejuvenation and maturation reached respectively a total width of 30 and 200 m (i.e. 13% of the fluvial corridor dominated by rejuvenation and 87% by maturation processes) along the transverse gradient from the main channel to the floodplain. Comparable biogeomorphologic dynamics implicating Populus nigra and Salix spp. were also described on the Platte River, Nebraska, USA [START_REF] Johnson | Woodland expansion in the Platte River, Nebraska, patterns and causes[END_REF]). These similar fluvial landscape trajectories on the River Tech and the Platte River attest an origin linked to specific effect and response traits of these engineering species (sensu [START_REF] Jones | Organisms as ecosystem engineers[END_REF]).

The transition analyses and Markov chain also suggested an increase in space and in time of the domination of biogeomorphologic resistive or cohesive forces over thirty years following a catastrophic flood in the Mediterranean context. In particular, as pointed out by Pie ´gay (1997) on south-eastern French piedmont rivers, such increase of the overall biogeomorphologic stability over time can be attributed to the synergy between succession stimulation within the disturbed zone on well hydrologically connected alluvial bars and slow but progressive biogeomorphologic maturation on less exposed areas. The biogeomorphologic succession dynamics identified on the River Tech suggest the important implications of the dichotomy observed within the immediate margins of active channels with a rapid and intensive stimulation of pioneer vegetation establishment and a slow but constant development of riparian forests on floodplain areas less exposed to flood events. The last high magnitude floods (T 20Á30 yr) observed on the River Tech since the 1980s did not produce significant rejuvenations of the biogeomorphologic succession which seems to have reached an attractor state corresponding to the domination by stabilized riparian forests. In a study undertaken in forested mountain river systems of the Pacific Northwest, USA, [START_REF] Beechie | Channel pattern and River floodplain dynamics in forested mountain river systems[END_REF] identified a threshold for biogeomorphologic rejuvenation associated to migration of river channels occurring preferentially at a bankfull width of 15Á20 m. Beechie and colleagues argued that larger channels are deep enough to erode below the rooting zone of banks densely colonised by vegetation. However, biogeomorphologic rejuvenation associated to lateral erosion and migration of the multiple channels with widths B20 m appears to be inhibited on the River Tech. Our results clearly suggest that the critical discharge needed to rejuvenate the biogeomorphologic succession at the corridor scale of the River Tech has to be larger than the one observed since 1988 with a recurrence interval T 20Á30 yr. It seems likely that only exceptional events with a return period ]100 yr, such as the 1940 flood, will be able to completely rejuvenate the biogeomorphologic succession on the River Tech.

Concluding remarks

The River Tech, a relatively natural Mediterranean fluvial system, was shaped by intermediate floods during the last sixty years which contributed to biogeomorphologic succession maturation. This river system demonstrated a high resilience over thirty years confirming, in the context of an intermediate disturbance regime in a Mediterranean setting, the hypothesis that such river systems tend to evolve mainly toward the ecological phase. These dynamics were initiated and reinforced by positive feedbacks of landform accretion and associated riparian vegetation succession. However, the evolution of the system from the geomorphologic toward the ecological phase and biogeomorphologic stability appeared to be non-linear with a threshold occurring thirty years after a major destructive flood. This threshold materialized a reinforcement of the biogeomorphologic cohesion at the scale of the corridor.

The associated effects of biogeomorphologic succession rejuvenation by floods in increasing biocomplexity and biodiversity are recognized [START_REF] Baptist | Assessment of the effects of cyclic floodplain rejuvenation on flood levels and biodiversity along the Rhine River[END_REF][START_REF] Gurnell | Effects of deposited wood on biocomplexity of river corridors[END_REF][START_REF] Pettit | Flood-deposited wood debris and its contribution to heterogeneity and regeneration in a semi-arid riparian landscape[END_REF] but are still difficult to evaluate. These effects on increasing landscape diversity were observed in this study on the River Tech during a period of thirty years following the flood of October 1940. Thus, restoring durably rejuvenation capacities for impacted rivers within the current anthropogenic and temperate bioclimatic settings appears today as a major but very complicated challenge. According to the results presented in this article, we suggest that progress in river restoration capacities will depend on our ability to achieve a better understanding of the reciprocal coupling between vegetation and hydrogeomorphologic dynamics and further quantifications of fluvial threshold dynamics in explicit relation with riparian vegetation effects and responses to hydrogeomorphologic disturbances in diverse bioclimatic and anthropogenic settings.

Figure 1 .

 1 Figure 1. Study reach location. Upstream limit: 42829?N, 2844?E, altitude 103 m a.s.l.; downstream limit: 42830?N, 2847?E, altitude 90 m a.s.l. The River Tech catchment (b) is located in the SE of France (a). The active channel and its floodplain are represented in (c). The flow direction is from W (source) to E (Mediterranean Sea).

  al.

Figure 4 .

 4 Figure 4. Maps of the studied river reach showing transition classes of maturation and rejuvenation including also areas with human activities and areas that shifted from human activities to open water or bare sediment. Map 1953 represents transitions between 1942 and 1953; map 1962 represents transitions between 1953 and 1962, and so far until map of 2000 which represents transitions between 1988 and 2000.

Figure 5 .

 5 Figure 5. Aerial photographs illustrating the stimulation of vegetation succession along the active channels within the study reach between 1942 and 1953. This propensity was observed on aerial photographs for the entire River Tech corridor.

Figure 6 .

 6 Figure 6. Probability of occurrence of the four different phases of the biogeomorphologic succession at time t'1 (e.g. 1953 corresponds to probabilities of occurrence between 1942 and 1953; 1962 corresponds to probabilities of occurrence between 1953 and 1962) (y axes) as a function of the distance to the active channels at time t (x axes).

