Structural elements enhanced by retention forestry promote forest and non-forest specialist bees and wasps

Abstract

Retention forestry promotes certain forest structural elements to enhance biodiversity. It is unclear however to what extent retention measures are suited to enhance the biodiversity of bees and wasps, and how relationships to structural elements promoted by retention may differ when habitat-based classifications are accounted for. Here, we analyze the abundance, diversity and species richness of forest and non-forest specialist cavity-nesting bees and wasps collected on 127 plots in the southern Black Forest, Germany. Our aim was to use habitat-based classifications, or groupings based on habitat occurrence of cavity-nesting bees and wasps to evaluate the effectiveness and importance of forest structural elements that are prioritized in biodiversity-focused conservation. We found that canopy cover, stand structural complexity and standing deadwood were principally important for abundance, diversity and species richness of bees and wasps, with differing responses among habitat classifications. Forest specialist biodiversity metrics and composition were related to forest structural variables indicating greater feeding and nesting resource availability, namely herb cover, standing deadwood and stand structural complexity. Non-forest specialist biodiversity metrics were related to primarily canopy cover and elevation while community composition was structured by only forest cover and understory species richness. Our results indicate the importance of considering habitat specializations of cavity-nesting bee and wasp communities for meaningful evaluation of retention forestry structural elements. The presence and arrangement of these forest elements can be altered by stand level management practices utilizing the cascading effects of structural changes, such as increasing herb cover and sun exposed standing deadwood via canopy opening, and high stump retention during tree harvesting.