14 Air pollution, Photosynthesis
and Forest Decline:
Interactions and Consequences
U. Heber, W. Kaiser, M. Luwe, G. Kindermann,
S. Veljovic-Javonovic, Z. Yin, H. Pfanz, and S. Slovik
14.1 Introduction
Industrialization has led, and is still leading, to the emISSlOn of large
quantities of sulfur dioxide and nitrogen oxides in many countries. Atmospheric photochemistry has added ozone, which is formed by the interaction
between light, NOz, and oxygen, and is one of the strongest oxidants
known. There are also other air pollutants with the potential to damage
plants, but since, according to Paracelsus, "the dose defines a poison", only
those pollutants can cause plant injury which occur at concentrations high
enough to produce appreciable diffusional fluxes into the plants, fluxes that
cannot be mastered by the chemical fluxes engaged in detoxifying entering
pollutants.
Forest decline has become a matter of public concern in many industrialized countries. Nevertheless, it has been, and still is, a matter of
dispute whether or not air pollution and forest decline are causally related.
Understanding possible interactions is complicated by the fact that agricultural productivity has remained high in countries where forest decline is
extensive. The multiplicity of factors acting in any ecosystem is another
reason why causal analysis of the phenomenon of forest decline, which
requires separation of individual factors, is so difficult to achieve. In this
situation, analysis must be based on a combination of fieldwork, laboratory
experiments under controlled conditions, which necessarily deviate from
those in the field, and, finally, on data integration and deduction.
In the following, we attempt to justify our conviction that, depending
on climatic conditions and exposure, ozone and S02 are important both
in triggering and causing, in part, the forest decline observed in many
mountainous areas of central Europe and Northern America. The main
emphasis will be placed here on a consideration of problems caused by S02,
because analysis of ozone effects has not yet progressed as far as that of SOz
effects. Nevertheless, owing to the chemical reactivity of ozone, problems
caused by ozone are well defined in principle, although much remains to be
done before quantitative conclusions can be drawn. For S02, a quantitative
approach appears to be possible already at present (Slovik et al. 1992a,b).
Although both SOz and ozone are known to affect photosynthesis of leaves
and Forest Decline:
Interactions and Consequences
U. Heber, W. Kaiser, M. Luwe, G. Kindermann,
S. Veljovic-Javonovic, Z. Yin, H. Pfanz, and S. Slovik
14.1 Introduction
Industrialization has led, and is still leading, to the emISSlOn of large
quantities of sulfur dioxide and nitrogen oxides in many countries. Atmospheric photochemistry has added ozone, which is formed by the interaction
between light, NOz, and oxygen, and is one of the strongest oxidants
known. There are also other air pollutants with the potential to damage
plants, but since, according to Paracelsus, "the dose defines a poison", only
those pollutants can cause plant injury which occur at concentrations high
enough to produce appreciable diffusional fluxes into the plants, fluxes that
cannot be mastered by the chemical fluxes engaged in detoxifying entering
pollutants.
Forest decline has become a matter of public concern in many industrialized countries. Nevertheless, it has been, and still is, a matter of
dispute whether or not air pollution and forest decline are causally related.
Understanding possible interactions is complicated by the fact that agricultural productivity has remained high in countries where forest decline is
extensive. The multiplicity of factors acting in any ecosystem is another
reason why causal analysis of the phenomenon of forest decline, which
requires separation of individual factors, is so difficult to achieve. In this
situation, analysis must be based on a combination of fieldwork, laboratory
experiments under controlled conditions, which necessarily deviate from
those in the field, and, finally, on data integration and deduction.
In the following, we attempt to justify our conviction that, depending
on climatic conditions and exposure, ozone and S02 are important both
in triggering and causing, in part, the forest decline observed in many
mountainous areas of central Europe and Northern America. The main
emphasis will be placed here on a consideration of problems caused by S02,
because analysis of ozone effects has not yet progressed as far as that of SOz
effects. Nevertheless, owing to the chemical reactivity of ozone, problems
caused by ozone are well defined in principle, although much remains to be
done before quantitative conclusions can be drawn. For S02, a quantitative
approach appears to be possible already at present (Slovik et al. 1992a,b).
Although both SOz and ozone are known to affect photosynthesis of leaves