Under and above the snow: Drivers of lichen diversity in subarctic birch forests

Lichens play a vital role in terrestrial and Arctic ecosystems. The subarctic mountain birch forests are among the most threatened habitat types in Fennoscandia and the lichens occupying them are a threatened species group. Snow has an impact on subarctic macrolichens, however, the ecology of lichen communities including the vast diversity of crustose lichens has not been studied. Here, we studied epiphytic lichen species and communities, including macrolichens and crustose lichens, in 74 sites in the subarctic birch forests of northern Finland. Our aim was to understand how ecological parameters, especially snow depth, temperature and humidity affect lichen species. We sampled the birch trees from the ground to a height of 160 cm in sections of 20 cm along the trunk. We used statistical modelling to assess the impact of climatic and environmental variables on the lichen species diversity and community composition. We found that the presence of the species was largely driven by temperature, with snow as a related factor. More specifically, most species were clearly more common and abundant either under or above the annual snow cover showing previously poorly known species-level adaptations to snow conditions. Furthermore, many lichen species only occurred in a single or few sections of the trunk, while few other species were very frequent and abundant. Considering that climate change affects temperature, snow conditions, and alters the composition of subarctic forests from birch to mixed, our results provide valuable insights to the ecology and threat status of lichen species in the region. This is further highlighted by the fact that we found several new species to the area. Most (75 %) of the lichen species that showed strong relationships to ecological parameters were crustose in growth form. This shows the need to include small, overlooked crustose lichens in community ecology studies.