COMPOSITION OF PRAIRIE PLANT-COMMUNITIES ON PRODUCTIVE VERSUS UNPRODUCTIVE SITES IN WET AND DRY YEARS

A natural productivity gradient was used to test questions about plant species composition, diversity, and sensitivity to environmental change of prairie vegetation within the tallgrass region. From 1986 to 1992 I monitored seasonal net aboveground production and species composition at four sites with soils that differed in texture and percent organic matter, pH, and concentrations of NH4, total N, K, Ca, Mg, and SO4. Four years of the survey featured above normal precipitation and 3 were drought years. August standing crop averaged 566 +/- 307, 419 +/- 143, 268 +/- 158, and 232 +/- 148 g . m(-2) at the four sites. Production generally increased with soil fertility (i.e., percent organic matter, total N, and K) and precipitation. The two more productive sites featured higher percentages of grass biomass, but legumes were rare. The site with the lowest soil N supported the consistently highest legume biomass and lowest grass biomass among sites. The least productive site displayed the highest percentage of composites. Species evenness, but not richness, was inversely related to August biomass for all sites. There were significant differences in production across years, as well as in percentages of grass, legume, and composite biomass. Total plot richness ranged from 24 to 40 species sampled per year at site 2 to 51-53 species at site 4, and tended to decline in the dry year 1989. Poor soils, although less productive overall, appear to prevent dominance by tall grasses and thereby maintain relatively more diverse spring and summer floras. Increased light availability near the soil surface probably enables the persistence of low-growing plants. Evenness, but not richness, varied among sites. The patterns of plant community composition have implications for restoration ecology as well as the design of prairielike perennial grain mixtures.