Information on ozone uptake by various tree species has been derived mainly from controlled-exposure studies of seedlings. Parallel measurements of foliar physiological processes in large trees are rare, as are measurements of local ozone concentrations within forest environments. Thus, our objectives were to characterize foliar ozone uptake in forest trees of different size and to identify a mechanistic linkage between small and large trees. Light-saturated net photosynthesis (Pn), stomatal conductance (gH2O), and ozone uptake (gO23) in seedling (< 2 m), sapling (2–5 m) and mature (9–21 m) black cherry (Prunus serotina Ehrh.), northern red oak (Quercus rubra L.) and red maple (Acer rubrum L.) trees were measured monthly over two growing seasons at sites at high (1240 m) and low (600 m) elevation in the Great Smoky Mountains National Park. Seasonal ozone dose to northern red oak and red maple in the upper crown of mature trees was approx. double that of respective understorey seedlings and saplings. Seasonal ozone dose to black cherry was up to six times greater in mature trees than in seedlings and saplings. The lower ozone dose to understorey trees was mainly a function of lower gH2o, although canopy gradients in ozone concentration were indicated. Differences in Pn and gH2O between small and large deciduous forest trees were linked to variation in nitrogen concentration expressed on a leaf area basis.