The vegetation composition and standing crop of 44 stands, representing a diversity of topographic sites, were examined in a Northern Great Plains mixed prairie to determine the distribution, diversity, and relative production of different C3and C4dominated plant communities. Cluster analysis identified seven species groups at the 40 similarity level; these have been identified as seven community types: high prairie, midprairie, low prairie, meadow, depression, mud flats, and marsh. Environmental variables which showed the highest significant correlation with the three axes of a modified Bray and Curtis indirect ordination were soil moisture regime, soil potassium, and soil nitrate. Analysis of delta;13C biomass values indicate that the greatest abundance (nearly 50) of C4biomass occurs in low prairie with roughly one-third of meadow and high prairie biomass contributed by C4species. C4biomass contribution appears greatest on soils relatively low to moderate in nitrate, potassium, phosphorous, and moisture, yet C4species cooccur with C3species where these resources are low. Both productivity and species diversity are related to topographic features but productivity does not appear to be functionally determined by the relative proportion or contribution of C4taxa. The greatest number of C4species occupy upland slopes and ridges and within these habitats C3and C4grasses may be distributed differently depending on slope exposure.
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