The impact of a road upon adjacent heathland vegetation: Effects on plant species composition

1. The effect of a road upon heathland vegetation was investigated at five sites adjacent to the main trunk road through the New Forest, Hampshire, UK, with nine supplementary sites adjacent to five minor roads. 2. There was enhanced growth of vascular plants, notably heather and grass species near the road, which was probably due to oxides of nitrogen from vehicle exhausts. There was a decrease in the abundance and health of lichens near the road. 3. There was an increase in the abundance of grasses in the heathland near roads, which may be due to the changes in relative competitive ability of plant species under conditions of eutrophication. 4. The extent of the edge effect in the heath was closely correlated with the amount of traffic carried by the road, with a maximum edge effect of 200 m adjacent to a dual carriageway. This should be taken into account when considering the environmental impact of new roads. Road widening with its associated increase in traffic can also be expected to impact on existing oligotrophic communities beyond the actual land-take. Such schemes should include buffer zones to protect oligotrophic communities or other measures to minimize the pollution input from vehicle exhausts to environmentally sensitive areas.