Environmental Engineering Reference
In-Depth Information
author noticed that phenolic substances affect plant
performance especially under acidic, nutrient-poor
soil conditions. In calcareous soils, most phenolic
compounds are rapidly metabolized by microbial
activity and adsorption is high.
In the Swedish boreal forest, the ground-layer
vegetation in late post-fire successions is frequently
dominated by dense clones of the dwarf-shrub
species
Empetrum hermaphroditum
, one of the most
widespread plants in the European arctic and boreal
biomes. The plants produce large quantities of
phenolics, in particular batatasin-III (a phenolic in the
compound class of stilbenes, C
6
-C
2
-C
6
), which is held
responsible for the strong negative effects exerted by
E. hermaphroditum
on tree-seedling establishment
and growth, microbial activity and plant-litter decom-
position rates, thus contributing to humus accumula-
tion and reduced nitrogen availability (Nilsson
et al.
1998). Further, this plant species is conspicuously
avoided by herbivores. Nilsson (1994) tried to deter-
mine the relative impacts of chemical inhibition (allelo-
pathy) and resource competition by
E. hermaphroditum
on seedling growth of
Pinus sylvestris
, by adding
finely powdered, pro-analysis activated carbon as an
adsorbent to the soil surface to remove the allelopathic
effect, while exclusion tubes were used to subject
seedlings to allelopathy in the absence of below-
ground competition by
E. hermaphroditum
. Both
allelopathy and root competition had a strong,
negative influence on seedling growth of the Scots
pine.
The interaction between allelopathy and fire is an
interesting issue. According to Zackrisson
et al.
(1996), with the prolonged absence of fire boreal for-
est can be dominated by
Picea abies
and
Empetrum
nigrum
, whereas fire at intervals of 50 -100 years is
conducive to domination by
Pinus sylvestris
and
the ground-layer species
Vaccinium vitis-idaea
and
Vaccinium myrtillus
in mesic and nutrient-poor sites.
Indeed, charcoal was shown to absorb phytotoxic
active phenolic metabolites from
E. hermaphroditum
solution, and wildfires were shown to play an import-
ant role in boreal forest. The authors proposed that
charcoal particles can act as foci for both microbial
activity (biodegradation) and chemical deactivation of
phenolic compounds through adsorption.
The role of allelopathy in ecological restoration is
largely unknown, but it cannot be ignored, because
5.3.3 Allelopathy
Allelopathy can be considered a form of interference
competition among plants. It is a unidirectional pro-
cess, the phenomenon of plants of one species negat-
ively affecting plants of other species, generally by
releasing toxic organic compounds. Examples are
tannins from
Pteridium aquilinum
and volatile oils from
Eucalyptus
species, substances that are supposed to have
an anti-herbivore function as long as the plant organ
is alive, and which in plant communities simultan-
eously may prevent changes in species composition
and thus inhibit vegetation succession. We follow the
definition given by Calow (1998), which is in concert
with the afore-mentioned definition of competition:
'Allelopathy is a form of interference competition,
brought about by chemical signals, i.e. compounds
produced and released by one species of plants which
reduce the germination, establishment, growth, survival
or fecundity of other species'. In contrast with what
have been named positive-feedback switches, which
means that individuals of a species modify their
abiotic environment in such a way as to promote their
own persistence in the area, these organisms benefit
from their own fitness promotion, thus also potentially
reducing the competitive abilities of other component
species. The classic work of Rice (1974) gives an
overview of the state of affairs as resulting from early
research on allelopathy among plants and micro-
organisms, in both terrestrial and aquatic systems. There
are many thousands of such compounds, but only
a limited number of them have been identified as
toxins involved in allelopathy, and even if they
potentially have detrimental effects they might not
cause allelopathy in reality.
Kuiters (1990) reviewed the role of phenolic sub-
stances in forest soils. They include simple phenols,
phenolic acids and polymeric phenols as condensed
tannins or flavonoids. Once released in the soil
environment, they influence plant growth directly by
interfering with plant metabolic processes and by
effects on root symbionts, and indirectly by affecting
site quality through interference with decomposition,
mineralization and humification. Effects of phenolics
on plants include almost all metabolic processes,
such as mitochondrial respiration, rate of photosyn-
thesis, chlorophyll synthesis, water relations, protein
synthesis and mineral nutrition. Interestingly, the
