Biodiversity in Rhododendron

Once Rhododendron has invaded an area few native plants survive. In woodlands only those trees which manage to grow above the level of the Rhododendron canopy will persist. These of course have a finite life span and on their death, there is no replacement because seedlings cannot become established under the lightless canopy. At this point, the Rhododendron 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 Rhododendron areas are essentially barren. Even where trees exist above the Rhododedron 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.

The Dormouse, one of Britain's most endangered species of mammal, has been severely affected by habitat loss. Some of this has been because Rhododendron has out-competed native plants and shrubs which are essential for the dormouse to survive.

The toxicity of Rhododendron 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 Rhododendron.  It is important to bear in mind that just because a species has been found on Rhododendron, it does not mean that it habitually lives there. It may have fallen or been blown into the Rhododendron 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 Rhododendron 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. Rhododendron 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 Rhododendron, 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 Rhododendron would also contain toxins derived from the foliage.

The flowers of Rhododendron 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 Rhododendron may be detrimental to competing native vegetation.

All of this means that areas dominated by Rhododendron have an exceedingly impoverished fauna in comparison to native habitats, both in terms of species and of biomass. If there is little eating the Rhododendron, 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 Rhododendron. 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:

1. The climax vegetation which consists of mature trees such as Oak or Ash.
   
2. An understorey of shorter, shade tolerant woody species such as Hazel or Holly.
   
3. A field layer which is typically of grasses and wild flowers.
   
4. 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, Rhododendron 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 Rhododendron, is responsible for the disappearance of 150 species from Britain in 100 years.

Habitat Restoration

Restoring areas which have been colonized by Rhododendron 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 Rhododendron 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 Rhododendron. 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.

Once Rhododendron has been removed and eradicated the toxic humus layer 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 Rhododendron 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 reinfestation figure of 15% per year after initial clearance