Étude des dynamiques végétales des hydrosystèmes du Haut-Rhône au cours des quarante dernières années par analyse comparative de cartographies de végétation

Through a comparative analysis of old and recent vegetation mappings, the study aims to highlight changes of vegetation dynamics over the last forty years at three different alluvial sites in the Upper Rhône. Two main factors are being taken into account: the consequences of the river regulation linked to hydroelectric works, and the expansion of invasive exotic species (IAS). Prior work was required to harmonise the nomenclatures used, which led us to group the habitats observed into major plant groupings: Hardwoods (HW) and Softwoods (SW) for woody stands; Hygrophilous Herbaceous (HYH), Mesophilous Herbaceous (MH) and Steppes and Dry Grasslands (SDG) for open stands. The results show an overall decrease in moisture conditions linked to flow regulation through a smaller active band, a reduction in HYH formations in two of the three studied sites, a decline in SW formations mainly in favour of HW formations and an increase of surface area of the SDGs in the two sites where they were initially present. The analysis of IAS is more delicate insofar as the nomenclatures do not take these species into account in a homogeneous manner between two dates. However, it can be seen that areas invaded by IAS were clearly identified in the old maps; the list of the main IAS has not really changed since then; and IAS clumps have mainly been established in former SW stands.