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River ecosystems are spatiotemporally and intimately tied to physicochemical and biological processes, driven by strong feedbacks between riparian vegetation dynamics and hydrogeomorphic processes and fluvial landforms. Climatic and hydrogeomorphic constraints to vegetation determine a naturally shifting habitat mosaic dynamism, fostering high habitat heterogeneity and biodiversity, and providing multiple ecosystem services to society. However, most European river systems have lost their inherent highly dynamic character after major human-induced impacts, such as river channelisation and altered flow and sediment regimes. In March 2019, the United Nations designated the period of 2021–2030 as the "Decade on Ecosystem Restoration", and river ecosystems will be a significant target. Consequently, river restoration practitioners will need robust decision-making tools to guide their deliberations and subsequent management actions. Recommendations are to avoid merely reproducing river features and instead restoring geomorphic, hydrological, and ecological processes, but river science has not fully understood yet how processes develop and interact following restoration interventions. Integrative modelling of feedback mechanisms between riparian vegetation dynamics and hydrogeomorphic processes is critical for making predictions that enable river managers to optimise the use of the natural self-regulation potential of riparian corridors whilst maximising human benefits. Today’s existing models, however, do not fully reflect the interactions between river hydraulics and vegetation succession. In particular, the role of vegetation needs to be included through its impact in modulating river landforms and their evolutionary trajectories. Here, we present the conceptual and methodological framework, preliminary results, and the perspectives of the NUMRIP project, funded by the French National Research Agency. Along the project, a numerical (cellular automata) model of fluvial landscape dynamics will be developed, integrating physical, biological, and human components. The project focuses on riparian vegetation, from individual plants to communities. It explicitly considers vegetation as a dynamic component of the system, both responding to and affecting hydrogeomorphic processes and fluvial landforms. Accordingly, NUMRIP builds upon the conceptual fluvial biogeomorphological succession model and recent advances in remote sensing techniques of plant-geomorphology interactions. The NUMRIP project will explicitly associate plant functional traits (e.g., physiological, morphological, and biomechanical characteristics) to hydrogeomorphic processes and fluvial landforms, using plant functional trait approaches, remote sensing- and numerical modelling techniques. The lower course of the Allier River (France) is used as a case study. It is one of the last remaining free meandering river segments in Europe, and thus, constitutes an opportunity to investigate riparian succession processes of a dynamic, temperate river system. Despite its natural character, it is also experimenting an increase of stability (i.e., a reduction in channel migration and progression/retrogression of vegetation patches), because of a concomitant decrease of high and moderate magnitude floods due to current global climate change. The model could be used as a research tool in river science as well as a decision support system for river managers. It will be able to predict potential future evolutionary trajectories of fluvial corridors, adjusting for example to a changing hydrological regime or river restoration works.

Riverbank erosion is linked to increasing risks in piedmont areas due to urbanization and hydromorphological alterations of rivers. Changes in riparian vegetation and in sediment dynamics modify aquatic macroinvertebrate communities. In this context, riverbank stabilization is a major issue in the conservation of stream ecosystem functioning. In this study, we aimed at assessing the impacts of riverbank stabilization techniques on the taxonomic and functional properties of benthic macroinvertebrate communities living in alpine mountain streams. For this purpose, the effects of four riverbank stabilization techniques (riprap, mixed, cribwall, and fascine) on the taxonomic richness and biological traits involved in the main ecological processes were tested and compared with natural bank conditions. Overall, the macroinvertebrate richness was lower in stabilized banks than in natural conditions, and communities welcomed on natural banks were significantly different from those found on managed banks. The biological trait composition such as “maximum potential size,” “life cycle duration,” “reproduction,” “feeding habits,” and “trophic status” differed significantly among riverbanks, especially between natural banks and the riprap, mixed, and cribwall trio. In a context of macroinvertebrate biodiversity and functional restoration, we can advocate the fascine technique as the most suitable technique tested.

Restoring the flooding-related disturbance regime by removing and setting back flood defenses (channel widening) is the most efficient strategy for recovering riparian plant communities in floodplains formerly impacted by human activities such as agriculture, mining, and forestry. Removing flood defenses is generally not socially accepted, and alternative recovery strategies must be explored. We assessed vegetation establishment on 33 sites in the floodplains of the Middle Ebro River and three of its tributaries (NE Spain) where restoration approaches applied in the last 20 years include channel widening, floodplain excavation, and abandonment of agriculture, gravel extraction, and hybrid poplar plantations, with and without plantation of native species. Using analysis of similarity and ordinations, we found that channel widening led to plant communities closely resembling those found on natural gravel bars, including new recruits of keystone tree species. Excavation of the floodplain as the restoration approach resulted in pioneer, non-strictly riparian communities. Abandonment of agricultural land or clearing of poplar plantations resulted in alternative stable states predominating, regardless of time elapsed since restoration and whether poles of native species were planted. However, forest-like communities relatively similar to mature, natural riparian references were attainable when hybrid poplars were allowed to resprout after clear-cutting, or after the human activities were abandoned and trees were not cut. Combining channel widening and assisted revegetation where flood defenses cannot be altered may partially reproduce a mosaic of habitats typical of natural floodplains.

