DELAYED IMPACT OF CHRONIC OZONE STRESS ON YOUNG DOUGLAS-FIR GROWN UNDER FIELD CONDITIONS

A 3-year study of the effects of low-level exposures of ozone (O-3) on the growth of young Douglas-fir (Pseudotsuga menziesii (Mirb.) France ) trees was undertaken during 1988-1990, using an open-air zonal air p ollution system (ZAPS) on the University of British Columbia campus. T he ZAPS provided daily O-3 enrichment of the air during a 14-h daytime period starting at 07:00 Pacific daylight savings time, resulting in 12 unique exposure treatments, with additional control plots exposed t o ambient air. Enrichment was provided for 92 days in 1988 and 101 day s in 1989. The minimum and maximum 12-h (09:00-20:59 Pacific daylight savings time) seasonal mean O-3 concentrations obtained were 18 and 41 ppb (nL/L) in 1988 and 27 and 66 ppb in 1989. Destructive harvests an d nondestructive measurements were taken at intervals throughout the 1 988 and 1989 exposure periods, and once in 1990. Variability within th e tree population and the relatively low levels of exposure resulted i n no significant effects on growth being observed until the latter par t of the second year of treatment. In the later harvests in 1989, anal ysis by simple linear regression showed significant reductions in the dry weight of second-flush growth as a result of O-3 treatment. The ef fect carried over into the third year, as revealed by small but signif icant reductions in leader length. In both cases, the use of nonlinear Weibull and gamma-type regression models resulted in improved fits to the data. Although young Douglas-fir trees are tolerant to modest inc reases in ambient O-3 in the short term, extended periods of such expo sures lead to adverse effects on long-term growth.