STRATEGIES FOR MIXED-GRASS PRAIRIE RESTORATION - HERBICIDE, TILLING, AND NITROGEN MANIPULATION

Large areas of North American prairie have been planted with grasses i ntroduced from Eurasia. We examined three strategies (herbicide, tilli ng, and nitrogen manipulation) for enhancing the establishment of seed lings of native species and suppressing the introduced grasses Agropyr on cristatum (crested wheat grass) and Bromus inermis (smooth brome). Plots (5 X 15 m) were subjected to one of three levels of tilling (non e, intermediate, complete) and four levels of nitrogen (none, intermed iate, high, and sawdust added to immobilize nitrogen). Treatments were applied in a factorial design with twelve treatments and ten replicat es. Seeds of 41 native species were drilled into the plots in May 1992 . Following the failure of seeds to establish in 1992, a subplot (5 X 13 m) within each main plot was sprayed with the herbicide glyphosate in April 1993. The nitrogen treatments were repeated in Spring 1993. I n August 1993, the density of native seedlings in sprayed subplots was 20 times that in unsprayed subplots. Within sprayed subplots, native seedling density and the cover of bare ground decreased significantly with increasing nitrogen availability. Plots receiving sawdust had sig nificantly higher mean cover of bare ground and significantly lower co ncentrations of soil available nitrogen. Native seedling density was s ignificantly higher in plots receiving the highest intensity of tillin g. The responses of native seedlings to all these factors point to the importance of neighbor-free establishment sites as a prerequisite for prairie restoration.