Development and Causes of Novel Forest Decline in Germany

Abstract

A clear distinction must be made between the decline and the death of certain forest species and forest areas, which we prefer to call “novel forest decline” (‘neuartige Waldschäden’), and the classical form of “smoke-damage”, caused mainly by sulphur dioxide. The new type of forest decline was first recognized in Southern Germany during the early to middle seventies. It is remarkable that this phenomenon has been concentrated on areas previously considered to be clean air regions. Other areas outside Southern Germany followed with significant delay. Forests at high altitudes have been especially endangered. The typical Symptoms of novel forest decline are not comparable to those caused by classical air pollutants such as sulphur dioxide, nitrogen oxides, chloride, fluoride, and heavy metals. The most common Symptom is chlorosis of needles exposed to sunlight, which is correlated with deficiency of nutrients, mainly magnesium.

The explanations of the various causes of this new kind of forest decline have undergone remarkable changes. Up to 1982 the terms “Waldsterben” and “acid rain” were used more or less as synonyms, and acid rain was considered to have caused acidification of soil, with mobilization of soluble aluminum ions affecting the root system. Thus trees become more vulnerable to additional stress factors, including other air pollutants. An alternative hypothesis introduced at the end of 1982 considered the effect of ozone and other photooxidants on the upper parts of the tree as the primary cause. Changes in membrane permeability resulting in an enhanced nutrient leaching, as well as reduced assimilate production affecting fine root growth, are major steps in a spiral of continuous decreasing vitality. This new explanation shifted the emphasis from sulphur dioxide to nitrogen oxides as the emissions of primary importance.

The following observations must be taken into consideration when arguing for or against competing hypotheses.

• 1. Against sulphur dioxide alone: In some of the most severely damaged areas ambient air concentrations of sulphur dioxide are extremely low with corresponding low sulphur contents in needles.

• 2. Against acid rain alone: Lichens proved to be sensitive to acid rain but show luxuriant growth, particularly in the most damaged areas.

• 3. Against influence of soil alone, including soil acidification: No clear correlation between degree of damage and soil condition or type of bedrock could be found on a broad scale. Severe damage is sometimes also occurring in calcareous soil.

The following lists experimental evidence to support the hypothesis that the combined effect of ozone and acid rain or fog is the primary factor in forest decline:

(1) inhibition of photosynthesis by ozone; (2) impairment of root development by ozone; (3) disturbance of allocation and translocation of assimilates by ozone; (4) seasonal variation of chlorophyll content and supply of essential nutrients to the needles; (5) enhanced leaching dependent on ozone-concentration, pH of rain or fog, previous damage of needles and age of needles; (6) relation between nutrient deficiency and increased photosensitivity; (7) disturbance by ozone of reactions controlled by phytohormones; (8) preference of occurrence of damage on soil with low nutrient status.

These arguments are the most appropriate for fir and spruce. Beech and pine (in this case Pinus nigra, var. austriaca (Hoess/Neum.)) may be severely damaged by the direct impact of ozone alone, without further interactions with soil, rain or fog. Although much progress has been made by identifying several possible causes of the novel forest decline, much more work is needed to understand the complex interactions of these causal variables.