PHOTOSYNTHESIS AND GROWTH OF 2 RAIN-FOREST SPECIES IN SIMULATED GAPS UNDER ELEVATED CO2

Two species common to the temperate rain forests of New South Wales, A ustralia (Doryphora sassafras and Acmena smithii) were grown for 2 wk in either ambient (350 mu L/L) or elevated (700 mu L/L) CO2 concentrat ions and low light (30 mu mol photons . m(-2). s(-1)) after which the seedlings were exposed for over 9 wk to a midday 2-h highlight period (1250 mu mol photons . m(-2). s(-1), maximum) to simulate a tree fall gap. For both species, plants grown in elevated CO2 had greater biomas s than plants grown in ambient CO2. However, relative increases in bio mass were greater in Acmena, which is an early-successional species, t han Doryphora, which is a late-successional species. Doryphora sassafr as also had greater reductions in photosynthetic efficiency, as measur ed by chlorophyll fluorescence techniques (F-v/F-m) upon exposure to t he high-light treatment than Acmena. Recovery in quantum efficiencies over time was observed for Doryphora, implying physiological acclimati on to the new light environment. Plants grown in elevated CO2 had lowe r values of F-v/F-m than plants grown in ambient CO2, but these differ ences between CO2 treatments were only significant for the late-succes sional Doryphora. Although exposure to the simulated tree fall gap dra matically increased the conversion of pigments of the xanthophyll cycl e, as well as increased the total pool size of xanthophyll cycle pigme nts relative to total chlorophyll concentration, there were no differe nces in either parameter between CO2 treatments. Leaves of Doryphora a nd those seedlings grown in elevated CO2 had greater starch concentrat ions than Acmena and those seedlings grown in ambient CO2, respectivel y. The reduction in quantum efficiencies for plants grown in elevated CO2 and exposed to a simulated tree fall gap is discussed in the conte xt of the importance of gap phase regeneration for species in rain for est ecosystems and the potential effects of global change on those pro cesses.