Climate warming is increasing fire severity in boreal forests and can alter forest structure and carbon (C) dynamics in Cajander larch (Larix cajanderi) forests of Siberia, which occur over C and ice-rich yedoma permafrost. Altered forest structure may impact understory vegetation through changing canopy cover, permafrost thaw depth, and soil temperatures. The primary objective of this study was to assess the long-term impacts of fire-driven changes in tree density on understory composition, diversity, and C pools and the underlying soil organic layer (SOL). Shrubs dominated low density stands, likely from reduced canopy cover and thaw depth, while mosses dominated high density stands. Consequently, understory C pools decreased from 415.46 to 158.87 g C m-2. Total SOL C pools remained unchanged as tree density increased. These findings suggest that fire-driven changes in tree density may alter understory composition and C pools, which could impact nutrient/water cycling and permafrost stability.