Complexities in understanding ecosystem response to ozone

Ecological risk assessment of O-3 impact requires consideration of many fac tors that, perhaps, are not of concern in human health risk assessment. The episodic nature of O-3 exposure, functional complexity of species assembla ges, and the broad spatial and temporal scales characteristic of natural ec osystems make ecological risk assessment extremely difficult. The majority of exposure studies using plants have examined the sensitivity of individua l species, growing under controlled conditions. Research has shown that ind ividuals growing in plant mixtures may not respond the same way to O-3 as w hen growing alone. In addition, other naturally occurring stresses can modi fy plant response to O-3. Understanding the effect of O-3 on natural system s and protecting vegetation resources represent significant scientific and regulatory challenges. Here we review several factors that need to be considered when evaluating e cosystem response to O-3. Then we briefly present two examples of controlle d seedling studies that were conducted to better understand mechanisms of t ree response to O-3. In the first example controlled exposure studies revea led responses in tree roots that led to hypothesis testing in the field in ponderosa. pine ecosystems. Field experiments have confirmed a similar resp onse in root biomass and carbohydrates across a natural O-3 gradient in S. California, suggesting at least a partial role for O-3 in the response. The second example illustrates the difficulty of understanding mechanistic int eractions to O-3 stress even in controlled chamber studies. The second exam ple also illustrates the difficulty of using chamber studies to understand responses in the field. While our knowledge of vegetation response to O-3 i s extensive and compelling, important questions remain about how to quantif y these effects in the field, assess their magnitude, and establish a suita ble standard that is protective of ecosystems.