Ongoing climatic changes potentially affect tree-line ecosystems, but in many regions the observed changes are superimposed by human activities. We assessed how the forest-tundra ecotone has changed during the last century in the Putorana Mountains, northern Siberia, an extremely remote and untouched area in Eurasia. A space-for-time approach was used to determine the spatio-temporal dynamics of forest structure and biomass along an altitudinal transect. From the closed larch forest to the upper tree line, the mean age of Larix gmelinii (Rupr.) decreased considerably from 220 to 50 years ago. At the current upper species line, there is a strong and successful germination of larch, with 1500 saplings per hectare, indicating an ongoing filling-in, a densification of a formerly open forest and an upslope shift of the tree-line position (approximately 30 to 50 m in altitude during the last century). The forest expansion coincided with large increases in winter precipitation during the 20th century. In contrast, tree growth rates were significantly positively related to summer temperatures, neither of which increased markedly. The total aboveground biomass decreased from approximately 40 t ha^-1 in the closed larch forest to 5 t ha^-1 at the tree line. Our study demonstrates that ongoing climatic changes lead to an upslope expansion of forests in the remote Putorana Mountains, which alters the stand structure and productivity of the forest-tundra ecotone. These vegetation changes are very probably of minor importance for aboveground carbon sequestration, but soil carbon data are needed to estimate the impact of the forest expansion on the total ecosystem carbon storage.