Relative roles of life-form, land use and climate in recent dynamics of alien plant distributions in the British Isles

Climate change may increase the number and distribution of alien plant species, where these taxa are better suited to future climates, comprise life-forms known to benefit from a warmer environment, and/or possess life-history traits that facilitate the rapid tracking of climate change. Sequential atlases of the British flora enable assessment of the role of climate in changes in native and alien plant distributions over the latter half of the 20th century, a period coincident with marked climatic warming. While the climate profiles (e.g. frost-sensitive) and life-forms (e.g. therophytes and geophytes) of aliens suggest a high likelihood of becoming more widespread under a warmer, drier climate, this is not evident from changes in distributions since the 1960s. The strong association of aliens with anthropogenic habitats and the tendency for species spread to consolidate occupancy within the current range rather than extending range boundaries, indicate that land-use change may be a driver of species distributions. Nevertheless, problems arising from the consolidation of a species range may warrant as much concern as any poleward expansion of its distribution. While certain species may respond more to future climates than others, changes in the alien flora as a whole will probably also reflect other drivers, such as land use, propagule pressure and human-mediated dispersal. The importance of land-use change in the spread of alien plants is expected to continue even in the face of future climate change, not only in the UK, but also worldwide.