A small colony of the arctic-alpine dwarf birch, Betula nana, has been discovered in Upper Teesdale, this being the only English station for the species. It is elsewhere in Britain confined to the Scottish Highlands. The plant associations with which the species occurs throughout its range are of two types, tundra vegetation in the north, and sub-alpine dwarf shrub vegetation in the south of its range. In Teesdale the species is confined to the sub-alpine dwarf shrub type although an open tundra type is also present. Sub-fossil leaf remains of B. nana have been found in Teesdale close to the living colony, in a fen peat. The leaf remains are associated with deposits later than the commencement of Zone Vila, thus indicating that the species has probably been continuously present in Teesdale since Atlantic times. The Late-glacial finding of the species in Lower Teesdale suggests that the present colony may have a continuity in Upper Teesdale from Late-glacial times to the present day (approximately 12,000 years).