Effects of Long-term Pollution Load on the Forest Ecosystem
The filter effect created by the surface structure of forests leads to a high pollutant impact and accumulation. Multiple effects on forest ecosystems are caused by the pollutant gas sulfur dioxide and its solid by-product sulfate. The intense international forest damage research which has been done since the beginning of the 80s has yielded completely new knowledge in the field. This has resulted in a very critical approach to the definition of pollution limits.
Direct damage impact from previously high SO2 concentrations were particularly noticeable in Berlin (c.f. Map 03.07, SenStadtUmTech 1996a) through reduction in lichen growth and through needle damage. A special method was developed during the long-term investigations of the monitoring program in order to measure and access the macroscopically recognizable needle damage (c.f. Meyer and Kalhoff 1996). The damage profiles were summarized as damage types of which damage type 3 (needles with clearly distinguishable ribbon and spot-shaped chloroses and necroses) can certainly be attributed to SO2 damage. Figure 3 shows the development of this kind of damage since 1986: parallel to the decreasing SO2 load, the incidence of type 3 needle damage also declined. The positive correlation with the average SO2 concentration from 1986-94 confirms the connection. The decline in this type of needle damage in Berlin coincides with the findings of Korsch and Jäger (1993) which also show a significant reduction in needle necroses in the Bitterfeld region within recent years.
Fig. 3: Temporal Development of Needle Damage Type 3 in Three Test Areas in the Forests of the Western Part of Berlin and the Average SO2 Concentration in Grunewald. The Average of all Test Trees with a Particular Damage Type is Shown (following Meyer and Kalhoff 1996). For 1992 there are no Needle Damage Data Available.
Despite these low concentrations of SO2 in 1995, there continues to be acid load of the Berlin forests which has been caused about 75 % by SO2. The acid inputs have been reduced as a result of the extensive pollution reduction measures taken, but not to the same extent as the reduction of SO2 concentrations. This is due to the fact that the calcium inputs, which serve as an atmospheric buffer against acids, have been reduced (c.f. Fig. 4). In addition, the nitrogen oxide pollutions, which contribute to a third of all acid formation have scarcely declined (c.f. Map 03.03, SenStadtUmTech 1997). The result has been a slight increase in acid precipitation from 1991 to 1994.
Fig. 4: Temporal Development of Acid Inputs, Atmospherically Buffered Acids and Calcium Inputs in kmol Ion Equivalents (IE) per Hectare and Year in Fields within Grunewald (following Fischer 1996)
These current acid input measurements still lie above the level of tolerable acid inputs for the Berlin forests, as defined by UN ECE "Critical Loads" for sustainable maintenance of the natural balance (c.f. Fig. 5).The value calculated shows what the forest soil's long-term acid neutralization capacity is. The acid neutralization capacity is determined by the ability of the soil, through erosion, to replenish basic cations (Ca, Mg, K). The same function is performed by the basic cations in dust which can also provide relief. This relief is reduced through the significant decline in calcium inputs as already mentioned.
Fig. 5: Development of Actual along with Tolerable Acid Inputs According to the UN ECE Concept of Tolerable Inputs (Critical Loads) in Ion Equivalents pro Hectare and Year from 1987 to 1995 (Example Grunewald). Actual Acid Inputs: Calculated Input Values in the Forest Stand.
There has been a decline in the pH value of rainwater (1984 - 94 from 4.7 to 3.9) (c.f. Pelz 1995) caused by the reduction in the atmospheric acid buffer (c.f. Fig. 6). The high acid level in rain and fog is also problematic for the forests. This can lead to increased acid damage to needle surfaces (pre-mature aging of the wax layer) and higher erosion of nutrients.
Fig. 6: Temporal Development of the pH Value in Precipitation in Berlin-Dahlem 1984 - 1995 (Pelz 1995, Pelz 1996)
The long-term acid inputs in the forest soil can be identified in the soil solution, which is a sensitive indicator of the soil-chemical condition. The soil solution from the forest long-term observation areas in Grunewald exhibit a high sulfate (SO4) concentration. With 85 % of the total anions, there is also a decisive effect on the ion-levels in the soil solution (c.f. Schlenther et al. 1995). This makes the high SO4-inputs of the past an important factor for the composition of the soil. Due to the continuing buffering of the sulfate-induced acidifications, increasing amounts of aluminum ions are released. These are toxic for roots. Only the pollution-effected calcium reserves in the soil prevent root damage. However, current acid inputs cause the gradual deterioration of these reserves. The "Critical Loads" concept which aims at long term soil quality, does not take this deteriorating reserve into account and therefore treats the current acid inputs from the standpoint of sustainability of the forest soil as intolerable.