Limitations and perspectives about scaling ozone impacts in trees

We review the need for scaling effects of ozone (O₃) from juvenile to mature forest trees, identify the knowledge presently available, and discuss limitations in scaling efforts. Recent findings on O₃/soil nutrient and O₃/CO₂ interactions from controlled experiments suggest consistent scaling patterns for physiological responses of individual leaves to whole-plant growth, carbon allocation, and water use efficiency of juvenile trees. These findings on juvenile trees are used to develop hypotheses that are relevant to scaling O₃ effects to mature trees, and these hypotheses are examined with respect to existing research on differences in response to O₃ between juvenile and mature trees. Scaling patterns of leaf-level physiological response to O₃ have not been consistent in previous comparisons between juvenile and mature trees. We review and synthesize current understanding of factors that may cause such inconsistent scaling patterns, including tree-size related changes in environment, stomatal conductance, O₃ uptake and exposure, carbon allocation to defense, repair, and compensation mechanisms, and leaf production phenology. These factors should be considered in efforts to scale O₃ responses during tree ontogeny. Free-air O₃ fumigation experiments of forest canopies allow direct assessments of O₃ impacts on physiological processes of mature trees, and provide the opportunity to test current hypotheses about ontogenetic variation in O₃ sensitivity by comparing O₃ responses across tree-internal scales and ontogeny.