Evidence for impacts of near-ambient ozone concentrations on vegetation in southern Sweden.

Substantial impacts of near-ambient ozone concentrations on agricultural crops, trees, and seminatural vegetation are demonstrated for southern Sweden. Impacts of ambient ozone levels (2-15 microL L(-1) hr annual accumulated ozone exposure over a threshold of 40 nL L(-1) [AOT40]) range from a 2%-10% reduction for trees (e.g., leaf chlorophyll, tree growth) up to a 15% reduction for crops (e.g., yield, wheat/potato). Visible leaf injury on bioindicator plants caused by ambient ozone levels has been clearly demonstrated. The humid climatic conditions in Sweden promote high rates of leaf ozone uptake at a certain ozone concentration. This likely explains the comparatively large ozone impacts found for vegetation in southern Sweden at relatively low ozone concentrations in the air. It is important that the future methods used for the representation of ozone impacts on vegetation across Europe are based on the leaf ozone uptake concept and not on concentration-based exposure indices, such as AOT40.