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Abstract
Ecological risk assessment of O3 impact requires consideration of many factors that, perhaps, are not of concern in human health risk assessment. The episodic nature of O3 exposure, functional complexity of species assemblages, and the broad spatial and temporal scales characteristic of natural ecosystems make ecological risk assessment extremely difficult. The majority of exposure studies using plants have examined the sensitivity of individual species, growing under controlled conditions. Research has shown that individuals growing in plant mixtures may not respond the same way to O3 as when growing alone. In addition, other naturally occurring stresses can modify plant response to O3. Understanding the effect of O3 on natural systems and protecting vegetation resources represent significant scientific and regulatory challenges. Here we review several factors that need to be considered when evaluating ecosystem response to O3. Then we briefly present two examples of controlled seedling studies that were conducted to better understand mechanisms of tree response to O3. In the first example controlled exposure studies revealed responses in tree roots that led to hypothesis testing in the field in ponderosa pine ecosystems. Field experiments have confirmed a similar response in root biomass and carbohydrates across a natural O3 gradient in S. California, suggesting at least a partial role for O3 in the response. The second example illustrates the difficulty of understanding mechanistic interactions to O3 stress even in controlled chamber studies. The second example also illustrates the difficulty of using chamber studies to understand responses in the field. While our knowledge of vegetation response to O3 is extensive and compelling, important questions remain about how to quantify these effects in the field, assess their magnitude, and establish a suitable standard that is protective of ecosystems.