Water potential, leaf conductance, growth, nitrogen content, and seedling survival of Wyoming Big Sagebrush (Artemisia tridentata ssp. wyomingensis) following defoliation of the herbaceous understory were assessed during two growing seasons. Precipitation was 107% and 63% of the long-term mean (283 mm) in 1989 and 1990, respectively, which presented an opportunity to study impacts during a drought and a non-drought year. Response of Artemisia was measured on a site seeded to Agropyron desertorum in the late 1960s, and a native site with Stipa thurberiana, Festuca idahoensis and Poa sandbergii in the understory. The two sites were analyzed as separate experiments. The dry year had a significant effect on plant water relations and growth of Artemisia on both sites. Pre-dawn water potentials averaged 1.23 MPa more negative on the seeded site and 1.22 MPa more negative on the native site in 1990 compared to the previous year. Mid-day water potential averaged 1.22 MPa more negative on the seeded site and 1.13 MPa more negative on the native site in 1990 compared to 1989. Morning leaf conductance in 1990 was 61% lower on the seeded site and 51% lower on the native site than in 1989. Mean afternoon leaf conductance in the drought year was 62% less on the seeded site and 63% less on the native site. Nitrogen content in current year's growth was reduced 29% on the seeded site and 18% on the native site from 1989 to 1990. Vegetative and reproductive shoot (stem plus leaves) weights were reduced by over 80% in 1990 compared to 1989. Other production variables showed similar reductions in the drought year. Nitrogen content was 11% greater in current year's growth from shrubs on control as opposed to defoliated plots on the native site in 1989. Shrubs on control plots had 8% heavier reproductive stems per unit of canopy, and 7% longer vegetative stems than shrubs on defoliated plots in 1989 on the seeded site. Shrubs on control plots had 18% fewer annual leaves per unit of vegetative shoot, but 12% more primary ephemeral leaves per vegetative shoot than shrubs on defoliated plots in 1990 on the seeded site. In 1990, shrubs on control plots on the seeded site had 11% more ephemeral leaves per vegetative shoot while shrubs on the control plots on the native site had 8% fewer ephemeral leaves per vegetative shoot. Large shrubs generally had more and heavier leaves, and longer and heavier stems than medium shrubs. Exceptions included large shrub lateral stems were 30% shorter than those of medium shrubs, and large shrubs had 37% fewer annual leaves per vegetative shoot than medium shrubs in 1989 on the seeded site. Drought had substantial negative impact on water relations and growth of Wyoming big sagebrush, while defoliation of understory vegetation had little effect. / Graduation date: 1993