[EN] Plant communities and dynamics can be characterized according to species composition or plant traits. Here, we used species composition and plant traits to compare their effectiveness in discriminating the biogeomorphological (involving reciprocal feedbacks between physical and biological processes) and ecological (mainly biologically driven) phases of the fluvial biogeomorphological succession (FBS) model. The comparison was done between two French rivers, the largely unchannelized lower Allier and the channelized middle Garonne. One reach representative of each river section was selected for the study. Within each river reach, we chose two contrasted study sites in terms of channel and floodplain dynamics: a reference site (least altered channel and floodplain dynamics) and an altered site (laterally stabilized by riprap and constrained). In the four study sites, we sampled vegetation in 402 plots of 4 m2. The 512 species identified in the plots were characterized in terms of plant traits (20) from a literature review. When comparing reaches in unconstrained ordinations and per mutational multivariate analyses of variance, both species composition and plant traits led to a similar identification of the biogeomorphological and the ecological successional trajectories. Nevertheless, the trait approach was less influenced by local and regional bioclimatic, hydrogeomorphological, and anthropogenic settings and thus produced a more comprehensive and general classification of the biogeomorphological and ecological phases of the FBS model. A lower than expected contrast between the four sites was found, because neither species composition nor plant traits could entirely characterize distinct successional trajectories occurring in our reference or altered sites. Furthermore, our results contributed to a better understanding of the multiple successional trajectories that can occur in midlatitude river corridors. It also showed that relating plant traits to their effects on fluvial landform dynamics remains a core challenge in explaining succession including feedback mechanisms between hydrology, morphodynamics, and vegetation dynamics., French National Centre of Scientific Research; French Ministry of Ecology, Environment, Sustainable Development and Planning

[EN] Within riparian corridors, Salicaceae trees and shrubs affect hydrogeomorphic processes and lead to the formation of wooded fluvial landforms. These trees form dense stands and enhance plant anchorage, as grouped plants are less prone to be uprooted than free-standing individuals. This also enhances their role as ecosystem engineers through the trapping of sediment, organic matter, and nutrients. The landform formation caused by these wooded biogeomorphic landforms probably represents a positive niche construction, which ultimately leads, through facilitative processes, to an improved capacity of the individual trees to survive, exploit resources, and reach sexual maturity in the interval between destructive floods. The facilitative effects of riparian vegetation are well established; however, the nature and intensity of biotic interactions among trees of the same species forming dense woody stands and constructing the niche remain unclear. Our hypothesis is that the niche construction process also comprises more direct intraspecific interactions, such as cooperation or altruism. Our aim in this paper is to propose an original theoretical framework for positive intraspecific interactions among riparian Salicaceae species operating from establishment to sexual maturity. Within this framework, we speculate that (i) positive intraspecific interactions among trees are maximized in dynamic river reaches; (ii) during establishment, intraspecific facilitation (or helping) occurs among trees and this leads to the maintenance of a dense stand that improves survival and growth because saplings protect each other from shear stress and scour; (iii) in addition to the improved capacity to trap mineral and organic matter, individuals that constitute the dense stand can cooperate to mutually support a mycorrhizal network that will connect plants, soil, and groundwater and influence nutrient transfer, cycling, and storage within the shared constructed niche; (iv) during post-establishment, roots form functional grafts between neighbouring trees to increase biomechanical and physiological anchorage as well as nutrient acquisition and exchange; and (v) these stands remain dense on alluvial bars until a threshold of landform construction and hydrogeomorphic disconnection is reached. At this last stage, intraspecific competition for resources (light and nutrients) increases, inducing a density reduction in the aerial stand (i.e., self-thinning), but root systems of altruistic individuals could remain functional via root grafting. Finally, we suggest new methodological perspectives for testing our hypotheses related to the occurrence of positive intraspecific interactions among Salicaceae trees in fluvial landform and niche construction through in situ and ex situ experiments., During the development of the ideas proposed in this article, Dov Corenblit had a chaire d'excellence position at the French National Research Centre (CNRS); Virginia Garafano-Gomez had a post-doctoral grant from the Universite Blaise Pascal (now: Universite Clermont Auvergne, Clermont-Ferrand, France), and Borbala Hortobagyi received funding for her Ph.D. (contrat de recherche) from the French Ministry of National Education, Higher Education and Research. We thank S. Luhmann-Steiger for helping us with the English editing, and the editors, D. Butler and It Marston, for handling our manuscript and their feedback.

AIM Within fluvial and coastal ecosystems world-wide, flows of water, wind and sediment generate a shifting landscape mosaic composed of bare substrate and pioneer and mature vegetation successional stages. Pioneer plant species that colonize these ecosystems at the land–water interface have developed specific traits in response to environmental constraints (response traits) and are able to modify habitat conditions by modulating geomorphic processes (effect traits). Changes in the geomorphic environment under the control of engineer plants often feed back to organism traits (feedback traits), and thereby ecosystem functioning, leading to eco-evolutionary dynamics. Here we explain the joint foundations of fluvial and coastal ecosystems according to feedback between plants and the geomorphic environment. LOCATION Dynamic fluvial and coastal ecosystems world-wide. METHOD Drawing from a pre-existing model of ‘fluvial biogeomorphic succession’, we propose a conceptual framework showing that fluvial and coastal ‘biogeomorphic ecosystems’ are functionally similar due to eco-evolutionary feedbacks between plants and geomorphology. RESULTS The relationships between plant traits and their geomorphic environments within different fluvial and coastal biogeomorphic ecosystems are identified and classified within a framework of biogeomorphic functional similarity according to three criteria: (1) pioneer plants develop specific responses to the geomorphic environment; (2) engineer plants modulate the geomorphic environment; (3) geomorphic changes under biotic control within biogeomorphic ecosystems feed back to organisms. MAIN CONCLUSIONS The conceptual framework of functional similarity proposed here will improve our capacity to analyse, compare, manage and restore fluvial and coastal biogeomorphic ecosystems world-wide by using the same protocols based on the three criteria and four phases of the biogeomorphic succession model.

