Effects on vegetation include visible injury, early senescence of leaves, and reduction of crop yield (PORG 1997). Experiments with open-top chambers in various parts of Europe (including the UK) show that exposure of plants to concentrations above 40 ppb for several weeks can reduce growth and the yield of sensitive crops species. However, it is difficult to translate this kind of information into effects on crops growing in the field and on natural communities. The most comprehensive information is available for wheat, and the evidence indicates that yields are reduced in some parts of Britain in high ozone years. For a discussion of the problems and challenges of assessing the effects on natural vegetation see Davison & Barns (1998).