INCREASING CO2 - COMPARATIVE RESPONSES OF THE C-4 GRASS SCHIZACHYRIUMAND GRASSLAND INVADER PROSOPIS

The woody C-3 Prosopis glandulosa (honey mesquite) and C-4 perennial g rass Schizachyrium scoparium (little bluestem) were grown along a grad ient of daytime carbon dioxide concentrations from near 340 to 200 mu mol/mol air in a 38 m long controlled environment chamber. We sought t o determine effects of historical and prehistorical increases in atmos pheric CO2 concentration on growth, resource use, and competitive inte ractions of a species representative of C-4-dominated grasslands in th e southwestern United States and the invasive legume P. glandulosa. In creasing CO2 concentration stimulated N-2 fixation by individually gro wn P. glandulosa and elicited in C-3 seedlings a similar relative incr ease in leaf intercellular CO2 concentration, net assimilation rate, a nd 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 th at was grown with S. scoparium was not affected by CO2 concentration. Stomatal conductance declined and leaf assimilation rates of S. scopar ium at near maximum incident light increased at higher CO2 concentrati on, but there was no effect of CO2 concentration on biomass production or whole-plant water use efficiency of the C-4 grass. Rising CO2 conc entration, especially the 27% increase since the beginning of the 19th century, may have contributed to more abundant P. glandulosa on C-4 g rasslands by stimulating the shrub's growth or reducing the amount of resources that the C-3 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.