Ragweed (Ambrosia) pollen transport and seed production in Finland during 1990–2022

Common ragweed (Ambrosia artemisiifolia) is one of the most harmful alien invasive species in Europe. Ragweed pollen is a potent aeroallergen, and can travel long distances in the atmosphere. In this research we studied pollen samples collected in Turku, Finland, during 1990–2022, to identify when ragweed pollen was first transported to the country and how frequently it has happened since, how much pollen has been transported to the country yearly, and whether it is possible to observe trends in airborne ragweed pollen occurrence in Finland during the past decades. We show that (1) ragweed pollen has been transported to Finland since the 1990’s, significantly earlier than previously thought, and (2) the long-distance transport episodes have often been more intense in the recent years. Ragweed pollen transports occur in the late summer or in the autumn, thus lengthening the pollen season in Finland. We also describe a case study where we show that ragweed is able to produce mature viable seeds in Finland. Our conclusion is that the significance of ragweed as an aeroallergen in Finland is increasing, and thus the situation needs to be regularly monitored.