Deriving ozone dose-response of photosynthesis in adult forest trees from branch-level cuvette gas exchange assessment

Branch-level gas exchange provided the basis for assessing ozone flux in order to derive the dose-response relationship between cumulative O₃ uptake (COU) and carbon gain in the upper sun crown of adult Fagus sylvatica. Fluxes of ozone, CO₂ and water vapour were monitored simultaneously by climatized branch cuvettes. The cuvettes allowed branch exposure to an ambient or twice-ambient O₃ regime, while tree crowns were exposed to the same O₃ regimes (twice-ambient generated by a free-air canopy O₃ exposure system). COU levels higher than 20 mmol m^-2 led to a pronounced decline in carbon gain under elevated O₃. The limiting COU range is consistent with findings on neighbouring branches exposed to twice-ambient O₃ through free-air fumigation. The cuvette approach allows to estimate O₃ flux at peripheral crown positions, where boundary layers are low, yielding a meso-scale within-crown resolution of photosynthetic foliage sensitivity under whole-tree free-air O₃ fumigation.