The response of plants to elevated CO2 : IV. Two deciduous-forest tree communities.

Tree saplings, two groups of three species from each of two deciduous tree communities, were grown in competition at three CO2 concentrations and two light levels. After one growing season, biomass was measured to assess the effect of CO2 on community structure, and nitrogen and phosphorus concentrations were measured for leaves, stems, and roots of all trees. Gas-exchange measurements were made on the same species grown under the same CO2 concentrations.Photosynthetic capacity (rate of photosynthesis at saturating CO2 and light) tended to decline as CO2 concentration increased, but differences were not statistically significant. Stomatal conductance declined significantly as CO2 increased. Nitrogen and phosphorus concentrations generally declined as CO2 increased, but there were some unexpected patterns in roots and stems. CO2 concentration did not significantly affect the overall growth of either community after one season, but the relative biomass of each species changed in a complex way, depending on CO2 light level, and community.