To the files
Even before the record summer of 2003, the then Federal Minister of Agriculture, Renate Künast, officially declared the forest dieback to be over. But after that, three quarters of all native tree populations were damaged. Would the absent "acid rain" - universal culprit of the diseased forest in 1982 - saved some thirsty plants from damage? And how is the forest doing today?
In 1984, the German forest went to the intensive care unit: The first forest damage report by the then federal government made the public aware of keywords such as crown thinning, acidified soil and acid rain. The trees were considered to be weakened or damaged in their vitality on around 50 percent of the forest area - this corresponds to four million hectares - and the trend is increasing. If the gloomy forecasts of the future had really come true, they could only allow for a conversation about the German forest today in a soft pluperfect.
But we can talk out loud in present tense, thank God. Lush green, the patient swells across the republic and, according to forest statistics, spreads by about 100 square kilometers every year. A worried look at the crowns still shows light spots, but the earlier explanations for this observation are no longer tenable today. Forest damage has a complex set of causes that is not easy to unravel.
Irreversible damage from air pollutants
One thing is certain: The forest was regionally - as the spruce trees on the wind-exposed western slopes of the Ore Mountains dramatically showed - severely and in some cases irreversibly damaged by emissions from thermal power plants. Since the beginning of industrialization, far more acids than bases have found their way into the air and permanently change the pH value of the atmosphere. The sulfur and nitrogen oxides, which are important in this context, dissolve in water droplets or form aerosols and finally fall on the forests as "acid rain". Bohemian trees have been hit hard, with rain recorded with pH levels hovering around 4.3 versus the rainwater standard of 5.6.
And yet the cause of this dying spruce, which was one of the figureheads of forest dying in the media, cannot be the blanket diagnosis for all conspicuous tree changes. Smoke damage to spruce trees already existed at the beginning of the 20th century, without the forest being declared dying at the time. Moreover, the spruce trees in the Erzgebirge or Fichtelgebirge have been growing again since the Czech and East German coal-fired power plants were modernized or shut down, albeit with liming aids and a new selection of varieties.
In the 1980s, the Munich forest botanist Otto Kandler compared tree photos from old forest reports with damage pictures that are now valid. What was considered good condition at the time, Kandler found, was now judged to be a "dying forest". Had perception changed, and where did the sudden public interest come from?
Fear of the future determined actions
In 1980, the "Global 2000" environmental study, with its devastating forecasts of the progressive scarcity of resources and environmental destruction, went straight to the heart of the feeling at the time. In numerous forms, nature became the focus of attention, the first forest damage report and especially the emotionally charged term "forest dieback" immediately made the rounds of the general public.
The abundance of research activities initiated in the following years and also the sum of the funds made available for them clearly demonstrate the importance attributed to the topic. Politically, the "dying forest" also had a number of things: The introduction of the "dual system", waste separation for the purpose of recycling previous waste, was pushed ahead massively. Waste incineration plants were then considered the main culprit in terms of air pollutant emissions and their use should be reduced through separate waste collection.
But just as none of the forest damage hypotheses put forward and intensively examined up to now have been able to turn out to be correct, the dual system is now also under massive criticism. A number of uncovered grievances, such as the export and disposal of "yellow bags" to other European countries, cast the theoretically good idea of waste separation in a very bad light today. Also, such a project on a large scale just doesn't seem feasible. Despite being informed, many citizens use the yellow sack as a rubbish bag with the corresponding contents, and the whole thing then ends up in landfill or incineration at considerable additional cost. But back to the forest:
Differentiated considerations
The assessment of crown thinning in the annual forest damage reports carried out to date is based on comparisons of the observed thinning with an assumed fictitious normal state. Yellowing of leaves and needles are also taken into account, and in the case of beech also branching anomalies. In 1998, Jörg Ewald and his colleagues from the Weihenstephan University of Applied Sciences found crown thinnings of around 25 percent in almost half of the spruce and 40 percent of the beech trees examined in a Limestone Alpine mountain forest far away from industrial use.
Deforestation? No, say the researchers. The condition of the crown is primarily dependent on the carbonate content of the soil, which in turn controls the availability of nitrogen, phosphorus and manganese, and - on the age of the trees. The older, the brighter; In addition, the trees are also affected by other naturally occurring stressors such as drought, says Ewald.
On calcareous sites, the all-clear can be given for the trees; a general connection between air pollutants and crown defoliation cannot be established here. The degree of crown thinning simply depends on natural site conditions. Of course, this statement does not apply to regional contexts such as the already mentioned smoke damage to spruces.
And what about lime-poor sites? The acid input from above seriously disturbs the already low neutralization ability of the soil and removes basic nutrients such as calcium, potassium and magnesium. If the soil pH value falls below a certain limit, aluminum and heavy metal ions eventually also break out of their compounds. Together with the nitrogen and sulfur compounds, they get from the air into the ground and surface water and finally into the food chain. That was and is being measured. But here, too, no forest has so far died of acid death. The absurd even: the trees react at least to the nitrogen charge with increased growth and with lush green leaves or needles. Forestry has been enjoying a higher wood yield per area for some time. What's the catch?
Focus on Nitrogen
Since 1985, sulfur oxides, the main cause of smoke damage observed in the 1980s, have been greatly reduced through the installation of catalytic converters and filters. But what about the nitrogen? Long-term measurements by the Federal Environment Agency also show a decrease here, but by no means to the same extent as with sulphur. Ammonia is mainly released into the air today through intensive agriculture, while traffic is the main source of nitrogen oxides.
In addition to the nitrogen input from the air, there is also a large load from agriculture in the form of fertilizer, which also gets into the ground and surface water with the rain and causes high nitrate and aluminum values in particular. The substances accumulate in the food chain, and the quality of the drinking water decreases. And it is no longer primarily the trees that are affected, but carnivores at the end of the food chain, including us.
Continue under observation
A forest dieback per se probably did not exist given the current state of knowledge. But simply closing the file would be too easy. Observations and measurements here on land are weakened or misinterpreted not least by massive silvicultural measures such as liming, new selection of varieties and knocking out weak trees.
Because the survey time is too short for the long-lived forest ecosystem, there is still a lack of really meaningful data. A reduced deep growth of the roots and thus a lower storm resistance of the trees could be related to acidic subsoil - the proof of this is still missing. And even the current lush growth does not give cause for unreserved joy: the forest is moving away from a he althy state of stability, in which it was previously maintained by the limited amount of nutrients available under natural conditions. No one can yet say what the consequences of this will be in the medium and long term. So the patient continues to deserve our attention.
And he himself is always good for surprises: Trees in highly acidic locations seem to have opened up a new source of nutrients for themselves, as Ernst Hildebrand and his team from the University of Freiburg found out. It is simple stones in the ground from which they elicit the missing ions with the help of their symbiosis partners, the mycorrhizal fungi. No model has predicted this before.
