Future ground-level concentrations of phytotoxic ozone are projected to grow in the Northern Hemisphere, at a rate depending on emission scenarios. We explored the likely changes in net ecosystem production (NEP) due to the increasing concentration of tropospheric ozone by applying a Generalized Additive Mixed Model based on measurements of ozone concentration ([O_3]) and stomatal ozone flux (FsO_3), at a mountainous Norway spruce forest in the Czech Republic, Central Europe. A dataset covering the growing period (May-August 2009) was examined in this case study. A predictive model based on FsO_3 was found to be marginally more accurate than a model using [O_3] alone for prediction of the course of NEP when compared to NEP measured by the eddy covariance technique. Both higher [O_3] and FsO_3 were found to reduce NEP. NEP simulated at low, pre-industrial FsO_3 (0.5 nmol m^{-2} s^{-1} ) was higher by 24.8% as compared to NEP assessed at current rates of FsO_3 (8.32 nmol m^{-2} s^{-1}). However, NEP simulated at high FsO_3 (17 nmol m^{-2} s^{-1}), likely in the future, was reduced by 14.1% as compared to NEP values at current FsO_3. The interaction between environmental factors and stomatal conductance is discussed in this paper.