Biomass reduction of juvenile birch is more strongly related to stomatal uptake of ozone than to indices based on external exposure

In order to test the hypothesis that ozone-induced limitation of biomass production in juvenile silver birch ( Betula pendula Roth) is driven by stomatal uptake of ozone (O 3) rather than external exposure, biomass reduction was related to the cumulative uptake of O 3 through stomata over an uptake cut-off threshold of x nmol O 3 m −2 s −1 (CUO> x), to the accumulated exposure to O 3 over a threshold of y nmol mol −1 during daylight hours (daylight AOT y) or during 24 h (24 h AOT y), and to the sum of daytime concentrations exceeding 60 nmol mol −1 (SUM06). The analysis included data from nine different experiments conducted in Sweden, Finland and Switzerland. Stomatal uptake of O 3 was estimated using a stomatal conductance ( g s) model including g s response functions for photosynthetic photon flux density, water vapour pressure deficit of the air and air temperature. Experiment-specific maximum g s ( g max) as well as g s in darkness ( g dark) were assessed through local measurements. Biomass reduction was more strongly related to CUO> x than to SUM06 and daylight or 24 h AOT y, but the difference between CUO> x and 24 h AOT y was small. The better performance of CUO> x was dependent on the use of site- and experiment-specific g max and g dark values, and there was a positive relationship between g max and biomass reduction per unit AOT40. Daylight AOT y and SUM06 could not account for the growth limiting impact of nocturnal O 3 uptake in the Swiss experiments. A sensitivity analysis revealed that the CUO> x estimates were largely insensitive to the estimate of the conductance for non-stomatal leaf surface deposition of O 3, as a result of turbulent conditions at the experimental plots. In summary, we conclude that CUO> x was more successful in accounting for the variation in biomass reduction in juvenile birch as compared to indices based on external exposure, if g max and g dark were locally parameterised.