Figure 7 .

 7 Figure 7. Markov chain analysis of the absorbent states over 1000 iterations corresponding to five approximately ten years periods between 1942 and 2000. Transition probabilities were calculated starting from the geomorphologic/pioneer phase.

Table 1 .

 1 Characterization of the flood regime for each time period investigated. Standard deviation is indicated for the average of flood peak discharges. The flood regime was highly contrasted between 1940 and 2000: a single catastrophic flood between 1940 and 1942; no floods between 1942 and 1953; a resumption of several intermediate to high magnitude floods between 1953 and 1962; a succession of high magnitude floods between 1962 and 1974; and high magnitude floods remaining frequent between two periods,1974Á1988 and 1988Á 2000.

	Period	Average of flood peak discharges (m 3 s (1 )	Flood peak discharge (m 3 s (1 )	Number of floods !300 m 3 s (1
	1940Á1942	250090	2500	1
	1942Á1953	50090	500	1
	1953Á1962	6189266	1000	5
	1962Á1974	7279331	1400	14
	1974Á1988	7249280	1130	7
	1988Á2000	7619299	1200	7

Table 2 .

 2 Evolution of the planform geometry of the River Tech determined from the quantification of different geomorphologic variables. Standard deviations are indicated for the channel mobility and the active channel mean width.

	Geomorphologic variable	1940Á1942	1942Á1953	1953Á1962	1962Á1974	1974Á1988	1988Á2000
	Channel mobility (m) Active channel surface area (m 2 )	25590 1 007 510	7 910 73 761	27939 412 029	90979 448 399	35942 235 791	21926 153 045
	Active channel mean width (m)	225983	1597	8 7 935	107956	51926	34918
	Braiding index (Brice 1960)	2.78	1.51	2.40	2.43	1.97	1.45
	Sinuosity index (Schumm 1963)	1.36	1.33	1.25	1.24	1.22	1.23

Table 3 .

 3 Biogeomorphologic succession dynamics and resulting fluvial landscape diversity. GEO/PION 0geomorphologic/pioneer phase;

	BIOGEO_HERB 0biogeomorphologic phase with herbs; BIOGEO_SHRU0biogeomorphologic phase with shrubs; ECOL0ecological
	phase; HA0human activity.
		1942	1953	1962	1974	1988	2000
	Total surface (m 2 )
	GEO/PION	1 007 510	73 761	412 029	447 711	235 790	153 045
	BIOGEO_HERB	86 771	285 000	571 106	38 378	169 46	15 015
	BIOGEO_SHRU	168 483	235 039	193 038	183 036	137 241	199 684
	ECOL	0.00	468 073	415 569	396 377	376 273	483 212
	HA	0.00	118 215	114 190	182 543	491 253	375 009
	Relative cover (%) with HA
	GEO/PION	79.79	6.25	34.57	35.87	18.75	12.48
	BIOGEO_HERB	6.87	24.15	4.79	3.08	1.35	1.22
	BIOGEO_SHRU	13.34	19.92	16.19	14.67	10.91	16.29
	ECOL	0.00	39.66	34.87	31.76	29.92	39.41
	HA	0.00	10.02	9.58	14.63	39.07	30.59
	Relative cover (%) without HA
	GEO/PION	79.79	6.95	38.23	42.02	30.77	17.99
	BIOGEO_HERB	6.87	26.84	5.30	3.60	2.21	1.76
	BIOGEO_SHRU	13.34	22.14	17.91	17.17	17.91	23.47
	ECOL	0.00	44.08	38.56	37.20	49.11	56.78
	Landscape diversity with HA
	Shannon (H?)	0.56	1.79	1.92	2.01	2.20	2.05
	Dominance (D)	0.70	0.23	0.22	0.19	0.15	0.21
	Landscape diversity without HA
	Shannon (H?)	0.56	1.05	1.35	1.33	1.24	1.15
	Dominance (D)	0.70	0.48	0.33	0.33	0.41	0.47
	Number of patches
	GEO/PION	119	14	151	142	142	83
	BIOGEO_HERB	127	178	86	70	33	22
	BIOGEO_SHRU	344	438	473	503	367	273
	ECOL	0.00	54	78	63	65	77
	HA	0.00	72	62	93	182	149
	Total	590	756	850	871	789

Table 4 .

 4 Markov Chain analyses of transitions between phases of the biogeomorphologic succession. The simulations were undertaken over 1000 iterations. The mean return interval corresponds to the number of iterations before the return to the initial biogeomorphologic phase; the mean stability corresponds to the number of consecutive occurrences of a specific phase of the biogeomorphologic succession. Standard deviations are indicated for the mean return interval and the mean stability.

	Phase	1942Á1953	1953Á1962	1962Á1974	1974Á1988	1988Á2000
	Relative frequency
	GEO/PION	0.00	0.48	0.45	0.23	0.11
	BIOGEO_HERB	0.00	0.04	0.04	0.03	0.00
	BIOGEO_SHRU	0.00	0.05	0.13	0.13	0.16
	ECOL	0.99	0.43	0.38	0.61	0.73
	Mean return interval
	GEO/PION	0.0090.00	2.4791.87	2.9492.31	6.6397.75	6.4195.53
	BIOGEO_HERB	0.0090.00	20.20914.20	22.10922.70	43.7932.10	1209187
	BIOGEO_SHRU	0.0090.00	19.50919.20	6.7295.08	5.6095.46	5.6098.49
	ECOL	1.0090.00	3.9393.71	4.2693.51	3.0792.61	2.691.54
	Mean stability
	GEO/PION	0.0090.00	3.1192.14	2.3192.10	2.0891.62	1.5390.66
	BIOGEO_HERB	0.0090.00	1.0090.00	1.0090.00	1.0090.00	1.0090.00
	BIOGEO_SHRU	0.0090.00	1.0090.00	1.0990.30	1.1390.35	1.2890.46
	ECOL	Absorbing state	2.7891.92	2.4092.09	4.0093.08	4.3595.07

