continuous decomposition, thus preventing phases of
humus accumulation and net mineralisation. Morgan et
al. (1992) and Brown (1992) demonstrated higher
mineralizationrates in litter of species mixtures (larch/
pine and pine/spruce, respectively) compared with
pure species, reflecting higher activity among decomposers
in litter mixtures. Other studies, however, contradict
these findings (Chapman et al., 1988).
Depending upon species-specific characteristics,
mixed forests may contribute to ecological stability by
increasing resistance and resilience and additionally
decreasing the potential magnitude of perturbation
owing to mineralization or acidification pushes as
defined by Ulrich ( 1987).
Mixed stands may have greater species diversity in
other compartments of the ecosystem. This might lead
to an increased resistance, especially in relation to better
controls of pests. It must be emphasised, however,
that this higher resistance is mainly due to the complexity
of energy flow developed through coevolution
among functional groups of organisms. Species
mixtures should support existing food web interactions
(Vasechko, 1983). Hence, the artificial creation of
more complex structures by mixing species, without
taking coevolutionary relationships into account, may
therefore not lead per se to higher system resistance
(Roberts and Tregonning, 1980).
Another aspect of system stability is the possibility
of increasing the potential depth of the rooting zone by
introducing species characterized by a deeper rooting
ability (Ulrich, 1987). Such species might work as a
pump of nutrients (mainly base cations) from deeper
soil horizons, thus increasing the weathering potential
and the storage capacity of the system and thereby both
resilience and resistance. Information about speciesspecific
capability to expand the rooting zone in relation
to soil characteristics are still very sparse, however.