Hollow beech trees identified as keystone structures for saproxylic beetles by analyses of functional and phylogenetic diversity

Abstract

The general importance of dead wood in European beech forests for species requiring high amounts of decayed wood of large diameter has recently been demonstrated using a functional approach. However, the effect of veteran trees, particularly of living hollow trees with mould, on functional diversity, is less understood. These trees are known to be a habitat for a few endangered and specialized arthropods and epiphytes. Their ecological role as a complex habitat has been assumed, but not yet formally tested. We compared the richness and functional and phylogenetic diversity of saproxylic beetle assemblages of vital beech trees, habitat trees (i.e. trees with partial bark loss, broken crowns or sporocarps) and hollow trees with mould. As expected, the richness of red-listed species increased from vital trees to habitat trees to hollow trees. When we controlled for species richness using null models, both functional and phylogenetic diversity were higher for hollow trees than for habitat trees, which can be explained by the habitat heterogeneity hypothesis. Single-trait analyses revealed that hollow trees promoted species requiring late decay stages, large diameters and shady habitats. This suggests that in beech forests, hollow trees not only promote the few specialists of hollow trees, but also play a superior role for species under pressure by current logging practices and as a keystone structure with high habitat diversity at one tree. We therefore urge forest managers and conservationists to monitor particularly the easy-to-identify hollow trees and the conspicuous species living in such trees, as useful umbrellas for a high-diversity dead-wood habitat.