No tropospheric ozone impact on the carbon uptake by a Belgian pine forest

Abstract. High stomatal ozone (O3) uptake has been shown to negatively affect crop yields and the growth of tree seedlings. However, little is known about the effect of O3 on the carbon uptake by mature forest trees. This study investigated the effect of high O3 events on gross primary production (GPP) for a Scots pine stand near Antwerp, Belgium over the period 1998–2013. Stomatal O3 fluxes were modelled using in situ O3 concentration measurements and a multiplicative stomatal model, which was parameterised and validated for this Scots pine stand. Ozone-induced GPP reduction is most likely to occur during or shortly after days with high stomatal O3 uptake. Therefore, a GPP model parameterised for days with low stomatal O3 uptake rates was used to simulate GPP during periods of high stomatal O3 uptake. Eventual negative effects of high stomatal O3 uptake on GPP would then result in an overestimation of GPP by the model during or after high stomatal O3 uptake events. The O3 effects on GPP were linked to AOT40 and PODy. Although the critical levels for both indices were exceeded in every single year, no significant negative effects were found of ozone on GPP and no correlations between GPP residuals and AOT40 and PODy were found. Overall, we conclude that no O3 effects were detected on the carbon uptake by this Scots pine stand.