Forest damage at Nusshard, Fichtelgebirge, Germany, in September 1989. In 1983, Zech and Pop diagnosed at this location, for the first time, Mg deficiency in young trees which was previously unknown in this area. During the next 10 years, the visible signs of Mg deficiency developed throughout the Fichtelgebirge. Mg deficiency does not necessarily lead to death of trees, but causes yellowing and early needle loss; combined with frost and drought it causes damage leading to thinning of crowns. With the death of a few individual trees the remaining trees have a better Mg supply. Thus, in the short term, the remaining trees recover but may die later. Thus, slowly, the stand of trees dies from stress, i.e. each year a few trees fall, so that in the end the stand has only a few trees which succumb to external factors. Bark beetles increased the rate of tree death as they attacked the relatively healthy trees rather than those suffering severe Mg stress. At the end of the 1980s, about 10,000 ha was affected in the Fichtelgebirge. Increased tree removal and liming helped to regenerate these forests; this occurred during the time when less S02 and SO4" was released due to changes in the law regulating combustion of coal. Critics doubted that forest dieback had anything to do with the amount of pollution in the atmosphere or that the trees were dying as a result of S02 and SO^". Rather they considered that it was due to complex forest "diseases" primarily related to factors in the soil that weakened the plant. The only way to confirm the cause would be to subject forest areas again to pollution, equivalent to values in the 1970s. The scientific investigations stimulated the "Advisory Council of the Federal Republic of Germany on Forest Decline" (Forschungsbeirat Waldschaden der Bundesregierung) in 1986 to conclude that, in all the different forms of forest dieback that were studied, air pollution played a significant role. Photo E.-D. Schulze
It is possible to demonstrate processes and interactions of organisms at the scale of the ecosystem in various ways (Mooney et al. 1991). These are:
• observation of natural catastrophes (El Niño);
• observation of the consequences of human intervention (clearing of forests, acid rain);
• experiments with radioactive labelled materials (e.g. release of 14C from atomic bomb tests);
• ecosystem experiments (fumigation with C02,
These "experiments" allow observations over areas differing in size (even globally) of the effects of climate change or of interference on ecosystems, or determination of the transport of particular substances in ecosystems. This is particularly important as scientifically planned experiments in ecosystems are not acceptable to the public or they are not practicable.
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