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 paramete rized with existing phenological, allometric, and growth data and used to p redict effects of ozone and drought on growth of a 53-year-old white fir (A bies concolor (Gord. & Gland.) Lindl. ex Hildebr) tree following a 3-year m odel 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 c arbon (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 t he 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 sm all reductions would probably not be detectable in the field. Results from both an open-top chamber experiment and a field survey indicated that reduc tions in C assimilation of white fir growing in elevated O-3 were much grea ter than 2.5%, but were not statistically different from control values. A simulated O-3 reduction in 0 assimilation of, 10% reduced total tree biomas s 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 % allocation to the branche s of white fir Although simulated reductions in total annual precipitation of greater than or equal to 25% reduced final tree biomass, the simulated reductions also reduced O-3 uptake and therefore reduced the O-3 response of white fir. How ever, a combination of low amounts of O-3 (2.5% reduction in C assimilation ) and drought (25% reduction in annual precipitation) synergistically reduc ed C gain of white fir more than either stress individually. Our simulation s predict that moderate drought (no more than a 25% reduction in total annu al precipitation) may not ameliorate the response of white fir to O-3 and t hat moderate amounts of atmospheric O-3 and drought could be more detriment al to white fir than either stress singly.