Effects of road type and urbanization on the diversity and abundance of alien species in roadside verges in Western Siberia

The spread of alien plant species by vehicles profoundly affects the roadside vegetation. Roads with high traffic densities in urban regions may facilitate the invasion of alien plants. The present study examined the effects of road type and distance to the city centre on native and alien plant species in both the aboveground vegetation and soil seed bank of road verges in originally dry steppe grasslands in the surroundings of Pavlodar, Western Siberia, Kazakhstan. This region is characterized by a recent change of land use and rapidly expanding urbanization. Vegetation surveys were conducted at 12 sites in equal numbers distributed along roads with different traffic densities (national and local roads) and at two distances to the city centre (city edge and rural surroundings). The seedling-emergency method was used to examine the soil seed bank at each site. We found a larger proportion of alien species along national roads (42% in the aboveground vegetation, 57% in the soil seed bank) than along local roads (20% and 44%, respectively), which can be explained by differences in traffic density, origin of vehicles and habitat conditions. More alien species were found in road verges at the city edge than in the rural surroundings along both road types, probably due to the spread of propagules from residential areas. Our study demonstrates that national roads are important pathways for the introduction of alien species in this Western Siberian region.