|Bradford, James - Jim|
Submitted to: International Rangeland Congress
Publication Type: Proceedings
Publication Acceptance Date: 4/18/2003
Publication Date: 7/26/2003
Citation: SIMS, P.L., BRADFORD, J.A. 2003. EVAPOTRANSPIRATION IN SOUTHERN PLAINS MIXED-GRASS PRAIRIES. INTERNATIONAL RANGELAND CONGRESS. pp. 1094-1096. Interpretive Summary: Grasslands are important components of semi-arid and arid rangeland ecosystems that cover more than 50 percent of the world's land area. The role rangelands play in the global carbon and water budgets is not well understood even though rangelands dominate the landscape. The objective of this research was to determine the relationship between efficiency of water-use and vegetation structure and plant growth on the mixed-grass prairie of the Southern Plains, an important livestock producing region in the Central U. S. One study area was a grass dominated mixed-grass prairie and the other study site was dominated by sand sagebrush with an under story of both warm-season and cool-season grasses. Water-use, as measured by evapotranspiration (ET) from 1995 through 1997 averaged about 660 mm and 1.8 mm/day on both sites. This indicates that the grass dominated site and the sagebrush dominated site used all of the 671 mm of annual precipitation regardless of the type of vegetation. Earlier studies also found that daily carbon dioxide uptake in this same prairie were closely tied to precipitation. However, the differences in the vegetation did result in seasonal differences in water-use, as well as plant growth, on these two mixed-grass prairie sites. The role of sand sagebrush in water-use needs further evaluation.
Technical Abstract: Grasslands are an important component of the temperate and arid rangeland ecosystems that comprise over 50 percent of the world's land area. The role rangelands play in the global C and water budgets is inadequately documented even though drought often limits carbon dioxide uptake and plant growth. The objective of this research was to determine the relationship between evapotranspiration (ET), vegetation structure, and seasonal carbon dioxide flux for two structurally different Southern Plains mixed-grass prairie sites; one dominated by warm-season grasses and the other dominated by Artemisia filifolia Torr. (sand sagebrush) with a complement of both warm-season and cool-season grasses. ET measured over the 1995, 1996, and 1997 sampling periods averaged 564 and 515 mm for the grass and sagebrush dominated mixed-grass prairie sites, respectively. Daily ET averaged 2.28 mm/day for the grassland and 2.30 mm/day for the sagebrush site. When extrapolated to an annual basis, ET on both sites averaged about 660 mm and 1.8 mm/day. Earlier reports found that daily carbon dioxide fluxes in this Southern Plains mixed-grass prairie were closely tied to precipitation since plant growth is primarily a function of soil water, radiation, and temperature. The similarity in total leaf area index (LAI) on these two sites would also indicate a similar potential for ET even though carbon dioxide uptake on grassland site was 0.70 g per meter square per day compared to 0.06 g per meter square per day for the sagebrush site. However, the diversity of species and species groups and their contribution to LAI resulted in seasonal differences in ET, as well as carbon dioxide flux, on these two mixed-grass prairie sites. In addition, the role of sand sagebrush in water-use cannot be fully evaluated from these data. Vegetation structure did affect the seasonal fluxes of both carbon dioxide and ET but the impact estimated on an annual basis was similar.