The forests in the mountain ranges in Europe have shown a
decline in recent years. In the early 1980's, 20 to 25% of the
forests were considered damaged. The damaged parts of these
forests were monitored and showed signs of improvement. There
were five main hypotheses for the forest decline. The hypotheses
are as follows:
1. High concentration of gaseous pollutants
2. Natural climate variation (e.g. drought)
3. Microorganisms or needle fungi
4. Soil cation leaching
5. Nutrient deficiencies
Studies to test these hypotheses were conducted on the damaged forests in the mountain ranges of northeast Bavaria. The gaseous pollutants, SO2, NOx, and ozone, reached levels that might cause damage to the spruce trees. However, the pollutants did not decrease the rate of photosynthesis in the spruce. The study of the pollutants also showed an interaction between needle yellowing and tree growth. It was, therefore, determined that gaseous pollutants do not cause any long-term effects on the spruce forests.
These gaseous pollutants are converted into particulates which are absorbed into the soil. These particulates, especially the nitrates, have caused the pH level of the soil to decrease in recent years. The reduced pH levels have caused the natural buffering mechanisms in the soil to change. These changes in the mechanisms redistributes the cations in the soil. The changes in the soil acidification and cation deficiency causes a reduction in the nutrients in the soil and the root depth to decrease. This is a problem for the spruce forests and could be one of the main reasons for their decline.
The increase of nitrates in the soil not only causes a cation deficiency, but also a high concentration of nitrogen in the tree itself. This excess nitrogen causes the needle biomass to grow at a faster rate. The new biomass exhausts the spruce's reserve supply of magnesium giving it a yellow color.
The study did not show any evidence that forest decline is being caused by microorganisms. Fungi was found in the needles but is so slow in spreading that it is believed to play only a small role in the spruce forest decline.
The nitrogen from nitrates seems to control the decline in spruce forests. It causes a decrease in pH levels in soil and an increase in soil cation leaching. These symptoms, as shown in the study, can significantly hurt the health of a spruce tree. If the nitrogen balance can become stable, yellowing symptoms and forest decline will slow and a lower growth rate will occur.
The results of this study support three of the five hypotheses. The high concentrations of gaseous pollutants do not show any long-term effects, but when converted into particulates, they contribute to many of the problems in the decline of spruce forests. The main problems that cause the decline and yellowing of the spruce are the decreased levels in pH and the cation deficiency. If the pH levels can be maintained and the cation deficiency controlled, the spruce forests can become healthy again. Once again, mankind is slowly beginning to see the effects caused from pollution on our planet, Earth.
Reference