Comparison of the response of red oak (Quercus rubra) seedlings and mature trees to ozone exposure using simulation modeling

Field studies have determined that the photosynthetic rates of mature northern red oak (Quercus rubra L.) trees are sensitive to ozone exposure whereas those of seedlings are not. We used a model of tree physiology to determine the consequences of these differences in photosynthetic response to carbon allocation and tissue growth in seedlings and mature trees over a 2-year period. In the seedling simulations at twice ambient ozone, only the total non-structural carbohydrate (TNC) storage pool was affected. The effects in the simulated mature tree were much greater, with large decreases predicted for TNC, fine root, leaf, stem, branch, and coarse root tissues. The model produced many ozone-induced responses in the mature tree that were similar or consistent with observations made in a field study, but the simulations overestimated the effect of twice ambient ozone on root TNC and growth. The discrepancy between field and simulated results suggests that the field study trees exposed to elevated ozone levels may use carbon at a reduced rate, particularly through reduced respiration.