Acknowledgements Á The authors thank the Direction de l'Environnement (DIREN) of Languedoc-Roussillon, the Agence de l'Eau Rho ˆne, Me ´diterrane ´e, Corse, and the Syndicat Intercommunal Tech Inter-Communal Syndicate (SIVU-Tech), France, for their financial and technical support. The field work and analyses for this paper were carried out while D. Corenblit prepared his doctoral thesis at the Univ. Toulouse III and ECOLAB Á Laboratory of Functional Ecology, UMR 5245 CNRS/UPS/INPT. We are grateful to the three anonymous reviewers for their comments and suggestions which helped to improve the manuscript.


Paul Cavaillé
email: paul.cavaille@irstea.fr

Fanny Dommanget

Nathan Daumergue

Gregory Loucougaray

Thomas Spiegelberger

Eric Tabacchi

André Evette

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

Keywords:

Erosion control of riverbank is frequently necessary to protect human investments situated along rivers. The technique chosen for such erosion control construction may have major impacts on biodiversity and on the functioning of river corridors. Even if there is agreement, that biodiversity should be one criterion for choosing embankments techniques little is known about whether such techniques can accommodate biodiversity.

We aimed to determine coleopteran and plant taxonomic diversities along a naturality gradient of riverbank protection systems, ranking from entirely civil engineering structures, through combined constructions (mixing civil engineering and bioengineering), to purely bioengineering structures.

Fifteen sites (five sites of each technique) were sampled in the Rhône-Alpes region (S.E. France). On each site, vegetation was sampled along three transects from the bottom to the top of the riverbank and flying beetles by trapping.

In total, we recorded 148 plant species and 78 beetle genera. We found significantly lower animal and plant diversities within civil engineering constructions than in the other two techniques. Diversities of both techniques tended to be higher, although not significantly, in combined techniques than in purely bioengineering ones. Furthermore, civil engineering structures were more subject to invasion by exotic plant species than the two other techniques. These results quantify and highlight the interest of bioengineering techniques compared to civil engineering in enhancing biodiversity and limiting invasive species techniques.

Introduction

In addition to its high level of biodiversity, riparian corridors supply multiple ecosystem services such as nutrient control, flood control, shading effect and social aspects. Riparian corridors have therefore been widely recognized as key component in landscapes [START_REF] Décamps | River networks as biodiversity hotlines[END_REF]. There is a strong causal linkage between biodiversity and ecosystem functioning [START_REF] Maestre | Plant species richness and ecosystem multifunctionality in global drylands[END_REF]. Biodiversity loss may induce a modification of natural ecosystem functioning and ecosystem services [START_REF] Mcneely | Biodiversity, Ecosystem Functioning, & Human Wellbeing: An Ecological and Economic Perspective[END_REF].

However, Western European riparian corridors and their vegetation have been widely affected by centuries of human use. River corrections and associated flood control have been primarily perceived has a great human achievement in environmental control. However, shortly after initial success such artificial landscapes spotlight a major conflict between riparian ecosystem conservation and human patrimony protection [START_REF] Pahl-Wostl | The importance of social learning in restoring the multifunctionality of rivers and floodplains[END_REF], as such river straightening has lead to a drastic loss of biodiversity in riparian systems. In urbanized areas as well as in agricultural landscapes, space available for riparian vegetation is often reduced to the riverbank only. Thus, organisms can hardly circulate along there remnants of the past riparian corridors especially when civil engineering erosion control structures are used [START_REF] Nilsson | Fragmentation and flow regulation of the world's large river systems[END_REF]. Such interruptions in the corridor mosaic may lower biological continuity by reducing circulation, refuge and feeding opportunities, and by increasing thermal contrasts during summer. Altered landscapes and disrupted corridor result in a global riparian ecosystem loss [START_REF] Poff | The natural flow regime: a new paradigm for riverine conservation and restoration[END_REF][START_REF] Tockner | Riverine flood plains: present state and future trends[END_REF]. Consequently, biodiversity conservation should be a strategy for practitioners to enhance ecosystem functioning [START_REF] Isbell | High plant diversity is needed to maintain ecosystem services[END_REF].

In addition, direct exposure to flood and high flood frequency lead to high propagule fluxes on bare soils and promote colonization by exotic species and subsequently invasion [START_REF] Décamps | Changes in the hydrological regime and invasions by plant species along riparian systems of the Adour River, France[END_REF]. Therefore, exotic species richness is significantly greater in riparian zones compared to upland sites [START_REF] Stohlgren | Riparian zones as havens for exotic plant species in the central grasslands[END_REF].

Habitat restoration has been clearly identified as a key element to reverse negative anthropic impacts on biodiversity and ecosystems functioning [START_REF] Helfield | Effects of river restoration on riparian biodiversity in secondary channels of the Pite River, Sweden[END_REF]. Public policy such as the European Water Framework Directive (WFD; 2000/60/EC) encourages a global "good ecological status" of aquatic environment including morphological conditions such as structure of the riparian zone. A study ordered in 2005 by the French Ministry of Ecology and Sustainable Development highlights that 51% of the surface water mass would not reach the objective of "good ecological status" by 2015 [START_REF] Ifen | L'environnement en France -Edition[END_REF]. Improving biodiversity on artificial riverbanks may act in favour of this objective and in particular if alternatives to civil engineering are used such as bioengineering techniques [START_REF] Li | Biotechnical engineering as an alternative to traditional engineering methods -a biotechnical streambank stabilization design approach[END_REF].

