This document provides a review of metrics and indices that have been developed to evaluate ozone concentrations in ambient air in terms of potential impacts to vegetation. In current scientific and regulatory terms, each metric or index is used in the establishment of "critical levels," which are numerical expressions of ozone exposure at or above which a negative efect on plant growth is expected, based on current knowledge. Critical levels are similar in philosophy to the critical loads used to express the amount of acidic deposition above which negative effects are expected (again, based on current knowledge).

It is not within the scope of this review to discuss or justify the scientific basis behind any of the metrics. More specifically, the scientific merit of any dataset used in the derivation o f the metrics discussed in this report is not discussed. The development and justification of the metrics have been presented in numerous reports, of which US EPA [7] and UNECE Convention on Long-range Transboundary Air Pollution [6] documents are particularly substantive.

Indices describing vegetation effects have been established for plant groups, namely agricultural crops, horticultural crops, semi-natural vegetation, and forests. Within each group, a subset of species has been evaluated for their responses to ozone, and it is on the basis of these responses that indices and their respective critical levels have been derived. From the perspective of the NSMWG and the Ozone Management Framework, the data and metrics developed from them that relate to tree growth are the most relevant, given the boreal forest setting in the CEMA management area. Hence, metrics applicable to forest environments or forest trees are emphasized in this review.

The main conclusions derived in 1996 on the basis of the National Crop Loss Assessment Network (NCLAN) studies, and reaffirmed by the US EPA [7] were: 1. Ozone effects in plants are cumulative; 2. Higher ozone concentrations are more important than lower concentrations in causing a plant response; 3. Plant sensitivity varies with time of day and developmental stage; and 4. Exposure indices that accumulate hourly ozone concentrations and preferentially weight higher concentrations better predict (statistically) plant responses than do indices based on average and peak concentrations.

Except for the concentration and averaging period indices, each of the metrics presented in this discussion are cumulative indices, in that they increase in value as ozone exposures increase over time. Indices that give greater weight to higher concentrations correlated well with plant responses [2, 4, 6, 7]. These cumulative indices also include consideration of other varibles, including light, soil moisture, and vegetation type and/or species. Each of these indices reflects an interest and effort to incorporate biologically-relevant factors and processes into the index.