Photosynthesis and growth of two rain forest species in simulated gaps under elevated CO₂

Two species common to the temperate rain forests of New South Wales, Australia (Doryphora sassafras and Acmena smithii) were grown for 2 wk in either ambient (350 μL/L) or elevated (700 μL/L) CO₂ concentrations and low light (30 μmol photons·m^-2·s^-1) after which the seedlings were exposed for over 9 wk to a midday 2-h highlight period (1250 μmol photons·m^-2·s^-1, maximum) to simulate a tree fall gap. For both species, plants grown in elevated CO₂ had greater biomass than plants grown in ambient CO₂. However, relative increases in biomass were greater in Acmena, which is an early-successional species, than Doryphora, which is a late-successional species. Doryphora sassafras also had greater reductions in photosynthetic efficiency, as measured by chlorophyll fluorescence techniques (F(v)/F(m)) upon exposure to the high-light treatment than Acmena. Recovery in quantum efficiencies over time was observed for Doryphora, implying physiological acclimation to the new light environment. Plants grown in elevated CO₂ had lower values of F(v)/F(m) than plants grown in ambient CO₂, but these differences between CO₂ treatments were only significant for the late-successional Doryphora. Although exposure to the simulated tree fall gap dramatically increased the conversion of pigments of the xanthophyll cycle, as well as increased the total pool size of xanthophyll cycle pigments relative to total chlorophyll concentration, there were no differences in either parameter between CO₂ treatments. Leaves of Doryphora and those seedlings grown in elevated CO₂ had greater starch concentrations than Acmena and those seedlings grown in ambient CO₂, respectively. The reduction in quantum efficiencies for plants grown in elevated CO₂ and exposed to a simulated tree fall gap is discussed in the context of the importance of gap phase regeneration for species in rain forest ecosystems and the potential effects of global change on those processes.