Forest connectivity and water stress: characterization and mapping of the impact of channel incision on riparian forest status through remote sensing

International audienceThe health of forests across the globe is a growing concern in a changing climate where droughts can lead to increased water stress. In a riparian context, forests rely on their hydrological connection to the river system. Connections can be affected by human pressures through channel incision and groundwater depletion as a result of dams or gravel mining in the riverbed. As such, assessing and monitoring the status and connectivity of riparian forests is important to better design preservation and restoration policies. In France, the Ain River is a tributary to the Rhône River where human activities in the 20th century have resulted in a riverbed incision. To assess the impact of the incision on riparian forest characteristics, airborne hyperspectral and LiDAR data were acquired over a reach including both incised and non-incised sub-reaches of the river. Coupled with field forestry surveys conducted by the French National Forestry Office, these remote sensing data were used to characterize each surveyed plot. Variations in species compositions within patches and individual preferences for drier or wetter soils can be linked to topographic information extracted from the LiDAR data, such as the elevation of each plot relative to the low flow water level. Canopy reflectance also discriminates fairly well between plots above or below the elevation separating the presence and absence of species related to wetter or drier soils through indices and spectral bands correlated with leaf water content. A map of the potential water stress in the riparian forest highlights an upstream – downstream connectivity gradient resulting from riverbed incision that has propagated downstream over the past 60 years