The study aimed to assess the effects of the potential replacement of dominant native plants by co-occurring exotic invasives on organic matter degradation in a riparian area. The questions were: i) Is there a relationship between the litter breakdown rate and species origin? ii) Is the chemical composition more relevant than species origin? iii) Does species origin influence the associated saprophagous invertebrate community? Within the riparian zone of the Garonne River (France), we conducted a litterbag experiment using pairs of native and exotic species selected in the stages of the successional gradient. Native and exotic species did not differ in their primary chemical composition and breakdown rate. Litter breakdown rates were primarily driven by the C:N ratio with no influence of species origin. The abundance and diversity of saprophagous invertebrates of exotic and native species did not differ, but the composition did. Only one plant litter was completely degraded much earlier than expected (next litterfall) while remnant litter were still present at this time for the other species. The replacement of a dominant native by an exotic invasive plant species does not generally result in noticeable changes in the litter breakdown rate, which mainly depends on litter composition. Litter from quickly degrading exotic species may cause a resource discontinuity in invaded areas stabilized by river regulation.

Patterns of native and exotic plant species richness and cover were examined in relation with ecosystem engineer effects of pioneer vegetation within the Mediterranean gravel bed river Tech, South France. The floristic composition was characterized according to two distinct vegetation types corresponding to two habitats with contrasted conditions: (i) open and exposed alluvial bars dominated by herbaceous communities; and (ii) islands and river margins disconnected from annual hydrogeomorphic disturbances and covered by woody vegetation. A significant positive correlation between exotic and native plant species richness and cover was observed for both vegetation types. However, significant differences in native and exotic species richness and cover were found between these two vegetation types. Higher values of total species richness and Shannon diversity were attained within the herbaceous vegetation type than within the woody type. These differences are most likely related to changes in local exposure to hydrogeomorphic disturbances driven by woody engineer plant species and to vegetation succession. A lower exotic species cover within the woody vegetation type than within the herbaceous type suggested an increase of resistance to invasion by exotic species during the biogeomorphic succession. The engineer effects of woody vegetation through landform construction resulted in a decrease of alpha (a) diversity at the patch scale but, in parallel, caused an increase in gamma (g) diversity at the scale of the studied river segment. Our study corroborates recent investigations that support the theory of biotic acceptance of exotic species by native species at the local scale (generally

International audience; Since several decades, American boxelder (Acer negundo) is replacing white willow (Salix alba) riparian forests along southern European rivers. This study aims to evaluate the consequences of boxelder invasion on understory community in riparian areas. We determined the understory species richness, composition and biomass in boxelder and white willow stands located in three riparian forests, representative of three rivers with distinct hydrological regimes. We investigated correlation of these variables to soil moisture and particle size, main soil nutrient stocks, potential nitrification and denitrification, tree canopy cover and photosynthetic active radiation (PAR) at the ground level. A greenhouse experiment was then conducted to identify the causal factors responsible for changes in the understory. The effect of soil type, PAR level and water level on the growth and the biomass production of Urtica dioica were examined. A lower plant species richness and biomass, and a modification of community composition were observed for boxelder understory in all sites, regardless of their environmental characteristics. The strongest modification that follows boxelder invasion was the decline in U. dioica, the dominant species of the white willow forest understory. These differences were mainly correlated with a lower incident PAR under boxelder canopy. The greenhouse experiment identified PAR level as the main factor responsible for the changes in U. dioica stem number and biomass. Our results indicate that adult boxelder acts as an ecosystem engineer that decreases light availability. The opportunistic invasion by boxelder leads to important understory changes, which could alter riparian ecosystem functioning.

1. A litter-bag experiment was undertaken in a pond on the margins of a large temperate floodplain in south-western France to assess the potential influence of the replacement of native by exotic riparian species on organic matter degradation. We determined initial litter chemical composition, breakdown rates and the invertebrate assemblages associated with the litter for five pairs of native dominant and exotic invasive species co-occurring at different stages along a successional gradient. 2. Litter chemical composition, breakdown rates and abundance and diversity of detritivorous invertebrates were similar for the exotic and native species overall. No overall changes in organic matter degradation can thus be predicted from the replacement of dominant natives by exotic invasives. Breakdown rates were primarily driven by the C⁄N ratio. 3. One invasive species (Buddleja davidii) showed significantly higher breakdown rates than its native counterpart (Populus nigra), resulting in the disappearance of leaf litter 6 months prior to the next litterfall. In some cases, therefore, invasion by exotic species may result in discontinuity of resource supply for decomposers.

Riparian vegetation responds to hydrogeomorphic disturbances and environmental changes and also controls these changes. Here, we propose that the control of sediment erosion and deposition by riparian vegetation is a key geomorphological and ecological (i.e. biogeomorphic) function within fluvial corridors. In a 3 year study, we investigated the correlations between riparian vegetation and hydrogeomorphic dynamics along a transverse gradient from the main channel to the floodplain of the River Tech, France. Sediment erosion and deposition rates varied significantly along the transverse gradient as a function of the vegetation biovolume intercepting water flow. These effects, combined with the extremely strong mechanical resistance of pioneer woody structures and strong resilience of pioneer labile herbaceous communities, Populus nigra and Salix spp., explain the propensity of biogeomorphic succession (i.e. the synergy between vegetation succession and landform construction) to progress between destructive floods. This geomorphological function newly identified as an ‘ecosystem function’ per se encompasses the coupling of habitat and landform creation, maintenance and change with fundamental ecosystem structural changes in space and in time. Three different biogeomorphic functions, all related to the concept of ecosystem engineering, were identified: (i) the function of pioneer herbaceous communities to retain fine sediment and diaspores in the exposed zones of the active tract near the water resource, facilitating recruitment of further herbaceous and Salicacea species; (ii) the function of woody vegetation to drive the construction of forested islands and floodplains; and (iii) the function of stabilised riparian forests to act as ‘diversity reservoirs’ which can support regeneration after destructive floods. Overall, this study based on empirical data points to the fundamental importance of sediment flow control by pioneer riparian vegetation in defining fluvial ecosystem and landform organisation in time and in space. Copyright © 2009 John Wiley & Sons, Ltd.

