Ozone pollution results in characteristic visible symptoms in some tree species. However, extreme care must be taken in the interpretation of these symptoms as they are superficially similar to those associated with senescence. Some species exhibit these symptoms more readily than others. Among commonly planted woodland trees in the UK, only beech and ash are ozone-sensitive in this context. An important point to note is that although a species may be symptomatic at relatively low ozone exposure, this may not result in poor health or yield loss. The converse is also true – non-symptomatic plants may suffer reduced growth and increased susceptibility to biotic and abiotic damage.
The ‘visible symptoms’ vary greatly between species, and for each species, they need to be validated by comparing symptomatic foliage from fumigated trees with foliage grown in unpolluted air. In some species, particularly conifers such as Aleppo pine (Pinus halipensis: a Mediterranean species) and Weymouth pine (Pinus strobus), diffuse chlorosis of one-year-old needles is associated with ozone pollution. Some broadleaf species such as beech and ash also show interveinal chlorosis initially, followed by ‘stippling’, or bronzing. Other species including many shrub species (eg Prunus spinosa – sloe and Viburnum lantana – wayfaring tree) show characteristic reddening, although care has to be taken to distinguish between this response to ozone pollution and the natural response of some plant species to produce red pigmentation (anthocyanins) in response to high light and other stresses.
Monitoring ozone injury in trees
Ozone injury is monitored as part of the UNECE ICP Forests Intensive Forest Health Monitoring network. To date, ozone-related symptoms have only been identified at one plot in the UK, where slight damage to Pinus strobus was observed in East Anglia. Across Europe as a whole, the observation of ozone-related damage has largely been restricted to southern and central regions, where ozone levels are high as a result of climate, altitude and photochemical precursor formation.
Visible injury assessment in the UK
Canopy access presents difficulties in the assessment of ozone-related injury, and in the UK, the assessment is carried out on samples collected for routine foliar chemical analysis. To augment this monitoring exercise, five widely distributed shrub and tree species have been selected to act as bio-indicators of the effects of ozone pollution:
- Goat willow (Salix caprea)
- Wayfaring tree (Viburnum lantana)
- Ash (Fraxinus excelsior)
- Sloe or blackthorn (Prunus spinosa)
- Hawthorn (Crataegus monogyna)
Assessments are made on south-facing foliage exposed to the sun at an accessible height (0-2 m). In addition to a widespread UK distribution, these five species are also well represented across much of Europe. Typical ozone-related injury to these species is given in the manual (PDF-359K) for ozone injury monitoring in the UK. During summer 2003, ozone injury was reported to wayfaring tree in Kent, and goat willow in Thetford forest. An extensive gallery of ozone related symptoms for a range of species is available.