ALLELOPATHIC INTERACTIONS IN A TEMPERATE FOREST SETTING BY HIGHER WOODY
PLANTS AND UNDERSTORY COMPONENTS
MATTHEW W. CARROLL
Colorado State University
Fort Collins, Colorado 80523
EN570 TERM REPORT
APRIL 30, 1994
INTRODUCTION
Within the plant kingdom there is constant competition for
resources which are considered limited from the perspective of the
plant, light, water and essential macro and micronutrients are a few of
these. In order to compete for these resources, many plants have
adopted strategies of chemical usage to acquire a greater proportion of
the available resources. One strategy associated with this chemical
usage has been termed allelopathy and is a strategy used by both aquatic
and terrestrial fauna. Because of numerous differences in the aquatic
and terrestrial ecosystems only one type of ecosystem will be focused
upon. In this case, the system which will be the focus of the paper is
a non-tropical, mountain type, temperate forest ecosystem characteristic
of the North American continent. Within this framework, the strategies
of the higher woody plants in terms of what chemicals are used and how
they are used to acquire a greater proportion of available resources in
the presence of a flourishing understory will be discussed. To do this,
first a definition of the forest ecosystem will be provided along with
its constituent parts, i.e. what tree's and understory plants make up
the system with some non-system species included to provide
clarification of some key points. Second, allelopathy will be defined
to provide a basic conceptual understanding of what chemicals and modes
of action are included in the term allelopathy. Third, a method of
separating allelopathic interactions from purely competitive actions
will be provided based upon Koch's postulates and research from current
literature. Next, the way a plant employs an allelopathic strategy in
competition for limited resources will be discussed. Then finally, the
affect allelopathy has on vegetation patterning.
FOREST ECOSYSTEM DEFINED
A forest ecosystem is defined as the interactions between the
biological community and various tree species and other organisms
comprising the system and the interactions of these organisms with their
current physical environment12. For simplicity, the system discussed is
a mixed forest system comprised primarily of pine, juniper, spruce and
oak with a thriving understory in mountains at the middle elevations.
Other trees such as beech and hackberry will also be used to illustrate
pertinant points and will be considered a part of the system. The
understory component of the system will consist primarily of grasses,
shrubs and seedlings. The fungal, viral, bacterial and invertebrate
portions of the ecosystem are not considered within the framework of the
paper and are therefore not discussed.
ALLELOPATHY DEFINED
The commonly accepted definition of allelopathy is the production
of biomolecules by one plant consisting mostly of secondary metabolites
which can affect another plant adversely or beneficially10. In this
context, it is important to view allelopathy from the perspective of the
plant or what strategy benefits the plant the most, it may be that
aiding another plant in germination could be in the plants best
interest, as would inhibiting the seed production of another plant.
Once viewed in this manner competition strategies within the forest
ecosystem add a level of subtlety which confuses an already complex set
of interactions.
Plants use a wide variety of chemical compounds in their chosen
allelopathic strategies. These compounds include alkaloids,
cyanohydrins, sulphides, flavaniods, terpenoids, steroids, phenolic
acids, aliphatic acids, glycosides, lactones, tannins, organic acids,
purines, nucleotides, cinnamic acid and sugars10,6. Again, most of
these chemical compounds are secondary metabolites produced by the
primary metabolic pathway. Compounds such as these enter the physical
environment of the system in several ways. Three common ways are
leaching from leaves or needles, volatilization, and exudation from
shoots and roots. In the forest ecosystem the concentration of
allelopathic compounds in the surrounding environment are variable and
can be seasonally based9. For example, the Hackbery tree produces a
phenolic compound found to inhibit seed germination, and was discovered
to exhibit much higher concentrations during the months of January and
April than at any other time of the year9.
