Botany online 1996-2004. No further update, only historical document of botanical science!

Air Pollution, Acid Rain, Damage to Forests.

"When the Viennese botanist A. KERNER von MARILAUN had an audience with the emperor in 1896, the monarch asked about the botanical gardens of the university, that had obtained new greenhouses within the last years. When the emperor heard that the conifers thrived well outside, he remarked, that his son in law, prince Leopold in Munich, was trying in vain to get the conifers in his park to thrive. KERNER added that the botanical garden housed around eighty different conifers and that their thriving was due to their location, that protected them from the smoke plague. The smoke in the cities, namely, would bring sulphurous acid to the trees, that would effect them like poison."

(from: E.M. KORNFELD: ANTON KERNER von MARILAUN. Leipzig, 1908)

Acid rain became a political emotive word during the 1980s. The visible damage of the vegetation became heavier every year. Conifers were especially affected, firs more than spruces. The damages were first noticed in Southern Germany as soon as the 1960s, but they were not brought into connection with the acid rain. The first reports about the consequences of acid rain were made during the early 1970s, fish and small organisms in the oligotrophic, almost unbuffered lakes of Scandinavia were dying. The pH of these lakes dropped to values around 3. At first, it was tried to lessen the damages by supplying lime, until it became clear that this method, that was later on also used in German forests, could only lessen the symptoms, but not remove the cause.

During the years following 1980, more and more damage of forests was observed in Central Europe. The severest and irreversible damage occurred in the former east block. Caused by the air pollution effects of the large Bohemian power plants, the forests of the wind-exposed heights of the Erzgebirge were completely destroyed, a real waldsterben or forest decay.

The situation was somewhat better in Germany, since many trees were sick, but only relatively few had died. By 1983, nevertheless, 30 to 40 percent of all conifers showed symptoms of disease. The extend became serious in the dry summer of 1983. The percentage of ill trees rose in 1984 and the illness started to effect deciduous trees, too.

During 1985 and 1986, the damages of the forests increased only slightly. Both years showed a rise by 2 percent. At this time, 54 percent of the total forest area, about 4 million hectare, were either impaired in their vitality or damaged. By then, it seemed as if the situation had stabilized somewhat, albeit on a very high level of damage. The slowing down of the damage of conifers is accompanied by a simultaneous increase in the damage of deciduous trees. The fir remains the most severely damaged species.

Among the deciduous trees, oak is damaged most. All forests are in large areas in a very feeble equilibrium. Especially the forests of the higher regions of the low mountain ranges and of the Alps are damaged. These areas are characterized by a frequent and abundant rain, relatively low annual average temperatures, long periods of frost and a high percentage of days with fog.

The symptoms are not uniform. Strongly damaged areas occur in the same regions as healthy wood, making the question whether further factors besides air pollution (acid rain) play a part seem reasonable. Both epidemics, like infection by viruses, mycoplasma or Rickettsia and infestation by parasites like insects (especially the bark-beetle), nematodes, and fungi were discussed. It became nevertheless clearer and clearer that none of these factors was a main cause of the extensive damage of forests. The damages caused by infections and parasites have since decades the same extend. In the case of the firs in the southern Black Forest that occurs in this wood almost only in monoculture, a natural ageing of the trees was discussed.

Besides environmental factors, genetic differences between damaged and not damaged populations of a species were analyzed. The analysis of the isoenzyme and alloenzyme pattern showed that the pattern of distribution of some of the damaged forests differs from the healthy ones. These results are alarming, since they show that only some genotypes survive in certain, polluted habitats resulting in a reduction of a species' gene pool.

The symptoms of an ill tree are manifold. The following symptoms were found in spruces and other conifers:

damages of the needles (yellowing and later dropping off),
damages of buds and young shoots,
damages of the bark,
damages of the wood,
anomalous growth, and
damages of the small roots.

The symptoms do usually occur independently of each other and differ from region to region. The spruce, for example, displays the following five types of damage (according to: Forschungsbeirat Waldschäden/Luftverunreinigungen, cited by K. SCHMITT, 1990):

yellowing of the needles in higher regions of the German low mountain range (Mittelgebirge)
thinning of the tree tops in the middle regions of the low mountain range (Mittelgebirge)
reddening of the needles of older forests in Southern Germany
yellowing in higher regions of the Limestone Alps
thinning of the tree tops in forests close to the seaside

Yellowing of the needles is often a consequence of lacking nutrients, especially a lack of magnesium or potassium usually in connection with a lack of nitrogen. Yellowing of the needles may lead to the death and thus the falling off of the respective needle, though regeneration and a renewed greening is possible. Reddening of the needles is often caused by fungus infection.

