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. & 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 (O(3)) exposure and drought on growth of white fir. The effects of O(3) 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 O(3) 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 O(3). In the O(3) simulations, an O(3)-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 O(3) were much greater than 2.5%, but were not statistically different from control values. A simulated O(3) 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 O(3) 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 O(3) uptake and therefore reduced the O(3) response of white fir. However, a combination of low amounts of O(3) (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 O(3) and that moderate amounts of atmospheric O(3) and drought could be more detrimental to white fir than either stress singly.