Air pollution problems are international in scale. All forests worldwide experience some degree of air pollution exposure above preindustrial levels. Atmospheric transport processes do not recognize geographic borders, but sources of pollutants, the pollutants of concern, and the speciﬁc effects of pollutants vary greatly depending on human cultural activities and natural climate patterns. For example, heavy-metal contamination is a result of poorly controlled mining and industrial emissions; when coupled with frequent rainfall, dispersion is minimized and local deposition is maximized. Deposition of suspended particles is most frequently a problem in forests in dry climates adjacent to agricultural areas where atmospheric conditions allow suspended particles to remain airborne for long periods of time. Ozone is a secondary pollutant formed from auto- mobile exhaust (nitrogen oxides) and volatile organic carbon from a variety of chemical, combustion, and natural processes. The reaction requires ample sunlight, thus ozone is a serious problem in urbanized areas in sunny climates. Air pollution effects on forests can, therefore, best be understood by looking at climate zone and the human cultural activities of agriculture, urbanization, and industrialization. Although there are many natural sources of air pollutants, such as vegetation ﬁres, windstorms, and volcanic eruptions, for the purposes of this article we shall focus on human caused or anthropogenic, sources of air pollutants and their effects on forest ecosystems.
[...] Air pollution effects Clearly, burning to remove vegetation alters the immediate landscape but the effects of smoke on ecosystems downwind have only recently been addressed. The huge ﬁres in Southeast Asia during the late 1990s and the annual burning of the cerrado grasslands in central Brazil offer examples of intentional and unintentional ﬁre effects on native forests. Serious increases in tropospheric ozone have been documented as a result of cerrado ﬁres. Concentrations measured are equivalent to those measured outside large urban centers (100–200 ppb). [...]
[...] Air pollution effects Ozone effects on semiarid forests are well documented in the Mediterranean climates of south-western North America, southern Spain, and Italy. Short- term exposures to ozone concentrations greater than 150 ppb can cause acute damage symptoms on many plant species. Long-term, chronic exposures to 50 ppb result in reduced growth of sensitive species and foliar mottling of many forest tree species. The primary sites of uptake and injury are the stomata of actively photosynthesizing leaves. When the stomata are open for gas exchange, ozone readily gains access to the stomatal cavity and mesophyll of foliage. [...]
[...] Air pollution concerns The effects of air pollutants on boreal forests are acute. The industries of the subarctic generate two primary pollutants of concern: sulfur dioxide and the generalized category of ‘metals.' These two pollutants have very different dispersal patterns and modes of action (that is, how they affect trees and forest ecosystems). Sulfur dioxide is a well-known byproduct of coal combustion, but it is also found in many minerals and is released during smelting. Once aloft, sulfur dioxide can be transported for hundreds of kilo- meters before depositing on foliage, soil, water, or other substrates. [...]
[...] Air pollution effect Mining and forestry operations in Russia have resulted in about 1 million hectares of ‘seriously damaged' forest and another 7 million hectares of ‘affected' forests. The Noril'sk mining complex is one of the best examples of the catastrophic effects of unregulated mining and smelting on boreal forests. Tree mortality due to sulfur dioxide extends for 200 km downwind of the complex and copper, cobalt, and nickel concentrations in soils are 1000 times higher than background levels up to 30 km downwind. [...]
[...] The study of multiple pollutant effects is an emerging topic that will ultimately improve our understanding of air pollution effects on natural ecosystems. Particulate pollutants occur in all parts of the world, but the low humidity and seasonal rainfall make suspended particulates a serious issue in semiarid ecosystems. The sizes of these pollutants can range from a few angstroms to several micrometers in diameter. The size affects physical behaviors such as travel distance and deposition as well as the pollutants' effects on biological organisms. [...]
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