Motor vehicles as vectors of plant species from road verges in a suburban environment

The objective of this study was to assess the potential contribution of car-borne flora to species dispersal in a suburban area in Flanders (Belgium). Therefore, motor vehicle-borne flora was evaluated monthly for a random sample of 20 cars between October 1988 and September 1989. At the end of the experiment 33 car-borne plant species, with a total of 690 seedlings were identified from mud, attached to these vehicles. There was considerable temporal variation in the mud mass attached to the vehicles and therefore also the size and composition of the vehicle-borne flora varied during the year. Because mud retention on cars is a crucial factor in seed dispersal, the number of emerging seedlings is negatively correlated with precipitation in the week preceding the sampling. The flora dispersed by vehicles consists mostly of species with small, light seeds and a persistent seed bank. Usually, they are pioneer species of nitrogen-rich habitats. The nature of the vehicle dispersed flora was compared with the local flora of road verges and differed significantly from this in, e.g., seed length, seed weight, ecological groups, Ellenberg indicator values for nitrogen supply, potential dispersal strategies and rarity. A larger resemblance with the flora of the zone immediately adjacent to the carriageway was found. We only found a low similarity to other motor vehicle-borne floras.