|Ainsworth, Elizabeth - Lisa|
Submitted to: Plant Cell and Environment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/23/2007
Publication Date: 9/1/2007
Citation: Wittig, V.E., Ainsworth, E.A., Long, S.P. 2007. 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 three decades of experiments. Plant Cell and Environment. 30(9):1150-1162. Interpretive Summary: The concentration of ozone in the lower atmosphere has risen substantially since the industrial revolution and is projected to increase over this century. This work summarizes the results of 3 decades of research on the effects of ozone on trees. We found that current levels of ozone reduce tree photosynthesis by 10% and reduce the conductance of water vapor by 13%. Conifer trees differ in their response to ozone from broadleaved trees, and aren't as sensitive to current ozone concentrations. However, future concentrations of ozone projected for later this century will decrease photosynthesis and conductance to water vapor in both conifers and broadleaved trees. These findings suggest that rising ozone, an often overlooked aspect of global atmospheric change, is progressively depressing the ability of forests to assimilate carbon and transfer water vapor to the atmosphere, with significant potential effects on terrestrial carbon sinks and regional hydrologies. This study is important for conservation of current forests and to ecosystem modelers who are examing the effects of future climates on tree productivity.
Technical Abstract: The surface concentration of ozone has risen from less than 10 ppb prior to the industrial revolution to a daytime mean concentration of approximately 40 ppb over much of the northern temperate zone. It 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 ozone on light-saturated photosynthetic CO2 uptake (Asat) and stomatal conductance (gs) in trees. In total 348 independent measurements of Asat from 61 studies, and 266 measures of gs from 55 studies were reviewed. Results suggested that the elevation of ozone that has occurred since the industrial revolution is depressing Asat and gs by 11% (CI 9 - 13%) and 13% (CI 11-15%) respectively, where CI is the 95% confidence interval. Gymnosperms were not significantly affected by contrast to angiosperms. Both drought and elevated CO2 significantly decreased the effect of ambient ozone. Younger trees (< 4years) were affected less than older trees. Elevation of ozone above current levels caused progressively larger losses of Asat and gs, including gymnosperms. Results are consistent with the expectation that damage to photosynthesis depends on the cumulative uptake of ozone into the leaf. Thus factors which that lower gs, lessen damage. Where both gs and ozone concentration were recorded, an overall decline in Asat of 0.21% per mmol m-2 of estimated cumulative ozone uptake (CU) was calculated. These findings suggest that rising ozone, an often overlooked aspect of global atmospheric change, is progressively depressing the ability of forests to assimilate carbon and transfer water vapor to the atmosphere, with significant potential effects on terrestrial carbon sinks and regional hydrologies.