Response of Colorado tallgrass prairie to fire, winter grazing, and nitrogen manipulatio

This study examined management alternatives for conservation of Colorado tallgrass prairie. Fire, winter grazing and nitrogen manipulation were compared in a three-way factorial design at two sites. Plots were treated and monitored from 1993 to 1996. Fire treatments were applied in April and May. Winter grazing allowed cattle between December and May. Nitrogen treatments included fertilization and reverse fertilization (carbon addition to immobilize nitrogen). Responses included changes in soil temperature, moisture and nitrogen; aboveground plant biomass, and plant species composition.;Nitrogen was expected to limit production and control vegetation responses to fire and winter grazing. Data suggested that moisture limited production and controlled vegetation responses to treatments. Plots that were irrigated, or winter-grazed, were cooler and wetter than other plots. Burned plots were warmer at the non-irrigated site.;Fire and reverse fertilization were both expected to reduce nitrogen and exotic cover. Reverse fertilization reduced ammonium and fire showed a similar trend. However, neither treatment affected native or exotic stem density or cover. Fire increased native species diversity. Reverse-fertilization reduced overall species diversity and forb stem density, but increased C$sb4$ grass cover.;Productivity was related both to resource availability, and cover frequency of the highly productive native C$sb4$ grasses that dominate tallgrass prairie. Winter grazing increased production at the irrigated site by increasing cover of native C$sb4$ grasses. Fertilization failed to stimulate production because it induced species shifts reducing native C$sb4$ grass cover. Fire increased biomass only at the irrigated site in the wettest year. Typical treatment responses for Colorado tallgrass were consistent with drought year treatment responses in eastern tallgrass prairie. At the non-irrigated site, winter grazing, like chronic grazing, reduced production and forb stem density. At the irrigated site, however, winter grazing differed from chronic grazing, increasing production, native species diversity, native cover, and C$sb4$ grass cover.