GROWTH AND PHYSIOLOGY OF ONE-YEAR-OLD POPLAR (POPULUS) UNDER ELEVATED ATMOSPHERIC CO2 LEVELS

The effects of elevated atmospheric CO2 concentrations on the ecophysiological responses (gas exchange, chlorophyll a fluorescence, Rubisco activity, leaf area development) as well as on the growth and biomass production of two poplar clones (i.e. Populus trichocarpa x P. deltoides clone Beaupre and P. x euramericana clone Robusta) were examined under open top chamber conditions. The elevated CO2 treatment (ambient + 350 mu mol mol-1) stimulated above-ground biomass of clones Robusta and Beaupre after the first growing season by 55 and 38 %, respectively. This increased biomass production under elevated CO2 was associated with a significant increase in plant height, the latter being the result of enhanced internode elongation rather than an increased production of leaves or internodes. Both an increased leaf area index (LAI) and a stimulated net photosynthesis per unit leaf contributed to a significantly higher stem biomass per unit leaf area, and thus to the increased above-ground biomass production under the elevated CO2 concentrations in both clones. The larger LAI was caused by a larger individual leaf size and leaf growth rate; the number of leaves was not altered by the elevated CO2 treatment. The higher net leaf photosynthesis was the result of an increase in the photochemical (maximal chlorophyll fluorescence Fm and photochemical efficiency Fv/Fm) as well as in the biochemical (increased Rubisco activity) process capacities. No significant differences were found in dark respiration rate, neither between clones nor between treatments, but specific leaf area significantly decreased under elevated CO2 conditions. (C) 1995 Annals of Botany Company