Ranchers and resource managers have become concerned about the role and spread of Cardaria draba (L.) Desv. (whitetop) in the sagebrush steppe of Northeastern Oregon. This area is an important natural resource for livestock production and big game winter range. An area near Keating, Oregon was selected to study the influence of environmental conditions on whitetop seed germination, mode of spread and species displacement by whitetop. Whitetop seeds were placed in the field at four different topographical positions (toe slope, mid-slope, north slope and south slope) and placed at three different locations in the soil profile (0, 1, and 3 cm). Seed placed on the toe slope position at 0 cm had the greatest germination. At the south aspect greater germination occurred at lower soil profile locations. Increases in whitetop populations were evaluated by establishing permanent transects in two locations in the study area. Whitetop shoot density increased during the three year study. Whitetop density increase was restricted to previously infested plots and was due to vegetative reproduction. In whitetop infested areas, crested wheatgrass density was less than in non-infested areas. Whitetop germination at different topographical positions was apparently related to moisture conditions. Once established, whitetop populations increase through vegetative reproduction. This phenomenon is reflected in a reduction of crested wheatgrass density and production as the level of whitetop infestation increases. / Graduation date: 1992