Environmental Engineering Reference
In-Depth Information
and Zobel (1996) tried to answer the question of
whether a symbiotic interaction with AM fungi can
help young plants resist the competition of older ones
with which they naturally coexist. They chose two com-
mon subordinates in the lowest sublayer of species-
rich calcareous grasslands;
Prunella vulgaris
as a
target species and
Fragaria vesca
as a neighbouring
species. In interspecific competition with old
F. vesca
,
the shoots of target plants were 22% greater when ino-
culated with AM fungi than when non-mycorrhizal.
Thus, if a young
P. vulgaris
plant has established itself
somewhere in a natural gap, the presence of AM fungi
might make intraspecific competition more severe, but
may decrease the strength of interspecific competi-
tion. Similarly, Kiers
et al.
(2000) suggested that
tropical mycorrhizal fungal (AM fungi) communities
have the potential to differentially influence seedling
recruitment among host species and thereby affect
community composition. Earlier, Grime
et al.
(1987)
demonstrated in a microcosm experiment that
14
C
could be transported through a mycorrhizal network
from dominant to subordinate species, which led to
an increase in biomass of the inferior competitors.
Mycorrhizal linkages were shown, indeed, to transport
15
N and
32
P within and between plant species (Chiarello
et al.
1982, Finlay
et al.
1988).
Orchids demand particular attention in the context
of restoration ecology. Dijk
et al.
(1997) reviewed
the nutritional relationships of orchid species. In the
first heterotrophic and subterranean phase of orchid
development, the growth of the protocorm is entirely
dependent on mycorrhizal fungi. The nutrient meta-
bolism of developing orchid individuals is adapted
to this symbiosis: reductions in orchid nitrogen
metabolism are permitted which can be considered
adaptations to the parasitic habit during at least this
phase. Mycorrhizal infection is restricted to subter-
ranean tissues only, to the subepidermal zone of the
protocorm and root parenchyma. After the initial
infection the development of mycorrhizae can easily
derail, and in symbiotic cultures a range of inter-
actions can be met from a loss of mycorrhiza via
normal mycorrhizal infections to pathogenic effects.
The primary function of mycorrhizal infection in the
juvenile phase lies in the transport of C compounds
to the developing seedlings. Translocation of sugars
towards protocorms has been demonstrated by radio-
active labelling in classic studies. Apart from inter-
fering with the carbon metabolism, mycorrhizal
infection has a pronounced influence on the uptake
of mineral macronutrients (P and N). These subtle pro-
cesses in the juvenile phase should have a chance before
we can ever expect adult and flowering orchids as a
result of restoration efforts.
Mycorrhizal fungi not only play an important role
in plant nutrition; they can also fulfil other functions
which may be important for survival in the long
term. Some species with a compact structure increase
resistance against pathogens, heavy metals and poly-
phenolic substances (Ozinga
et al.
1997). There may
be a trade-off between protecting functions as a
compact structure and efficiency in nutrient uptake.
Moreover, mycorrhizal fungi and their positive effects
on plant performace may benefit from the effects of
mycophagous animals such as the order Collembola
if the latter feed preferentially on non-mycorrhizal fungi
in the rhizosphere (Gange 2000). It has also been shown
that the presence of Collembola in the soil can lead
to a decrease in reproduction of the aphid
Myzus per-
sicae
when feeding on
Trifolium repens
(Scheu
et al.
1999). Still another mechanism by which Collembola
might positively affect AM fungi is through spore
dispersal.
4.4.2 Plant-pollinator interactions
Plant-pollinator interactions can be considered a
non-symbiotic mutualism. The most basic evolution-
ary outcome that is common across both plants and
pollinators is the efficiency of both in exploiting
what is for each a valuable or critical resource. But
the mutualism is neither symmetrical nor cooperative.
The mutual exploitation interest may be skewed
towards a consumer-resource relationship between the
two parties. One common manifestation is oppor-
tunism and flexibility on the part of pollinators
toward plants, and vice versa.
In their review on 'endangered mutualisms', Kearns
et al.
(1998) pointed out the phenomenon that over
90% of 250,000 modern angiosperm species are
pollinated by animals. Among the nearly 300,000
flower-visiting animal species are insects, birds, bats
and small marsupials. Habitat fragmentation and other
effects of land use, like agriculture, grazing, herbicide
and pesticide use, and the introduction of non-native
