Use of a single-tree simulation model to predict effects of ozone and drought on growth of a white fir tree

A physiologically based, single-tree simulation model, TREGRO, was parameterized with existing phenological, allometric, and growth data and used to predict effects of ozone and drought on growth of a 53-year-old white fir (Abies concolor (Gord. and Glend.) Lindl. ex Hildebr.) tree following a 3-year model simulation. Multiple experimental simulations were conducted to assess the individual and interactive effects of ozone (O₃) exposure and drought on growth of white fir. The effects of O₃ were imposed as reductions in carbon (C) assimilation of 0, 2.5, 5, 10, and 20%. Drought was imposed as 0, 10, 25, and 50% reductions in total annual precipitation. The results of the simulations were compared with the effects of O₃ on white fir seedlings grown in the presence and absence of ozone in open-top chambers and with a field survey of white fir trees subjected to a gradient of O₃. In the O₃ simulations, an O₃-induced reduction in C assimilation of 2.5% reduced total tree biomass and branch total nonstructural carbohydrate (TNC) content by < 7%. Although quantifiable in simulation experiments, such small reductions would probably not be detectable in the field. Results from both an open-top chamber experiment and a field survey indicated that reductions in C assimilation of white fir growing in elevated O₃ were much greater than 2.5%, but were not statistically different from control values. A simulated O₃ reduction in C assimilation of ≥ 10% reduced total tree biomass by 7% and branch TNC by 55%. Results from the field survey indicated that branch elongation was reduced in response to increased O₃ concentration, corroborating the simulated response of reduced C allocation to the branches of white fir. Although simulated reductions in total annual precipitation of ≥ 25% reduced final tree biomass, the simulated reductions also reduced O₃ uptake and therefore reduced the O₃ response of white fir. However, a combination of low amounts of O₃ (2.5% reduction in C assimilation) and drought (25% reduction in annual precipitation) synergistically reduced C gain of white fir more than either stress individually. Our simulations predict that moderate drought (no more than a 25% reduction in total annual precipitation) may not ameliorate the response of white fir to O₃ and that moderate amounts of atmospheric O₃ and drought could be more detrimental to white fir than either stress singly.