Issues in scaling tree size and age responses to ozone: A review

Ozone is probably the most important regional air pollutant currently affecting forest trees. The vast majority of information regarding ozone effects on trees has come from experiments with seedlings, where controlled concentrations of ozone were administered in indoor or outdoor chambers for periods lasting from several days to one or more growing seasons. However, it is difficult to extrapolate, or scale, results from experiments on seedlings to larger forest trees. Mature trees typically differ from seedlings in morphological, phenological, and physiological characteristics that may profoundly influence response to ozone exposure. Also, it is possible that ozone exposure dynamics may differ for seedlings growing near the ground compared to exposure within the forest canopy. Complicating the ability to scale ozone responses from seedlings to canopy trees are differing interpretations of what is meant by ozone sensitivity. Differences in ozone sensitivity between mature trees and seedlings may occur because of differences in: ozone uptake (avoidance), compensation for injured tissues, internal properties that oppose the production of tissue injury (defense), and repair of tissue injury. Of these mechanisms, avoidance is commonly measured in scaling studies whereas compensation, defense, and repair are not. A review of studies that have compared stomatal conductance between trees of different size and age showed that large trees typically have lower stomatal conductance than seedlings, suggesting greater avoidance of ozone uptake by large trees, but there are notable exceptions to this apparent trend. For tree species where response to ozone and stomatal conductance has been compared between large and small trees, the tree size class with the greatest stomatal conductance showed the greatest detrimental leaf physiological reponse to ozone. This finding supports the hypothesis that differences in ozone response between different-sized trees of the same species occurs, at least in part, because of differences in ozone uptake. For most of these species (Picea rubens, Pinus ponderosa, Prunus serotina, Sequoiadendron giganteum), large or old trees had lower stomatal conductance and lower ozone foliar injury than small or young trees, whereas for Quercus rubra, large trees had greater stomatal conductance and greater ozone foliar injury than small trees. Drawing firm conclusions from published scaling studies is complicated by wide variation in study methodology and how 'seedling' and 'mature tree' are defined. To facilitate synthesis and improve prediction of ozone impacts on trees, we recommend that future ozone scaling research examine the occurrence and effectiveness of avoidance, compensation, defense, and repair in trees of different sizes with consistent methodology, on a suite of species that vary in ecological and physiological characteristics. We also recommend that future research examine the possible consequences of these mechanisms to long-term ecological success for trees of different sizes in the presence of other biotic and abiotic stresses.