Trembling aspen (Populus tremuloides Michx.) is sensitive to tropospheric ozone (O3) as determined by visible foliar symptoms, accelerated foliar senescence and premature abscission, degradation and change in composition of epicuticular waxes, decreased photosynthesis and chlorophyll, and decreased aboveground and belowground growth. The species is highly variable in O3responses as some clones are similar in sensitivity to Bel W3 tobacco and other clones are tolerant to moderate levels of O3. We have, therefore, hypothesized and presented evidence for natural selection of O3-tolerance in aspen populations. This hypothesis has been criticized, however, as improbable because O3is thought to be a fairly weak selection force and because changes in tree population are thought to occur over very long time periods, longer than tropospheric O3has been known to be a problem. To shed more light on this argument, in 1994 we established a set of research field trials using clones of known origin and previously determined O3sensitivity at three sites in the Lake States region with differing O3profiles (Rhinelander, Wisconsin-low O3; Kalamazoo, Michigan-moderate O3; and Kenosha, Wisconsin-high O3). In this paper, we present evidence of changes in the relative volume d2h growth of clone 259 (O3-sensitive) compared to clone 216 (O3-tolerant) of -0.1%, -44.2%, and -62.8% at the low, medium and high O3sites at age 5. In addition, relative survival of the clone 259 compared to 216 was -11.0%, -6.8%, and -38.4% at the low, moderate, and high O3sites. Actual survival rates at the high O3site were 78.2% for clone 216 and only 48.2% for clone 259. Our results suggest that very rapid and significant changes in competitive ability and fitness can occur under ambient levels of O3in the lower Great Lakes region for aggrading forests. These results are consistent with the hypothesis that O3is inducing natural selection for O3tolerance in aspen. © 2003 Elsevier Ltd. All rights reserved.
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