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Few people who visit Britain's countryside when
Rhododendron ponticum is in flower can comprehend the
damage that has been caused to our native flora and fauna by
this exotic Victorian introduction. (Although there are
hundreds of species of Rhododendron, it is only R
ponticum which is known to be a problem.)
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The plant is
responsible for the destruction of many native
habitats and the abandonment of land throughout
the British Isles. The reason for this is
simple. Where conditions are suitable, R
ponticum will out compete most native
plants. It will grow to many times the height of
a person, allowing very little light to
penetrate through its thick leaf canopy. This
effectively eliminates other competing native
plant species which are unable to grow due to
insufficient light. This in turn leads to the
consequent loss of the associated native
animals. |
Introduction to Britain
Rhododendron ponticum
is native to countries in the western and eastern
Mediterranean such as Spain, Portugal and Turkey and also
occurs eastwards to Asia. It is not native to Britain, but
was first introduced in the late 18th Century. It became
especially popular on country estates in Victorian times,
providing ornamental value, as well as cover for game birds.
Recent research indicates that British Rhododendron
ponticum originates from Spanish and Portuguese
introductions.
R ponticum thrives
in milder, wet climatic conditions, where there are poor,
acidic soils. Unless established stands are constantly kept
in check, they will expand into adjacent areas, rapidly
eliminating the majority of native plant species. The
twentieth century demise of many former country estates and
the consequent lack of control of remaining R ponticum,
has allowed this species to invade large areas of the
British countryside.
Rhododendron Invasion
R ponticum invades
areas both vegetatively and via seed. Established plants
spread by lateral horizontal growth of the branches. A
single plant may eventually end up covering many metres of
ground with thickly interlaced, impenetrable branches. Where
the horizontal branches touch the ground, they will root,
continually extending the area of R ponticum
cover.
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It
is worth noting that because of its extremely
lateral growth form, R ponticum plants
are capable of extending well into areas which
otherwise would not be suitable for their
growth. For example, R ponticum is
capable of dominating large areas of wetland
with its canopy, while the main stem and roots
of the plant are well back on suitably dry land.
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For the same reason, streams can
become completely overgrown and shaded out by
R ponticum
growing on the banks. This severely affects animal life in
the stream. Fish such as trout depend upon invertebrates
which fall off native bankside vegetation for 80% of their
food.
R ponticum
seeds are tiny and hence wind dispersed. Each flower head
can produce between three and seven thousand seeds, so that
a large bush can produce several million seeds per year. Of
course not all the seeds will grow successfully, but given
the right conditions, a good many will germinate.
Seedlings have difficulty becoming
established in areas where there is already continuous
ground cover from native plants. Establishment is best in
disturbed areas where the native vegetation has been in some
way disrupted, providing an opening in the plant cover. The
seedlings also germinate well in areas covered in moss. This
may be because of the water holding capacity of the mosses.
Specific
mycorrhizal
(fungal) associations with the roots of
R ponticum
plants provide great competitive advantage and allow the
plants to flourish in nutrient-poor soils. The mycorrhizae
are specific to ericaceous plants. This includes heathers.
R ponticum
therefore has a huge advantage over other non-ericaceous
plants which may not have their own mycorrhizal network.
This may also explain why the plant is so successful at
colonizing heathlands, as the heathers which exist there
have already established suitable mycorrhizal networks.
Toxicity of Rhododendron
Potentially toxic chemicals,
particularly 'free' phenols, and diterpenes, occur in
significant quantities in the tissues of plants of
Rhododendron species. Diterpenes, known as grayanotoxins,
occur in the leaves, flowers and nectar of Rhododendrons.
These differ from species to species. Not all species
produce them, although Rhododendron ponticum does.
These toxins make Rhododendron
unpalatable to most herbivores. Phenols are most
concentrated in the young tissues, such as young emergent
leaves and buds. This provides a primary defense against
herbivores, before the tissues have acquired the added
deterrent of physical toughness found in older tissues.
Young emergent leaf buds have the additional protection of a
sticky exudate which also contains phenols. This physically
discourages small invertebrates from eating the buds,
because they get stuck in the exudate. Its poisonous nature
must act as a further discouragement.
