To better manage invasive populations, it is vital to understand the environmental drivers underlying spatial variation in demographic performance of invasive individuals and populations. The invasive common ragweed, Ambrosia artemisiifolia, has severe adverse effects on agriculture and human health, due to its vast production of seeds and allergenic pollen. Here, we identify the scale and nature of environmental factors driving individual performance of A. artemisiifolia, and assess their relative importance. We studied 39 populations across the European continent, covering different climatic and habitat conditions. We found that plant size is the most important determinant in variation of per-capita seed and pollen production. Using plant volume as a measure of individual performance, we found that the local environment (i.e. the site) is far more influential for plant volume (explaining 25% of all spatial variation) than geographic position (regional level; 8%) or the neighbouring vegetation (at the plot level; 4%). An overall model including environmental factors at all scales performed better (27%), including the weather (bigger plants in warm and wet conditions), soil type (smaller plants on soils with more sand), and highlighting the negative effects of altitude, neighbouring vegetation and bare soil. Pollen and seed densities varied more than 200-fold between sites, with highest estimates in Croatia, Romania and Hungary. Pollen densities were highest on arable fields, while highest seed densities were found along infrastructure, both significantly higher than on ruderal sites. We discuss implications of these findings for the spatial scale of management interventions against A. artemisiifolia.

Common ragweed (Ambrosia artemisiifolia) is an invasive alien species indigenous to North America. Ragweed is a big threat to agriculture and has a serious impact on human health. The most important area with ragweed occurrence within Europe is Pannonian Plain in Central Europe. This research introduces for the first time the unique, direct broad-scale survey of A. artemisiifolia in relation to real-life occurrence and infestation in the field. To understand the distribution of ragweed in the territory the geospatial analysis was applied to create 2D map in ArcGIS environment. The field survey was undertaken during summer 2014 in the Žitný ostrov (Slovakia). The investigation revealed that spatial occurrence of A. artemisiifolia is not homogeneous and there is also striking territorial heterogeneity of infestation rate. Ragweed was observed at 80 (47.2%) out of 169 sites. The infestation of arable land (mostly stubbles) varied from weak infestation, 24 sites (1-30 plants*10 m-2) to heavy infestation, 26 sites (˃80 plants*10 m-2). The research on associated plant communities was also carried out. Stubble field’s vegetation was found to be highly species poor, on an average only 2.5 species*10 m-2. Among 40 recorded species the most frequent (noticed at 25% sites) were Datura stramonium, Chenopodium album, and Mercurialis annua. This broad-scale survey provides an overview and useful information on considerations needed to make decisions about ragweed control and potential future expansion. The presented study offers also inventory of ragweed frequency over the large area.