Bioengineering techniques for erosion control consists in the uses of living plants in order to mimic natural vegetation able to resist intense scouring [START_REF] Gray | Biotechnical and Soil Bioengineering Slope Stabilization -A Practical Guide for Erosion Control[END_REF][START_REF] Norris | Species Selection for Soil Reinforcement and Protection[END_REF][START_REF] Evette | Les végétaux ont du génie[END_REF]. Bioengineering applied to riverbanks aims at combining both artificial and natural components to provide protection at the entire bank level [START_REF] Evette | Using ecological and biogeographical features to produce a typology of the plant species used in bioengineering for riverbank protection in europe[END_REF]. In addition to their erosion control capacity, bioengineering structures are supposed promoting the recovery of indigenous species and to ensure better plant coverage than artificially reconstructed embankments using civil engineering techniques [START_REF] Pezeshki | Factors governing survival of black willow (Salix nigra) cuttings in a streambank restoration project[END_REF]. One of the arguments given for choosing bioengineering structures instead of civil engineering ones is that the first promote the re-establishment of a functional and self-sustainable ecosystem; however, to our knowledge, no quantitative scientific evidences have been produced to support this idea. Until now socio-economical issues are the main reasons to whether protect or not. River and land managers' final decision on which riverbank protection technique will be used is mainly based on several crucial points such as hydro-engineering aspects, bed material and watercourse bank stability [START_REF] Schiechtl | Water Bioengineering Techniques for Watercourse Bank and Shoreline Protection[END_REF]. However, no quantitative ecological aspect is for the moment taken into account in the decision line.

To date, many scientific studies on bioengineering structures along rivers focus on methodological aspects such as structures, construction methods, mechanical resistance and the choice of species (Breton et al., under review), compared to the few works which are dedicated to biodiversity subsequently related to erosion control structures on watercourse banks. Since 2002 and the early work on the environmental and ecological advantages of bioengineering structures [START_REF] Li | Biotechnical engineering as an alternative to traditional engineering methods -a biotechnical streambank stabilization design approach[END_REF], few studies have used quantitative methods to investigate relation between biodiversity and erosion control structures. Some works are dealing with plant species and habitat diversity assessment but remain study cases on very few sites [START_REF] Li | Soil bioengineering and the ecological restoration of riverbanks at the Airport Town, Shanghai, China[END_REF][START_REF] Sudduth | Effects of bioengineered streambank stabilization on bank habitat and macroinvertebrates in urban streams[END_REF]. On the other hand, studies have been done on riverbank restoration by removing bank fixation or protection and comparative biological survey on restored and non-restored sites were made [START_REF] Helfield | Restoration of rivers used for timber floating: effects on riparian plant diversity[END_REF][START_REF] Januschke | Effects of stream restorations on riparian mesohabitats, vegetation and carabid beetles[END_REF]. Though there is a big knowledge gap concerning quantitative biological comparison of riverbank techniques for erosion control and their impact on ecosystem properties [START_REF] Pahl-Wostl | The importance of social learning in restoring the multifunctionality of rivers and floodplains[END_REF]). To our knowledge, no study has yet demonstrated the relationship between materials and techniques used for erosion control and associated changes in biodiversity on the constructions.

The aims of this study were (i) to evaluate and compare animal and vegetal biodiversity on three riverbank techniques for erosion control and (ii) to assess the capacity of different riverbank protection techniques to limit invasion of exotic species. The applied aim of this study was to integrate ecological aspects into the decision line by providing ecological data. We provide guidelines to practitioners on what ecological conditions they can expect on the riverbank technique chosen in order to develop strategies linked to riverbank management and allow restoration of ecosystem goods and services. We hypothesize that civil engineering techniques do not promote biodiversity as good as bioengineering techniques.

Materials and methods

Sites

We assessed biological diversity on five replicates of three riverbank techniques (Fig. 1):

-Civil engineering embankments: riprap protection (rocks used to stabilize shorelines, against scour and erosion) noted as "Mineral". -Mixed embankments combining civil engineering techniques (riprap at the lower part of the bank) and bioengineering techniques (cuttings and woody plantation at the upper part) noted as "Mixed". -Completely bioengineering embankments: (willow fascines at the lower part of the bank with cuttings and woody plantation at the upper part) noted as "Vegetal".

These three techniques were considered as three level of a naturality gradient. Naturality can be defined by the amount of living vegetation used in the construction as classification key criterion [START_REF] Brunel | Sources of water used by natural mesquite vegetation in a semiarid region of northern Mexico[END_REF][START_REF] Werdin-Pfisterer | Soil amino acid composition across a boreal forest successional sequence[END_REF]. Mineral structures were considered as man-made environments with little plant coverage, different from the environment found on natural mountain riverbank. Mixed methods show intermediate plant cover with revegetation on half of the bank. Bioengineering structures are completely vegetal and therefore are expected to show the highest vegetation cover and thus the highest level of naturality. These three engineering techniques therefore constitute three distinct levels on a naturality gradient.

We selected fifteen riverbanks along six distinct French prealps rivers belonging to the Rhone catchment area. A large and intense prospection at territorial units for river management organizations was performed to list enough comparable banks. Table 1 lists sites characteristics. As altitude profoundly affects ecological community composition, study sites were selected at a narrow range of 270 m (between 200 and 470 m asl). Only sites between 20 and 30 m long and with an established vegetation were chosen for analysis. All the study sites were located in the foothills of the French Alps characterized by highly fragmented habitats.