The mode of action of the allelopathic compound is often very
subtle and hard to determine correctly but several have been researched
quite well. One reason the mode of action is so hard to determine is
due to the symptoms the plant produces. These are often secondary in
nature, chlorosis, wilting ect., and are hard to diagnose due to the
limited means a plant has to express stress. Several modes of action
attributed to alleopathic compounds include effecting cell elongation
and ultrastructure of roots by inhibition of cell division and
accumulation of lipid globules in the cytoplasm of root cells9. Another
mode of action is the allelopathic effect on membrane permeability
which causes an alteration in water and ion permeability of the
cytoplasmic membrane or in the case of Aescin, a triterpeneglycoside,
induced leakage of ribonucleotide material, nucleosides, and pentose
from the species Ophiobolus graminis.9 Yet another mode of action which
can occur is the interference in protein synthesis and changes in lipid
and organic acid metabolism9. Cinnamic acid was found to interfere in
the mechanism of protein synthesis while ferulic acid diverted acetate
into lipid synthesis instead of Kreb's cycle and other pathways leading
to amino acid and protein synthesis. Other modes of action include
disruption of mineral transport, inhibition of enzymatic activity,
inhibition of germination, inhibition of seedling growth, mutations and
disruption of nutrient uptake are all considered viable means of
allelopathic interference9. Table 1 shows a conceptual diagram of
allelochemical trasport in the environment10.
DISTINGUISHING ALLELOPATHY FROM COMPETITION
In order to properly present allelopathy, it must first be
presented in a way which distinguishes it from the affects of plant
competition which in many ways can resemble an allelopathic affect.
There are only so many ways a plant can express symptoms of stress,
chlorosis, wilting, flagging, abnormal growth, mutations, who can say
for sure by just looking at dying/declining plants that they are failing
due to allelopathic interactions or competition without all the bells
and whistles associated with allelopathy. In 1983 in an article by E.P.
Fuerst and A.R. Putnam it was suggested that some consistent methodology
should be developed and followed to provide proof to the researcher that
he was indeed studying allelopathy or competition5,3,6. Fuerst and
Putnam suggested the following methodology for proving competition5.
One, identify symptoms of interference, two, demonstrate that the
presence of an agent is correlated with reduced utilization of resources
by the suspect, three, demonstration of which resources are the limiting
resources in the environment, four, simulation of the interference in
the absence of the agent by reducing the supply of resources to levels
which occur during interference5. For proof of alleopathic interference
Fuerst and Putnam offered the following. One, identification of the
symptoms of interference, two, isolation, assay, characterization and
synthesis of the toxin, three, simulation of the interference by
providing the toxin as it would be provided in nature and four,
quantification of the release, movement and uptake of the toxin5. All
eight of the proposed criteria are based on Koch's postulates for
dealing with complex interactions5. For the purposes of this paper
examples dealing with allelopathic interactions were selected to match
as closely as possible Fuerst's and Putnam's criteria for allelopathic
inhibition, although some examples will be lax in this regard. Added to
this exacting criteria, it has been suggested that researchers use
either the additive or substitutive design methodology to demonstrate an
allelopathic mechanism of plant interference3.
An example of the difference between allelopathy and competition
can be seen in an article on the affects of vine competition on
availability of light, water, and nitrogen to a tree host Liquidambar
Styraciflua 4. It was determined that two species of vines, Lonicera
japonica and Parthenocissus quinquefolia which use Liquidambar
Styraciflua as a host significantly reduced the amount of light and
nitrogen available to the tree4. From the test results the authors
concluded that the affects of light reduction caused by the vines were
not responsible for the reduced growth response of tree's infected with
the vines. Rather, data suggested that the reduced growth response was
primarily due to the significant loss of leaf nitrogen by below ground
competition for nitrogen with the vine species4. The authors data
included the chemical characteristics of the soil and found no evidence
of volatile compounds or chemical exudates within the soil matrix4
(Table 2). If this case had been hypothetically attributed to
allelopathic interactions, it would be logical to assume a compound not
normally found in the soil would be present as an indicator that the
vines were inhibiting Liquidambar Styraciflua's nitrogen uptake ability
in some manner.
