The report summarizes scientific information about forest ecosystems and their relationship to climate change and air pollution, with special emphasis on forests in Europe, North America and China. The contributions reflect the current state of science, but also uncertainties and gaps in scientific knowledge.

Climate change effects on forest soils will occur directly on soils or indirectly through effects on tree growth. Increases in soil temperature will cause an increase in soil respiration and decomposition and many increase rates of mineral weathering and ion exchange reactions. Forest soils represent a major sink for C and increased productivity in response to elevated atmospheric carbon dioxide will lead to an increase in c storage in forest ecosystems. Changes in forest productivity will have significant effects on forest soils, included water use, soil moisture status and nutrient cycling.

Two modeling approaches were applied to evaluate the potential risks of increasing atmospheric carbon dioxide concentrations and possible climate change on the vegetation cover of the Alpine region. The first model is a stochastic forest succession simulator, the second is a static regression type model. The questions of the study were as follows: (1) Which temporal vegetation changes might occur under given carbon dioxide and climate scenarios? (2) Which region of the Alps might be most susceptible to a vegetation change?

Tropospheric ozone is the most important single air pollutant of importance to forests in eastern United States. Both broadleaf and needled trees may suffer premature foliar senescence following even low ozone exposure years. Genetic sensitivity within species is likewise prevalent.

The area of productive woodland in the UK has been increasing steadily since 1950, generally at about 20-30 thousand hectares per year. The current forest estate covers 2,1 million hectares, the annual growth in the total stored carbon in wood is 2,0 million tonnes. The annual UK output of carbon dioxide from the burning of fossil fuels is 162 million tonnes.

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.

Visible chlorotic injury and reduced foliar biomass are found in Pinus ponderosa and P. jefreyi in the mountains of southern California and the west slope of Sierra Nevada. Reduced tree growth and accelerated rates of forest succession has been documented in this area, with ozone resistant, shade tolerant species replacing P. ponderosa. There may also be some interactions among ozone, drought stress, insects and fungal pathogens.

Destruction of forests leads to the release of carbon dioxide stored in the wood and also usually results in major releases of carbon dioxide stored in soil. Annual releases of carbon dioxide from destruction of tropical forests was estimated at about 1650 million tonnes per year.

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.

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.