The ecological role of Fraxinus for species diversity in floodplain forests

Abstract

To assess how different tree species contribute to overall diversity and system function, we sampled 99 trees by fogging in Elbe floodplain forests and collected 182,179 arthropods, of which 25,861 (14.2%) were beetles. Ninety percent of all beetle individuals and 65% of the beetle species were associated with the canopy. Beetles of the herb and soil layer contributed 35% to canopy richness, but represented only 8% of all individuals. Beetle communities differed significantly in diversity, structure and functional composition between tree species, suggesting different roles in maintaining diversity. Two types of communities could be distinguished: 1) those with high proportions of specialists that formed a predictable framework, characterised by high alpha diversity and low within-beta diversity such as those on Quercus robur and Ulmus laevis, and 2) those on Fraxinus excelsior with predominantly non-specific beetles, characterised by high alpha and high beta diversity. We found that the distribution of the most abundant zoophagous and second most abundant phytophagous guilds in the beetle- and ordinal communities was similar for all trees but differed significantly between tree species. This suggests strong top-down control of herbivore populations. These results indicate that a comprehensive assessment of tree canopy diversity is necessary and overdue to understand the ecological processes that ensure the functioning of forest ecosystems and the provision of ecological services.