Since plants utilize CO��� as the substrate for photosynthesis, terrestrial plants may be directly affected by increasing levels of CO��� in the atmosphere. Plants native to the sagebrush steppe are predicted to increase in growth in response to elevated CO��� through increased water use efficiency and higher photosynthetic rates. This study examined the interactions between edaphic factors and CO��� in order to determine how species native to the sagebrush steppe may respond to elevated CO���. The objectives of these experiments were to: 1. determine if Sitanion hystrix, Artemisia tridentata ssp. wyomingensis, and Stipa thurberiana alter their growth and physiology in response to CO��� and soil temperature. 2. determine if Sitanion hystrix and Artemisia tridentata ssp. wyomingensis alter their growth and physiology in response to CO��� and water stress. Two experiments were conducted using environmentally controlled chambers. In the first experiment, Sitanion hystrix, Artemisia tridentata and Stipa thurberiana were exposed to ambient (374 ppm) or high (567 ppm) CO��� conditions and low (13��C) or high (18��C) soil temperature. After four months in the chambers, plants were harvested and plant material was divided into shoots, roots, and leaves. Results from the first experiment demonstrated that carbon dioxide and soil temperature modified the growth of these species. Sitanion hystrix increased its shoot and root weights at elevated CO��� when grown under low soil temperatures. Artemisia tridentata had lower plant weights under elevated CO��� and 18 ��C soil temperature than plants grown at ambient CO��� and 13��C. Shoots of Stipa thurberiana were responsive to soil temperature and roots were responsive to CO��� at 18��C. In the second experiment, Sitanion hystrix and Artemisia tridentata were exposed to ambient (371 ppm) or high (569 ppm) CO��� and well-watered or water stressed conditions. Results indicated that there were no interactive effects betweeen CO��� and water stress with respect to plant growth or physiology. CO��� increased water use efficiency in S. hystrix and increased water use efficiency of A. tridentata at the beginning of the experiment but had no interactive effects with water stress on growth or photosynthesis. Results suggested that the effect of CO��� on plant growth and productivity of the sagebrush steppe is dependent upon the soil temperature to which the plants are exposed. Differences between species in their response to CO���, soil temperature, and water stress were also apparent in this experiment. These controlled environment studies should pave the way for field studies in the sagebrush steppe in order to determine if differences in carbon allocation, resulting from changes in CO��� and soil temperature, are realized in the field. Alterations in carbon allocation may potentially alter the competitive relationships between species and influence successional processes in the sagebrush steppe. / Graduation date: 1997