For what, when, and where is green-tree retention better than clear-cutting? A review of the biodiversity aspects

Green-tree retention cutting (GTR) is a modification of traditional clear-cutting, aimed at better consideration of biodiversity. We reviewed 214 North American and European studies to answer whether, and under which circumstances, GTR meets its objectives: ‘lifeboats’ species over the regeneration phase, provides microhabitats for old-forest species in re-established forest stands and for disturbance-phase species on the recent cuts, and enhances species’ dispersal by increasing landscape connectivity. To answer these questions is complicated, partly because the target taxa differ regionally and due to research biases: 81% of the studies have been carried out in North America, 82% have been short term, and the objective of improving landscape connectivity has not been studied. A meta-analysis of GTR effects on species richness and abundance of different taxa indicated no negative responses, but birds and ectomycorrhizal fungi benefited most. Compared with clear-cutting, GTR lowered the harvest-related loss of populations or individuals in 72% of studies, and it nearly always improved the habitat for disturbance-phase insects and birds on the cuts and for forest species in the regenerated stand. Lifeboating was most successful for ectomycorrhizal fungi, epiphytic lichens and small ground-dwelling animals, and least successful for bryophytes and vascular plants. Retention tree species always contributed to the success of GTR, followed by tree density (65% of cases) and the spatial arrangement of the trees (50%); the influence of forest type is likely, but insufficiently studied. Ectomycorrhizal fungi, epiphytes, birds, and wood-dependent beetles may be suitable indicator taxa for measuring the success of GTR. For future research, we encourage clearly objective oriented studies of relevant taxa, spatially explicit landscape perspectives, and long-term (including retrospective) studies.