Vegetation

Vegetation was assessed using the contact point method in June and July 2009. This method encompasses the vertical organization of the vegetation (tree, shrub and herbaceous layers). Plant species diversity and frequencies were estimated using a 2-m-long stick with a diameter of 1 cm. Measurements were taken every metre along three 20 m transects placed parallel to the shore one located at the water line (Transect 1), one in the middle (Transect 2) and one at the top (Transect 3) of the riverbank embankment (sixty measurements per site). Vegetation was identified to the species level using various identification keys and flora [START_REF] Rameau | Flore Forestière Franc ¸ aise[END_REF][START_REF] Lauber | Flora Helvetica, second edition[END_REF][START_REF] Aeschimann | Flora Alpina[END_REF]. Exotic species were defined on the basis of established lists relevant to the area [START_REF] Muller | Plantes invasives en France[END_REF], a list of exotic invaders in Switzerland (Landolt and Bäumler, 2010) and a list established for the administrative district of Isère [START_REF] Gentiana | Les espèces envahissantes de l'Isère[END_REF].

Fauna

Coleopterans were identified to genus (Insecta, Coleoptera). This order, and more specifically the Carabids, is used as proxy for habitat diversity, hydromorphological processes and habitat health status [START_REF] Boscaini | River ecotones: carabid beetles as a tool for quality assessment[END_REF][START_REF] Kleinwaïchter | Impact of modified groynes on Carabid beetles[END_REF]. Moreover, morphological attributes of Coleopterans can provide information on their functional and trophic status [START_REF] Van Looy | Ground beetle habitat templets and riverbank integrity[END_REF]. Flying coleopterans were trapped using two Flora ® "Yellow Well" traps, located at the two extremities in the middle of the bank (transect 2) and placed at the canopy level. Traps were set in June and July 2009 and remained in the field for seven days. Insects were identified using a binocular microscope and appropriate reference works [START_REF] Picard | Faune de France (tome 20): Coléoptères Cerambycîdae[END_REF][START_REF] Paulian | Faune de France (tome 38): Coléoptères Scarabéides[END_REF][START_REF] Hoffmann | Faune de France (tome 44): Coléoptères Bruchides et Anthribides[END_REF][START_REF] Guignot | Faune de France (tome 48): Coléoptères Hydrocanthares[END_REF][START_REF] Balachowsky | Faune de France (tome 50): Coléoptères Scolytides[END_REF].

Statistical analysis

Differences in plant species and coleopteran genera richness, and in frequency and number of exotic species among the three types of development was assessed using Kruskal-Wallis tests. In case of a significant main effect, paired Mann-Whitney tests were carried out to detect differences between each technique. Overall statistical risk was assessed using the Holm's correction [START_REF] Holm | A simple sequentially rejective multiple test procedure[END_REF]). An inter-class principal component analysis (PCA) (Doledec and Chessel, 1987) was carried out to analyze the overall plant pattern. A Monte-Carlo test was processed to detect significant differences in floristic composition on structures using different engineering techniques [START_REF] Metropolis | The Monte Carlo method[END_REF]. All tests were performed using R statistical language (R, 2005) with ade4 package [START_REF] Thioulouse | ADE-4: a multivariate analysis and graphical display software[END_REF] for multivariate analysis.

Results

Vegetation

We identified 126 plant species from the 15 sampled sites. Plant species richness differed along the naturality gradient (Kruskal-Wallis test: p-value = 0.006, Table 2). "Vegetal" or "Mixed" techniques having higher species plant diversity compared to "Mineral" technique (Mann-Whitney U test with Holm correction: p-value = 0.024 and 0.036, Table 2). There was no significant difference between mean species richness on "Vegetal" and "Mixed" techniques although diversity tended to be higher on the latter, especially in the middle and at the bottom of the bank (Fig. 2).

Floristic composition differed significantly along the naturality gradient (Monte-Carlo test: p-value = 0.003). The distribution and spacing of the three spherical clusters (mn: Mineral; mx: Mixed; v: Vegetal) on the PCA factorial map provides information on the variability of floristic compositions (Fig. 3). However, some species contribute more than others to the segregation of the three sets, e.g. multiple Salicaceae species (Salix viminalis L., Salix purpurea L., Salix triandra L., Salix fragilis L., Salix myrsinifolia Salisb. and Salix pentandra L.) characterized "Vegetal" technique. "Mineral" technique was characterized by species like Buddleja davidii Franch., Humulus lupulus L., Robinia pseudoacacia L., Urtica dioica L. and Parthenocissus quinquefolia L. "Mixed" technique was mainly characterized by the presence of Cornus sanguinea L., Calamagrostis epigeios L., Salix incana L., Viburnum opulus L. and Lonicera xylosteum L. (Fig. 4). The inertia of the inter-class analysis represents 18.19% of the total variance among the sites. In addition, 66.16% of the information explained by this inter-class PCA was explained by axis 1. Salicaceae species as well as shrubs like C. sanguinea and V. opulus mainly explained axis 1 component on the positive side of the axis, and to a lesser extent by species like B. davidii and R. pseudoacacia on the negative side. Axis 2 expressed 33.84% of the information. In the upper part of this axis, the discriminatory species were mainly: S. viminalis, S. purpurea and S. triandra. In the lower part, C. sanguinea, C. epigeios, S. incana and V. opulus were the discriminatory species (Fig. 4).

The area of the ellipse provided information about intratechniques plant species variability (Fig. 3). "Mixed" and "Vegetal" techniques show comparable variability of plant species composition, higher than that of "Mineral" technique.

