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 fl uvial 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 fl oodplain of the River Tech, France. Sediment erosion and deposition rates varied signifi cantly along the transverse gradient as a function of the vegetation biovolume intercepting water fl ow. 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 fl oods. This geomorphological function newly identifi ed 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 identifi ed: (i) the function of pioneer herbaceous communities to retain fi ne 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 fl oodplains; and (iii) the function of stabilised riparian forests to act as 'diversity reservoirs' which can support regeneration after destructive fl oods. Overall, this study based on empirical data points to the fundamental importance of sediment fl ow control by pioneer riparian vegetation in defi ning fl uvial ecosystem and landform organisation in time and in space.
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.
ResultsThe 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 conclusionsThe 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.
Aim To contribute to the development of a macroevolutionary framework for riparian systems, reinforcing conceptual linkages between earth surface processes and biological and ecological processes.
Location Riparian systems.Methods Literature review leading to an original proposition for perceiving the functioning of riparian systems in a new and different way.Results Riparian systems provide diverse landforms, habitats and resources for animals and plants. Certain organisms, defined as 'ecosystem engineers' , significantly create and modify the physical components of riparian systems. Many studies have highlighted such engineering effects by animals on riparian systems, but the identification and understanding of the effects and responses of plants within fluvial corridors have emerged only recently. The modulation of matter, resources and energy flows by engineering plants helps establish characteristic sequences of fluvial landform creation and maintenance associated with synergetic ecological successions. We relate this process to the concept of niche construction, developed mainly by evolutionary biologists. Feedbacks between adaptive responses of riparian plants to flow regime and adjusting effects on biostabilization and bioconstruction are discussed in the context of niche construction at the scale of ecological succession and the evolution of organisms.
Main conclusionsOur conceptualization forges an integrated approach for understanding vegetated fluvial systems from a macroevolutionary perspective, for elucidating riparian ecosystem dynamics and potentially for establishing long-term stream conservation and restoration strategies.
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