Mapping Ragweed Presence in Europe Using Top-down and Bottom-up Approaches

Background: Common ragweed (Ambrosia artemisiifolia), originating from North America, is a key invasive species in Europe that produces large amounts of allergenic pollen. The SMARTER network (Sustainable Management of Ambrosia artemisiifolia in Europe), funded as a COST Action during 2012-17, address issues relating to ragweed management. A key aspect of the successful management is the accurate mapping of ragweed. This study provides the results of this mapping from the SMARTER network.
Methods: Two methods for mapping ragweed were applied. Method 1 is the so-called top-down approach where infested ragweed habitats are based on a combination of expert knowledge, land cover data and calculated pollen integrals, here from 313 sites distributed over Europe. The infestation for each site is calculated and then spatially interpolated to European scale on a harmonized 10km x 10km grid. Method 2 is a bottom-up approach using geographically referenced ragweed occurrences, and abundances where available, in selected countries. These are also interpolated to the 10km x 10km grid. Each gridded data set is investigated and they are compared numerically.
Results The pollen based map shows the largest ragweed infestation in Eastern Ukraine and parts of Russia. Large infestations are also found in the western part of the Pannonian Plain, particularly parts of Croatia and Serbia. France and Italy have substantially smaller areas of ragweed with high infestation and some infestation was identified in the European part of Turkey. Two maps of ragweed occurrences and abundances are provided for Serbia and Austria. Both countries show substantial variations throughout the country, with high densities in the Northern parts of Serbia and in eastern parts of Austria.
Discussion & Conclusion: The two approaches depict a similar spatial distribution of ragweed in these two countries, suggesting that both types of inventories are suitable as background data for managing ragweed. In cases where ragweed occurrence data are not available, the high congruence of the two approaches allows for the reliable prediction of high ragweed densities using the top-down approach.