A hitchhiker from the beach: the spread of the maritime halophyte Cochlearia danica along salted continental roads.
The increase in road networks facilitates the dispersal of many species of plants along roadsides. In these special habitats, the use of deicing salt can provide suitable habitat conditions for the establishment of stress-tolerant halophytes. This study investigates the spread of an alien halophyte Cochlearia danica in continental Europe. This species is native to the Atlantic shores of Europe, has already spread in many countries along roadsides and was recently discovered in Hungary. We performed a literature review to track the European spread of this species, and investigated the Hungarian occurrences in detail. Then we determined the ability of this species to adapt to local soil conditions by means of soil analyses and germination tests using 19 different NaCl concentrations and alkaline soils. To estimate the rate of spread, we estimated the size of the four Hungarian populations in 2016 and 2017, and at the same time we measured the number of flowering stems, number of flowers in an inflorescence, number of seeds per fruit and seed mass. Cochlearia danica is recorded growing along roadsides in eight countries in continental Europe. Literature data indicate a rapid spread of this species along European roads, of 62-65 km/year. In Hungary this species is recorded at four roadside localities characterized by a high soil salt content. The relationship between NaCl concentration and percentage germination followed a sigmoidal curve. Germination tests revealed a significant negative effect of NaCl concentration on germination above 0.5% NaCl, but germination occurred even on extremely saline substrates with a 2% NaCl concentration. The area of the largest Hungarian population decreased by more than 99% and that of the second largest population increased by more than 30% between 2016 and 2017. Even though this species can adapt to high salinity in the soil, these rapid and marked changes in population size indicate that the population dynamics of this species may depend on interactions between the amount of local precipitation and soil type. Our study indicates that we should expect further occurrences of C. danica along roads.