Boreal and Arctic sparsely vegetated siliceous boulders, stones or gravel screes occurring over base-poor substrates that harbour acidophilous plant communities. They are of diverse origin, uneven distribution through the region and often subject to continuing natural disturbance through rock falls, freeze-thaw or coastal erosion and deposition. The vegetation typically consists of lichens and bryophytes with different growth forms dominating different microhabitats, eg crustose and foliose lichens and small-cushion forming bryophytes on the sides of boulders, and fruticose lichens and mat forming bryophytes in the hollows between blocks. Where vascular plants find enough soil between blocks, they contribute a sparse cover.