Interaction of ozone pollution and light effects on photosynthesis in a forest canopy experiment

Ozone pollution may reduce net carbon gain in forests, yet data from mature trees are rare and the effects of irradiance on the response of photosynthesis to ozone remain untested. We used an open‐air system to expose 10 branches within the upper canopy of an 18‐m‐tall stand of sugar maple (Acer saccharum Marsh.) to twice‐ambient concentrations of ozone (95nmol mol⁻¹, 0900 to 1700, 1 h mean) relative to 10 paired, untreated controls (45nmol mol⁻¹) over 3 months. The branch pairs were selected along a gradient from relatively high irradiance (PPFD 14.5 mol m⁻² d⁻¹) to deep shade (0.7mol m⁻² d⁻¹). Ozone reduced light‐saturated rates of net photosynthesis (A_sat) and increased dark respiration by as much as 56 and 40%, respectively. Compared to sun leaves, shade leaves exhibited greater proportional reductions in A_sat and had lower chlorophyll concentrations, quantum efficiencies, and leaf absorptances when treated with ozone relative to controls. With increasing ozone dose over time, A_sat became uncoupled from stomatal conductance as ratios of internal to external concentrations of carbon dioxide increased, reducing water‐use efficiency. Ozone reduced net photosynthesis and impaired stomatal function, with these effects depending on the irradiance environment of the canopy leaves. Increased ozone sensitivity of shade leaves compared to sun leaves has consequences for net carbon gain in canopies.