It may safely be assumed that the soils of Central European forests are not in their best state. In contrast to agriculture, it, too, is harvested in forests, but hardly ever fertilized. The soil was consequently continuously depleted of minerals like magnesium and trees are thus more liable to become diseases.

Tree and wood damages are indeed based on a complex bundle of causes that is not easily analyzed. Much worse is, that positive feedback circuits occur, so that little disturbances that may be unimportant by themselves cause disastrous effects in combination with other conditions. Burning of fossile fuel led to a severe change of the atmosphere's redox potential. Before industrialisation, oxidative and reductive processes within the earth's atmosphere were in equilibrium. The extend of oxidation of fuel caused by industrialisation is higher than the rates of oxidation measurable in biological systems. Besides increasing the atmosphere's amount of carbon dioxide, this led, too, to its increase in sulphur dioxide, sulphur trioxide, nitrogen oxides, nitrous acid, nitric acid and other compounds. These gases were still emitted in 1989, even though a decrease of the rate of increase was observed due to political measures. The situation remained tense, because the burning of polyvinylchloride, a major component of plastic produces HCl. Moreover, it is well possible that TCDD is a product of burning plastic. Here, too, political measures have started to induce changes.

The increase in carbon dioxide is primarily of no importance in the following considerations, but it is of eminent impact in the world-wide changes of the climate (greenhouse effect). The increase in the other mentioned components is severe when taking the turnover ratios and the capacity of the atmosphere into consideration. An absorption by the oceans can be neglected as it would take around 1,000 years. Water and the substances dissolved in it remain in the atmosphere for about nine to ten days. Dissolving of the above mentioned oxidation products in the water droplets leads to a dissociation and thus to the production of protons. The consequence is a drastic drop in the pH. Besides acids, bases, too, find their way into the atmosphere. They are, nevertheless, until today almost completely of natural origin. Among them are especially ammonium compounds and carbonates.

Drops or aerosols of differing composition like ammonium hydrosulphate, ammonium disulphate, ammonium nitrate, etc. are generated together with their acidic equivalents. A further source of damage is the increase in ammonia due to intensive livestock farming, since nitrogen compounds cause at least locally strong eutrophication of the soil. They do thus add to the growth of trees, but at the same time the leaves or needles of the thus fertilized trees become more liable for pests, dryness, and frost. Beside this, they cause a small, but additive fertilization with nitrogenous compounds of naturally poor formations like oligotrophic grassland communities or dwarf shrub heaths, that happen to be the habitats of most of nowadays protected plants sensitive against over-fertilization.

As long as only carbon dioxide and water occur in the atmosphere in equilibrium, rain water has a pH of 5.6. If the compounds introduced above are dissolved, too, it can drop as low as 4.3. During the nine to ten days that water remains in the atmosphere, these compounds can travel hundreds to thousands of kilometers. Around two third of the sulphurous and nitrogenous compounds enter earth again by means of rain water, the remaining third is deposited again by 'dry deposition', i.e. undiluted. The proton surplus in the rain water causes an enhanced breakdown of rocky soils, the rate of degradation rises (rapid destroying of cultural monuments and sandstone sculptures in cities close to often used roads). The washing out of the soil of cations is enhanced, among them aluminium and heavy metal ions that are toxic for plants and other organisms. In other words: the acid rain causes an indirect damage of organisms. In addition, the corrosion of rock and the washing out of cations like magnesium and calcium ions causes a further acidification of the soil. The soil pH can thus drop to 3.9, a pH that hardly any plant survives. The uncoupling either in time or in place of production and mineralization worsens the proton balance further (for example under intense agricultural or forestry use of the soil). Water systems become acidic, if the water that runs into them has run through forest soil before.

