Long‐term vegetation changes of treeless heath communities in northern Fennoscandia:links to climate change trends and reindeer grazing

Abstract Question: In recent decades, high‐latitude climate has shown regionally variable trends towards warmer and moister conditions. These changes have been predicted to cause afforestation or shrubification of open tundra, increases of warmth‐demanding southern species and plant groups favoured by increased moisture, and decline of species and habitats that are dependent on snow cover. In this study, we explore temporal changes in northern tundra upland plant communities along regional gradients and in local habitats. We ask how vegetation changes are linked with long‐term trends in regional climate and grazing pressure. Location: Northern Europe. Methods: In 2013–2014, we resurveyed a total of 108 vegetation plots on wind‐exposed and snow‐protected tundra habitats in three subareas along a bioclimatic gradient from the northern boreal to the arctic zone. Vegetation plots were originally sampled in 1964–1967. We related observed vegetation changes to changes in temperature, precipitation and grazing pressure, which all showed regionally variable increases over the study period. Results: We found a significant increase of the evergreen dwarf shrub Empetrum nigrum subsp. hermaphroditum in snow‐protected communities and a prominent decrease of lichens throughout the study area. No evidence for extensive tree or larger shrub (Betula spp., Salix spp. or Juniperus communis) encroachment despite climatic warming trends was found. Among studied communities, most pronounced changes in vegetation were observed in snow‐protected boreal heaths on small isolated uplands, where community composition showed low resemblance to the original composition described decades ago. Changes in plant communities correlated with changes in summer and winter temperatures, summer precipitation and reindeer grazing pressure, yet correlations varied depending on region and habitat. Conclusions: Northern tundra uplands vary in their resistance to on‐going climate change and reindeer grazing. Isolated treeless heaths of boreal forest–tundra ecotone appear least resistant to climate change and have already shifted towards new community states