Fauna

We identified 42 distinct genera of flying Coleoptera from the 14 sampled sites (every traps have been lost for one site from the "Vegetal" technique). Fig. 5 indicates the mean numbers of flying Coleoptera genera collected in Flora ® yellow traps for each three techniques. "Vegetal" supported a mean of 12.2 different genera of coleopterans, "Mixed" ones supported 7.2 and "Mineral" techniques supported 3.0 genera. A genius names list was included in supplementary files.

Riverbank techniques significantly influenced the number of coleopteran genera (Kruskal-Wallis p-value = 0.006, Table 2). "Mixed" and "Vegetal" techniques were significantly richer than "Mineral" technique (Mann-Whitney U test with Holm correction: p-value = 0.03 and 0.038; Table 2).

We observed many genera associated with Salicaceae including various saproxylic Cetonia species with larvae growing in decomposing wood (Cetonia aurata, Potosia cuprea bourginii). There were also Cerambycidae species like Lamia textor, of which the larvae develop in willow stumps and roots, Necydalis major in the high cavities of trees, and Aromia moschata, of which the larvae develop in rotting stumps and branches. Many herbivorous coleopteran species were identified, especially Variimorda ssp. (often seen on Apiaceae), Hoplia farinosa (which feeds on pollen), Oedemera ssp. (the adult of which is herbivorous and the larva xylophagous). Some were associated with Salicaceae like C. aurata, Chaetocnema aridula and Altica aenescens. Predatory species were also identified, including the carnivorous Adrastus limbatus and Orchesia micans.

Exotic plant species

Although exotic plant species were sometimes found on sites that have been constructed using bioengineering, they were not among the ten dominant species observed in the study. In contrast, two exotic species, B. davidii and R. pseudoacacia, belong to the ten most abundant species on "Mineral" structure (100% of observed exotic species). Fallopia sp. was the main genius represented on "Mixed" structure (83% of observed exotic species) and on "Vegetal" structure (100% of observed exotic species). Overall exotic species richness varied from 1.75 to 2.2 between the techniques (Fig. 6) and did not differ significantly between the three techniques (Kruskal-Wallis test p-value = 0.308; Table 2). Invasive species frequency was significantly different according to the technique (Kruskal-Wallis test p-value = 0.027; Fig. 7), reaching a maximum on mineral developments (42.5) compared to vegetal (10) and mixed (13). Frequency of invasive species was significantly different between "Mixed" and "Mineral" techniques but not between "Vegetal" and "Mixed" techniques because of high variability on "Vegetal" developments (from f = 0.34 to f = 22.71; Mann-Whitney U test with Holm correction: p-value = 0.063 and 0.045; Table 2). Exotic invasive species were different according to the technique.

Discussion

Vegetation diversity

This survey provides a first assessment of plant species and Coleoptera genera richness associated with three categories of riverbank protection techniques. Based on field observations, we highlight significant differences in plant diversity between conventional civil engineering and bioengineering. In accordance with our hypothesis, significantly fewer plant species were found construction using on "Mineral" structures than on either of the two others probably due to the substrate in "Mineral" structures that physically limits colonization by plants. In contrast, we observe no significant differences in plant species richness between "Vegetal" and "Mixed" techniques. The higher number of plant species were found on "Mixed" structures compared to "Vegetal" ones may be explained by a large proportion of willows from the fascines installed at the water line which show an important growth rate; willows are part of fast-growing species usually used riverbank bioengineering [START_REF] Adam | Le génie végétal, Un manuel technique au service de l'aménagement et de la restauration des milieux aquatiques[END_REF]. This willow canopy rapidly develops a dense coverage that can partially inhibit colonization of the understory by herbaceous plants. For the "Mixed" technique, the presence of vegetation free riprap at the bottom of the bank restricts colonization and coverage by Salicaceae species to the upper part. Riprap at the interface between the aquatic environment and the first willow cuttings provide open space available for colonization by halophytic species [START_REF] Chen | Effects of size variation and spatial structure on plastic response of plant height to light competition[END_REF]. This lower part constitutes a different habitat, distinct from that offered by the "Vegetal" technique where woody species, mostly willows, predominate. Species typically found on this "Mixed" structures are (i) typical wetland species like Tetragonolobus maritimus, Juncus acutiflorus, Carex pseudocyperus and Phragmites australis, (ii) meadow species like Holcus mollis, (iii) edge species and species of woodland fringes like Ligustrum vulgare, L. xylosteum and Crataegus monogyna and (iv) trees from alluvial zones like Populus nigra and Populus alba. Most of those species were not found on "vegetal" structures because of the predominance of willows due to a lack of light and space. Finally, periodic sedimentation fills in the gaps between the blocks and creates a loam-rich substrate favourable for colonization by helophytic species. Our results are therefore consistent with other studies where the presence of some litter and loam increases both the density and diversity of pioneer plants [START_REF] Langlade | Effet de l'accumulation de limon sur la colonisation végétale d'un banc de galets en rivière[END_REF].

