Upgrading of forest edges: impacts of orientation and site fertility Wide, species-rich forest edges are rare in Switzerland. For some years now, the ecological improvement of forest edges has therefore been encouraged by the federal government. In order to make optimal use of the limited resources, we need a good knowledge of the site factors that influence the success of upgrading treatments. We studied 48 improved forest edges in Canton Aargau to understand the impacts of orientation and site fertility on the success of the improvement measures. The results showed that both orientation and site fertility had a significant impact on the quality of the enhanced forest edges. On south-facing and nutrient-poor sites, the ecological quality of the improved forest edges was significantly better than on nutrient-rich sites oriented towads other directions. On sites which were not south-facing fertility had no significant effect on the ecological quality of the improved forest edges. On nutrient-poor sites, the risk that problematic or invasive plant species become dominant after upgrading treatments was much smaller than on nutrient- rich sites. The orientation of the forest edge, on the other hand, had no significant effect on that risk. In the future, therefore, forest edge enhancement projects should preferably be carried out on south-facing and nutrient-poor sites. On sites with other expositions or with high fertility, successful upgradings are less likely but not impossible.