Impact of windthrow and salvage-logging on taxonomic and functional diversity of forest arthropods

Windthrow is recognized as the most important driver in European forest dynamics and its importance is likely to increase with climate change. Typically, windthrown timber is salvaged for economic and phytosanitary reasons. This markedly affects the natural development of the disturbed areas, in particular by removing important dead wood resources. For a sustainable and ecologically sound management of disturbance effects, more knowledge is needed on the impact of windthrow and salvage logging on animal species communities.

We monitored various arthropod taxa (spiders and insects) two and five years after the storm Lothar in 1999 in salvaged and unsalvaged windthrows as well as in adjacent intact forests. Basing on a comprehensive data set with 1276 species and 228,718 individuals, species diversity, abundance and community composition were compared. Species richness and abundance of most taxonomic and functional groups (pollinators, saproxylics and predators) in the windthrow habitats clearly differed from those in the intact forests. On average, twice as many species were present in windthrows as in the forest. Windthrows also supported more red-listed beetles (mainly saproxylics) than the intact forest and more habitat indicator species (mainly Heteroptera and Aculeata) were found in windthrow areas.
No difference in species diversity was found between salvaged and unsalvaged windthrows. However, similarity analyses showed that the communities of certain taxonomic and functional groups differed between the two salvaging treatments. A combination of unsalvaged and salvaged windthrows in intact forests increased local species richness approximately 2.5 times relative to that in the forests alone. Therefore after large-scale windthrows, a mosaic of salvaged and unsalvaged windthrow patches fosters high forest biodiversity levels.