The role of microclimate in determining the sensitivity of Quercus rubra L. to ozone

Microclimate has been suggested as a factor to consider when scaling ozone responses of forest trees from the seedling to tree level. To test whether microclimate influences the response of seedlings to ozone. 1-yr-old Ouercus rubra L, seedlings were grown in the shaded upper or lower canopy of 31-yr-old trees in open-top tree chambers, or in more exposed open-top seedling chambers, and received subambient (cumulative SUM.M06* dose of (0 ppm-h), ambient (cumulative SUM06 dose of l9 ppm-h) or twice ambient (cumulative SUMU6 dose of 135 ppm-h) exposures to ozone for one growing season. Seedlings exposed to warmer temperatures and greater PAR within the seedling chambers had greater biomass, total leaf nitrogen, second foliar flush net photosynthesis, leaf conductance and dark respiration than seedlings grown in either location within the tree canopies. However, microclimate did not influence the responses to ozone. Net photosynthesis, leaf conductance and apparent quantum yield of first flush leaves of seedlings exposed to the same leaf microclimate as trees were reduced between 16% and 20% by ozone treatment. Ozone induced larger reductions in the net photosynthesis, leaf conductance and apparent quantum yield of tree leaves. These results indicate that, although microclimate influenced seedling growth and physiology, microclimate did not play a dominant role in determining the sensitivity of Q. rubra seedlings to ozone.