1. Dominant species can act as a biotic filter in structuring plant communities by constraining the establishment and survival of subordinate species. The effect of intraspecific trait variability of dominant species on the functional response of subordinate species, however, is not well understood. 2. We quantified intraspecific variation in four functional traits of 26 subordinate species established in an experimental grassland established with two population sources (i.e., cultivars and local ecotypes) of three dominant grasses (Sorghastrum nutans, Andropogon gerardii, and Schizachyrium scoparium) and three pools of subordinate species (each from one origin) within each of the dominant grass source treatments. 3. Twenty of the 26 subordinate species exhibited intraspecific trait variability for one trait or more in response to dominant species population source, and variation among population sources of the dominant species was non-random. Dominant grass population source affected intraspecific variability in functional traits of multiple subordinate species. Cultivar sources of the dominant grasses and some of the subordinate species that established with them had higher and generally more variable functional leaf area and leaf nitrogen content compared to local ecotypes of the dominant grasses and the subordinate species that established with them. 4. Synthesis. This study provides evidence that intraspecific trait variability in dominant species acts as an inner, biotic filter to constrain niche availability and dimensionality affecting trait variation of subordinate species during community assembly.