Whilst biological organisms adapt to the environment, earth surface processes and landforms evolve as a result of physicochemical processes, and as the result of the activity of certain living organisms defined as ‘ecosystem engineers’. The importance of long- and short-term impacts on geomorphic structures and processes by ecosystem engineers appears to be underestimated. Recent recognition of complex abiotic–biotic feedbacks in nature has resulted in a convergence of approaches in ecology and geomorphology. Present biogeomorphic knowledge supports the hypothesis that abiotic–biotic feedbacks create characteristic modulated patterns of earth surface landforms, adjusting according to biological evolution in the long term and to ecological succession in the short term. In this context, natural selection of organisms and ecological successions are considered to have the potential, in some cases, for extension to the physical world, including earth surface landforms. This perspective aims to contribute to the disruption of the ‘classical’ dichotomy between abiotic–biotic compartments because it emphasizes reciprocal adjustments (i.e., feedback mechanisms) between living organisms and abiotic environment dynamics. The extended evolutionary perspective, that is intended to feed back to ecology and evolutionary biology, indicates the potential for change in our deep understanding of geomorphology to reflect evolutionary and ecological succession theories.

Seed inputs were studied from January 2001 to August 2002 along a tranverse successional gradient of a riparian zone within the mid-reaches of the Garonne River (SW France). Seeds were trapped monthly at ten sites distributed along the gradient. In total, 117 000 seeds belonging to 296 species were collected, among which 145 were exotic species. Seed inputs were overall dissimilar to the extant vegetation; however, similarity among seed input samples was higher than similarity among the extant vegetation samples. Exotic seed inputs resulted from distinct modes of dispersal in comparison to the native ones. Exotic guilds also comprised more short-lived species. A significant positive linear relationship was observed between exotic and native species richness for both seed inputs and extant vegetation. Flood events were responsible for species richness peaks in seed inputs. They also contributed towards peaks in exotic species and individuals. However, most exotic seeds were recruited during low water period, with a significant lag in time compared to native inputs. Species composition, densities of species and individuals as well as the percentages of exotic species distinguished the infrequently agricultural zone from the frequently disturbed riparian zone. The latter zone was the most invaded both in terms of seed input and extant vegetation.

We studied the riparian vegetation on the agricultural floodplain of the Middle Garonne River (SW France) in order to compare native and exotic plant community patterns. In total, we investigated the vegetation of 1,824 plots during four seasons along 50-m-long transects delineated transversely from the banks of five types of water bodies (subsystems) differing by their exposure to natural and to human-induced disturbance. Exotic species accounted for 21% of the total of 726 species identified. We characterized native and exotic plant communities of each subsystem on the basis of Grime's ecological strategies, species lifespan, species richness, and species cover. The communities of each subsystem were compared with respect to the landscape structure (35 patch types) and to their distribution according to the distance to the bank of each water body, the distance to the main channel of the river, the annual duration of inundation, the total plant cover, the total species richness, and the proporti...

Little is known about changes in the composition of dead wood jams along rivers and the possible consequences of any such changes on the river ecosystem. Although tree zonation along the upstream-to-downstream continuum is weak and highly variable from a system to another, a clear transition appears in the piedmont zone, which is reflected by transitions in dead wood sources as well as for dead wood transport, storage and decomposition processes. In this paper, we focus on large lowland rivers of southwestern France, where riparian vegetation is increasingly fragmented, reduced in area and/or is entirely replaced by planted forests (poplar plantations). The amount and the potential role of dead wood is practically unknown in these rivers. One reason is that French legislation obliges landowners and public service managers to remove all material from the stream in order to maintain unobstructed river flows. The other reason is that unlike pristine streams in northern regions, these rivers have been regulated for several decades (Adour River) or even for several centuries (Garonne River). The vegetation component of the managed riparian landscape has changed in particular as a result of i) a decrease in stream dynamics, ii) the replacement of natural forests by planted ones, and iii) the invasion of natural communities by introduced woody species. The possible consequences of biological invasions on the role of dead wood jams are discussed in light of: i) a local study of wood jams along a moderately modified system; ii) changes observed in the composition of trees along the Adour River over the past 10 years; iii) a regional case study involving two chosen species. Whereas white willow populations are declining along streams in southwestern France, the box-elder, introduced from the United States, has spread extensively in the last two decades. Statistical models would suggest that competitive pressures are limited between these two species, boxelder is expected to replace white willow in the near future as a consequence of an increase in river regulation and global warming. This can be expected to have important consequences on dead wood dynamics, and on the management of woody debris, especially since trends indicate a replacement of softwood species by hardwood species.

We examined how restoration of riparian vegetation has been implemented and evaluated in the scientific literature during the past 25 years. A total of 169 papers were read systematically to extract information about the following: 1) restoration strategies applied, 2) scale of monitoring and use of reference sites, 3) metrics used for evaluation, and 4) drivers of success. Hydro-geomorphic approaches (e.g., dam operations, controlled floods, landform reconfiguration) were the most frequent, followed by active plant introduction, exotic species control, natural floodplain conversion and grazing and herbivory control. Our review revealed noteworthy limitations in the spatio-temporal approaches chosen for evaluation. Evaluations were mostly from one single project and frequently ignored the multi-dimensional nature of rivers: landscape spatial patterns were rarely assessed, and most projects were assessed locally (i.e., ≤meander scale). Monitoring rarely lasted for more than six years and the projects evaluated were usually not more than six years old. The impact of the restoration was most often (43%) assessed by tracking change over time rather than by comparing restored sites to unrestored and reference sites (12%), and few projects (30%) did both. Among the ways which restoration success was evaluated, vegetation structure (e.g., abundance, density, etc.) was assessed more often (152 papers) than vegetation processes (e.g., biomass accumulation, survival, etc.) (112 papers) and vegetation diversity (78 papers). Success was attributed to hydro-geomorphic factors in 63% of the projects. Future evaluations would benefit from incorporating emerging concepts in ecology such as functional traits to assess recovery of functionality, more rigorous experimental designs, enhanced comparisons among projects, longer term monitoring and reporting failure. © 2015 Elsevier Ltd., This research was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme.

