290
U. Heber et al.
combination of N02 and S02 actually appears to be more toxic than would
be expected on the basis of individual toxicities (Heath 1980; Wellburn
1984; Yin 1990). It is not known whether this experimental observation can
be extrapolated to the lower levels of different pollutants experienced simultaneously by plants in the field. Oren and Schulze (1989) introduced the idea
of nutritional disharmony to explain effects of increased N availability on
forest decline. By stimulating growth, nitrogen increases cation demand,
adding to the problems caused by the necessity to supply cations as counterions of airborne sulfate.
For the combination of ozone and S02, rational explanation of enhanced
toxicity as observed by Heggestad and Bennet (1981) and Yin (1990) has
been proposed by Slovik et al. (1992b). If ozone cannot be completely
detoxified in the apoplasm before reaching the plasmalemma, increased
plasmalemma permeability caused by ozonization of unsaturated fatty acid
residues could lead to cation losses exacerbating already existing S022dependent cation deficiency.
14.5.7 Interactions with Climatic Conditions
The primary sites of forest damage are mountainous areas where plants are
exposed to harsh climatic conditions. Drought can in principle be protective
by limiting the direct impact of air pollutants through stomatal closure.
However, drought also represents a stress. Added stress is unlikely to
relieve a stress produced by air pollutants during prolonged exposure.
Rather, it is likely to result in damage which neither stress alone would
cause. This may be rather general. S02 is known to increase the frost
sensitivity of plants (Davison and Bailey 1982). Frost periods decrease the
tolerance of spruce trees to S02 (Slovik et al. 1992a). Low temperatures will
also decrease the rate of metabolic detoxification and repair reactions.
14.6 Tolerance Limits
At elevated concentrations, air pollutants are toxic not only for plants, but
also for man. Legislative bodies and concerned organizations have defined
limits of tolerance for both (for Germany: Bundesimmissionsschutzgesetz;
Jager 1989). It appears that legal limits are a result of both damage risk
assessment and compromise. Control of industrial and household emissions
is costly. Main problems in defining (or redefining) limits of tolerance are
that different organisms possess different capacities for detoxification, and
that tolerance of a particular organism may vary with its physiological state
and the season. For S02 pollution in exposed forest sites with severe climatic
conditions, a general tolerance limit recommended by IUFRO (International
U. Heber et al.
combination of N02 and S02 actually appears to be more toxic than would
be expected on the basis of individual toxicities (Heath 1980; Wellburn
1984; Yin 1990). It is not known whether this experimental observation can
be extrapolated to the lower levels of different pollutants experienced simultaneously by plants in the field. Oren and Schulze (1989) introduced the idea
of nutritional disharmony to explain effects of increased N availability on
forest decline. By stimulating growth, nitrogen increases cation demand,
adding to the problems caused by the necessity to supply cations as counterions of airborne sulfate.
For the combination of ozone and S02, rational explanation of enhanced
toxicity as observed by Heggestad and Bennet (1981) and Yin (1990) has
been proposed by Slovik et al. (1992b). If ozone cannot be completely
detoxified in the apoplasm before reaching the plasmalemma, increased
plasmalemma permeability caused by ozonization of unsaturated fatty acid
residues could lead to cation losses exacerbating already existing S022dependent cation deficiency.
14.5.7 Interactions with Climatic Conditions
The primary sites of forest damage are mountainous areas where plants are
exposed to harsh climatic conditions. Drought can in principle be protective
by limiting the direct impact of air pollutants through stomatal closure.
However, drought also represents a stress. Added stress is unlikely to
relieve a stress produced by air pollutants during prolonged exposure.
Rather, it is likely to result in damage which neither stress alone would
cause. This may be rather general. S02 is known to increase the frost
sensitivity of plants (Davison and Bailey 1982). Frost periods decrease the
tolerance of spruce trees to S02 (Slovik et al. 1992a). Low temperatures will
also decrease the rate of metabolic detoxification and repair reactions.
14.6 Tolerance Limits
At elevated concentrations, air pollutants are toxic not only for plants, but
also for man. Legislative bodies and concerned organizations have defined
limits of tolerance for both (for Germany: Bundesimmissionsschutzgesetz;
Jager 1989). It appears that legal limits are a result of both damage risk
assessment and compromise. Control of industrial and household emissions
is costly. Main problems in defining (or redefining) limits of tolerance are
that different organisms possess different capacities for detoxification, and
that tolerance of a particular organism may vary with its physiological state
and the season. For S02 pollution in exposed forest sites with severe climatic
conditions, a general tolerance limit recommended by IUFRO (International