Artemisia tridentata Nutt. ssp. vaseyana (mountain big sagebrush) grows at slightly higher, cooler, and drier sites than does A.t. ssp. tridentata (basin big sagebrush). Natural hybrids between the two subspecies are found on a single east-facing hillside in Salt Creek Canyon near Nephi, Utah, where the parent populations are separated by 85 m in elevation and 1.1 km linear distance along the transect. In 1993, three gardens were established with seedlings from five populations along the transect planted in each garden. Stem water potential was measured at the site using a Scholander Pressure Bomb. Gas exchange respiration was measured with an infra-red gas analyzer. Plant stress was assessed with a chlorophyll fluorescence meter and carbon isotope ratios. Mineral concentrations in soil and plant tissue were measured using Inductively Coupled Plasma spectroscopy and spectrophotometric methods. Calorimetry was used to measure rates of metabolic heat loss and respiration on the same tissue. Significant differences were observed among the gardens whatever the source of origin and among the plant sources in whichever garden they were grown. Acclimation, showing phenotypic plasticity, occurred with change of season. Both nature and nurture have an influence. Sagebrush plants from all sources grow well at cool temperatures (when moisture is abundant), but are stressed and growth ceases at temperatures much above 30 C (when moisture is scarce). Sagebrush can become dormant to withstand high summer temperatures, but may have no mechanism for slowing growth during very cold conditions, and thus may be damaged if unprotected (Nelson and Tiernan 1983). Metabolic distinctions are made among closely related populations of plants grown on a single hillside in environments with only slight differences, showing physiological adaptation to very small environmental differences.

Key words: Artemisia tridentata, hybrids, respiration, subspecies, temperature