Erosion control is a major issue in the Prealps region since piedmont is subject to both intense flood hazards and anthropic pressure. Riverbank protections may have major impacts on local ecosystem functioning and ecological corridor continuity. This study aimed to estimate the effects of the types of riverbank protection technique (from pure riprap to pure bioengineering) on the taxonomic and ecological composition of plant communities in comparison with unmanaged riverbanks as the referential system. Thirty-eight embankments were sampled in the foothills of the French and Swiss Alps. Four distinct riverbank techniques were analyzed and natural young willow stands were chosen as the referential system. At each site, vegetation was sampled along three transects from the waterline to the top of the riverbank. Plant communities were characterized using biological group composition (growth forms and life history, life strategies and distribution in space and time) and functional diversity indices (MFAD, FDc and wFDc). We identified 177 distinct plant species on 38 sites. Higher species richness levels were observed on bioengineered banks (from an average of 12 species recorded on ripraps to 27 species recorded on bioengineered banks) strongly dominated by Salicaceae species, especially for fascine and cribwall banks. Functional analyses of plant communities highlighted significant differences among bank types (p-value: 0.001) for all selected biological groups. Competitive - ruderal strategy, rooting shoots, stems or leaves that lie down or break off, and unisexual - dioecious, as well as pioneer plants and low shrubs (

This paper analyzes the spatial patterns of plant species richness observed in 1989 and in 1999 alongside the Adour River (Southwest France), focussing on: (1) upstream-to-down-stream changes, (2) mid-term changes, and (3) differences in observed patterns among various plant functional groups. The purpose of the research was to analyze how the proportions of invading species and/or ruderal species within riparian corridors can be related to regional environmental settings. About half of native and exotic species were ruderals. However, the proportion of competitors was higher within the native pool. No correlation was found between exotic and native functional groups. The longitudinal distribution of native and exotic ruderal species were correlated together, but they were not correlated with the total species richness. Converse to observations at the local scale in other studies, these patterns seem to be driven more by factors co-varying with the size of the river and by climatic factors than by hydrological disturbance-related factors. The role of the various plant groups studied and potential consequences on ecosystem function are discussed in the context of an increasing susceptibility of our ripanan corridors to invasions by nonriparian species. Management perspectives are discussed for the purpose of biodiversity conservation.

The main impacts of riparian vegetation on hydrological processes are briefly reviewed in order to highlight needs and perspectives for research and management goals. This review is based upon three distinct influences of riparian vegetation on hydrological processes: (i) the control of runoff, i.e. the physical impact of living and dead plants on hydraulics, (ii) the impact of plant physiology on water uptake, storage and return to the atmosphere, and (iii) the impact of riparian vegetation functioning on water quality. Riparian vegetation influences runoff through complex hydraulic interactions during baseflows as well as overbank flows. The contribution of fine vegetational structures to landscape hydrological roughness needs to be considered in relation to the spatial complexity (patchiness, vertical stratification, rhizosphere) and temporal variability (phenology, succession) of plant communities. With the exception of some woody species, the uptake, storage and return of water to the atmosphere is poorly known for riparian communities, and therefore the assessment of the regional hydrological importance of the riparian corridor remains difficult to estimate. Although better understood than the above two influences of riparian vegetation on hydrological processes, there are still a number of unresolved issues concerning the role of riparian vegetation in controlling water quality. In particular, little is known about the coupling of microbial and vegetational functions in nutrient cycling and the dynamics of carbon release from coarse and fine plant debris. The influence of vegetation complexity and plant diversity on both qualitative and quantitative aspects of water cycling remains an important area for future research. Fundamental and management issues are identified in relation to the use of riparian vegetation as a model and as a tool.

Summary 1. Riparian structure and function were considered from a longitudinal perspective in order to identify multiscale couplings with adjacent ecosystems and to identify research needs. 2. We characterized functional zones (with respect to vegetation development in association with various biogeochemical processes) within geomorphological settings using a delineation based upon erosional, transitional and depositional properties. 3. Vegetation dynamics within the riparian corridor are clearly influenced substantially by hydrological disturbance regimes. In turn, we suggest that vegetation productivity and diversity may widely influence riverine biogeochemical processes, especially as related to the consequences of changing redox conditions occurring from upstream to downstream. 4. However, surface and groundwater linkages are the predominant controls of landscape connectivity within riparian systems. 5. The importance of riparian zones as sources and sinks of matter and energy was examined in context of structural and functional attributes, such as sequestering or cycling of nutrients in sediments, retention of water in vegetation, and retention, diffusion or dispersal of biota. 6. The consequences of interactions between different communities (e.g. animals and plants, micro-organisms and plants) on biogeochemical processes are notably in need of research, especially with respect to control of landscape features. Multiscale approaches, coupling regional and local factors in all three spatial dimensions, are needed in order to understand more synthetically and to model biogeochemical and community processes within the river-riparian-upland landscape of catchments.

International audience; 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.

