Effects of silvicultural treatments on forest biodiversity indicators in the Mediterranean

Biodiversity maintenance is a key management objective and a requisite for sustainable forestry. Research efforts on the effects of forest management on biological diversity are therefore increasingly needed, particularly in regions such as the Mediterranean that have been comparatively less studied in this respect. We analysed the effects of different regeneration and stand improvement treatments on six forest biodiversity indicators (snags, mature trees, shrub abundance, shrub species richness, tree species richness and tree species diversity) in the Mediterranean region of Catalonia (NE Spain) by analysing a set of 9808 plots from the Third Spanish National Forest Inventory comprising both managed and unmanaged stands.Managed stands had significantly fewer large-diameter trees (with the exception of Quercus suber forests) and snags than unmanaged ones. While clearcutting decreased tree species richness, stands with selection cutting had higher shrub and tree species richness and tree species diversity, which is consistent with the intermediate disturbance hypothesis, which states that diversity is highest at intermediate disturbance levels. Stand improvement treatments increased the number of large-diameter trees, tree species richness and diversity (cleaning and thinning), and shrub species richness (pruning), while no significant negative effect was found for any of the other indicators.Our results suggest that preventing silvicultural disturbances may not be the best solution for conserving and enhancing biodiversity in the Mediterranean forests of Catalonia, where many stands have high tree density that impedes the establishment of a variety of plant species and the transition to more developed stages. Selection cutting may be an appropriate and sustainable regeneration treatment for Mediterranean forests in this respect, but their practical implementation should avoid the systematic harvesting of the highest quality and largest trees, which is still common in many private forests in Catalonia.