For forests the AOT40 is summed for April to September daylight hours inclusive, while for arable crops it is summed from June to August inclusive of daylight hours. There are two AOT40 critical levels for semi-natural vegetation. For semi-natural vegetation dominated by annual, the AOT40 is calculated over three months, and for those dominated by perennialsan AOT40 period from mid April to mid October is used (see table below). In APIS we present data based on the critical level of AOT40 5000 ppb hours (based on semi-natural vegetation communities dominated by perennials, 6 month growing season mid April- mid October). The alternative critical level may also be relevant. Daylight hours are defined as when solar radiation exceeds 50 W m-2. This is when vegetation's stomata should be open and so it will be absorbing ozone through them as it respires. The calculation of AOT40 is based on ozone concentrations at the top of the canopy as described in Section 3.4.2 of the CCE report.