Ozone effects on red oak root dynamics
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Many research projects concerning the possible deleterious effects of ozone on forest health have been conducted on individual tree species. The common goal of these projects has been to identify mechanisms of damage by ozone, and then extrapolate research results to forests. Results from seedling studies are used to parameterize process-based tree growth models which are used to project mature tree responses to different levels of ozone. This approach has been criticized because nothing is known about differences in seedling and mature-tree responses to ozone. Another problem is that few projects have examined the effects of ozone on below ground processes; therefore, very little data exists for parameterizing the models. In order to address the problem of scaling seedling results to mature trees, and increase our level of understanding of ozone effects on below ground processes, an ozone fumigation experiment on northern red oak seedlings and mature trees was conducted. It was hypothesized that carbon reallocation to replace foliage damaged by ozone would decrease fine-root production and turnover. The red oak trees and seedlings were fumigated for three years with three levels of ozone (subambient, ambient, and 2X ambient) in open-top chambers. After two seasons of exposure, 2X ozone (0.082 ppm 7hr-mean conc.) reduced mature- tree cumulative net fine-root production and turnover by 31 and 41 %, respectively, relative to ambient ozone (0.042 ppm 7hr-mean conc.). For the same time period, ozone had no effect on seedling cumulative fine-root turnover; fine-root production was 25% higher under ambient ozone relative to subambient and 2X ambient ozone. During the summer, 1994, mature tree BUE was reduced by 2X ozone. Decreased fine-root production, turnover, and BUE under 2X ozone for the mature trees indicates that ozone can alter the dynamics of belowground carbon allocation in mature red oak. Since the seedlings were not sensitive to ozone, use of seedling results for modelling purposes may underestimate mature tree responses to ozone.
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