The global exposure of forests to air pollutants

The tall, aerodynamically rough surfaces of forests provide for the efficie nt exchange of heat and momentum between terrestrial surfaces and the atmos phere. The same properties of forests also provide for large potential rate s of deposition of pollutant gases, aerosols and cloud droplets. For some r eactive pollutant gases, including SO2, HNO3 and NH3, rates of deposition m ay be large and substantially larger than onto shorter vegetation and is th e cause of the so called "filtering effect" of forest canopies. Pollutant i nputs to moorland and forest have been compared using measured ambient conc entrations from an unpolluted site in southern Scotland and a more polluted site in south eastern Germany. The inputs of S and N to forest at the Scot tish site exceed moorland by 16% and 31% respectively with inputs of 7.3 kg S ha(-1) y and 10.6 kg N ha(-1) y(-1). At the continental site inputs to t he forest were 43% and 48% larger than over moorland for S and N deposition with totals of 53.6 kg S ha(-1) y(-1) and 69.5 kg N ha(-)1 y(-)1 respectiv ely. The inputs of acidity to global forests show that in 1985 most of the areas receiving > 1 kg H+ ha(-1) y(-1) as S are in the temperate latitudes, with 8% of total global forest exceeding this threshold. By 2050, 17% of global forest will be receiving > 1 kg H-1 ha(-1) as S and most of the increase i s in tropical and sub-tropical countries. Forests throughout the world are also exposed to elevated concentrations of ozone. Taking 60 ppb O-3 as a concentration likely to be phytotoxic to sen sitive forest species, a global model has been used to simulate the global exposure of forests to potentially phytotoxic O-3 concentrations for the ye ars 1860, 1950, 1970, 1990 and 2100. The model shows no exposure to concent rations in excess of 60 ppb in 1860, and of the 6% of global forest exposed to concentrations > 60 ppb in 1950, 75% were in temperate latitudes and 25 % in the tropics. By 1990 24% of global forest is exposed to O-3 concentrat es > 60 ppb, and this increases to almost 50% of global forest by 2100. Whi le the uncertainty in the future pollution climate of global forest is cons iderable, the likely impact of O-3 and acid deposition is even more difficu lt to assess because of interactions between these pollutants and substanti al changes in ambient CO2 concentration, N deposition and climate over the same period, but the effects are unlikely to be beneficial overall.