Extensive forest declines during the last decade,
especially in Europe but also in other parts of the world,
have initiated the discussion on the stability of forest
ecosystems. Initially the forest decline discussion
focused on single causes such as acid rain, ozone, climatic
extremes, pests, etc. Today most scientists look
upon the so-called ‘novel forest decline’ as a decrease
in the stability of the ecosystem, caused by a number
of partly interacting stresses, some of which are anthropogenic
(Ulrich, 1987, 1989; Fiihrer, 1990).