Evaluating retention forestry 10 years after its introduction in temperate forests regarding the provision of tree-related microhabitats and dead wood

The individual or grouped retention of habitat trees in managed multiple-use forests has become an approach used to protect biodiversity-related structural attributes typically found in old close-to-nature forests. This study focuses on the effect of one such retention approach in the managed forests of Baden-Württemberg, Germany, ten years after its introduction. Specifically, we asked: (1) How effective are habitat tree groups (HTGs) at providing large living trees (LLTs > 80 cm DBH), tree-related microhabitats (TreMs), and dead wood?, and (2) which tree and stand variables have the greatest influence on the occurrence of TreMs? For this purpose, we inventoried 326 HTGs and 94 reference plots in forests dominated by the most widely occurring native conifer and broadleaf tree species, silver fir ( Abies alba ) and European beech ( Fagus sylvatica ). In accordance with our hypotheses, LLTs and TreMs were significantly more abundant in HTGs than in reference plots in both forest types. More importantly, when retaining 5% of the forest area as HTGs (a common retention level), old forest attributes such as woodpecker cavities, rot-holes or exposed heartwood increased significantly at the stand level while the volume of LLTs almost tripled, and volume of snags increased by 25%. However, quantities of these two attributes remain below minimum thresholds recommended in the scientific literature. A conversion of 15–25% of the stand area into HTGs is needed to increase the stand level abundance of TreMs such as concavities, exposed sapwood, or crown dead wood significantly in the short term. At the single-tree level, tree diameter (DBH), tree species, vitality and neighborhood competition had a significant influence on modeled TreM abundance. At the stand level, TreM occurrence increased with stand age and amount of snags, whereas TreM richness declined with stand density. Ten years after introducing the retention approach, forest stands with HTGs comprised significantly more important structural attributes than those without. Selecting HTGs with high stand volume or low tree density that also include snags, a mix of tree species, LLTs, and some low-vitality habitat trees could further improve this practice.