Todays knowledge of the damages of the forests that became apparent during the 1970s and 80s can supply us with the following cause-effect scheme, that points at the causal relationships and the synergistic effects of single factors:

1. Primary harmfull substances: air pollution in the form of gas or dust, especially sulphur dioxide, nitric oxides, fluor, ozon, peroxides, and heavy metalions. Chemical transformations are accelerated by temperature and light (photooxidation). The effects on plants are:

destroying of leaf organs and bark
destroying of wax layers

2.Acid rain develops as a consequence of the dissolving of the gases named under 1. in atmospheric water. It effects plants either directly or via the soil. The results are:

changes of the soil: the nutrients of the upper soil layers are washed out, eutrophication occurs as a consequence of the addition of nitrogenous compounds. Toxic minerals are set free. The degree of the damage depends on the type of soil.

interference with the absorption of the plant due to changes in the soil chemistry in addition to root damages.

unbalanced nutrients as a consequence of higher amounts of biologically usable nitrogenous compounds.

3. Weather impacts (dry periods, too little rain, too much rain):

an risen temperature leads to an increased transpiration and potentially to a lack of water in the plant
Too little rain causes waves of acidification in the soil and thus damages of the roots

4.Symptoms that the trees display as a consequence of the damages:

damages in the fine roots. The exchange between tree and mycorrhiza is impaired as is the ability to take up nutrients.
Wet heartwood (found in firs, rotting of the heartwood)
lack in nutrients in combination with a lack of water leads to a dying of leaves and needles
growth impairments
a declining ability to resist frost, infections, pests, etc.
a damage of all physiological performances that causes finally the death of the tree.

Damage to forests or waldsterben (forest decay) ? The latter term especially is emotionally charged. It is quite fitting in the example of the forests of the Erzgebirge mentioned at the beginning of this chapter. German forests are characterized better by the term damage of forests. We know far too little about the ecosystem wood or forest yet, but it should be noticed that a strong loss of leaves or needles does also have the effect of more light at the soil level. As a consequence, the soil vegetation may change. Species that are sensitive to light and drying out like some mosses may be replaced by a changed spectrum of species characterized mostly by grass species. This new vegetation influences the micofauna and the soil conditions. An increased rate of transpiration of these plants leads to a faster drying out of the soil and causes even more damage to the already ill trees.

The research studies performed as fast as possible during the 1980s analyzed mostly the behaviour of single trees. It showed that they have abundant reserves and a high ability to regenerate. If all practized and planned measurements of protection are indeed kept, the trees will have a real chance of regeneration. Under these conditions, the ecosystem wood or forest is able to return to a stable equilibrium. In this context, ecologically sensible afforestations are important, too. New monocultures would program further damages. In the late 1980s the damages were still reversible. Long-term disruptions of the ecosystem wood occurred only, if at least one of the following four criteria were fulfilled:

  1. Climatic changes. The European ice-ages destroyed the wood several times for thousands of years and replaced it by a cold semidesert

  2. Destroying of the soil. Erosions by strong rain in mountainous areas that resulted in blank rock prevent a new growth of forest. In the best case, the forest is replaced by shrubbery (the karst landscape in the former Yugoslavia is an example).

  3. Long-term wetting or drying out of the soil that do usually prevent the growth of forests. (Swamp vegetation and swamp forests are exceptions).

  4. Strong chemical changes of the atmosphere that effect the soil. This has been the case in the damages of forests and the waldsterben discussed above.

How do the German forests look in 1995? Is the general waldsterben a mere construct? asks the ecologist H. ELLENBERG from Göttingen, Germany. He refers to incomplete reports and weaknesses of the annual reports about the forests. The published data are only rarely based on real losses of leaves or needles. Instead, they are based on estimations of the foliage density if compared to photographic series of trees with differing foliage densities. Since only one standard series is used per tree species, these estimations do not take the sometimes very strong effects the climate and the soil have on the foliage of perfectly healthy trees, too, into consideration. A low density was therefore often erroneously interpreted as a sign of damage. The general waldsterben remained a hypothesis. The German botanist O. KANDLER from Munich came to the same conclusion using other arguments. He pointed out that many forests in Central and Western Europe showed high rates of growth increase even through the 1960s to 1980s. It cannot be denied that local to regional damages of forests are a feature that occurred at all times. The studies that started at the beginning of the 1980s (the annual inquiry of the damage of forests of the German ministry of nutrition, agriculture, and forests) displayed fluctuations in all studied tree species, but no constant increase of higher stages of damage.

© Peter v. Sengbusch - Impressum