Towards repeated clear-cutting of boreal forests - a tipping point for biodiversity?

Boreal forests are important carbon sinks and host a diverse array of species that provide important 14 ecosystem functions. Boreal forests have a long history of intensive forestry, in which even-aged 15 management with clear-cutting has been the dominating harvesting practice for the past 50–80 16 years. As a second cycle of clear-cutting is emerging, there is an urgent need to review the effects 17 of repeated clear-cutting events on biodiversity. Clear-cutting has led to reduced numbers of old 18 and large trees, decreased volumes of dead wood of varied decay stages and diameters, and altered 19 physical and chemical compositions of soils. The old-growth boreal forest has been fragmented 20 and considerably reduced. Here, we review short- and long-term (≥50 yrs) effects of clear-cutting 21 on boreal forest biodiversity in four key substrates: living trees, dead wood, ground and soil. We 22 then assess landscape-level changes (habitat fragmentation and edge effects) on this biodiversity. 23 There is evidence for long-term community changes after clear-cutting for several taxa: epiphytic 24 lichens; saproxylic fungi, bryophytes and insects; epigeic bryophytes; soil snails, bacteria, and 25 ectomycorrhizal fungi. Long-term declines in species richness were found for saproxylic fungi, 26 bryophytes and true flies. Still, for the majority of taxa, long-term effects of clear-cutting are not 27 well understood. On the landscape level, reduced connectivity to old-growth forests has negative 28 effects on several species of fungi, lichens, bryophytes and insects, notably among red-listed 29 species. Furthermore, altered microclimate near clear-cut edges negatively affect epiphytic lichens 30 and epigeic arthropods, implying complex effects of habitat fragmentation. Repeated cycles of 31 clear-cutting might pose even stronger pressures on boreal forest biodiversity due to continued 32 fragmentation of old-growth forests and accumulation of extinction debts. Examining the broad 33 effects of forestry on biodiversity across the boreal biome is crucial. First, to increase our 34 knowledge of long-term and landscape-level effects of former clear-cutting. Second, to gain a 35 better understanding of how forestry will affect biodiversity and, subsequently, ecosystem 36 functioning, with repeated cycles of clear-cutting.