Integrating disturbance and colonization during rehabilitation of invasive weed-dominated grasslands

Developing ecological principles applicable to invasive plant management is central to implementing sustainable strategies. We tested portions of a potentially useful successional-based management framework to further our understanding of the relationship between disturbance and colonization during revegetation of invasive weed-dominated grasslands. We hypothesized (1) intermediate wheatgrass density and biomass would be greatest at highest seeding rates, (2) control and tillage procedures that increase the availability of safe sites would increase wheatgrass abundance, and (3) spotted knapweed density and biomass would be lowest in treatments with highest wheatgrass density and biomass. Treatments included three disturbance levels: (1) no disturbance, (2) application of glyphosate, and (3) fall tillage. Colonization treatments were seeding intermediate wheatgrass of 0, 500, 2,500, and 12,500 seeds m−2. Treatments were factorially applied in a randomized complete-block design with four replications at each of two sites located in Montana. Density and biomass of intermediate wheatgrass and spotted knapweed were sampled in 1997 and 2001. At both sites, seeding 2,500 or 12,500 seed m−2 increased wheatgrass density over that of the nonseeded control in 1997. The highest seeding rate produced almost three times as many wheatgrass plants as 2,500 seeds m−2 that year. By 2001, only the highest seeding rate produced wheatgrass densities greater than that of the nonseeded control at Bozeman. Seeding rates higher than 500 seeds m−2 yielded greater wheatgrass biomass than the nonseeded control with or without either tillage or glyphosate. At the highest seeding rate, tillage or glyphosate doubled intermediate wheatgrass biomass compared with no disturbance. Spotted knapweed generally had lower biomass where intermediate wheatgrass density and biomass was highest. One approach to rehabilitation is to design disturbances that favor desired species and then use high seeding rates that overwhelm the pool of available propagules and occupy a high percentage of safe sites. Nomenclature: Glyphosate; intermediate wheatgrass, Elytriga intermedia (Host) Nevski; spotted knapweed, Centaurea maculosa Lam.