Many riparian ecosystems in European temperate regions have lost their inherent, highly dynamic character due to human-induced impacts such as river channelization and flow regulation. The lower course of the Allier River (France) is one of the last remaining free meandering river segments, and thus, constitutes an opportunity to investigate riparian succession processes of a dynamic, temperate river system. We analyzed (i) the dynamics of the succession phases based on eight sets of aerial images of the riparian corridor over the last five decades (1967-2014), (ii) the dominant succession trajectories and (iii) their relation with the Allier River flow regime. Results revealed that the study site was characterized by a shifting habitat mosaic between 1967 and 2005, whilst floods did not change the overall habitat composition of the riparian corridor but their distribution in space. After the year 2005, progression and retrogression processes have been drastically reduced, with an increase of stability (i.e. no more channel migration and neither progression nor retrogression of patches), and a concomitant reduction of high and moderate magnitude floods. This study depicts a clear threshold of trajectory change in succession processes since the beginning of the 21^st century and facilitates a better understanding of future trajectories under current global climate change.

Pioneer riparian trees such as Populus nigra L. which establish on alluvial bars within dynamic riparian corridors strongly influence fluvial geomorphology by trapping sediments and constructing landforms during floods. The engineering effects (changes in the physical state of the habitat by organisms) of P. nigra on alluvial bars depend on its biomass and its exposure to mechanical stress. P. nigra has a strong phenotypic plasticity that enables individuals to adapt their morphological and biomechanical traits, according to the local hydrogeomorphic conditions. The comprehension and quantification of the variation of morphological and biomechanical response trait attributes of P. nigra populations according to their exposure to mechanical stress is fundamental to better understand why riparian plants are capable to impact fluvial geomorphology. In an empirical in situ study, we quantified the relation between response trait attributes of P. nigra and its exposure to three different levels of mechanical stress. At a highly exposed bar-head, plants clearly developed response traits such as small flexible stems and a strong root system which favour higher mechanical resistance, while at the less exposed bar-tail plants developed taller, less flexible stems and finer root systems. Plants that established in the lower reach of the chute channel developed some common trait attributes in comparison to the bar-tail population and some other trait attributes which were common to the bar-head population. Poplar plants which established on bar-tails favoured bioconstruction, and thus are potentially faster disconnected from hydrogeomorphic disturbances. These results further suggest that fine scale biogeomorphic feedbacks have an influence on larger scale processes within the fluvial corridor requiring hierarchical biogeomorphic bottom-up and top-down cross scale studies for a better understanding of complex biogeomorphic fluvial ecosystems.