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Increasing CO2: Comparative Responses of the C4 Grass Schizachyrium and Grassland Invader Prosopis

Polley, H. Wayne, Johnson, Hyrum B., Mayeux, Herman S.
Ecology 1994 v.75 no.4 pp. 976-988
C4 plants, Prosopis glandulosa, Schizachyrium scoparium, aboveground biomass, belowground biomass, biomass production, carbon dioxide, carbon dioxide enrichment, grasses, grasslands, invasive species, leaves, legumes, net assimilation rate, nitrogen fixation, nutrient use efficiency, seedlings, shrubs, stomatal conductance, water use efficiency, Southwestern United States
The woody C₃ Prosopis glandulosa (honey mesquite) and C₄ perennial grass Schizachyrium scoparium (little bluestem) were grown along a gradient of daytime carbon dioxide concentrations from near 340 to 200 μmol/mol air in a 38 m long controlled environment chamber. We sought to determine effects of historical and prehistorical increases in atmospheric CO₂ concentration on growth, resource use, and competitive interactions of a species representative of C₄—concentration of C₄—dominated grassland in the southwestern United States and the invasive legume P. glandulosa. Increasing CO₂ concentration stimulated N₂ fixation by individually grown P. glandulosa and elicited in C₃ seedlings a similar relative increase in leaf intercellular CO₂ concentration, net assimilation rate, and intrinsic water use efficiency (leaf net assimilation rate/stomatal conductance). Aboveground biomass of P. glandulosa was not altered by CO₂ concentration, but belowground biomass and whole—plant water and nitrogen use efficiencies increased linearly with CO₂ concentration in seedlings that were grown alone. Biomass produced by P. glandulosa that was grown with S. scoparium was not affected by CO₂ concentration. Stomatal conductance declined and leaf assimilation rates of S. scoparium at near maximum incident light increased at higher CO₂ concentration, but there was no effect of CO₂ concentration on biomass production or whole—plant water use efficiency of the C₄ grass. Rising CO₂ concentration, especially the 27% increase since the beginning of the 19th century, may have contributed to more abundant P. glandulosa on C₄ grasslands by stimulating the shrub's growth or reducing the amount of resources that the C₃ required. Much of the potential response of P. glandulosa to CO₂ concentration, however, appears to be contingent on the shrub's escaping competition with neighboring grasses.