Contrôles naturels et anthropiques de la structure et de la dynamique des forêts riveraines des cours d'eau du bassin rhodanien (Ain, Arve, Drôme et Rhône)

Abstract : Floodplain forests present a complex and diversified mosaic of vegetation units, which result from a combination of physical and temporal gradients, biotic interactions and human pressures. Qualitatively, the links between vegetation and hydro-morphological processes have been explored by several authors, but some aspects remain poorly documented (Chapter 1). In this thesis, we analyse, quantitatively, the impact of the hydro-morphological evolution of the channel on water and sediments fluxes, and thus on floodplain forests of Rhodanian hydrosystems.

At first, a temporal framework for examining the vegetation dynamics during the second part of the 20th century is developed (Chapter 2). At the landscape scale (Chapter 2-A), human control is fundamental to both the evolution of the surfaces and the structural characteristics of the corridor. Thus, we show that abandonment of forestry and pastoral practices can lead to an increase in floodplain vegetation cover (Ain River). Conversely, human activities can reduce forested cover and lead to its fragmentation (Arve River). Where lateral channel mobility occurs, the diversity of the landscape mosaic is increased. However, we highlight a strong geographical decoupling between the internal zone of the corridor and the external one. Finally, the transitory nature of the structure and diversity are discussed. At the individual scale (Chapter 2-B), radial growth analysis of ash along the Rhône River and its tributaries also show that hydro-morphological and human processes are the dominant controls. The altitude of a site is the most important control on the degree of hydrological connection; on the Ain River a threshold has been identified between a flow of 750 m3.s-1 and 1000 m3.s-1. Furthermore, the spatial pattern of sediment deposits can greatly affect ash growth. Indeed, analysis of the root systems shows a double control: an edaphic control (pebble layer as an impenetrable horizon) and hydrologic control (high groundwater level). Obviously, process and pattern are strongly linked and our results show that bed mobility, by modifying topographical pattern connection and sediment dynamics, changes ecological conditions within the floodplain. In highly human-influenced systems, human activities alter the distribution of temporal and spatial fluxes.

Second (Chapter 3), we worked at a smaller timescale and at the community level. For both the understory strata of post-pioneer forests of the Ain River (Chapter 3-A) and pioneer communities of Drôme River, we demonstrate that ecological conditions are influenced not only by the spatial position of a unit, but also by the geometrical evolution of the bed. This evolution depends on reach dynamics: bed mobility on the Ain River; embankment on the Drôme River. For riparian communities, contrasts in the degree of connection and disturbance result in different community composition, structure, regeneration and diversity. We highlight the role of fine sediment dynamics within forested ecosystems. Thus sediment erosion and deposition is a main process in term of disturbance impact. In pioneer communities, sediment grain size and thickness act as an important filter for ligneous species selection. We also show how fine sediment dynamics are controlled by reach-scale processes.

The examples developed in this work show a geographical pattern of key factors linked with hydro-morphological conditions and human influences. They constitute a significant argument for promoting a consistent management strategy for fluvial systems and floodplain forests, with practices that consider the evolution of all hydrosystem components (bed, groundwater etc.). For this purpose, after noting the specificities of floodplain forests, a strategic framework is proposed that addresses the spatial coherence of forests, functional diagnosis, the choice of management, taking into account the hydro-morphological context, site rehabilitation, and the legal and financial tools available (Chapter 4).

Finally, the results obtained within the framework of this thesis make it possible to discuss the concepts of physical connection (hydrological and morphological), human pressures, and riparian path (Conclusion).
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Submitted on : Tuesday, April 18, 2006 - 11:35:22 AM
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Simon Dufour. Contrôles naturels et anthropiques de la structure et de la dynamique des forêts riveraines des cours d'eau du bassin rhodanien (Ain, Arve, Drôme et Rhône). Géographie. Université Jean Moulin - Lyon III, 2005. Français. ⟨tel-00012155⟩

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