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 (O3) rather than external exposure, biomass reduction was related to the cumulative uptake of O3 through stomata over an uptake cut-off threshold of x nmol O3 m−2 s−1 (CUO>x), to the accumulated exposure to O3 over a threshold of y nmol mol−1 during daylight hours (daylight AOTy) or during 24 h (24 h AOTy), 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 O3 was estimated using a stomatal conductance (gs) model including gs response functions for photosynthetic photon flux density, water vapour pressure deficit of the air and air temperature. Experiment-specific maximum gs (gmax) as well as gs in darkness (gdark) were assessed through local measurements. Biomass reduction was more strongly related to CUO>x than to SUM06 and daylight or 24 h AOTy, but the difference between CUO>x and 24 h AOTy was small. The better performance of CUO>x was dependent on the use of site- and experiment-specific gmax and gdark values, and there was a positive relationship between gmax and biomass reduction per unit AOT40. Daylight AOTy and SUM06 could not account for the growth limiting impact of nocturnal O3 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 O3, 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 gmax and gdark were locally parameterised.