Figure 1 . Relationship between three-month AOT40 measured at the site and the biomass of the ozone-sensitive ( NC-S ) white clover ( relative to the ozone-resistant ( NC-R ) biotype ) for ( a ) all data from ICP Vegetation sites , ( b ) mean data ( +- one standard error ) for five geographical regions : NE = Northern Europe , ACE = Atlantic Central Europe , CCE = Continental Central Europe , EM = Eastern Mediterranean and WM = Western Mediterranean .