STEM AND LEAF GAS EXCHANGE OF TWO ARID LAND SHRUBS

Gas exchange studies were conducted on two shrub species found in cool shrub‐steppe communities of the American West, big sagebrush ( Artemisia tridentata subsp. tridentata Nutt.) and broom snakeweed ( Gutierrezia sarothrae [Pursh] Britt. and Rusby), with a goal of evaluating characteristics and relative contributions of green stem and leaf material to total shoot CO 2 exchange at different temperatures. Variations in tissue temperature exerted a pronounced effect on CO 2 exchange—net photosynthesis and dark respiration—of green stems and leaves of both species. Definite temperature optima of net photosynthesis were noted, and dark respiration rates consistently increased with increases in temperature. Green stems of both species exhibited sizable dark respiration rates, although stem rates at all temperatures were lower than corresponding leaf rates. Artemisia tridentata did not exhibit sizeable green stem net photosynthesis even under conditions of optimal temperature and water availability, and leaf net photosynthesis rates were much lower than those of G. sarothrae. However, A. tridentata in general possessed a greater leaf biomass than G. sarothrae. Green stems of G. sarothrae exhibited considerable rates of net photosynthesis under both optimal and sub‐optimal temperature and water availability conditions. A higher optimum temperature of net photosynthesis was noted for stems than for leaves of G. sarothrae. The adaptive significance of these interspecific differences in CO 2 exchange characteristics is discussed.