GROWTH, SURVIVAL COMPETITION, AND CANOPY CARBON-DIOXIDE AND WATER-VAPOR EXCHANGE OF 1ST YEAR ALFALFA AT AN ELEVATED CO2 CONCENTRATION

Alfalfa was grown in field plots at the current CO2 concentration (350 mumol mol-1 = c350) and at 350 mumol mol-1 above the current concentr ation (= c700). Alfalfa and weed growth, and canopy water vapor (E) an d carbon dioxide exchange (F) were determined for the first year. Alfa lfa yield summed for the three harvests.in the first year was greater for the c700 treatment in two of the years studied, but significantly less in a third year. Weed growth was unaffected. Survival of alfalfa plants was greater at c700 for years in which there was substantial mo rtality, even when yield was not increased by the c700 treatment. In s pite of a persistent reduction in leaf conductance to water vapor (g(l )), total canopy conductance (g(c)) to water vapor did not differ betw een CO2 treatments when averaged over years, because of compensating c hanges in canopy leaf area. CO2 efflux (F) at night per unit of ground area was consistently less in the c700 treatment even when daytime CO 2 uptake was higher. Hence the periodic harvesting of alfalfa crops do es not necessarily allow elevated CO2 to cause persistent growth stimu lation nor reduced water use.