Patterns of plant species richness and community composition were studied in diverse riparian zones. Eight rivers in south-western France, one river in north-western USA and four rivers in south-eastern Spain were compared. A brief review is presented of factors that have been singled out as controlling diversity along rivers. The proportions of plant group classified according to life-span, morphology, ruderality and location along the rivers were analysed. Both proportions of plant traits and total species richness differed considerably between rivers. Semi-arid rivers were more influenced by local patterns than the other rivers. The patterns exhibited by true riparian species and invading, essentially non-riparian species, showed the importance of the relationships between the river and its close surroundings. The mean proportion of ruderals remained roughly the same in all rivers, suggesting some regulatory mechanisms independent of the general environmental settings. The longitudinal patterns of species richness in riparian plant communities of the oceanic rivers could be related to the intermediate disturbance hypothesis, whereas semi-arid rivers showed very irregular patterns of species richness along their courses, as a consequence of the irregular water availability. Exotic and ruderal species were more sensitive than other groups to the longitudinal zonation of the rivers and to environmental factors influenced by humans. Also, the permanency of superficial water appeared as a major factor in the control of riparian community structure in semi-arid regions.

Invasibility of riparian plant communities was estimated by the percentage of alien species found along the Adour River (Southwest France) and along Lockout Creek, McKenzie River, and Willamette River (Central Cascades, Oregon, U.S.A.). At the patch scale, the invasibilities of riparian plant communities were compared between one exceptionally rich site of the Adour River and patches selected in the Hoh and Dungeness watersheds (Olympic Peninsula, Washington, U.S.A.). Alien species represented 24% of 1396 species for the Adour and 30% of 851 species for the McKenzie. They represented 24% of 148 species for the Hoh drainage and 28% of 200 species for the Dungeness drainage. Similar trends were found along the Adour River and along the McKenzie River for changes in total number of species per site and in percentages of alien species per site. These trends may be related to the intermediate disturbance regimes and to the physical structure of the riparian corridors. Climatic and human factors are also involved in these longitudinal changes. Positive linear relationships were found between the total number of species and the percentage of aliens observed in each site. At the patch scale, most of the sampled communities contained alien species. Although mature vegetative patches appeared to be invasible, young communities contained more alien species than older ones. For entire corridors, a positive linear relationship was found between total species richness and percentage of alien species in each patch type for the richest site of the Adour River. This may be partially explained by landscape features considered in a successional context. We suggest the use of empirical rules, and stress the importance of riparian systems for monitoring the conservation of local and regional species pools are suggested. Se estimo la propension de las comunidades de plantas riberenas a la invasion a partir del porcentaje de especies invasoras encontradas a lo largo del Rio Adaour (Sudoeste de Francia) y a lo largo del arroyo Lookout/Rio McKenzie/Rio Willamette (Cascadas Centrales, Oregon, EUA). A escala de porche una la propension de la comunidad de plantas riberenas a las invasiones fue comparda, entre un sitio excepcionalmente rico en el Rio Adour y parches seleccionados en las cuencas de Hoh y Dungeness (Peninsula Olimpica, Washington, EUA). Las especies invasoras representaron un 24% de las 1396 especies de Adour y un 30% de las 851 especies de McKenzie. Estas tambien representaron un 24% de las 148 especies de la cuenca de Hoh y un 28% de las 200 especies de la cuenca de Dungeness. Tendencias similares fueron encontradas a lo largo del Rio Adour y del McKenzie, para los cambios en el numero total de especies y en el porcentaje de especies invasoras por sitio. Estas tendencias podrian estar relacionadas a los regimenes intermedios de perturbacion y a la estructura fisica de los corredores riberenos. Los factores climaticos y humanos tambien se encuentran involucrados en estos cambios longitudinales. Se encontro una relacion lineal positiva entre el numero total de especies y el porcentaje de especies invasoras observado en cada sitio. A una escala de parche, la mayoria de las muestras de la comunidad contenian especies invasoras. Sin bien, los parches vegetativos maduros parecieron ser propensos a la invasion, las comunidades mas jovenes presentaron mas especies invasoras que las comunidades mas antiguas. En cuanto a los corredores en su totalidad, se encontro una relacion lineal positiva entre la riqueza total de especies y el porcentaje de especies invasoras en cada tipo de parche para el sito mas rico del Rio Adour. Esta relacion puede ser explicada parcialmente por las caracteristicas del paisaje, consideradas dentro de un contexto sucesional. Se sugiere el uso de reglas empiricas y la importancia de los sistemas riberenos para el monitoreo de la conservacion de grupos de especies locales y regionales.

Study of the impacts of biological invasions, a pervasive component of global change, has generated remarkable understanding of the mechanisms and consequences of the spread of introduced populations. The growing field of invasion science, poised at a crossroads where ecology, social sciences, resource management, and public perception meet, is increasingly exposed to critical scrutiny from several perspectives. Although the rate of biological invasions, elucidation of their consequences, and knowledge about mitigation are growing rapidly, the very need for invasion science is disputed. Here, we highlight recent progress in understanding invasion impacts and management, and discuss the challenges that the discipline faces in its science and interactions with society., The workshop that spawned this paper was sponsored by CEFE-CNRS, the University of Montpellier (UM2) and the Région Languedoc Roussillon. P.P. was supported by grant no. P504/11/1028 (Czech Science Foundation), long-term research development project no. RVO 67985939 (Academy of Sciences of the Czech Republic), institutional resources of Ministry of Education, Youth and Sports of the Czech Republic, and acknowledges the support by Praemium Academiae award from the Academy of Sciences of the Czech Republic. D.A.W. was supported by a Wallenberg Scholars award, E.G.B. by the Spanish Ministry of Science (projects CGL2009-12877-C02-01 and CSD2009- 00065), M.V. by the Spanish Ministry of Science projects CGL2009-7515 and CSD2008-00040 and the Junta de Andalucıía RNM-4031, and F.C. by the French ANR (2009 PEXT 010 01)., info:eu-repo/semantics/publishedVersion

Changes in the hydrological regime affect the phenomenon of invasion by plant species along riparian systems. The dynamics of exotic (non-native) and native species were examined at five sites that differed in exposure to hydrological disturbance (floods) during three consecutive years (medium, wet and dry years). When considering the disturbance gradient, exotic plants were favoured by direct exposure to floods (main channel) and by high flood frequencies. The response to year to year changes in hydrology was rapid for both native and exotic communities. However, the exotic plants responded more rapidly and were favoured by a dry year. A general framework including human and natural factors involved in invasions by exotic plants along rivers is presented.

