Ozone has been recognized as an important cause of damage to crops since the 1950s. Damage to trees was first identified in the 1960s and is now known to be widespread in both North America and Europe. Most impact studies have emphasized the importance of determining growth losses attributable to ozone and as a result have concentrated on species of commercial importance. This is illustrated by the critical loads approach to ozone risk assessment in Europe, which is currently based on the AOT40 - 10 ppmh threshold. At higher levels, it has been argued that a 10% growth reduction occurs in European beech (Fagus sylvatica). Such an approach suffers from a number of serious limitations, not least the widespread impacts on ecosystems that may occur at lower ozone exposures and the very poor quantitative basis for setting this threshold.
In Europe, there has been increasing emphasis on the conservation and management of species without any direct economic importance. This has arisen from a growing environmental awareness of the general public. The trend has been accelerated by the perceived environmental benefits of the large amounts of land that has been taken out of agricultural production (as a result of the "set-aside" policy of the European Union) and the public concern about the ecological and environmental impacts of industrial forestry. In agricultural landscapes, hedgerow species and "weed" species are being looked at as important parts of the agricultural ecosystem. In particular, weed species are an important part of the food chain for the wildlife present in such ecosystems.
In forests, much greater emphasis is being given to the authenticity of the forest ecosystems. Particular emphasis is being given to ecosystem management techniques such as continuous cover forestry and the furthering of natural regeneration. In theory, timber production is no longer the single most important priority, and multiple-purpose forestry is currently in fashion. As a result, many more components of the forest ecosystem are being examined.
These shifts in emphasis have not yet been reflected in ozone impact studies. Despite emerging knowledge of the sensitivity to ozone of many herb and shrub species, such species are rarely included in ozone risk assessments; until recently, the widespread occurrence of visible injury induced by ozone had passed unnoticed. The situation is even worse for animal species, with virtually nothing known about ozone impacts on any species other than humans. Effects can be anticipated on the basis of studies done on human health, but how these effects can be incorporated into ecological risk assessments remains unclear.