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ent (Marschner 1994). Some nutrients (particularly nitrogen) participate more in the growth process than others.

The model of a nitrogen-cation interaction with limited cation supply resulting from soil acidification observed in forest decline could also be related to other observed cation deficiencies, particularly K deficiency on bogs, Mn and Fe deficiency on limestone, and the rarer Ca deficiency.

With the decrease in S02 deposition resulting from regulations controlling emissions from large electric power plants, input of sulfur sank and thus the rate of soil acidification also (Fig. 3.5.4; Ulrich 1994). With reduced S and N input, loss of Ca decreased. This together with so-called compensation liming, which was supposed to balance the acid input, has resulted in partial recovery of soils with respect to base saturation.

| Fig. 3.5.6. Schematic representation of the processes that lead to forest decline. (After Schulze and Lange 1990)

0 100 200 300 400 500 600 700 Stem volume per area In 1963 a (m^ ha"1)

0 100 200 300 400 500 600 700 Stem volume per area In 1963 a (m^ ha"1)

0 500 1000 1500 2000 2500 3000 Stand density In 1963 ■ (trees ha"1)

| Fig. 3.5.5. The effects of management on forests. The figure shows that logging in the 1980s left free-standing single trees. With less S pollution and greater N the remaining individual trees grew faster. Despite fewer trunks and a smaller leaf area, the volume of growth per stand remained the same. (Mund et al., 2002)

Heavy clearing reduced the density of trees in declining forests of the Fichtelgebirge (Germany) far below the recommended values of yield tables (Fig, 3.5.5). Thus, the cation supply per tree increased and, together with the high N deposition and with the higher light availability, growth of individual trees improved. Despite the reduced density of stands wood growth per area was eventually maintained.

Reducing the sulfur stress does not solve the problem of forest damage, as there is obviously more than one path of damage and complicated interactions occur between pollutants and organisms (Fig. 3.5.6; Schulze 1989). With the shift from S to N deposition other types of damage occurred, particularly to deciduous trees where insect damage increased. Tropospheric ozone concentrations are still damaging, particularly for herbaceous plants with high stomatal conductance (FBW 1989).

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