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Title: Vulnerability of crops and native grasses to summer drying in the U.S. Southern Great Plains

Author
item YASEEF, RAZ - Lawrence Berkeley National Laboratory
item BILLESBACH, DAVE - University Of Nebraska
item FISCHER, M. - Lawrence Berkeley National Laboratory
item BIRAUD, S. - Lawrence Berkeley National Laboratory
item Gunter, Stacey
item Bradford, James
item TORN, MARGARET - Lawrence Berkeley National Laboratory

Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2015
Publication Date: 8/31/2015
Publication URL: http://handle.nal.usda.gov/10113/61426
Citation: Yaseef, R., Billesbach, D., Fischer, M.L., Biraud, S.C., Gunter, S.A., Bradford, J.A., Torn, M.S. 2015. Vulnerability of crops and native grasses to summer drying in the U.S. Southern Great Plains. Agriculture, Ecosystems and Environment. 213:209-218.

Interpretive Summary: The Southern Great Plains are characterized by a mixture of different land uses, predominantly winter-wheat and grazed pasture lands, with relatively small areas of other crops; native rangelands is the vegetation on the majority of the Southern Great Plains. Recent droughts and predictions of increased drought, especially during the summer months, in the Southern Great Plains raise concern about the function of these ecosystems. We measured carbon, water, and energy fluxes over cultivated cropland for 10 years, and over lightly grazed prairie and new switchgrass fields for 2 and 4 years, respectively. Growing-season precipitation showed the strongest control over carbon uptake by any system, but the precipitation use efficiency varied between them: Grasses (prairie and switchgrass) needed at least 14 inches of precipitation during the growing season to become carbon sinks, while crops needed only 4 inches. In summer, high temperatures increased water loss through evaporation and led to higher likelihood of dry soils. Therefore, summer-growing native prairie species and switchgrass experienced more seasonal droughts than spring-growing wheat. The reduction in carbon uptake resulted mostly from a decrease in forage production, and to a lesser extent, from increased plant water use. Management practices for crops were effective in suppressing were loss and plant carbon loss after harvest and control of secondary growth, and increased carbon uptake during the growing season. In light of future projections for wetter springs and drier, warmer summers in the Southern Great Plains, our study indicates increased vulnerability in native ecosystems and summer crops over time.

Technical Abstract: The Southern Great Plains are characterized by a fine-scale mixture of different land cover types, predominantly winter-wheat and pasture lands, with relatively small areas of other crops, native prairie, and switchgrass. Recent droughts and predictions of increased drought (especially during the summer months) in the Southern Great Plains raise concern for these ecosystems. We measured ecosystem carbon, water, and energy fluxes with eddy-covariance systems over cultivated cropland for 10 years, and over lightly grazed prairie and new switchgrass fields for 2 and 4 years, respectively. Growing-season precipitation showed the strongest control over carbon uptake, but the precipitation use efficiency varied between ecosystems: Grasses (prairie and switchgrass) needed at least 350 mm of precipitation during the growing season to become net carbon sinks, while crops needed only 100 mm. In summer, high temperatures enhanced evaporation and led to higher likelihood of dry soil conditions. Therefore, summer-growing native prairie species and switchgrass experienced more seasonal droughts than spring-growing wheat. The net reduction in carbon uptake resulted mostly from a decrease in gross primary production, and to a lesser extent, from increased respiration. Management practices for crops were effective in suppressing evapotranspiration and decomposition after senescence (harvest and removal of secondary growth), and increased carbon uptake during the growing season (fertilization and decreased water loss). In light of future projections for wetter springs and drier (and warmer) summers in the Southern Great Plains, our study indicates increased vulnerability in native ecosystems and summer crops over time.