Fauna diversity

Flying Coleoptera were more diverse on "Vegetal" (12.2 genera on average) and "Mixed" (7.2 genera on average) structures which have more complex vegetation and greater floristic diversity than on "Mineral" (3.0 genera on average) structures. Our results are consistent with other studies which show that the abundance of Coleopteran species can be related to the presence of different vegetation strata, from grasses through bushes to trees [START_REF] Burel | Landscape structure effects on carabid beetles spatial patterns in western France[END_REF]. In general, insect populations--especially Carabidae--depend on vegetation structure and heterogeneous habitats [START_REF] Verdonschot | Ground beetles (Coleoptera: Carabidae) within a vegetation gradient in the Millingerwaard[END_REF]. Along river corridors, hotspots of biodiversity are principally situated in regions where habitat diversity is high [START_REF] Tews | Animal species diversity driven by habitat heterogeneity/diversity: the importance of keystone structures[END_REF]. Such habitat heterogeneity primarily reflects gradients of disturbance, sediment grain size, moisture and fertility that are tightly linked to river dynamics that recurrently renews the habitat mosaic and drives succession. In particular small and medium floods create more spatial variability and therefore more heterogeneity [START_REF] Pollock | Plant species richness in riparian wetlands -a test of biodiversity theory[END_REF][START_REF] Helfield | Restoration of rivers used for timber floating: effects on riparian plant diversity[END_REF].

Although "Mixed" structures harbour greater biodiversity in terms of plant species richness (30 species on average), more genera of flying Coleoptera were found on "Vegetal" structures (12.2 genera on average). This suggests that the plant species identity has a major impact on insect diversity. Salix species probably attracts many nectar-feeding insects by providing an important nectar source available early in the year (in January and February) when other nectar sources are scarce [START_REF] Newsholme | Willows: The Genus Salix[END_REF]. The high number of coleoptera associated with Salicaceae found in this study converges with Newsholme's observations. Vegetal cover and biomass were higher on "Vegetal" structures compared to the two others what could explain the higher coleopteran diversity on "Vegetal" structures. Moreover that shore zones contain large number of predators and scavengers including carabid beetles that feed on wrack or carrion that has been deposed on the shore [START_REF] Kleinwächter | Distribution patterns of terrestrial and aquatic invertebrates influenced by different groyne forms along the River Elbe (Germany)[END_REF]. We can assume that "Vegetal" and "Mixed" structures support more vegetation compared to "Mineral" structures and can thus produce more wrack and induce higher animal drowning mortality. In general, shore zones are an important feeding zone for many species including carabid species [START_REF] Strayer | Ecology of freshwater shore zones[END_REF].

Invasive species

This study did not reveal any significant difference in the number of exotic plants between "Mineral", "Mixed" and "Vegetal" techniques (respectively 1.75; 2.2 and 1.8). The occurrence of exotic plant species has been often related to the intensity of human activity [START_REF] Pyšek | Disentangling the role of environmental and human pressures on biological invasions across Europe[END_REF]. All techniques studied involve heavy human intervention like earthmoving or digging activities. It can be assumed that the soils were all subject to an equivalent degree of perturbation and comparable propagule pressure explaining why the different techniques harbour a similar number of exotic plant species. In contrast, this study shows that the frequency of exotic species was higher on "Mineral" structures than on the other techniques. Exotic plants species identified in this study are characterized by high growth rates which gave them a competitive advantage in pioneer habitats [START_REF] Call | Analysis of spatial patterns and spatial association between the invasive tree-of-heaven (Ailanthus altissima) and the native black locust (Robinia pseudoacacia)[END_REF][START_REF] Tallent-Halsell | The invasive Buddleja davidii (butterfly bush)[END_REF]. Moreover the relative abundance of invasive plants could be explained by biotic interactions, notably related to competition [START_REF] Levine | A meta-analysis of biotic resistance to exotic plant invasions[END_REF]. The presence of competitors on "Mixed" and "Vegetal" structures could restrict the vigour and inhibit the propagation of exotic species what is in particular seen on bioengineering structures; where it is very likely that the high density of willow restricted the performance of exotic species. Interestingly, B. davidii was the only species on "Mineral" technique and Fallopia sp. was the only invasive species on "Vegetal" technique. As mineral structures could experience higher temperatures during sunny weather than vegetalized banks, high temperature on mineral techniques could be here hypothesized as an environmental filter for species sensitive for heat stress. B. davidii is a Mediterranean and tropical species, can be supposed to stand high temperatures [START_REF] Tallent-Halsell | The invasive Buddleja davidii (butterfly bush)[END_REF][START_REF] Watt | Using species niche models to inform strategic management of weeds in a changing climate[END_REF]. Colonisable substrata available can also be an explanation; Fallopia sp. rhizomes need more space to develop and end up in a viable organism.

Perspectives and applications

The applicative purpose of this study is to provide useful information to decision-makers for planning sustainable management strategies of mountain riverbanks. Along small rivers, riverbank protections should be plant instead of self-recovery of natural vegetation in order to promote a rapid vegetal cover and to limit exotic species invasion. The bioengineering techniques using willows controls erosion, limit colonization by exotic heliophilous species and provides food resources and habitat to entomofauna.

Salicaceae are particularly adapted to riverbank restoration techniques. The morphological (deep roots, dense coverage), mechanical (flexible wood) and physiological (rapid growth, production of large amounts of nectar and pollen) properties of this genus limit erosion and ensure a good return to naturality [START_REF] Schiechtl | Water Bioengineering Techniques for Watercourse Bank and Shoreline Protection[END_REF][START_REF] Frossard | Le génie végétal pour la lutte contre l'érosion en rivière: une tradition millénaire en constante évolution[END_REF]. However, such restoration techniques should always include a large diversity of species and growth forms. Thus, creating different plant strata (grasses, bushes, and trees) ensures habitat diversity. River restoration programme could also aim at increasing habitat diversity. For instance, it is possible to promote the recruitment of helophytic species by creating interface areas between the water and the bank, free of bushes and trees and where sediment can be deposed to facilitate the accumulation of a colonisable substrate. "Mixed" technique with riprap at the base of the bank and bioengineering techniques on the upper part represent a trade-off between vegetal covering and habitat diversity.

