CIIAPTER 2 Forests and the Changing Chemical Composit ion of the Atmosphere

The chemical composit ion of the atmosphere is changing due to the trends in the f luxes of, for example, CO2, SO2, and NO, that are emitted from industr ial and cnergy combustion processes into the atmosphere. Forest ecosystems are unl ikely to react in any single, universal way, but rather there may be a large variabi l i ty of ecological reactions both in t ime and in space. This variabi l i ty is part ly due to the concentrat ion patterns of the emitted compounds. Also, in condit ions of a given load of pol lutants the ecological response may vary according to the ecosystem characterist ics. At this stage i t is extremely dif f icult to actual ly quanti fy the possible reac-t ions of forest ecosystems, so al l quanti tat ive scenarios should be examined cautiously because of this uncertainty. Quanti tat ive scenarios are not use-less, however, because they provide a systematic way of ranking and evaluating the dif ferent factors of uncertainty. That, in turn, improves our understanding of the phenomena. In this chapter we highl ight three issues, al l deal ing with the responses of forests to the trends in the chemical composit ion of the atmo-sphere. The f irst issue is that of forest damage currently being observed in Europe. Acid precipitat ion, the stress due to sulfur dioxide, ozone, and heavy metals, and the excess amount of deposit ing nitrogen compounds are conlmon denominators in this phenomenon. The two remaining issues are related to CO, emissions. The so-cal led CO, fert i l izat ion effect is due to the obvious possibi l i ty that the rate of tree photosynthesis wil l general ly increase as the air is given increasing amounts of one of the main substrates to photosynthesis, i.e., carbon dioxide. The greenhouse effect is related to CO, and other "greenhouse gases", which al l have a tendency to increase Forests and the Changling Chemicd Composition ol the Atmosphere I larth surface temperatures. The possible cl imatic warming in the air would affect the ecology o [ forests. 2.1. European Forest Damag