Phenological Variation in Ambrosia artemisiifolia L. Facilitates Near Future Establishment at Northern Latitudes.

The invasive weed Ambrosia artemisiifolia (common ragweed) constitutes a great threat to public health and agriculture in large areas of the globe. Climate change, characterized by higher temperatures and prolonged vegetation periods, could increase the risk of establishment in northern Europe in the future. However, as the species is a short-day plant that requires long nights to induce bloom formation, it might still fail to produce mature seeds before the onset of winter in areas at northern latitudes characterized by short summer nights. To survey the genetic variation in flowering time and study the effect of latitudinal origin on this trait, a reciprocal common garden experiment, including eleven populations of A. artemisiifolia from Europe and North America, was conducted. The experiment was conducted both outside the range limit of the species, in Sweden and within its invaded range, in Croatia. Our main hypothesis was that the photoperiodic-thermal requirements of A. artemisiifolia constitute a barrier for reproduction at northern latitudes and, thus, halts the northern range shift despite expected climate change. Results revealed the presence of a north-south gradient in flowering time at both garden sites, indicating that certain European populations are pre-adapted to photoperiodic and thermal conditions at latitudes up to, at least, 60° N. This was confirmed by phenological recordings performed in a region close to the northern range limit, the north of Germany. Thus, we conclude that there exists a high risk for establishment and spread of A. artemisiifolia in FennoScandinavia in the near future. The range shift might occur independently of climate change, but would be accelerated by it.

Annual variation of airborne pollen in the city of Vinkovci, northeastern Croatia.

Pollen concentration in the atmosphere of Vinkovci (northeastern Croatia) has been analyzed using a Hirst-type volumetric spore trap. During the year 2005, 58 pollen types were recorded with the sum of annual totals of 14,011. The pollen spectrum reflected the floristic diversity of the region. Non-arboreal pollen predominantly contributed to the total pollen sum with a percentage of 81.80%. The main pollen producers characterized by allergenic pollen were: Betula, Quercus, Fraxinus, Populus, Pinus, Urticaceae, Ambrosia, Poaceae, Plantago and Artemisia. Urticaceae was the most frequent and most abundant pollen type accounting for 46.58% of the total annual pollen, followed by Ambrosia (19.66%) and Poaceae (11.01%).