Understanding the response of understory vegetation to fire disturbance is vital to biodiversity conservation and management of boreal forests. We surveyed understory vascular plant richness and composition, and measured related environmental variables along a toposequence within three successional stages, initial(3 years post-fire), early(13 years post-fire) and late(>100 years post-fire)successional stages. Using permutation multivariate ANOVA and nonmetric multidimensional scaling, we analyzed how understory species richness and composition change as time-since-fire proceeds, and their correlative relationships with environmental variables. Species richness and composition showed significant differences among the three successional stages. Understory species richness and abundance were significantly associated with time-since-fire,topographic position, elevation and organic layer depth. Among these variables, time-since-fire had the strongest effect and topographic position was the second major factor on affecting understorycommunity assembly. In addition, time-since-fire overwhelmed the effects of soil pH in the initial successional stage and gravimetric soil moisture in early and late successional stages on understory species composition Understanding the response of understory vegetation to fire disturbance is vital to biodiversity conservation and management of boreal forests. We surveyed understory vascular plant richness and composition, and measured related environmental variables along a toposequence within three successional stages, initial (3 years post-fire) , early (13 years post-fire) and late (> 100 years post-fire) successional stages. Using permutation multivariate ANOVA and nonmetric multidimensional scaling, we analyzed how understory species richness and composition change as time-since-fire proceeds, and their correies relationships with environmental variables. With these patterns showed significant differences among the three successional stages. Understory species richness and abundances were significantly associated with time-since-fire, topographic position, elevation and organic layer depth. -fire had the strongest effect and topographic position was the second major factor on affecting understorycommunity assembly. In addition, time-since-fire overwhelmed the effects of soil pH in the initial successional stage and gravimetric soil moisture in early and late successional stages on understory species composition