A 7-yr study was conducted to investigate successional dynamics of herbaceous vegetation after control of western juniper (Juniperus occidentalis Hook.) trees with picloram (4-amino-3,5,6-trichloropicolinic acid) pellets. Downy brome (Bromus tectorum L. # BROTE) rapidly dominated areas under dead tree canopies with an accompanying yield increase from almost none to about 1400 kg/ha within 4 yr after treatment. In the interstitial zone between tree canopies, a dramatic shift in herbaceous species occurred with tree control. Annual broadleaf species decreased, downy brome increased, and medusahead [Taeniatherum asperum (Sim.) Nevski # ELYCM] gradually became dominant. Total yield of herbaceous vegetation in the interstitial zone increased dramatically after picloram treatment, especially where few shrubs occurred. Available soil moisture, litter accumulation and decay after picloram treatment, and N in the surface layer of the soil affected responses of herbaceous vegetation, especially under dead tree canopies. Responses of annual weedy species to western juniper control emphasize the need for comprehensive manipulation of all components of the vegetation when range improvement is attempted.