Performance of ambrosia artemisiifolia and its potential competitors in an experimental temperature and salinity gradient and implications for management

As it produces large quantities of allergenic pollen that has a serious effect on human health Ambrosia artemisiifolia (common ragweed) is among the most noxious invasive plant species in Europe. It is most widespread in southern parts of Eastern and Central Europe and likely to spread into other regions. Thus its suppression is highly desirable. To identify species with a potential to suppress this invasive species, we carried out a growth chamber experiment to investigate the performance (height and weight) of A. artemisiifolia and 10 native Central-European species (six grasses and four herbaceous plants), growing in similar habitats and hence potential competitors of ragweed. Two factors were investigated in order to determine the processes that are likely to affect the future spread of A. artemisiifolia in Central Europe, i.e. increasing temperatures and winter treatment with salt of roads that serve as dispersal corridors. The plants were reared at five temperatures ranging from 10 to 26 °C and three levels of salinity. The height and weight of A. artemisiifolia increased with increasing temperature over the whole range of temperatures tested, with most native species growing best at 22 °C. This indicates that A. artemisiifolia will perform better in a warming climate and its spread will be facilitated by the poor performance of native species growing at suboptimal temperatures. As the largest differences in size between A. artemisiifolia and the native species were recorded at 10 and 14 °C we recommend that native seed mixtures are sown at ragweed invaded sites early in spring, or the previous autumn, to provide the competitors with a growth advantage at lower temperatures when A. artemisiifolia plants are still small and thus competitively weak. With respect to the other factor tested, A. artemisiifolia was suppressed similarly by high salinity as most of the native species tested, which indicates that the ragweed spread along roads is not primarily facilitated by its high tolerance of salinity. Different tolerances of native species to salinity indicates that this should be reflected in the selection of species for roadside seeding.