The pioneer vegetation of five distinct littoral zones along the middle Adour River (SW France) was investigated during three successive low water periods. There was a high year to year variability in the hydrology of the river during the period of study with a normal (1987), a wet (1988), and a dry year (1989). Changes in population and life forms density of both native and exotic species were analyzed. Communities were species rich when 45% of the 428 species present in the study area (2-km-long stretch) were found in the 10 × 10 m plots. Most of the species were annuals, but perennials were also abundant. The exotic component represented nearly 20% of the species and of the total number of individuals. There was high spatial variability in pioneer communities; sites of the main river channel differed from those along abandoned arms in species composition and population densities. The communities responded to changes in the hydrology of the preceding growing season. Sharp year to year differences were found in population and species density. The nature of the hydrological change drove those changes. Plots located in the oldest arms of the river showed a greater sensitivity to flooding conditions than those located in young littoral zones, with an increase in hygrophilous species. Alternatively, plots adjacent to active river channels showed greater sensitivity to drought conditions, as the density of short-lived and exotic species increased. Comparatively, exotics appeared to be more sensitive to the type of hydrological changes than natives. Warmer years also affected native plants, which reacted like invasive species. Relationships between the observed changes, the level of hydrological disturbance, and the recent history of each site are discussed in this paper with the possibility of using introduced species as descriptors of climatic and hydrologic changes. Key words: riparian vegetation, pioneer community, community structure, biological invasions, hydrological perturbation, variability.

International audience; 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.

The interstitial fauna of an alluvial pond on the floodplain of the Adour River, South-west France was sampled over a period of 13 months. The pond was connected to the adjacent river through phreatic flow and its water level varied in an annual cycle related to the level of the river. Monthly samples were taken from fixed points along five transects across the pond. The littoral zone was sampled between +20 and −80 cm from the substrate surface. From the seasonal variations in the distribution and abundance of invertebrates, the dynamics of their colonization and decolonization cycles could be elucidated and related to the variations in the groundwater level. Eight different colonization/decolonization cycles were determined, each one being characterized by a ‘typical’ common species. These included both passive and active movements in response to the changes in water level. Many taxa were shown to have behavioural strategies which altered according to the stages of their life cycles. Most aquatic invertebrates were adapted to move into the most stable conditions at a given time of the year. Certain terrestial species were also shown to tolerate short periods of immersion.

International audience; The structure and function of alluvial Highly Dynamic River Systems (HDRS) are driven by highly variable hydrological disturbance regimes, and alternate between resistant, metastable states and resilient, transitional states. These are in turn subject to influences of feedback loops within hydrogeomorphic and biological processes. Here we consider how resistance and resilience largely determine HDRS ecosystem trajectories and how these characteristics can be modified by natural and anthropogenic processes. We review the mechanisms by which biodiversity can affect both resistance and resilience and introduce a conceptual framework that incorporates some unique HDRS characteristics. We suggest that resilient and resistant patterns frequently coexist in the active tract of these river systems, and that this coexistance promotes the return of metastable states after major disturbances. In contrast, highly resistant and poorly resilient patterns dominate at their external boundaries. The loss of these natural dynamics resulting from direct and indirect human impacts causes deviations to resistance and resilience patterns and therefore to HDRS trajectory. We propose that understanding the role of interactions between biological and physical processes that control resistance and resilience is crucial for system restoration and management.

1. We propose a model of plant strategies in temperate fluvial hydrosystems that considers the hydraulic and geomorphic features that control plant recruitment, establishment and growth in river floodplains. 2. The model describes first how the disturbance gradient and the grain-size of the river bed load affect the relative proportion of erosion and deposition processes, and how the frequency of flood disturbance affects the intensity of such processes. 3. Secondly, the model predicts plant strategies according to direct and indirect effects of floods (disturbances through erosion versus deposition processes, and associated nutrient excess or limitation). 4. The relevance of the model as a prediction tool is discussed. Some proposals are made to validate the model, and traits are proposed that should be considered in future research for improving the predicting value of the model.

The concept of continuity/discontinuity is applied to the riparian vegetation of the corridor of the River Adour (S.W. France), in order to precisely define longitudinal structure, and to test the degree of floristic continuity of the fluvial axis. The measure of floristic connectance along the river course is based on presence/absence data, and is applied to successive stretches of the river, at various resolution levels. This analysis shows that the River Adour corridor cannot be assumed to be floristically continuous. The observed discontinuities may correspond to two types of change in the riparian vegetation: zones of slow change (high level of floristic connectance) or zones of sharp change (low level of floristic connectance).

Whilst earth surface landforms evolve largely as a result of the activity of certain living organisms, the biologically driven geomorphic changes have evolutionary consequences on organisms and ecological communities. Recognition of complex biotic–abiotic feedbacks may lead to deep changes in our understanding of geomorphology. Copyright © 2007 John Wiley & Sons, Ltd.