Grazing animals are discouraged from
eating Rhododendron foliage because of its toughness and
unpalatability. The unpalatability is learned and cases of
poisoning may result in animals such as sheep and
cattle, if they ingest sufficient quantities because of
extreme hunger or inexperience. The general toxicity of
Rhododendron to herbivores means that it cannot usually be
controlled by grazing.
Cases of human poisoning are also
known. Most are caused by the consumption of honey produced
from Rhododendron flowers. This is known as 'Mad
Honey Disease', or 'Honey Intoxication'. Cases of this
have been recorded from as far back as 400 BC. It results in
relatively short-lived intestinal and cardiac problems and
is rarely fatal. The severity of symptoms depends on the
amount of contaminated honey consumed. It is worth thinking
carefully about the siting of bee hives if Rhododendron is a
prominent feature of the area.
Inhibitory Effects of Rhododendron
There is some evidence for
allelopathic interactions (the production of adverse effects
on other species) between Rhododendron and other plants.
This may include the inhibition of germination, or of
establishment of the seedlings of competing species. Direct
poisoning is
a
possibility. As noted above, the tissues of
Rhododendron contain significant quantities of phenols and
other potentially toxic chemicals. There is also evidence
for the prevention of mycorrhizal development in roots of
the seedlings of competing plant species. Research and
debate in this field is on-going.
Biodiversity in Rhododendron
Once R ponticum has
invaded an area, few native plants survive. In woodlands
only those trees which manage to grow above the level of the
R ponticum canopy will persist. These of course,
have a finite life span. On their death, there is no
replacement because seedlings cannot become established
under the lightless canopy. At this point, the R
ponticum completely
dominates the area. This is particularly noticeable in
Silver Birch woodlands as this species is short lived.
Once the native plants have
disappeared, the animals which rely upon them either
directly or indirectly for food cannot survive. Thus R
ponticum areas are essentially barren. Even where trees
exist above the R ponticum
canopy, species such as woodland butterflies disappear. This
is because the caterpillars of most woodland butterflies can
only feed on the wildflowers and grasses which are found in
the glades and rides of well managed woodland.
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The Dormouse, one of Britain's most endangered
species of mammal, has been severely affected by
habitat loss. Some of this has been because
R ponticum
has out-competed native plants and shrubs which are
essential for the dormouse to survive. |
The toxicity of
R ponticum
and its alien status means that there are few animal species
associated with it in Britain. However, a number of
herbivorous invertebrate species have been linked to it.
Most of these are not
restricted or specific to
R ponticum. It is
important to bear in mind that just because a species has
been found on R ponticum, it does not mean that it
habitually lives there. It may have fallen or been blown
into the R ponticum from
neighbouring native plant species. The canopies of trees can
contain vast numbers of individuals of different
invertebrate species. It is common for these to fall or be
blown off to locations other than their chosen habitat.
Indeed the air can contain an astonishing variety and number
of small organisms, including invertebrates. Spiders are
even able to balloon hundreds of miles in air currents.
Biomass (the total weight of individuals) of the species
is also important. One individual of a species does not
imply an association, whereas a plant covered in a great
many individuals would tend to indicate some sort of
relationship.
Occasionally a few
R ponticum
leaves can be found which have obviously been eaten by an
insect. However the toxicity of the leaves means that this
may well have resulted in the subsequent death of the
individual(s) concerned. R ponticum growth is in no
way controlled by herbivores feeding on it in great numbers.
Even if the odd, isolated herbivore can occasionally be
found on R ponticum, there is certainly little in
the way of biomass to feed carnivores such as other
invertebrates and birds. There is also the added likelihood
that any herbivore feeding on R ponticum
would also contain toxins derived from the foliage.
The flowers of
R ponticum are very
attractive to insects, particularly Bumble Bees. In the main
flowering period of May/June, the exotic showy blooms
monopolize the attentions of pollinating insects, virtually
to the exclusion of all others. This means that the flowers
of native plant species in the vicinity suffer from a lack
of pollinating insects. As a result they may not
successfully set seed. This is yet another way that R
ponticum may be detrimental
to competing native vegetation.
