Assessing the importance of intersections between linear transportation infrastructures and fluvial corridors on plant diversity: first results on road bridges

The TGB (ITTECOP program) project aims to identify the role of intersections between linear transportation infrastructures and fluvial corridors for regional and local plant diversities. As a first step, we studied 19 sites distributed along the Adour River (335 km long) and along the Garonne River (650 km long) in SW France. On each site, plant community composition and abundance was assessed from 240 0.5x1 m plots distributed among 3 zones (from inner to outer corridor side) at proximity (test) and far from (reference) the bridges, in both fluvial and road corridors. The purposes of the study were to: 1) understand how each corridor generates or integrates the regional biodiversity gradient; 2) assess possible differences between bridge and reference subsites; 3) identify, from plant traits, the mechanisms involved in a possible disruption or reinforcement of the biological connectivity along the river corridor gradient. About 1800 plant species were identified at the total, with a maximum species density of 73 sp/m² along rivers and 49 sp/m² along roads. Unexpectedly, the total number of species found on roadsides was similar to the number observed along riversides, and the Garonne River system, more heavily impacted by human activities for several centuries, showed similar total species richness than the more natural Adour River system. These results are analyzed and discussed on the basis of biological trait distributions and on landscape patterns, with recommendations for integrated management of roads and rivers.

plant diversity, road corridors, river corridors, bridges, landscape patterns, regional to local scale