1. Denitrification in floodplain soils is one of the main biological processes emitting and reducing nitrous oxide, a greenhouse gas, and the main process responsible for the buffering capacity of riparian zones against diffuse nitrate pollution. 2. The aim of this study was to measure denitrification rates under a wide range of current climatic conditions and hydrological regimes in Europe (from latitude 64°N to latitude 42°N and from longitude 2°W to longitude 25°E), in order to determine the response patterns of this microbial process under different climatic and hydrological conditions, and to identify denitrification proxies robust enough to be used at the European scale. 3. Denitrification activity was significant in all the floodplain soils studied whatever the latitude. However, we found an increase in rates of an order of magnitude from high to mid latitudes. Maximum rates (above 30 g N m?2 month?1) were measured in the maritime conditions of the Trent floodplain. These rates are similar to mineralisation rates measured in alluvial soils and of the same order of magnitude as the amount of N stored in herbaceous plants in alluvial soils. 4. We used Multivariate Adaptative Regression Splines to relate the response variable denitrification with five relevant predictors, namely soil moisture, temperature, silt plus clay, nitrate content and herbaceous plant biomass. 5. Soil moisture, temperature, and nitrate were the three main control variables of microbial denitrification in alluvial soils in decreasing order of importance. 6. The model developed for denitrification with interaction effects outperformed a pure additive model. Soil moisture was involved in all interactions, emphasising its importance in predicting denitrification. 7. These results are discussed in the context of scenarios for future change in European hydrological regimes.

Special issue: Sediment Management in River Systems: A Need to Assess Changing Processes in the Long Term and at a Large Scale; International audience; Hydrogeomorphic processes within alluvial river systems create, maintain and degrade riparian habitat. The dynamic interactions between water, sediment, aquatic-terrestrial landforms and biotic elements control the functional processes and biodiversity patterns within the riparian zone and, thus, contribute directly to their ecological integrity and societal value. Numerous researchers from different disciplines publish work on the physical, biological, economic and societal functions of the riparian zone within various physiographic areas. The present paper aims to review the hydrogeomorphic processes of unconfined alluvial channel-floodplain rivers within the temperate zone. These processes and their interactions with the biotic environment provide the basis for understanding the physical as well as the ecological functioning of fluvial hydrosystems. The review focuses mainly on the European context, but major advances in riparian research from other continents are also considered. Rehabilitation and management strategies for the riparian zone are summarized and recommendations for further research conclude this review. Copyright © 2005 John Wiley & Sons, Ltd.

The objective of this work is to evaluate plant species origin importance for biological invasion consequences, through the analysis of the influence of native and exotic dominant plant species for ecosystem functioning. Five pairs of native dominant species (Agrostis stolonifera, Rubus caesius, Populus nigra, Urtica dioica et Salix alba) and exotic invasive species (Paspalum distichum, Fallopia japonica, Buddleja davidii, Impatiens glandulifera et Acer negundo) were compared for litter breakdown process and primary production. A more detailed analysis evaluates the consequences of A. negundo invasion for riparian forests. Our results indicate that the lack of coevolution between exotic species and the organisms of recipient areas has little implications for ecosystem functioning, even if exotic invasive species can be more efficient in some ecological processes. No general pattern can be drawn for functional consequences of native dominant species replacement by exotic invasive ones.; Dans le contexte des invasions, l'objectif de ce travail est d'évaluer dans quelle mesure l'origine des espèces végétales conditionne le fonctionnement d'un écosystème en situation de dominance. Cinq paires d'espèces autochtones dominantes (Agrostis stolonifera, Rubus caesius, Populus nigra, Urtica dioica et Salix alba) et introduites envahissantes (Paspalum distichum, Fallopia japonica, Buddleja davidii, Impatiens glandulifera et Acer negundo) ont été comparées en milieu riverain pour les processus de production primaire et de dégradation des litières. Ces études ont été complétées par une analyse détaillée des conséquences de l'invasion des ripisylves par A. negundo. Nos résultats montrent que si les espèces introduites envahissantes peuvent être plus efficaces dans la réalisation de certains processus écologiques, l'absence de coévolution entre les espèces introduites et les organismes des milieux récepteurs n'a pas d'implication systématique pour le fonctionnement des écosystèmes.

The objective of this work is to evaluate plant species origin importance for biological invasion consequences, through the analysis of the influence of native and exotic dominant plant species for ecosystem functioning. Five pairs of native dominant species (Agrostis stolonifera, Rubus caesius, Populus nigra, Urtica dioica et Salix alba) and exotic invasive species (Paspalum distichum, Fallopia japonica, Buddleja davidii, Impatiens glandulifera et Acer negundo) were compared for litter breakdown process and primary production. A more detailed analysis evaluates the consequences of A. negundo invasion for riparian forests. Our results indicate that the lack of coevolution between exotic species and the organisms of recipient areas has little implications for ecosystem functioning, even if exotic invasive species can be more efficient in some ecological processes. No general pattern can be drawn for functional consequences of native dominant species replacement by exotic invasive ones.; Dans le contexte des invasions, l'objectif de ce travail est d'évaluer dans quelle mesure l'origine des espèces végétales conditionne le fonctionnement d'un écosystème en situation de dominance. Cinq paires d'espèces autochtones dominantes (Agrostis stolonifera, Rubus caesius, Populus nigra, Urtica dioica et Salix alba) et introduites envahissantes (Paspalum distichum, Fallopia japonica, Buddleja davidii, Impatiens glandulifera et Acer negundo) ont été comparées en milieu riverain pour les processus de production primaire et de dégradation des litières. Ces études ont été complétées par une analyse détaillée des conséquences de l'invasion des ripisylves par A. negundo. Nos résultats montrent que si les espèces introduites envahissantes peuvent être plus efficaces dans la réalisation de certains processus écologiques, l'absence de coévolution entre les espèces introduites et les organismes des milieux récepteurs n'a pas d'implication systématique pour le fonctionnement des écosystèmes.