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UBC Theses and Dissertations

The effects of ozone air pollution on plant competition Penney, Kathy Coreen


Low levels of ozone air pollution have been shown to cause reductions in plant growth. It has been proposed that these reductions could translate into effects on a plant's competitive ability, and hence, on the inter-relationships of plants in a community. Experiments were carried out to determine if there was an effect of low levels of ozone on intra- and inter-specific competition of barnyardgrass (BYG), redroot pigweed (RPW) and green foxtail (GFT). Three ozone treatments (a filtered air control, and 75 ppb/7 hr and 150 ppb/3.5 hr daily) were applied to monocultures and binary mixtures in replacement and additive series. The two ozone treatments represent the same ambient dose. The effects of each on the plants studied were compared and contrasted. The order of competitive ability of the three species was found to be BYG > RPW > GFT. There were significant differences in the interaction of the species between the three ozone treatments. BYG experienced significant intra-specific competition only in the two ozone-added treatments; GFT was significantly affected by its own density only in the control. In contrast, RPW experienced significant intra-specific competition effects in all treatments. BYG benefitted significantly from the presence of GFT in the control. A similar positive effect of GFT density on BYG yield is seen in the 150 ppb/3.5 hr treatment. It is suggested that this localized positive allelopathic effect may be due to the upward transport of a volatile compound released by GFT, or disseminated through the soil. All inter-specific competitive relations of GFT and RPW were significant in all ozone treatments. RPW appeared to experience the most ozone effects of the three species studied. RPW and GFT in replacement series mixtures showed over-yielding in the control treatment, equal replacement in the 75 ppb/7 hr treatment and under-yielding in the 150 ppb/3.5 hr treatment. This interaction of ozone and competition treatments between RPW and GFT was significant for root dry weight per pot. BYG yield was significantly enhanced in the 75 ppb/7 hr ozone treatment over the control and 150 ppb/3.5 hr treatments. This may reflect an acclimation of BYG plants to the low (0.01-0.04 ppm) ambient background levels of ozone. The concept of an appropriate control treatment is discussed. Although the 75 ppb/7 hr and 150 ppb/3.5 hr treatments represent the same ambient dose, the species used in this study reacted quite differently to the two treatments. Overall, the more acute 150 ppb/3.5 hr dose had a more detrimental effect on the yield variables measured for RPW and GFT, and the 75 ppb/7 hr treatment had a significant enhancing effect on BYG yield over that of the control and the 150 ppb/3.5 hr dose. An examination of the size frequency distributions of the three species confirms that BYG is competitively dominant, whereas the GFT size distributions do not appear to be sensitive to competitive suppression. RPW displays a very skewed size frequency distribution under all treatments. It is suggested that RPW has inherent genetic variability for a wide size distribution in the populations studied. There were no ozone effects on the size frequency distributions of any species studied.

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