Old trees as a key source of epiphytic lichen persistence and spatial distribution in mountain Norway spruce forests

Habitat loss and fragmentation can negatively impact the persistence of dispersal-limited lichen species with narrow niches. Rapid change in microclimate due to canopy dieback exposes species to additional stressors that may limit their capacity to survive and colonize. We studied the importance of old trees as micro-refuges and microclimate stability in maintaining lichen survival and diversity. The study was situated in mountain Norway spruce (Picea abies) forests of the Gorgany Mountains of the Ukrainian Carpathian mountain belt. Lichens were collected on 13 circular study plots (1000 m²). Dendrochronological methods were used to reconstruct age structure and maximum disturbance event history. A linear mixed effects model and general additive models were used to explain patterns and variability of lichens based on stand age and disturbance history for each plot. Tree age was the strongest variable influencing lichen diversity and composition. Recent (<80 years ago) severely disturbed plots were colonized only by the most common species, however, old trees (>200 years old) that survived the disturbances served as microrefuges for the habitat-specialized and/or dispersal limited species, thus epiphytic lichen biodiversity was markedly higher on those plots in comparison to plots without any old trees. Most species were able to survive microclimatic change after disturbances, or recolonize disturbed patches from surrounding old-growth forests. We concluded that the survival of old trees after disturbances could maintain and/or recover large portions of epiphytic lichen biodiversity even in altered microclimates.