MAXIMIZATION OF ABOVEGROUND GRASSLAND PRODUCTION - THE ROLE OF DEFOLIATION FREQUENCY, INTENSITY, AND HISTORY

Production of tallgrass prairie vegetation was measured on experimenta l plots in which defoliation intensity and frequency were manipulated by mowing and using movable exclosures on areas chronically grazed by cattle. Defoliation history largely controlled whether or not defoliat ed plants overcompensated (exhibited enhanced production compared to u ndefoliated controls) for tissue removal. Plants on chronically grazed sites only compensated for foliage removed by grazers. Production on plots mowed prior to the year of measurement was similar to that on ch ronically grazed sites, while previously unmowed plots exhibited subst antial aboveground overcompensation. Aboveground production was maximi zed by the most frequent mowing treatment and by intermediate mowing h eights. Nitrogen and phosphorus concentrations and amounts in abovegro und tissues were increased by mowing and grazing. Current mowing regim e was more important than mowing history in determining nitrogen conce ntrations except very early in the growing season. Effects of grazing and mowing on belowground biomass were inconsistent, but frequent mowi ng appeared to limit accumulation of belowground N reserves and biomas s. In North American grasslands, overcompensation is a nonequilibrium plant response to grazing. Photosynthate that would be stored as reser ves and used for root growth and flower and seed production instead is used to replace lost leaf area, thereby resulting in higher foliage p roductivity. However, under chronic grazing or mowing, vegetation is p revented from maintaining high nutrient and water uptake capacity (lar ge root biomass) and accumulating reserves that allow overcompensation responses.