Forest Scientists must recognize that numerous diseases and insect occurrences as well as more subtle environmental stresses are prevalent as causes of changes in forest health. Their interactions often lead to "natural" declines of individual tree species or site-species declines of multiple species. The role of air pollution should be carefully investigated in an integrative sense with these other endemic and sometimes epidemic outbreaks of biotic agents and environmental stresses.

The potential effects of long-term exposures to widespread low but raised concentrations above natural levels in Central Europe are still being discussed. This uncertainty results from an inadequate mechanistic understanding of the influence of air pollutants and other environmental factors of trees.

The results of numerous investigations in Europe on potential relationships between forest decline caused by immissions and the wood quality of diseased trees consistently showed the technical properties of wood form diseased trees to be unaffected. Wood from diseased trees can be used without reservation.

Physiological processes of western trees are effected by ozone at concentration over 80 ppb, depending on the duration of the exposures and environmental conditions. At a single fascicle level short-term ozone exposures can cause reduction, no change or increase of stomatal conductance and net assimilation rate. Two seasons of exposures at twice level ozone concentrations caused a significant reduction of stomatal conductance and pigment concentrations in foliage.

Long-term changes in forest soils are characterised by decreases in soil pH, exchangeable base cations, percent base saturation and accumulation of heavy metals. Acidic deposition effects on forest soils can be demonstrated experientially either in the laboratory or in the field by stimulated acid treatments.

Atmospheric pollutant can cause direct effects mediated by foliage and indirect effects mediated through soil. Biotic and abiotic factors can confound relationships among nutrient cycling, growth loss and mortality and air pollution. Changes in the soils could result from acidic deposition. Changes could include increased fertility as a result of sulphur and nitrogen input to soils that are deficient in these elements or decreased fertility through ion leaching or mobilization of toxic substances as aluminium.

The soil biota plays a critical part in the functioning of forest ecosystems. Acidification and heavy metals have the greatest direct effects on the soil biota. However pollutants can have indirect effects through their influence on tree root systems. Acidic deposition can decrease the incidence of mycorrhizal infection, but soil pH and concentration of nitrogen and certain forms of sulphur are also important.