Scenario Analysis with the Dutch Acidification Systems (DAS) Model: An example for forests and forest soils

Abstract Within the framework of the Dutch Priority Programme on Acidification, the DAS model has been developed. This model aims at evaluating the effects of acidification abatement strategies on a number of receptor systems in the Netherlands and describes the entire causality chain from emissions to effects in a regionalized way. Effects of three emission/deposition scenarios on forest soils and forests are described. The emission/deposition scenarios are based on political measures as announced in the Netherlands Environmental Policy Plan + for the period 1990-2000. For the period after 2000, three deposition targets have been chosen. Emission reductions between 1990 and 2000 cause a reduction of the average deposition to the Netherlands of about 50%. This reduction is mainly caused by reduction of Dutch agricultural NH 3 emissions, although considerable reductions are also foreseen for NO, and S0 2 emissions in the Netherlands and other European countries. Deposition reductions lead to a fast increase of the pH value and decrease of the Al 3+ concentration and Al 3+ Ca 2+ ratio in the soil solution, decreasing the risk for indirect forest damage. The exceedance of a critical Al 3+ concentration of 0.2 mol c .m' 3 and a critical molar Al 3+ /Ca 2+ ratio of 1.0 reduces from about 75% and 65% of the considered forest area at present to about 40% and 30% in the year 2000. When further deposition reductions are established, the exceedance of these parameters will become negligible in the year 2050. When deposition is kept constant after 2000, the critical Al 3+ concentration and Al 3+ /Ca 2+ ratio will still be exceeded in a considerable area of forest soil in 2050. As a result of the retention of nitrogen in organic matter, continued mineralization and leaching of nitrate may remain a serious problem in areas with high nitrogen deposition several decades after imposing emission reductions. The scenario analysis for forests indicates that the nitrogen content of needles is reduced significantly. However, when the deposition is kept constant after 2000, a critical nitrogen content of 1.8% in needles is still exceeded in forests in areas with high nitrogen deposition. In these areas, the reduction of nitrogen and potential acid deposition leads to a net increase of needle mass and wood production. However, in relatively unpolluted areas, reduction of nitrogen emission will decrease needle mass and wood production. Direct effects of S0 2 and 0 3 appear to be less important in this context than soil mediated, indirect effects. The final part of the paper gives a brief description of future model developments.