Field measurements of photosynthesis, water-use efficiency, and growth inAgropyron smithii (C3) andBouteloua 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 inB. 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 inB. gracilis andA. smithii, respectively. Water-use efficiencies in both species were similar or slightly higher forB. 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 asB. 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 inA. 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.