The development leading to forest decline is characterized by the accumulation of acidity deposited in the soil from the very beginning of soil formation, and by increasing deposition rates, first of sulfate and nitrate (from 1900 on), later also of ammonium (from 1960 on). Up to 1960 the accumulation of acidity in soils occurred mostly in the form of aluminum sulfate, i.e. with only limited leaching losses of exchangeable Ca and Mg. From 1960 on, the leaching of these cations was greatly increased. In the highly acidified soils nitrification was delayed and the NH₄/NO₃ ratio in the soil solution changed in favor of ammonium. This initiated uptake of ammonium by trees and resulted in excess nitrate remaining in the soil solution. The presence of nitrate in soil solution in turn increased cation leaching, decreasing soil pH, and turned soil chemistry from accumulation of aluminum sulfates into their mobilization. Thus, along with the continuing stimulation of needle growth by N deposition, the deposition of ammonium and nitrate has greatly influenced the development within the ecosystem and determines the type of tree damage.

   In addition to the forest decline phenomena, acid deposition over long periods of time has also influenced species composition of the herbaceous layer in forests. Falkengren-Grerup (1986) found that a large number of nitrophilic species have increased their cover on a majority of sites, while species indicating calcareous soils decreased in cover, and species indicating acid soil conditions increased. This shows that not acidity alone but also nitrogen is an important factor in explaining changes in vegetation. It appears that while N deposition led to an increase of nitrophilous plants acidification of soils led to a decrease in species richness (Bürger, 1988; Rost-Siebert,1986). Ellenberg (1985) demonstrated for the flora of central Europe that the largest number of endangered species represents those which occupy habitats of low N supply. Thus the red lists of endangered species are in fact a documentation of increased nitrification of ecosystems. The effect on the flora of a region seems to be stronger than the effect of soil acidification alone.

   The change in vegetation is paralleled by a change in soil fauna; mainly earthworms and species that need Ca for their skeleton disappear with acidification (Hartmann et al. ,1989).

   Ulrich (1987) developed a theory on how ecosystems will change with acidification of the soil from a steady state of high resilience by acidification processes to a new steady state of low resilience, and how these processes may be reversed by soil weathering (Fig. 5.9). N-deposition adds an additional dimension to this scheme, namely a change in flora and fauna towards nitrophilous species.