All of this means that
areas dominated by R
ponticum have an exceedingly impoverished fauna in
comparison to native habitats, both in terms of species and
of biomass. If there is little eating the R ponticum,
then it follows that there are few or no carnivores eating
these herbivores and so also, few top carnivores. Song birds
which feed on either seeds or invertebrates are reduced to
trying to survive in smaller numbers by feeding in areas
above or adjacent to the R ponticum.
Once the song bird populations decline so do species such as
sparrowhawks which predate upon them.
Many broadleaf woodlands are
capable of supporting a complex and interlinked rich
diversity of plant and animal life. This is because there
can be as many as four distinct layers of vegetation. These
are:
- The climax vegetation which
consists of mature trees such as Oak or Ash.
- An understorey of shorter, shade
tolerant woody species such as Hazel or Holly.
- A field layer which is typically
of grasses and wild flowers.
- A ground layer of mosses and
lichens.
Crucially, each layer
supports a whole range of invertebrates which in turn
support the larger and larger animals which make up the food
chain. To put the value of native plants in perspective, a
single species such as Willow, can support over 400
different species of insect and mite. There may well be
hundreds or even thousands of individuals of each of the
species present representing a large biomass.
However it is not just
woodlands which have suffered. Heathlands, which have
declined by 75% in just the latter half of this century,
have also been seriously affected. Once again,
R ponticum dominates
many such areas and as a consequence, the heathland plants
and animals originally present have disappeared. It is
hardly surprising therefore to find that habitat
destruction, including that caused by R ponticum,
is responsible for the disappearance of 150 species from
Britain in 100 years.
Habitat Restoration
Restoring areas which
have been colonized by
R ponticum is not just a matter of cutting the
vegetation. The plant is notoriously difficult and expensive
to actually kill. The leaves are waxy and herbicide
treatment must include a chemical additive to help break
this surface down. Even then, where R ponticum
is well established with a large root system, such herbicide
treatment usually has to be done over several years. This is
because herbicides do not translocate well through the
plant.
Techniques such as mist
spraying or tractor application are usually out of the
question in woodlands because of the risk to trees.
There are a variety of
techniques for the mechanical removal of
R ponticum.
These usually employ a tractor or tracked swing shovel with
a rotary flail mounted on a moving hydraulic arm. In
sensitive conservation areas such techniques may well not be
appropriate. Such mechanical devices often leave a thick
layer of smashed Rhododendron on the ground which may have
to be removed using expensive manual labour.
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Once
R ponticum
has been removed and eradicated the toxic humus
layer
(left) still
remains. Where this has built up over a number
of years the resulting thick mat prevents
natural regeneration. The only solution is to
remove the layer. This often cannot be done by
mechanical means and thus may require several
hand operations to effect conditions which
favour regeneration. However, even then,
reinfestation can easily occur from the millions
of seeds which will have been produced over the
years. Thus areas where
R ponticum
has been present require careful monitoring over
a number of years. Prompt action must be taken
at a relatively early stage to prevent
re-establishment. |
Further problems exist
because any serious infestation is likely to affect
neighbouring areas which may not be in the same ownership.
Unless these areas are cleared, the seeds as well as the
limbs of the plant will encroach. Given time, a single plant
can cover 100 metres² and grow to more than 10 metres high.
This should be taken into account when making out a
management plan. Each site is different but where there has
been a significant presence of this plant and where it is
present in adjacent areas, it is prudent to budget for a
re-infestation figure of 15% per year after initial
clearance.
In summary:
R ponticum is an
introduced species. It is highly invasive where conditions
suit it. R ponticum destroys habitats and thus
whole colonies of native plants and animals disappear.
Because it is so expensive to control and physically
prevents access, land has been abandoned. However, such
areas can be restored but re-infestation must be prevented.
Clearance of an area of R ponticum is but the first
stage of restoration. Subsequent control measures may take
several years to eradicate the R ponticum. Even
then a viable seed bank may exist for many years. On some
sites eradication may not be possible because of
re-infestation from adjacent uncontrolled areas.
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