Finally yet importantly, bioengineering structures represent a good alternative in the French Environmental Law, as administrative procedures are simpler (Decree n • 93-743 of the law 92-3 article 10). For civil-engineering structures, an environmental impact assessment is required what brings the administrative delay to 6-9 months to obtain a construction permit.

Conclusion

The intensification of human disturbance along mountain river lead to a dramatic decrease in riparian biodiversity. Maintaining biological continuity between small remnant refuge zones is a priority for the conservation of biodiversity threatened species [START_REF] Miller | Restoration, reconciliation, and reconnecting with nature nearby[END_REF]. The present study shows the capacities of restoration ecology and, more specifically, bioengineering, to promote the recovery of coleopteran and plants of riparian forests. In addition, "Mixed" and "Vegetal" embankment techniques enhance connectivity between different habitats and thus contribute to the restoration of ecological corridors. Increased connectivity between fragmented landscape patches is likely to increase genetic diversity within modified landscapes [START_REF] Neaves | Landscape discontinuities influence gene flow and genetic structure in a large, vagile Australian mammal, Macropus fuliginosus[END_REF].

In a further step, this study should be extended to the aquatic compartment. Benthic invertebrates seems to be a good indicator of naturality with their diversity correlated to the amount of organic substrate (wood and roots) on the banks [START_REF] Sudduth | Effects of bioengineered streambank stabilization on bank habitat and macroinvertebrates in urban streams[END_REF]. The questioning could also be raised to a functional analysis of the restored ecosystems [START_REF] Lavorel | Predicting changes in community composition and ecosystem functioning from plant traits: revisiting the Holy Grail[END_REF][START_REF] Hooper | Effects of biodiversity on ecosystem functioning: a consensus of current knowledge[END_REF].

Fig. 1 .

 1 Fig. 1. Schematic representation of riverbank techniques studied. (a) Civil engineering embankments: riprap protection, noted as "Mineral". (b) Mixed embankments combining civil engineering (riprap at the lower part of the bank) and bioengineering techniques, noted as "Mixed". (c) Completely bioengineering embankments: willow fascines at the lower part of the bank with cuttings, noted as "Vegetal".

Fig. 2 .

 2 Fig. 2. Mean plant species numbers for each riverbank protection technique according to their positions on the bank. Different letters indicate a statistical difference between samples. Error bars indicate standard errors. The "All transect" bar represent the mean of the total number of plant species found for each technique.

Fig. 3 .

 3 Fig. 3. Inter-class analysis of vegetation on three riverbanks protection technique. mn: Mineral; mx: Mixed; v: Vegetal. The distribution and spacing between the three spherical clusters provides information on the variability of floristic compositions between the different techniques. The size of the ellipse provides information about intra-technique plant species variability.

Fig. 4 .

 4 Fig. 4. Spatial distribution of all species encountered on the three riverbanks protection technique. Species codes refer to the first three letters of the genius and the species names. Percentages of variance observed are given on each axis. A correspondence table is given into the supplementary files.

Fig. 5 .

 5 Fig. 5. Coleoptera mean genus numbers in yellow traps on the three riverbanks protection technique. Different letters indicate a statistical difference between samples. Error bars indicate standard errors.

Fig. 6 .

 6 Fig. 6. Exotic mean species numbers on the three riverbanks protection technique. Different letters indicate a statistical difference between samples. Error bars indicate standard errors.

Fig. 7 .

 7 Fig. 7. Exotic mean species frequencies on the three riverbanks protection technique. Different letters indicate a statistical difference between samples. Error bars indicate standard errors.

Table 1

 1 Characteristics and localizations of embankments.

	Site code	Construction type	Locality	River	Latitude	Longitude	Construction work year	Altitude (m asl)
	6V	Vegetal	Les Echelles	Guiers vif	45 • 26'19.15"N	5 • 45'36.75"E	2005	393
	35V	Vegetal	Villard Bonnot	Vorz	45 • 15'01.29"N	5 • 54'01.62"E	2006	242
	7V	Vegetal	St Geoire en Valdaine	Ainan	45 • 28'02.24"N	5 • 39'40.90"E	2004	381
	38V	Vegetal	Le Grand Serre	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	Les Echelles	Guiers vif	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	St Geoire en Valdaine	Ainan	45 • 28'02.24"N	5 • 39'40.90"E	2004	381
	35Min	Mineral	Villard Bonnot	Vorz	45 • 15'01.29"N	5 • 54'01.62"E	2006	242
	37Min	Mineral	Chateauneuf de Galaure	Galaure	45 • 14'09.59"N	4 • 58'30.83"E	2004	257
	38Min	Mineral	Le Grand Serre	Galaure	45 • 15'45.62"N	5 • 06'29.65"E	2007	370
	39Min	Mineral	Cluse	Arve	46 • 04'13.98"N	6 • 33'18.72"E	2004

Table 2

 2 Probability values of the Kruskal-Wallis test and Mann-Whitney test for vegetation survey, flying coleoptera and invasive species.

	Statistical tests	Pair wise comparison	Vegetation survey	Aerian coleoptera	Invasive species
			Probability values	Probability values	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 *

* Indicate statistical significant differences (p-value < 0.05). ** Indicate highly significant differences (p-value < 0.01).

Acknowledgements

This work would not have been possible without the help of the Eco-Erosion Unit of the Mountain Ecosystem Research Unit of Irstea, Grenoble and the University of Toulouse, CNRS EcoLab, Toulouse. The French-Swiss Interreg IV Project Geni'Alp and the Agence de l'Eau Rhone-Alpes are thanked for providing funding.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ecoleng. 2012.12.105.