Delayed impact of chronic ozone stress on young Douglas-fir grown under field conditions

Abstract

A 3-year study of the effects of low-level exposures of ozone (O3) on the growth of young Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) trees was undertaken during 1988–1990, using an open-air zonal air pollution system (ZAPS) on the University of British Columbia campus. The ZAPS provided daily O3 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 to ambient air. Enrichment was provided for 92 days in 1988 and 101 days in 1989. The minimum and maximum 12-h (09:00–20:59 Pacific daylight savings time) seasonal mean O3 concentrations obtained were 18 and 41 ppb (nL/L) in 1988 and 27 and 66 ppb in 1989. Destructive harvests and nondestructive measurements were taken at intervals throughout the 1988 and 1989 exposure periods, and once in 1990. Variability within the tree population and the relatively low levels of exposure resulted in no significant effects on growth being observed until the latter part of the second year of treatment. In the later harvests in 1989, analysis by simple linear regression showed significant reductions in the dry weight of second-flush growth as a result of O3 treatment. The effect carried over into the third year, as revealed by small but significant 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 increases in ambient O3 in the short term, extended periods of such exposures lead to adverse effects on long-term growth.