Field measurements of photosynthesis, water-use efficiency, and growth in Agropyron smithii (C₃) and Bouteloua gracilis (C₄) in the Colorado shortgrass steppe

Field measurements of gas exchange and growth were conducted on a C₃ grass, Agropyron smithii, and a C₄ grass, Bouteloua gracilis, in order to further establish the adaptive significance of the C₄ pathway under natural conditions. Maximum rates of leaf area expansion in tillers and maximum seasonal photosynthesis rates of both species occurred during the cool, early summer month of June. The occurrence of maximum growth and photosynthesis in B. gracilis during this cool period was apparently related to its occupation of warm microenvironments next to the ground surface. As temperatures increased during the midsummer, photosynthesis rates decreased to 47% and 55% of the seasonal maximum in B. gracilis and A. smithii, respectively. Water-use efficiencies in both species were similar or slightly higher for B. gracilis during June, the period of maximum growth. By mid-July, however, leaves of the C₃ grass, A. smithii, exhibited water-use efficiencies approximately half as high as B. gracilis. These differences in water-use efficiency were the result of differences in stomatal conductance, rather than differences in daily CO₂ uptake rates which were similar in both species. The results demonstrate that in certain environments there are no offset periods of growth and maximum photosynthesis during the growing season in these C₃ and C₄ species. The greater amounts of daily water use in A. smithii during the midsummer might contribute to its much greater abundance in lowland sites in the shortgrass steppe. The C₄ grass, B. gracilis, occurs in dry upland sites in addition to the more mesic lowland sites.