ALLELOPATHIC USE BY TREE'S AND UNDERSTORY PLANTS
Within the boundaries of the forest system, no single understory
or tree species can produce the wide range of compounds identified as
having allelopathic properties. Although use of some chemical compounds
may overlap, each tree species uses those compounds which benefit it the
most. Several examples of allelopathic use by trees will be discussed,
these tree's are the Cherry Bark Oak(Quercus falcata var.
pagodaefolia)9, the Red Pine(Pinus densiflora)7, the Black Pine(Pinus
thumbergii)7 and two understory species, Athyrium filex-femina 8 and
Vaccinium myrtillus,8 which inhibit the germination of the spruce Picea
abies.8. Allelopathic use by Cherry Bark Oak and two other oak species
was first noticed by Hook and Stubs in 19679. The scientists noticed the
lack of a thriving understory around three species of oak, one of which
was the Cherry Bark Oak, in an area where the growing conditions were
good with a thriving understory community set some distance away from
the tree species. In the immediate area surrounding the Cherry Bark
Oak, seed germination and seedling growth of same tree species along
with other plants in the area was greatly reduced in the soil around the
tree. In addition to the hypothesized presence of toxins in the soil,
Hook and Stubs discovered that cold water leachates from the crowns of
the oak were found to inhibit the growth of another tree species
Liquidambar styraciflua known by the common name sweetgum. The
primary toxin isolated by Hook and Stubs as being the responsible
compound involved in inhibiting seed and seedling growth is salicylic
acid9. This acid is theorized to enter the environment through the
leaching action of water falling from the oak crowns during rainfall9.
Although the allelopathic affect is attributed to salicylic acid, it is
only one factor in the suppression of competitive understory growth and
other tree species. Other factors include, physical suppression of seed
germination by oak leaf litter, strong competition by oak roots against
other plants, oak root production of an allelopathic compound which
inhibits seed germination and lastly, the production of water soluble
toxins by green oak foliage which restricts seedling growth9. The
compound exuded from the roots is unknown9.
Red pine(Pinus densiflora) and Black pine(Pinus thumbergii) are
two of three pine species which have attracted notice for their
allelopathic properties7. Like the Cherry Bark Oak, the Red and Black
pines were observed to have similar and sparse understory growth around
the trees, with a denser and more varied growth in areas not strongly
populated by the tree species. In several experiments, aqueous
extracts of the Red pine were taken from fresh leaves, fallen leaves and
roots and were tested for inhibitory affects using petri dishes and pots
with soil to test the extracts7. These extracts were tested against
other Red pines and common species within the forest system along with
species of plants found outside the forest system occupied by the pine
species7. (TABLE 3,4,5,6) Those species native to the system showed
much higher rates of germination than those species normally found
outside the system thus providing a plausible explanation for very
similar understories amongst systems containing the pine species. This
is verified by a second test involving soil germination and growth of
interior and exterior plant species in the presence of the aqueous
extract7.
Once the tests showed a causal affect between the reduced and
similar understory growth and the pine species, the compounds were then
identified as best was possible. Allelopathic identification of the
agents responsible were done by paper chromatography, TLC, HPLC and Gas
chromatography7. Table (7) shows identification of compounds extracted
from pine leaves, while table (8) shows a gas chromatagraph trace with a
chart identifying those chemical compounds found in Pinus thunbergii..
Among all the compounds detected, Phenolic acids and Benzoic acids were
found to be more common than gentistic, gallic and caffeic acids with
the compounds being more prevalent and uniform in concentration when
nutrient availability was low. Phenolic compounds such as benzoic acid,
cinnamic acid and coumarins were identified as the most important
phytotoxic agents in the temperate ecosystems. It has been theorized
that Phenolic acids present in pine leaves may play a vital role in the
allelopathic inhibition of understory species7.
Trees are not the only strategists in the game of allelopathic
inhibition, understory species have been long thought to use chemicals
as a means of gaining advantages in the forest ecosystem8. A very good
example of allelopathic interference by understory growth are the two
shrub species Athyrium filex-femina and Vaccinium myrtillus which
inhibit the germination of the spruce species Picea abies 8. In the
normal conifer forest ecosystem Picea abies were found to have problems
regenerating naturally, although the spruce experiences severe
parasitism of their seed it was determined that this was not enough to
prevent the natural regeneration of the tree. After carefull study it
was theororized that the understory species Athyrium filex-femina and
Vaccinium myrtillus were responsible for producing a secondary compound
which inhibits the spruces ability to germinate seeds. The two shrub
species were selected based on the following criteria. First both
species provide extensive cover in the understory growth stands and
second both develop very well in the presence of Picea abies . Auto-
intoxification as is known to occur in fir-beech-spruce succession was
considered a possibility also but at the time was not considered as the
primary contributor. In order to determine whether the shrub species
were responsible for the inhibition of spruce seed germination chemical
compounds were first isolated from the plants by drying them and using a
system of gutters to collect their leachates after natural rain
occurrences and immediately sterilizing the solutions8. Extracts of the
spruce needles were also prepared in a similar manner. After collection
of the aqueous extracts fractions containing phenolic acids, flavanols,
flavones, and anthocyans were created by extraction with ethyl ether8.
