LIMING AS A MITIGATION TOOL IN GERMANY DECLINING FORESTS - REVIEWING RESULTS FROM FORMER AND RECENT TRIALS

The so-called new types of forest damage in Germany are frequently related to 'acid rain' and enhanced atmospheric N deposition resulting in accelerated soil acidification and induced nutritional disturbances. To stop or reverse these effects liming is seen as an important mitigation tool. As forest liming is not new in Germany, the analysis of older liming trials indicated that liming generally leads to a long-term decrease of soil acidity, improvement of cation exchange capacity, base saturation, content of exchangeable Ca (when dolomite is used, also of Mg) and Ca/Al (Mg/Al) ratio. However, an increase of forest productivity due to a faster turnover of organic matter, was not achieved. Because of this fact and the observation that enhanced NO3 leaching losses may occur from limed soils, liming practices were practically discontinued in the 1970s. Owing to awareness of the new type of forest damage, the goals of liming were altered. Liming was now aimed at 'compensating' further acidic deposition at the soil surface and eventually reducing soil acidification and by these means increasing forest vitality. Nevertheless, the NO3 constraint of liming remained, particularly when considering that nowadays, most of the German forests are impacted by relatively high atmospheric N deposition rates. Furthermore, it was found that liming can cause acidification of the subsoil and the displacement of heavy metal ions. Liming appears to stimulate fine root development in the uppermost soil layers, increasing the danger of frost and drought damage. As forest liming may be associated with ecological risks, a careful analysis concerning the need of liming should be carried out for each treatment area. Besides soil pH values, humus content, site history, actual atmospheric N deposition, N uptake capacity and nutritional status of the stand, forest floor vegetation as well as heavy metal load should be examined.