The ecological role of Fraxinus for species diversity in floodplain forests

Little is known about the importance of different tree species in maintaining diversity and function. To address this, we fogged 99 trees (30 Fraxinus excelsior, 30 F. pennsylvanica, 21 Quercus robur, 12 Ulmus laevis and 6 Tilia cordata) in Elbe floodplain forests, resulting in 182,179 arthropods including 25,861 (14.2%) beetles. Ninety-two percent of all beetles and 65% of all species were canopy associated. Tourist beetles of the ground contributed 35% to the arboreal richness but only 8% to overall abundance, suggesting little influence on trophic interactions. The highest alpha and beta diversity was recorded on Fraxinus trees, mainly due to the high number of tourists in low abundance. On the contrary, Quercus or Ulmus were characterised by large numbers of associated species. The beetle communities, and even more so the entire communities of all orders, were dominated by predators, suggesting strong top-down control of herbivore populations. Phytophages were significantly more abundant on Quercus and Ulmus, while they represented less than 10% on the ashes. F. excelsior is a keystone species, and this includes the ability to maintain biodiversity and communities with different functional profiles. As a result of ash dieback and drought, F. excelsior has been severely reduced in these forests. Our extensive data show that F. excelsior and F. pennsylvanica are similar in many ecological features due to their taxonomic relationship, resulting in a similar diversity and functional composition of arthropod communities, indicating that F. pennsylvanica can at least partially fill the ecological role of the common ash.