Cajander larch (<i>Larix cajanderi</i> Mayr.) forests of the Siberian Arctic are experiencing increased wildfire activity in conjunction with climate warming. These shifts could affect post-fire variation in the density and arrangement of trees and understory plant communities. To better understand how understory plant composition, abundance, and diversity vary with tree density, we surveyed understory plant communities and stand characteristics (e.g., canopy cover, active layer depth, and soil organic layer depth) within 25 stands, representing a density gradient of similarly-aged larch trees that established following a 1940 fire near Cherskiy, Russia. Understory plant diversity and mean total plant abundance decreased with increased canopy cover, which was also the most important variable affecting individual species’ abundances. In general, tall shrubs (e.g., <i>Betula nana</i> subsp. <i>exilis</i>) were more abundant in low-density stands with high light availability, and mosses (e.g., <i>Sanionia</i> spp.) were more abundant in high-density stands with low light availability. These results provide evidence that post-fire variation in tree recruitment affects understory plant community composition and diversity as stands mature. Therefore, projected increases in wildfire activity in the Siberian Arctic could have cascading impacts on forest structure and composition in both overstory and understory plant communities.