Lateral differentiation and the role of exotic species in roadside vegetation in southern New Zealand

Roadside vegetation was surveyed across the southern part of the South Island, New Zealand. Samples were taken at 10 km intervals along 750 km of selected roads that provided a climatic gradient from semi-arid to hyperoceanic conditions and which crossed both, areas of farmland, where the native vegetation has been replaced by an anthropogenic plant cover consisting almost entirely of introduced species, and areas of managed native tussock grassland and native forest. Contiguous plots, placed in four zones parallel to the road, were used to examine any lateral differentiation of vegetation. Variation in floristic composition in all four zones was associated with variation in rainfall, continentality, altitude, and the presence of forest. In all sites there was a distinct change in species composition from the outer verge to the inner roadside. The vegetation of the zone nearest to the road showed weaker correlation with altitude and stronger correlation with continentality, a marked increase in short-lived exotic species, and a greater proportion of the more continental and weedy vegetation types than the vegetation of the outermost verge. This supports the hypothesis of anthropogenic continentality of road-shoulders. The most frequent species on the road-shoulders are those exotic species that transgress climatic barriers in their native continents. This suggests that, globally, the range of such species is liable to expand, particularly in the habitat-complex provided by roadsides.