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 (O-3) effects within the canopy of adult forest trees. However, this principle may not hold in general regarding biochemical interactions between O-3-impacted branches exposed inside cuvettes and neighbouring crown parts under the unchanged ambient O-3 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 O-3 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 O-3 sensitivity. On such grounds, also validation and improvement of stomatal O-3 uptake modelling becomes feasible. The branch-level approach, however, does not substitute whole-tree free-air O-3 fumigation and related flux assessments, as branches in view of representativeness and boundary layer characteristics represent one stage in scaling O-3 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.