Populus nigra L. establishment and fluvial landform construction: biogeomorphic dynamics within a channelized river
Résumé
Populations of the riparian pioneer species Populus nigra L. which establish on alluvial bars within river channels
modulate sediment dynamics and fluvial landforms. Dense cohorts of P. nigra have colonized gravel point bars along the channelized
River Garonne, France, during the last 20 years and have enhanced the vertical, lateral and longitudinal development of the
bars. For this period, the geomorphic characteristics of two wooded point bars on this laterally stable river are closely linked to
the spatial distribution and intensity of establishment and resistance of different cohorts of P. nigra. Furthermore, P. nigra colonization
dynamics were controlled by engineer effects of this same species. This relationship is illustrated by a significant correlation between
key geomorphic and biological variables measured in situ and characterized with a set of four aerial photographs taken between
2000 and 2010. The development of wooded point bars, which are discrete biogeomorphic units, over the studied period, appear
to result from a specific biogeomorphic positive feedback of matter aggregation and vegetation establishment related to sediment
trapping and stabilization by pioneer engineer plants.We propose a conceptual model of biogeomorphic unit construction for channelized,
lateral stable rivers.We consider the resultant biogeomorphic units as functional from an ecological point of view because P.
nigra enhances at the cohort scale (i) its own inherent capacity to resist hydrogeomorphic disturbances, and (ii) its resilience capacity
as a result of successful colonization, especially downstream of mature poplar stands.