A 3.86 m core of peat and organic lake mud from a polygonal peatland in the Lena River valley of Siberia was radiocarbon dated and analyzed for pollen, plant macrofossils, chrysophyte stomatocysts, stable isotopes, and charcoal. At around 7200 BP, a shallow lake or open-water wetland supported diverse aquatic macrophytes. The site had transformed initially into a richer fen with Carex, Comarum palustris, and Drepanocladus and later a poorer fen with Sphagnum which persisted until around 3000 BP. Fire may have been responsible for silt being blown onto the peatland, which changed the hydrological and geochemical conditions for development of the poor fen. Ice accretion led to an increase in the height of the centre of the polygon and expansion of Sphagnum peatland . 18O values become progressively more enriched, which reflects more direct input of summer precipitation waters and less groundwater during this period. Finally, the peatland surface was elevated sufficiently to limit water and nutrient supply, thereby allowing Ericaceae and Betula to grow at the coring site. Fire burned the peatland surface and may have exaggerated the extremely slow rate of peat accumulation. Fire may also be a factor in maintaining the open Larix dahurica forest in the region today, while climate may be contributing to reducing postfire regeneration. Fire and climate together may be controlling the character and composition of forests near tree line in the Lena River valley of this part of Siberia.