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 xnmol O₃m^-2s^-1 (CUO>x), to the accumulated exposure to O₃ over a threshold of ynmolmol^-1 during daylight hours (daylight AOTy) or during 24h (24h AOTy), and to the sum of daytime concentrations exceeding 60nmolmol^-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 (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 24h AOTy, but the difference between CUO>x and 24h AOTy 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 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 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.