A Site-Specific Analysis of the Implications of a Changing Ozone Profile and Climate for Stomatal Ozone Fluxes in Europe

In this study we used eight sites covering the major climates of Europe to investigate the implications of a future climate (2ºC warmer and 20% drier) and a changing ozone profile (increased background concentrations and reduced peaks) on stomatal ozone fluxes of three widely occurring plant species. In future climates, small increases in background ozone concentrations over the course of a growing season could have significant impacts on the annual accumulated stomatal ozone uptake, even if peak concentrations of ozone are reduced. Predicted increases in stomatal ozone uptake were larger at sites from northern and mid-Europe than those from southern Europe, and showed a strong relationship with latitude. At the sites used from central and northern regions of Europe, including the UK and Sweden, climatic conditions were highly conducive to stomatal ozone uptake by vegetation during the summer months and therefore a small increase in daily mean ozone concentration of 3 - 16% during this time of year (from increased background concentrations, reduced peaks) would have a large impact on stomatal ozone uptake. In contrast, during spring and autumn, the climatic conditions can limit ozone uptake for many species. Although small increases in ozone concentration during these seasons could cause a modest increase in ozone uptake, for those species that are active at low temperatures, a 2°C increase in temperature would increase stomatal ozone uptake even in the absence of further increases in ozone concentration. For some southern regions of Europe, where temperatures are close to or above optimum for stomatal opening, then an increase in temperature of 2°C could limit further stomatal ozone uptake by enhancing stomatal closure during the summer months, whereas during the spring, when many plants are actively growing, a small increase in temperature would increase stomatal ozone uptake in southern regions. Ozone values as low as 10 ppb can contribute to stomatal ozone uptake above the threshold for accumulation of the Phytotoxic Ozone Dose (PODY) of 1 nmol O3 m-2 s-1 for forest trees and semi-natural vegetation. Therefore, impacts of ozone pollution could occur at any time of year when plants are actively growing and not just during the periods of peak ozone concentrations.