LONG-TERM GROWTH OF ALFALFA AND ORCHARD GRASS PLOTS AT ELEVATED CARBON-DIOXIDE

Alfalfa (Medicago sativa L.) and orchard grass (Dactylis glomerata L.) plots were exposed to ambient or ambient plus 350 cm(3) m(-3) carbon dioxide concentrations at Beltsville, Maryland, U.S.A. Replicate plots were established in different years and fertilized annually. We repor t here data for the second and third years after establishment. There has been no increase in the yearly production of either species at the elevated carbon dioxide concentration after the first season. In orch ard grass, reduced growth at the high carbon dioxide concentration in the spring offset growth stimulation in the summer. Weed growth was co nsistently increased by carbon dioxide enrichment, but weed species co mposition was unaffected. Leaf photosynthetic capacity was reduced by the high carbon dioxide concentration in both crop species, as was lea f nitrogen content. Canopy carbon dioxide uptake was slightly higher i n the elevated carbon dioxide treatments, consistent with the increase d weed growth. In alfalfa, elevated carbon dioxide significantly reduc ed canopy carbon dioxide efflux at night for the same daytime uptake r ate and temperature. The growth conversion efficiency estimated from e lemental composition of the tissue was not substantially altered by ca rbon dioxide treatment in either crop species, indicating little effec t of carbon dioxide treatment on the respiratory cost of tissue synthe sis. Canopy conductance to water vapour averaged 23% less at high than at low carbon dioxide in the orchard grass plots, and 14% less in the alfalfa plots. This was consistent with the smaller short-term respon se of conductance to carbon dioxide concentration in the alfalfa plots . It is concluded that a warm climate and fertile soil does not guaran tee a persistent response of production to elevated carbon dioxide con centration in these herbaceous perennial species.