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UNDERSTANDING ECOSYSTEM RESPONSE TO OZONE STRESS
Citation:
Andersen, C P. UNDERSTANDING ECOSYSTEM RESPONSE TO OZONE STRESS. Presented at Indicators in Health and Ecological Risk Assessment, 5th NHEERL Symposium, Research Triangle Park, NC, June 6-8, 2000.
Description:
Ecological risk assessment of ozone impact requires consideration of many factors that, perhaps, are not of concern in human health risk assessment. The episodic nature of ozone exposure, functional complexity of species, and 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 ozone as when growing alone. In addition, other naturally occurring stresses can modify plant response to ozone. Understanding the effect of ozone on natural systems and protecting vegetation resources represent significant scientific and regulatory challenges.
I will present two examples to illustrate the difficulty of evaluating ecosystem response to ozone. The first example will demonstrate how difficult it is to use individual plant exposure studies to predict how competition affects plant response to ozone. In mixtures of ponderosa pine and blue wildrye grass, ozone did not significantly reduce growth except when the grass was present. Carbon/nitrogen (C/N) ratios in pine needles decreased with ozone in the absence of grass competition, while C/N ratios increased in the presence of both grass and ozone. The significant interaction suggests that N acquisition and utilization in response to ozone is altered by the presence of a grass competitor, demonstrating the difficulty of using individual plant studies to evaluate ozone sensitivity. The second example describes a series of experiments that show the difficulty of understanding belowground changes in response to ozone. Belowground ecosystems are comprised of complex foodwebs of interacting organisms responsible for processing carbon, nitrogen and other essential nutrients. Ozone was found to reduce allocation of carbon belowground, decrease allocation to mycorrhizal fungi, and increase root exudation. The result is that ozone may alter carbon movement to soil foodweb organisms, which could alter carbon and nutrient cycling in ecosystems. Such a response in the field is difficult to identify and characterize due to our poor understanding of soil ecosystems in general. While our knowledge of vegetation response to ozone is extensive and compelling, important questions remain about how to quantify these effects, assess their magnitude, and establish a suitable standard that is protective of ecosystems.