Influence of historical and present landscape patterns on plant communities of roadfield boundaries

For many years now, agricultural landscapes have been characterized by an increase in the surface area of arable crops and an enlargement of road networks, leading to a loss of semi-natural habitats. Road verges, which are generally composed of four elements – i.e. the safety zone, the berm, the ditch and the embankment – represent potential favourable remnants for many plant species. While present landscape composition and configuration influence taxonomic composition of plant communities in road verges, past landscape pattern may also explain present observed communities via the soil seed bank. It is known that plant species respond differently to changes of landscape pattern; as an example, specialist species of particular habitats are expected to be more sensitive to changes than generalist species. While many studies have investigated the relationship between past landscape composition and present plant communities on grassland habitats, only a few have focused on road verges and arable field margins. In this context, we addressed the following question: How past and present landscape patterns influence the present diversity and composition of plant communities in road-verges and adjacent field margins?

We sampled the berm, the embankment and the field margin in 190 road-field boundaries situated in Central-Western France (i.e. 1 plot of 10 m2 per element, 570 plots in total). We characterised past (1980) and present (2011) landscape covers within a buffer of 1000 m radius around each sampled road-field boundary. Through an original methodology, based on semi-automatic digitalisation of aerial photographs, we produced a past land cover map, while we combined several databases in order to build up the present land cover map. Both past and present maps were created using Arcgis software. In both the maps we distinguished the following habitat types: semi-natural grasslands, forest, vineyard and orchard, annual crops and roads. In each buffer and for each date, we computed the relative surface of grasslands and forest, the Shannon’s diversity index and the edge density (landscape variables). Within the study area, the grassland area decreased between 1980 and 2011 (i.e. a loss of 37%), while annual crops area and arable field size increased. Multiple regression analyses were used in order to investigate the influence of landscape variables on plant species richness and diversity of the berm, the embankment and the field margin for each date. Canonical and co-inertia analyses were used to study relationships between landscape variables and the species composition of berms, embankments and field-margins for each date.

Results allow assessing the effect of past Vs present land cover on plant biodiversity of intensively managed road verges.

Road verge; Berm; Embankment; Field margin; Historical landscape; Present landscape; Plant communities