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Increasing evidence points to the major impact of intraspecific variability in species characteristics on community assembly. Contrasting theories predict that high trait variability might either steepening dominance orders or promote diversity. Here, we try to test these predictions with understorey plants in a Polish mixed temperate forest. We focus on four important traits (leaf dry mass LDW, plant height SH, specific leaf area SLA, and number of flowers N F) of the small balsam Impatiens parviflora , the dominant species in this forest system. We relate trait variability to community species richness and abundances, as well as to soil characteristics. Trait variability was high irrespective of species richness. Contrary to theory, abundances of non- I. parviflora species significantly decreased with increasing variance in I. parviflora trait expression. Trait variability did not significantly influence species composition and was not significantly correlated with soil characteristics. Our results emphasis the need to study trait variability in terms of appropriate statistical standards as provided by the allometric variance - mean relationship. We argue that an individual based study of local trait variability might return important insights into the composition and assembly of local communities. • Intraspecific trait expression of forest understorey plants is locally highly variable. • Plant species richness is unaffected by intraspecific trait variability. • Community composition is unaffected by intraspecific trait variability. • Plant abundances decrease with increasing intraspecific trait variability. [ABSTRACT FROM AUTHOR]