Stand-scale potential of production forests for lichen diversity: a hemiboreal perspective

It is still poorly documented how the role of production forests for sustaining biodiversity varies depending on the management techniques used, particularly in the case of species-rich and poorly detectable taxon groups. We studied lichens in hemiboreal forest landscapes in Estonia to understand (i) the stand-scale potential of production forests in maintaining the full assemblages present in old-growth forests and natural wildfire areas; (ii) limiting factors for lichen species and assemblages in production forests. The Estonian production forests are known to have relatively high habitat quality despite the dominant clear-cutting practices: their medium-intensity ‘semi-natural forestry’ approach has maintained tree species diversity through predominantly natural regeneration and through retention and low-intensity thinning practices. We sampled stand-scale full lichen assemblages and measured stand structure in 125 2-ha plots representing three successional stages (old growth; mature managed forests; clearcut sites) and both clear-cut and wildfire origin. The full dataset (378 species, including non-lichenized calicioid fungi and lichenicolous fungi) comprised three-fourths of the respective species pool known to inhabit forests in Estonia. Our main finding was that approximately half of the total species pool can inhabit production forests only if specific substratum legacies (e.g., deciduous trees, dead wood) are favored during management. In turn, half of these species tend to be nationally rare and their populations in production forests may be crucially important (currently distributed in rather similar proportion between production forests and reserves). The other half of the total species pool comprises species that regularly inhabit production forests of highly variable habitat quality (35% of all species) or are restricted to natural forests (15%). The post-disturbance successional differences among lichens assemblages were very clear, and this was only partly dependent on tree retention practices that determined the presence of 152 epiphyte species in early successional stands. Additionally, one third of species regularly inhabiting production forests were confined to older stands where their presence depends on rotation age or the spread of uneven-aged silviculture. Managing for tree species diversity within stands was also very influential while the diversity among stands was restricted to two main assemblage types (pine forests and spruce-deciduous mixed forests). In conclusion, production forests did not form one ‘habitat’ that could be used for analysing conservation solutions across landscapes and much more attention should be paid on the particular management regimes used or available.