Slow recovery of bryophyte assemblages in middle-aged boreal forests regrown after clear-cutting

Clear-cutting followed by even-aged forestry is transforming forests around the globe. There is growing concern that considerable parts of the native forest biodiversity will not be able to re-colonize these new stands before the next clear-cutting. The development of species assemblages during the full forestry rotation period must be understood in order to assess the need for management adaptations and to get a basis for their design. Knowledge is accumulating from studies of permanent plots before and shortly after clear-cutting, but for later stages only comparative studies have been published (space-for-time substitutions). In this study, I combined this comparative approach with direct monitoring of the pace of assemblage recovery in boreal stands regrown after clear-cutting half a century ago (treatment stands). I found little re-colonization in assemblages of mosses and liverworts between an initial survey to a resurvey 15 years later in 0.1-ha permanent plots of upland and stream-side forest. The assemblages of the treatment stands were still significantly different from those in matched old control forests that had never been clear-cut. The treatment stands had significantly fewer species of liverworts and of the substrate-based species subgroup “wood or bark”, and the six most negatively affected species were liverworts more or less specialized to this substrate. The only significant recovery recorded over the 15 years was for the “rocks or boulders” subgroup in upland stands, probably related to a shadier and moister climate resulting from canopy development. During the inter-survey period, some of the upland treatment stands were thinned. All disfavored subgroups recovered less in thinned than in not thinned upland stands, most likely as a result of a return to lighter and drier microclimates and direct mechanical disturbance. The incomplete and slow recovery halfway into the forestry rotation period calls for action. Adaptation of thinning for conservation has rarely been implemented in boreal forest management, but has a large potential. To facilitate re-colonization by disfavored liverworts and mosses growing on wood or bark and/or under shaded and moist conditions, I suggest retention of unlogged patches during thinning and addition of coarse deadwood on the ground in these patches. Such measures would also favor re-colonization of other late-successional species.