Detection of the flavonols was done through the use of specific
chealatiion of the compound by aluminum. Analysis of the phenolics and
anthocyans was accomplished through the use of a high-preformance liquid
chromatograph8.
Results of the tests indicated that all of the extracts caused
significant delay and interference in the germination of the spruce
seeds(Table 9). From these results an order of potential phytotoxicity
was established, P. abies > A.filex-femina > V. myrtillus, and this
suprisingly indicated that the leachates associated with spruce needles
are indeed involved in the suppression of spruce germination8. When
these results are combined with heavy seed parasitism it is of little
wonder that the spruce is having problems in natural regeneration. It
could be hypothesized that the spruce prevents nearby germination of its
seeds in order to avoid competition with its own progeny in a nutrient
limited area such as the sub-alpine region where it commonly forms large
monocultures.
Another example of understory allelopathic interaction occurs in
the Salvia spp. which have been shown to give off volatile terpenes in
sufficient quantities that it inhibits the growth of other plants around
it2. The mode of entry was found to result from terpenes dissolving in
the cutin of their leaves. In another species, Helianthus rigidus , it
was shown that toxins exudated from the roots were in part responsible
for the fairy ring phenomena commonly seen in meadows, grasslands and
forest clearings2. It should be also noted that not all allelopathic
interactions are negative, in a study on the Allelochemic Control of
Biomass Allocation11 using extracts from Acacia cyclops on two other
plant species, Protasparagus capensis and Eriocephalus racemosus, it
was found that while all plants tested showed a sublethal phytotoxic
response, the two species Protasparagus capensis and Eriocephalus
racemosus showed basal stimulation11.
VEGETATION PATTERNING AND ALLELOPATHIC INFLUENCE
Because allelopathy can benefit other plants as well as harm them,
it plays a logical role in influencing the patterning of understory
vegetation as well as being able to affect plant succession within the
system. Quarterman(1973)9 worked on Cedar glade communities and
reported that forbe distribution was sharply delimited and attributed
this to allelochemicals being able to affect species distribution and
zonation within the cedar-glade systems9. The forbes Quarterman
reported working with were Arenaria patula, Leavenworthia spp., Sedum
pulchellum, dominated in spring and winter, while Cyperus inflexus and
Talinum calcaricum dominated in the summer. All of the previous plant
species characterized zonal area 1. Zonal area 2 is characterized by
two grasses Sporobolus vaginiflorus and Aristida longespica, a legume
Petalostemon gattingeri and a moss Pleurochaete squarrosa . After
testing aqueous extracts from all of the species Quaterman found a
widespread occurance of leachable germination inhibitors among the
species and determined that Petalostemon gattingeri influences the
distribution pattern of Arenaria patula by production of inhibitors at
all times of the year thus preventing germination during the optimal
times of reproductive potential9.
CONCLUSION
It can not be denied that allelopathy still requires a great deal
study as to exactly how it affects plants intracellularly by benefiting
or harming them in some fashion. To accompany this, more research is
needed on the beneficial aspects of allelopathy in the forest ecosystem
instead of concentrating on the more easily seen harmful affects.
Allelopathic inhibition can be caused by numerous compounds which tend
to be species particular with every plant using that which it has
evolved to be most beneficial to it. Within the forest ecosystem
allelochemical interactions play a much larger role than previously
thought, affecting growth , germination, plant succession, and
vegetative patterning of the forest ecosystem. The species selected as
examples proved that higher woody species use allelopathic interactions
to inhibit understory growth and thus acquire a larger share of the
available resources. This was seen in the results of the Red and Black
pine tests where concentrations of allelochemicals increased when
nutrient availability was limited. Separation of allelopathic affects
from competition is necessary in order to see the causal agent operating
in the forest system as is sound methodology suggested by Fuerst and
Putnam, based on Koch's postulates5.
References
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