Aggregated retention protects trees against wind, but not against disease: a long-term study in mixed forests

Retention forestry can help achieve multiple objectives in production forests, but its effectiveness is often low due to high mortality of the trees retained. We assessed the potential of improving tree survival in a low-level retention system in mixed forests by comparing the retention of multiple species of solitary trees (dispersed retention) and two approaches to aggregated retention. We annually monitored 58 dispersed-retention sites (since 2001) and 21 aggregated-retention sites (since 2013) in Estonia. Eight-year total mortality was 45% for solitary trees (and 1%–4% annually thereafter) but only 8% for tree groups; special planning for wind protection provided little further reduction. These estimates do not include dieback-affected Fraxinus excelsior L. that had distinct mortality dynamics independent of the retention pattern. Ulmus spp. also died frequently within the groups due to a dieback disease. Mixed-species tree groups enabled partial retention of Picea abies (L.) H. Karst that has extremely poor survival when retained solitarily. Likely, ecological costs of aggregated retention include some loss of microhabitat quality of individual trees. An optimal retention strategy could combine tree groups (maximizing wind protection and patch integrity) and individual trees (maximizing tree-scale biodiversity qualities), which collectively would also spread the risks of diverse mortality agents.