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Title: ABOVE- AND BELOWGROUND RESPONSES OF C3-C4 SPECIES MIXTURES TO ELEVATED CO2 AND SOIL WATER AVAILABILITY

Author
item Derner, Justin
item Johnson, Hyrum
item Kimball, Bruce
item Pinter Jr, Paul
item Polley, Herbert
item Tischler, Charles
item BOUTTON, THOMAS - TEXAS A&M UNIVERSITY
item LAMORTE, ROBERT - FORMER USDA-ARS
item Wall, Robert
item Adam, Neal

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2003
Publication Date: 3/15/2003
Citation: DERNER, J.D., JOHNSON, H.B., KIMBALL, B.A., PINTER JR, P.J., POLLEY, H.W., TISCHLER, C.R., BOUTTON, T., LAMORTE, R., WALL, R.J., ADAM, N.R. ABOVE- AND BELOWGROUND RESPONSES OF C3-C4 SPECIES MIXTURES TO ELEVATED CO2 AND SOIL WATER AVAILABILITY. GLOBAL CHANGE BIOLOGY. 2003.

Interpretive Summary: Increasing concentration of atmospheric carbon dioxide (CO2) is a global change that affects plant performance, with greater responses often observed in cool-season (C3 photosynthetic pathway) than warm-season (C4 photosynthetic pathway) plants. Unfortunately, most previous experimental studies have evaluated either cool- or warm-season plant responses to elevated CO2 in single species settings. As a result, little is known how plants perform with elevated CO2 in mixtures containing both cool- and warm-season plants. We evaluated the influence of elevated CO2 on growth of cotton (cool-season) and sorghum (warm-season) plants grown in mixtures. Cotton, but not sorghum, growth was reduced in mixtures compared to control plants grown in single species plantings. Elevated CO2 did partially offset the reduction in growth of cotton in mixtures, however. These results indicate that elevated CO2 may influence competitive abilities of cool- and warm-season plants in mixtures. Because of this, elevated CO2 likely will affect changes in vegetation composition in natural ecosystems.

Technical Abstract: We evaluated the influences of CO2 [Control, ~370 umol-1; 200 umol mol-1 above ambient applied by free-air CO2 enrichment (FACE)] and soil water (Wet, Dry) on above- and below-ground responses of C3 (cotton, Gossypium hirsutum) and C4 (sorghum bicolor) plants in monocultures and two density mixtures. In monocultures, CO2 enrichment increased height, leaf area, above-ground biomass and reproductive output of cotton, but not sorghum, and was independent of soil water treatment. In mixtures, cotton, but not sorghum, performance was generally reduced compared to monocultures, across both CO2 and soil water treatments, implying that sorghum was the superior competitor. Density did not affect individual plant performance of either cotton or sorghum across the other treatments. Total (cotton + sorghum) leaf area and above-ground biomass in low-density mixtures were similar between CO2 treatments, but increased by 17-21% with FACE in high-density mixtures, due to a 121% enhancement of cotton leaf area and a 276% increase in biomass under FACE treatment. Total root biomass in the upper 1.2m of the soil was not influenced by CO2 or by soil water in monoculture or mixtures; however, under dry conditions we observed significantly more roots at lower soil depths (>45 cm). Sorghum roots comprised 81-85% of the total roots in the low density mixture and 58-73% in the high-density mixture. CO2-enrichment partly offset negative effects of competition on cotton in both low- and high-density mixtures by incresing above-ground biomass, with a greater relative increase in the high-density mixture. As a consequence, CO2-enrichment increased total above-ground yield of the mixture at high density. Individual plant responses to CO2 enrichment in global change models that evaluate mixed plant communities should be adjusted to incorporate feedbacks for interspecific competition. Future field studies in natural ecosystems should address the role that a CO2-mediated increase in C3 growth may have on subsequent vegetation change.