Abstract
The deposition of strong inorganic acids in forests leads to low pH values and high concentrations of Al3+ in the soil.
Forest liming neutralizes acidity and leads to an increase
in base cations. Additionally, surface organic layers decay
more rapidly and stored nutrients are mobilized. The
ground vegetation responds to liming with an increase in
nutrient-loving species and a decline in acidophytes. The
mycorrhizal fungi and soil fauna species composition change
dramatically. Tree roots, at least initially, retreat from the
topsoil. It takes several years for the base cations to reach
the deeper soil horizons (up to about 30 cm) and they rarely
go deeper.
Since 1990 sulfur deposition has greatly declined. However
N deposition rates remain at high levels (conifer stands > deciduous
> open areas). High N inputs continue to contribute
to soil acidification and at the same time cause the eutrophication
of forest ecosystems which are naturally N-limited.
The increased rate in forest growth results in an increased
need for other nutrients. In many forests, the critical deposition
threshold (“critical load”) of approximately 10 to 20
kg N ha -1 yr -1 has been exceeded. In stands where the soils
are supersaturated with nitrogen, the nitrate which is not
retained by the humus or assimilated by the forest vegetation
leaches into the ground water.
Up until today in Baden-Wuerttemberg, Rhineland-Palatinate,
Hesse, Lower Saxony, North Rhine-Westphalia, Thuringia
and recently in Saxony-Anhalt large areas of forest
have been limed using three to four tons of dolomitic lime per
hectare per decade. The aim is to prevent the ongoing acidinduced
degradation of clay minerals in order to increase
forest vitality. Often P and / or K are added to the lime.
Bavaria, Brandenburg and Mecklenburg-Western Pomerania
either do not lime their forests or only do so in a very
restricted manner. In these states the central European tree
species beech, spruce, Scots pine, fir and oak growing there
are no less vital than elsewhere. These tree species grow in
a wide range of ecological soil types. Analyses of leaves and
needles show that a small, but adequate nutrient supply exists
even in the most acidic forest soils.
Today it is not soil acidification but eutrophication (and
climate change) which are the main threats to forest ecosystems,
especially for oligotrophic communities on sandy
soils. Liming in eutrophic forests counteracts acidification
and eventually leads to deeper rooting. But it also increases
nitrogen availability and other limiting nutrients and adds to
the process of eutrophication. Therefore, the results evaluating
of the effects of liming are ambivalent. The problem can
only be solved by a reduction in N deposition.
From a nature conservation standpoint it is of particular
concern that liming is done on oligotrophic forest communities
which are naturally low in base substrates. These habitats need to be protected from the addition of lime. In mesotrophic clay soils susceptible to acidification, liming can be occasionally tolerated. The enrichment with P and K means fertilisation and is therefore not acceptable. In order to weigh the impacts of forest liming, unlimed control plots should be established. In the light of today’s relatively high forest growth rates further funding for forest liming should be reassessed.
