Road de-icing salt and the invasion of verges by halophytes
The halophytes invading roads are mostly from maritime habitats and their invasion appears to be a response to the heavy use of de-icing salt which began in the 1960"s. The most widespread species, Puccinellia distans now occurs extensively in the north and east of England and Wales. Other maritime species occur mostly near the coast and include Aster tripoli , Cochlearia officinalis, Puccinellia rraritirna and Sperqularia marina. Experiments and observations indicated that seed of invading populations is carried from the coast to roadsides on vehicles and once there is swept along in vehicular slipstreams. Rates of invasion, estimated by comparing the present distribution with that in 1975, were highest for species adapted to disturbed saline sites. Levels of salt in roadside soils varied between sites. Well drained soils had lower salinities than clay soils, some of which had salinities higher than previously recorded on roadsides in Britain. There were peaks in winter during salt application and at some sites in early summer due to evaporation. Soil salinity levels proved to be significantly correlated with salt application rates, site exposure to the prevailing wind and traffic volume. Species occured in distinct zones with maritime species confined to the saline road margin. This confinement was partly due to canpetition as glycophytes eliminated mritime species from non-saline experiRental plots but was also due to a requirement for salt by the maritirre species when growing on roadside soil. 2 Salt affected the resistance of maritime species to stress. For example the application of salt increased the freezing resistance of four of the species but decreased it in one. In trials using roadside soil protection from drought increased the survival of maritime species not treated with salt. Treatment with nutrients and other solutions containing ions easily assimilated by plants overcame the requirement f or salt. Two of the maritime grasses, Puccinellia distans and P. maritima proved to be suitable for reclaiming areas of roadside verge damaged by de-icing salt.