Two distinct subpopulations of wood bison Bison bison athabascae inhabit the Mackenzie Bison Sanctuary and adjacent areas: the ML subpopulation which is increasing and inhabits the periphery of the bison range, and the MBS subpopulation which is stabilizing and inhabits the core area of the bison range. This system provided the unique opportunity to examine how the plant community responded to an erupting, indigenous herbivore population. The standing crop of sedges and grasses in willow savanna habitats located in the core area of the range (MBS) was consistently lower than that of willow savannas located at the periphery (ML). The difference in standing crop between areas appears to be a direct result of different grazing pressures between the two areas. Net primary production was similar between MBS and ML indicating that the vegetation in MBS showed at least some partial compensation in response to grazing. The species composition of willow savannas differed between areas. Willow savannas located in MBS showed an increase in more unpalatable or less preferred species compared to willow savannas located in ML. Forages were of similar quality between the areas.