Nitrogen and sulphur are essential elements for all organisms. Although the ecological importance of their natural biogeochemical cycles has long been recognized, little emphasis has been placed until recently on the ecological consequences of the perturbation of these cycles by human activities. A realization of the scale of these perturbations and of their potential ecological effects has just begun to focus attention on them. In particular, the spectacular forest decline in industrial countries during the 1980s and the 1990s focused the attention on the dramatic impact of acidic mists and rains largely caused by SO2 pollution. SO2 concentrations sharply increased as a result of developing smelting activities in the immediate pre-industrial era. In the second half of the twentieth century, changes in legislation, fuel usage and combustion technology resulted in the increasing importance of high level emissions from power stations. This resulted in increased dispersion of SO2 and much lower concentrations in formerly polluted districts. These changes in fuel usage and combustion technology are one of the factors resulting in increasing atmospheric nitrogen deposition, but may also have exacerbated the dispersion of acidic pollutants such as NO—. The successive waves of nitrogen and sulphur pollution cannot, therefore, be looked upon as being entirely independent of one another. Long-term changes induced by sulphur pollution may still be occurring in ecosystems as their responses to nitrogen are initiated and these changes interact with one another (Lee 1998). It is the impact of these ecological changes on bryophytes that are the focus of the next section.
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