292
U. Heber et al.
acceptable average. The obvious conclusion is that legal (or recommended)
tolerance limits for S02 must be changed in accordance with the changing
sensitivity of plants. They should be lower during the winter when evergreen
plants are more sensitive than in the summer. If this is difficult to implement,
permissible annual averages must be reduced. Depending on growth rates
of spruce which provide information on the extent of possible reductive
detoxification of S02 and on cation availability, Slovik et al. (1992b) calculated S02 immissions between 2 and 5 ppb S02 (annual means) to be
tolerable. This is below international and German tolerance limits. Depending on growth rates, the annual cation demand has been calculated to
be doubled in the presence of S02 levels between 16 and 40 ppb (annual
average). Doubling the cation demand on soils where growth is nutrientlimited must, in the long term, have disastrous consequences on forests.
However, it would appear that increasing the nutrient supply by proper
application of fertilizers or by liming could reduce or even stop the decline
of spruce forests (cf. Lange et al. 1989b).
14.7 Conclusions
In the middle and eastern part of Europe, forest decline is particularly
apparent in alpine regions, the Black Forest, the Bavarian Forest, the
Thuringian Forest, the Harz, the Fichtelgebirge, the Ore Mountains
(Erzgebirge), the northern and northeastern part of Czechoslovakia, and in
Upper and Lower Silesia (Riesengebirge). Except for the Alps and the
Black Forest, where ozone appears to predominate, S02 pollution is frequent
in these locations. Although the evidence for direct involvement of air
pollutants in forest decline is overwhelming, the different distribution of
individual air pollutants, differing soil qualities, and climatic conditions
make it impossible to attribute forest decline to a particular offender. Rather
it appears that forest decline is caused by a complex combination of different
factors, including not only air pollutants but also natural stress factors such
as drought and frost. Ozone and S02 decrease cellular viability, thereby
increasing the susceptibility to natural stress factors. Decreased forest health
cannot be explained on the basis of direct effects of air pollutants on
photosynthesis. Rather, the photosynthetic apparatus appears to be well
protected against ozone by its location in cells and against N02 and S02 by
antioxidative defenses. These are particularly efficient in scavenging radicals.
The cellular pH-stat is effective in regulating cellular pH values during influx
or production of acid as long as bases are available or protons of the acid
can be exchanged for cations. Decline is inevitable and leaves or needles
are shed when increased cation demands cannot be met. Of particular
importance in this respect is the decreased availability of nutrient cations
from poor soils or soils exposed to acid leaching in situations where cations
U. Heber et al.
acceptable average. The obvious conclusion is that legal (or recommended)
tolerance limits for S02 must be changed in accordance with the changing
sensitivity of plants. They should be lower during the winter when evergreen
plants are more sensitive than in the summer. If this is difficult to implement,
permissible annual averages must be reduced. Depending on growth rates
of spruce which provide information on the extent of possible reductive
detoxification of S02 and on cation availability, Slovik et al. (1992b) calculated S02 immissions between 2 and 5 ppb S02 (annual means) to be
tolerable. This is below international and German tolerance limits. Depending on growth rates, the annual cation demand has been calculated to
be doubled in the presence of S02 levels between 16 and 40 ppb (annual
average). Doubling the cation demand on soils where growth is nutrientlimited must, in the long term, have disastrous consequences on forests.
However, it would appear that increasing the nutrient supply by proper
application of fertilizers or by liming could reduce or even stop the decline
of spruce forests (cf. Lange et al. 1989b).
14.7 Conclusions
In the middle and eastern part of Europe, forest decline is particularly
apparent in alpine regions, the Black Forest, the Bavarian Forest, the
Thuringian Forest, the Harz, the Fichtelgebirge, the Ore Mountains
(Erzgebirge), the northern and northeastern part of Czechoslovakia, and in
Upper and Lower Silesia (Riesengebirge). Except for the Alps and the
Black Forest, where ozone appears to predominate, S02 pollution is frequent
in these locations. Although the evidence for direct involvement of air
pollutants in forest decline is overwhelming, the different distribution of
individual air pollutants, differing soil qualities, and climatic conditions
make it impossible to attribute forest decline to a particular offender. Rather
it appears that forest decline is caused by a complex combination of different
factors, including not only air pollutants but also natural stress factors such
as drought and frost. Ozone and S02 decrease cellular viability, thereby
increasing the susceptibility to natural stress factors. Decreased forest health
cannot be explained on the basis of direct effects of air pollutants on
photosynthesis. Rather, the photosynthetic apparatus appears to be well
protected against ozone by its location in cells and against N02 and S02 by
antioxidative defenses. These are particularly efficient in scavenging radicals.
The cellular pH-stat is effective in regulating cellular pH values during influx
or production of acid as long as bases are available or protons of the acid
can be exchanged for cations. Decline is inevitable and leaves or needles
are shed when increased cation demands cannot be met. Of particular
importance in this respect is the decreased availability of nutrient cations
from poor soils or soils exposed to acid leaching in situations where cations