Differential impact of clearcut and insect outbreak on boreal lichens and bryophytes 50 years after disturbance

Lichens and bryophytes are ubiquitous in terrestrial habitats, sensitive to environmental changes, and they contribute to ecosystem functions and biogeochemical cycles. Intact forest ecosystems host distinct biotic communities that are associated with microhabitat and structural diversity at stand and landscape scales. In the boreal region of eastern Canada, insect outbreaks and clearcutting (the dominant timber harvesting method) are common disturbances. However, while insect outbreaks from native species can maintain ecological continuity, clearcutting disrupts it and homogenizes forest structure. We compared lichen and bryophyte communities between old-growth forest stands and 50 years-old stands regenerated from clearcuts and insect outbreaks. From these communities, liverwort and deadwood specialist species richness was higher in insect outbreak and old-growth than in clearcut forest stands. Stand type explained 10.3 % and 7.0 % of the variability observed in liverwort and deadwood specialist community composition, respectively. Clearcut stands were responsible for most of this variability since communities had less unique species and lacked some of the most common species found in old-growth and insect outbreak stands. Indeed, many species associated with deadwood and ecological continuity appeared unable to recolonize forest stands between planned intervals of clearcut rotations (i.e. ~50 to 60 years in our study area). However, deadwood volume did not differ between stand types and could not explain any part of the observed variability in the composition of communities. We suggest that forests regenerating from insect outbreaks are a suitable alternative for biological conservation in regions where old-growth forests are rare, especially for liverworts and deadwood specialists. • Liverworts and deadwood specialists are the most sensitive to clearcuts. • Insect outbreak and old-growth stands host highly similar communities. • Current deadwood volume did not influence the composition of communities. • Successive clearcut rotations may lead to biodiversity erosion. • Insect outbreaks present an alternative to old-growth for biological conservation.