COS 271-3 - Bridges affect plant composition along roads and rivers

Abstract: Plant communities along linear systems such as rivers corridors and, to a lesser extent, road corridors have been widely studied. However, how vegetation responds when linear corridors intersect in the landscape remains largely unknown. Roads intersect with rivers at bridges. As nodes between roads and rivers, bridges could increase biological connectivity due to local increase of plant spread by human activity (traffic, angling and management practices such as mowing), or act as barriers for species as a result of local environmental changes (e.g., flood protection structures around bridges). Anthropogenic disturbances can affect the water and air flow, soil moisture, air temperature and light, leading to changes in community composition, e.g., assembly of species with different ecological and dispersal strategies, colonization of invasive species. To examine the influence of bridges on the roadside and riverside plant communities, we used a dataset with more than four thousand quadrats along the Adour and the Garonne rivers from the Central Pyrenees to the Atlantic Ocean (SW France). We tested for changes in 1) community diversity metrics, 2) dominant strategies based on community averages of functional indicators related to response to disturbance, resource acquisition, and propagule dispersal syndromes, and 3) percentage of invasive species between bridge and control sites, while also controlling for climate, land cover, soil and other environmental characteristics. We also tested the role of spatial structure, including directionality, driving plant community variation using spatial modelling methods (symmetric and asymmetric Moran’s Eigenvector Maps).

Preliminary analyses showed that species composition is different between bridge and control sites. Beta diversity among quadrats within sites decreased downstream of the bridge. We interpret this as the bridge disturbance causing convergence in community composition by increasing habitat homogenization. At the bridge, we found species with preference for higher soil temperature and less soil organic matter on roads, and for lower soil moisture on both roads and rivers. We conclude that several disturbance types are more intense at the bridge and can change roadside and riverside plant communities accordingly. Directional spatial variables (unidirectional up-stream to down-stream) were more important in explaining plant composition than spatial variables without directionality. This first attempt to clarify how the intersection among roads and rivers can affect plant communities will help to solve the ambivalent (connection vs barrier) role of bridges for plant diversity within corridors, and will inform conservation strategies.