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Summary Report No. 56

Wachstum von Kiefern-Ökosystemen in Abhängigkeit von Klima und Stoffeintrag - Eine regionale Fallstudie auf Landschaftsebene

M. Erhard (December 1999)

An analysis on landscape level was performed to investigate the growth of Scots pine stands (Pinus sylvestris L.) in the research area of "Dübener Heide" under the impact of rapidly changing pollution loads and to evaluate their further development.

Central to the work was a spatial database, installed using a geographical information system (GIS). This database enabled the statistical analysis of relationships and correlations between the growth of the pine stands, the influence of pollution impacts and the natural site characteristics. The results of emission and immission modelling gave detailed evaluation of the pollution load over the investigated area.

The spatial information database was linked with a process-based growth model called FORSANA. The resulting regional model was used to simulate forest growth on stand level for variable time periods. The plausibility of the simulation results of the model was checked for the influence of different patterns of pollution using the data available for the investigated area. Simulation runs were also made with these data to estimate the further development of the pine stand under various climate and emission scenarios.

The theoretical background of the work was provided by the results of forest decline research, considering especially the hypothesis of SO2-effects, nitrogen saturation and acidification on tree growth. For the analysis of cumulative as well as concentration effects and for the testing of the forest growth model a time span of approximately 20 years was used.

The pollution pattern in the research area showed a quite rapidly change from high SO2-immissions and high nitrogen inputs linked with a good base cation supply, to a pattern with low SO2-immissions with ongoing high nitrogen loads and an increasing soil acidification.

The results of the statistical analysis show the main cause of the variation in growth rates in the research area was atmospheric SO2-concentration. At the beginning of the investigated time span, the stands already showed different growth patterns according to previous damage. These differences are closely related to the average gradient of SO2-immissions. The strong decrease of atmospheric SO2-concentrations led a to decreasing load in all areas as well as to an equalisation of the pollution gradient. Present values are slightly above levels described in the literature as harmless for forest trees. The gradient of SO2-concentrations may not influence the further growth, if no effects occur resulting from prior loads. However, because of the prior damage differences in yield will still be recognised in the next decades.

The high immissions of fly ash, especially in the western part of the research area, led to accumulation of base cations in the soils. The acidifying effects on the sulphate immission caused by high SO2-concentrations, was buffered by the alkaline component of the fly ash especially on the areas near the emitter. As well as this chemical reaction, the immission of fly ash also had an effect on the improvement of the base cation supply to the trees. This also may have compensated for some direct damage caused by the SO2 load.

Measures for the reduction of fly ash emissions during a constant high level of SO2-emissions led to increased potential of acidification in the precipitation, and subsequently to a (re)acidification in soils. This process already started in the 1980th. The leaching of sulphate from soils in the western part of the research area is mainly buffered by calcium, while in the eastern part, pedogeneous aluminium is already mobilised. There was however no indication of yield losses or damage in the investigated pine stands.

Despite the strong decrease of sulphate immissions after 1989 it is nontheless to be expected that the tendency of acidification in the soils will continue. This is seen as a result of the ratio between the acidifying and basic compounds in precipitation, and the soil sulphur content which is still high. Furthermore, the combination of enhanced nitrogen input and a lowered cation supply or a high concentration of aluminium compounds in the soil solution, leads to a higher risk for tree growth and forest stand stability in the subsequence of nutrient imbalances and aluminium toxicity. These kinds of problems are expected first in the eastern part of the area, at sites which presently are nitrogen saturated but already suffer from depletion of base cations.

The combination of strongly reduced SO2-immissions and high nitrogen input led to a strong increase in growth rates. This is why yields in the research area are somewhat higher than those in the other parts of eastern Germany. High nitrogen inputs resulted also in high eutrophication of forest soils. Subsequently a substantial amount of the soil in the research area is extensively saturated with nitrogen. Nitrogen immission rates are still high. Reductions are not forseen at least not in the near future. For that reason a further increase in the nitrogen content of the soils must be assumed. There are first indications of humus disintegration and subsequent nitrogen release from soils at several sites. The most probable effect of this process would be increased nitrogen leaching into the groundwater. The direct negative effect of enhanced nitrogen concentrations in soil on the growth rate of the stands is related to the base cation supply, and therewith to the development of soil acidifiation. At present, the eutrophication gradient overlies the previous gradient of SO2-impact.

As well as nitrogen originating from traffic and other sources the further development of agriculture close to the investigated area will be of essential significance in the research area especially at its margin sections. An important measure for the maintenance of the nitrogen capacity and the protection of base cation levels in soils, is the introduction of deciduous species into the pure pine stands.

Concerning the relationship between growth rate and site conditions, a correlation could be shown between yield and the potential exchange capacity for base cations in older stands only. Because of small differences in soil status in the research area, significant effects on yields can only recognised over several decades. Furthermore there is evidence that, because growth rates are enhanced by nitrogen input and SO2-reduction site characteristics relevant for the maintenance of the water cycle become increasingly the limiting factor of growth, especially for younger stands and may be the main driving force for the differentiation of yield in future.

Problems for estimation of further development in the research area could rise from uncertainties in the driving forces as well as from the changing of the pollution pattern in-between the time span of simulation. With the later growth factors get increasing influence which are not implemented in the model sufficiently. Especially the physiological mechanism of soil acidification and the direct nitrogen uptake through the canopy are not implemented in process models yet, because they are insufficiently known. Therefore these important growth parameters must be evaluated using empirical risk factors.

The results of the simulation runs with FORSANA show positive effect with respect to the growth of the stands. In general stemwood production seems to benefit relatively more than total yield. Reduction of the SO2-immissions has a specific effect on the leaf area. As indicated already by the analysis of the natural site characteristics the processing of the scenarios also made clear the sensitivity of the stands to water deficiency. It is shown that the already relatively low average precipitation leads to a negative effect on the growth of the stands independently of any pollution impact, because of drought stress. This lead to the question of whether the competition of the ground vegetation for water and nutrients could prevent the positive development of the forest stands. Because of this combination of enhanced growth rates and low precipitation the research area is quite sensitive to potential changes in precipitation and therefore quite vulnerable in the context of climate change.

Finally it is concluded, that the positive effects of immission reductions until now could be proofed statistically as well as by the simulations with the process based model. With respect to immission reduction, results of the simulation runs show also a further positive development. The decrease in pollution impact is opposed to some other risks. From the simulation runs resulted an increased probability of drought stress. Further risks, which are rising from the nitrogen saturation of the soils and the soil acidification and affect future stand development, are interpreted empirically from existing data, because of lacking model implementation.

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