Managing production stem density and proximity near retention trees in even-aged stands: a conservation knowledge gap with insights from Sweden

Green tree retention is intended to sustain important habitats for biodiversity in production forests by retaining living trees during harvest. As stands mature, the proximity and density of production stems surrounding retained trees often increases, raising questions about their optimal management for biodiversity. We searched the scientific literature for studies assessing mid- to late-rotation even-aged managed forests that quantified how stem management surrounding individual retention trees affects richness, abundance, or community composition of species. Of the 3655 studies analyzed, we found 23, but only three assessed production stem density near retention trees. These three studies, conducted in Sweden with pedunculate oaks (Quercus robur) retained in Norway spruce (Picea abies) forests, indicate that removing stems under or beyond the oak crown enhances vascular plant and saproxylic beetle abundance and richness. While findings suggest potential biodiversity benefits from adjusting the management of surrounding trees, they also reveal a significant knowledge gap in mature stands. Management guidelines are already provided by government agencies, forest companies, or certification organizations in at least four countries regarding how production trees should be managed near retention trees. We therefore place our findings within the context of understanding the extent of evidentiary support that underlies such guidance.