Feedbacks between the riparian Salicaceae and hydrogeomorphic processes: A quantitative review

Riparian vegetation is a geophysical driver influencing the formation of fluvial landforms through interactions with the flow field, sediment entrainment, transport and deposition processes. At the same time, hydrogeomorphic processes regulate the riparian vegetation community by exerting selective pressures and setting the physical template on which vegetation develops. Many of the species ascribed to the Salicaceae family bear a set of traits that makes them extremely well suited to riparian habitats, allowing this family to out-compete other riparian woody genera to become the dominant woody-vegetation family in the Northern hemisphere and to quickly expand beyond their native range. The complex entanglement between the riparian Salicaceae and hydrogeomorphic processes has been acknowledged by several conceptual models, but the physical process thresholds affecting Salicaceae are perceived as uncertain. This uncertainty partly reflects the fact that, despite the publication of quantitative observations by many researchers, to date there has been no attempt to collate and compare these results in a comprehensive review. We aim to contribute to filling this gap by summarising findings from a large array of published studies. In particular, we focus on quantification of the physical processes that trigger riparian vegetation responses, emphasising the Salicaceae. The first section of the review defines a conceptual hierarchy of the processes entwining Salicaceae and hydrogeomorphic processes and the second part considers Salicaceae feedbacks on fluvial processes. These are followed by a third, more developed, section, focussing on findings from published studies concerned with hydromorphic process feedbacks on the Salicaceae. In contrast to previous research that has conceptualised vegetation-fluvial processes, this review is concerned with identifying explicit thresholds that can be used to support predictions of river system functioning and to parameterize numerical models that attempt to replicate riparian vegetation and hydrogeomorphic process interactions.