Stomatal uptake and non-stomatal ozone removal by a mixed Oak-Hornbeam mature forest in the Po valley: results of the ECLAIRE long-term campaign

In the framework of the ÉCLAIRE Project of the 7th EU framework programme a joint field campaign and a following long-term one had been conducted at an Oak-Hornbeam mixed mature forest in the Po Valley (Bosco Fontana, Mantova, Italy). Inside the forest, a 42 m tall micrometeorological tower was installed to measure the mass (O3, CO2, H2O) and energy exchange between the ecosystem and the atmosphere. The focus of this work will be on the ozone flux measurements which started in June 2012 and are currently running. The ecosystem behaved as a relevant sink for tropospheric ozone, both in summer and in winter, with an average monthly ozone removal of 5.8 kg ha-1 and a total annual deposition of more than 70 kg ha-1. The stomatal flux, estimated by deriving the bulk stomatal resistance through the inversion of the Monteith equation. The maximum stomatal fraction was observed in July 2013 (55%) and the minimum in October 2012 (13%). A strong interannual variability about the stomatal deposition between summer 2012 and 2013 was observed. The phytotoxic ozone doses taken up by the plants were respectively 13.5 mmol m-2 in 2012 and 29 mmol m-2 in 2013. The non-stomatal deposition, obtained as a residual between the total and the stomatal ozone fluxes, was investigated too. In the joint field campaign Neimitz et al. (2013) found that half of the non-stomatal deposition, was due to NO soil emission, which were on average above 100 mg N m-2 s-1. A minor part of the non-stomatal fluxes (6%) was attributed to isoprene reactions with ozone. A large part of the non-stomatal deposition (44%) was not explained. An investigation on this part, with particular attention on the deposition on non-transpiring surfaces, will be showed. Additional measurements of O3, NO and NO2 concentrations along the tower profile, allowed to estimate the ozone storage inside the trunk space. The ozone storage can be seen as a part of the deposition flux which is temporarily stored into the trunk space. Taking it into account, the daily course of the non-stomatal deposition significantly changed, with important consequences for non-stomatal interpretation and for proper modelling of the non-stomatal processes. An estimation of the ozone impact on the whole ecosystem was assessed by looking at the departure from the carbon dioxide balance closure. First results of CO2 exchange seem to indicate a net emission of CO2 from the ecosystem, as it should not be expected from a healthy mature forest ecosystem where, typically, the carbon balance is zero. Again, a general limitation of the net carbon uptake has been observed at high ozone stomatal fluxes. Finally, the great differences observed in the two years highlight the need for long time series of flux measurements. Moreover, this kind of studies will help in raising the awareness of the policy makers on the ecosystem services offered by the vegetal ecosystems.