The potential of common ragweed for further spread: invasibility of different habitats and the role of disturbances and propagule pressure

The infilling of existing suitable habitats within a landscape after establishment is of critical importance for the final outcome of a plant invasion, yet it is an often overlooked process. Common ragweed, Ambrosia artemisiifolia, is an invasive annual species in Europe causing serious problems due to its highly allergenic pollen and as an agricultural weed. Recent studies have modelled the broad-scale distribution of the species and assessed future invasion risk, but for predicting the expected outcome of ragweed invasion we also need a mechanistic understanding of its local invasion success. We conducted a field experiment to investigate the invasibility of eight common non-arable habitat types and the role of soil disturbance in central Hungary, in the hot spot of ragweed invasion in Europe. Seed addition alone resulted in negligible amount of ragweed biomass, except for sites where disturbance was part of the present management. Soil disturbance alone resulted in ragweed at those few sites where ragweed seeds were present in the seed bank, related to farming in recent decades. When disturbance and seed addition were combined, ragweed emerged in all habitat types and reached high biomass in all habitat types except for closed forests. As our experiment showed that most habitat types have high invasibility when disturbed, we conclude that ragweed has a high potential for further spread, even in this heavily infested region. Management should focus on preventing seed dispersal and eradicating establishing populations where ragweed is still absent, while reducing soil disturbance may be needed to avoid ragweed emergence in infested sites. This latter may require a reconsideration of land-use practices in infested regions