To what extent do current and projected increases in surface ozone affect photosynthesis and stomatal conductance of trees? A meta-analytic review of the last 3 decades of experiments

The surface concentration of ozone ([O₃]) has risen from less than 10 ppb prior to the industrial revolution to a day-time mean concentration of approximately 40 ppb over much of the northern temperate zone. If current global emission trends continue, surface [O₃] is projected to rise a further 50% over this century, with larger increases in many locations including Northern Hemisphere forests. This review uses statistical meta-analysis to determine mean effects, and their confidence limits, of both the current and projected elevations of [O₃] on light-saturated photosynthetic CO₂ uptake (A_sat) and stomatal conductance (g_s) in trees. In total, 348 measurements of A_sat from 61 studies and 266 measures of g_s from 55 studies were reviewed. Results suggested that the elevation of [O₃] that has occurred since the industrial revolution is depressing A_sat and g_s by 11% (CI 9-13%) and 13% (CI 11-15%), respectively, where CI is the 95% confidence interval. In contrast to angiosperms, gymnosperms were not significantly affected. Both drought and elevated [CO₂] significantly decreased the effect of ambient [O₃]. Younger trees (<4 years) were affected less than older trees. Elevation of [O₃] above current levels caused progressively larger losses of A_sat and g_s, including gymnosperms. Results are consistent with the expectation that damage to photosynthesis depends on the cumulative uptake of ozone (O₃) into the leaf. Thus, factors that lower g_s lessen damage. Where both g_s and [O₃] were recorded, an overall decline in A _sat of 0.21% per mmol m^-2 of estimated cumulative O ₃ uptake was calculated. These findings suggest that rising [O ₃], an often overlooked aspect of global atmospheric change, is progressively depressing the ability of temperate and boreal forests to assimilate carbon and transfer water vapour to the atmosphere, with significant potential effects on terrestrial carbon sinks and regional hydrologies.