Impact of traffic-borne dispersal of invasive Asclepias syriaca L.in Special Nature Reserve Deliblato sands

Abstract

The effectiveness of roadside corridors for the migration of invasive plant species has often been linked to altered site conditions along roads due to increased disturbance, thus reducing the competitive strength of the native roadside flora. The invasion histories of some roadside plant species indicate that, in addition to site-related changes, traffic also promotes invasiveness. Asclepias syriaca (common milkweed) is a fast-growing competitor with effective clonal spreading and seed dispersal ability. A single common milkweed plant can produce a few thousand seeds with long, white flossy hairs that can easily be carried by wind. A. syriaca is common along the borders of the protected area - Deliblato sands, and is spreading into this area, mostly along the roads. Impact of traffic and accompanying human activities as dispersal vectors on seed deposition of A. syriaca in Deliblato sands has been investigated and our results demonstrate that, besides altered site conditions and abundance along particular Special Nature Reserve border, traffic-borne dispersal is an important cause of the higher incidence of A. syriaca on roadside verges. There was significantly higher number of A. syriaca plants along roads with heavier traffic. Also, higher frequency of road use results in higher incidence of new establishment of A. syriaca. Transport and human activities around roads can also explain the occurrence of isolated founder populations and discontinuous distributional patterns of common milkweed in this area.