The natural decline of a virgin high-altitude Pinus sylvestris forest during the Little Ice Age (approximately AD 1300–1850) was studied in the Swedish Scandes. Methods included 14C and cross-dating of wood remnants, soil analyses, year-ring chronologies and age structures. A closed pine forest started a gradual decline in the 11th century or somewhat later, which proceeded until the mid-19th century. Long-term failure of regeneration and premature death of trees are postulated to have caused the forest dieback, mainly owing to a negative shift of the carbon budget, including weather-induced loss of photosynthetic tissue and root disfunction. Temperature measurements on the study site stressed its current marginality. Especially thermal conditions in and close to the ground were deduced to be decisive. The decline of the forest is considered to indicate a long-term lowering during the Little Ice Age of the summer (June-September) mean temperature by 1°C or more, compared with the 1931–1960 mean.