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Wildfires are a naturally rare phenomenon in subarctic tundra ecosystems. Climate change triggers feedback loops that probably increase fire frequency and extent in those regions. Fire can change ecosystem properties of the Arctic tundra. However, long-term effects of fire on vegetation dynamics are still poorly understood.We studied soil and vegetation patterns of three fire scars (>44, 28 and 12 years old), situated at the northern border of the forest tundra ecozone within the Yamalo-Nenets Autonomous Okrug in Western Siberia, Russia.Lichen cover was lower on burnt compared with unburnt plots, while bryophyte and shrub cover was higher. Those effects were still apparent more than four decades after fire.Betula nana showed enhanced growth of individual plants after burning, indicating increased vitality and growth potential, due to modified ecosystem processes after fire: While the active layer and soil temperatures returned to levels comparable with unburned plots after 44 years, shrub growth was still enhanced. This reveals a strong fire legacy effect and can reinforce shrub encroachment with far reaching impacts on the ecosystem.

Wildfires are one of the main factors for landscape change in tundra ecosystems. In the absence of external mechanical impacts, tundra plant communities are relatively stable, even in the face of climatic changes. In our study, lichen cover was degraded on burnt tundra sites, which increased the permafrost thaw depth from 100 to 190 cm. In old fire scars (burnt 1980 – 1990) of the forest-tundra, vegetation cover was dominated by trees and shrubs. The soil temperature on burnt forest-tundra sites was higher in comparison to conditions of the unburnt control sites and permafrost was was not found at a depth of 2-2,3m. Dynamics of the Normalized Difference Vegetation index (NDVI) from 1986-2020 reveal that immediately after fires, vegetation recovered and biomass increased due to the development of Betula nana shrubs. In old fire scars of the forest-tundra (burnt 1980-1990), a significant increase in NDVI values was evident, in contrast to the unburnt tundra vegetation where this trend was less pronounced. We conclude that "greening" in the north of Western Siberia may occur due to fire-induced transformation processes. The role of wildfires in the advance of the treeline to the north, driven by climate change and active economic development of the Arctic, will gradually increase in future.

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