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Japanese brome (Bromus japonicas Thunb.) is an annual grass that has invaded thousands of hectares of Northern Great Plains rangelands. We studied the effect of Japanese brome on the current year's increase in biomass in a plant community in the Northern Great Plains dominated by western wheatgrass [Pascopyrum smithii Rydb. (Love)]. In our experiment, brome seedlings were either removed or left in place in replicated l-m2 plots. Above-ground biomass of western wheatgrass increased (891 to 1,095 kg ha-1 ) with the removal of Japanese brome. However, total above-ground biomass decreased (1,873 to 1,334 kg ha-1) when brome was reduced in early spring (708 to 12 kg ha-1). Increased biomass of western wheatgrass resulted from increases in the density of tillers and not in the weight of each tiller. Since the effect of removing brome did not vary among the combinations of site and year, similar outcomes can be expected over a wide array of environmental conditions, such as among years with variable April to late-June or mid-July precipitation or stands with varying percents of brome and western wheatgrass. Thus, we conclude that the presence of annual Japanese brome reduces the biomass of important grasses in the Northern Great Plains.