Difference in ozone uptake in grassland species between open-top chambers and ambient air

Exposure-response data from open-top chamber (OTC) experiments are often directly applied to ambient air (AA) conditions. Because microclimatic conditions are modified and pollutant uptake by plants may differ (i.e. 'chamber effect'), there is concern about the influence of OTCs on these relationships. In addition, AA concentrations are often measured at a height which differs from canopy height and correction for the concentration gradient (i.e. 'gradient effect') is necessary. To quantify the relative contribution of plant characteristics and microclimatic factors to these effects, ozone uptake by horizontal leaves at the top of the canopy was calculated for plants grown in OTCs or AA by using a resistance analogy model. Data from an OTC experiment in 1996/97 for six species typical of productive grasslands were used. Ozone concentration inside OTCs was set equal to the concentration measured at a height of 3 m above ground (C(z(ref))) or at canopy height (C(0)). The gradient effect resulted in a 16-27% lower average C(0) than C(z(ref)), depending on species. The main determinant of the chamber effect was a systematic difference in leaf-to-air vapour pressure deficit between OTCs and AA which affected stomatal resistance and ozone uptake. In case of monocultures both effects were species-specific. In species mixtures the gradient effect differed between mixing ratios, whereas the chamber effect was species-specific. Because of the inter-specific difference in the chamber effect on ozone uptake, it is concluded that ozone effects on species mixtures differ systematically between OTCs and AA. The data underline that extrapolation of ozone flux-response relationships from OTC experiments must be based on canopy-level ozone concentrations, and that these relationships should be applied only to single species under microclimatic conditions similar to those prevailing in the experiment.