Effectiveness of solitary retention trees for conserving epiphytes: differential short-term responses of bryophytes and lichens

Green-tree retention (GTR) on clearcuts is an attempt to mimic natural disturbances and provide habitat for species that are generally absent in clear-cut stands, but its efficacy for sustaining biodiversity is poorly known. We studied (i) the total cover and vitality of lichens and bryophytes on four common tree species in three locations (centre and edge of GTR cuts and adjacent forest) and (ii) the composition of and damage to various epiphytic species on European aspen (Populus tremula L.) and birches (Betula spp.) in Estonia during 2 postharvesting years. Bryophytes on all tree species throughout the GTR cuts were severely unhealthy (60% of shoots desiccated, on average); lichens were much more robust (2% of thalli bleached or broken), particularly at the edges of harvested areas and on aspen and European ash (Fraxinus excelsior L.; hereinafter referred to as ash). Most lichen damage appeared to be unrelated to logging (the damaged species were also affected in forests). Aspen hosted many more species, including those of conservation concern, than birch. If tree species, size, and bark texture are carefully considered, GTR could be a successful tool for conserving lichens, particularly many microlichens on aspen and ash. However, bryophytes on solitary trees were generally unhealthy, at least in the short term.