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 ([O(3)]) 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 [O(3)] 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 [O(3)] on light-saturated photosynthetic CO(2) 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 [O(3)] 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 [CO(2)] significantly decreased the effect of ambient [O(3)]. Younger trees (<4 years) were affected less than older trees. Elevation of [O(3)] 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 (O(3)) into the leaf. Thus, factors that lower g(s) lessen damage. Where both g(s) and [O(3)] 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.