Branch cuvettes as means of ozone risk assessment in adult forest tree crowns: Combining experimental and modelling capacities

The branch autonomy principle has been referred to extensively for using branch cuvettes as a technique of studying ozone (O₃) effects within the canopy of adult forest trees. However, this principle may not hold in general regarding biochemical interactions between O₃-impacted branches exposed inside cuvettes and neighbouring crown parts under the unchanged ambient O₃ regime. After reviewing relevant cuvette studies conducted to date, we will provide evidence that cuvette-exposed branches may serve, given awareness of outlined pre-requisites and restrictions, as surrogates for examining the crown-level response of trees to elevated O₃ regimes. Such a conclusion is based on the defence metabolism of branches, which seems to be autonomous to some extent from neighbouring crown sections. Cuvette studies may, therefore, be used to derive dose response functions as measures of O₃ sensitivity. On such grounds, also validation and improvement of stomatal O₃ uptake modelling becomes feasible. The branch-level approach, however, does not substitute whole-tree free-air O₃ fumigation and related flux assessments, as branches in view of representativeness and boundary layer characteristics represent one stage in scaling O₃ flux between leaf and tree level. Branch level-based flux scaling should be backed, therefore, by independent trunk sap-flow assessment techniques that offer derivation of FO₃ at the whole-tree level.