Day of the Rhododendron

Background Information

The story told in this programme is about ecology, the study of living organisms in relation to their natural, or in this case somewhat un-natural, environment. The story is set in Snowdonia National Park (Parc Cenedlaethol Eryri), which is the second largest National Park in England and Wales with an area of 2171 km², situated in the north-west of Wales. The Welsh name Eryri means 'place of the eagles'.

Students will need to be familiar with a number of related terms:

• Environment: the surroundings of an organism and everything that could possibly influence it. In the case of the rhododendron this will include the weather conditions on the hillsides of Snowdonia National Park and the plants and animals already living there.
• Habitat: where the organism lives, in this case the Bodnant gardens and the Snowdonia hillsides.
• Population: which in biology always refers to a particular species, in this case Rhododendron Ponticum.
• Monoculture: the growth of a single plant species to the exclusion of all others.
• Community: includes all the plants and animals living in a habitat. The danger here is that the rhododendron is so prolific that all other plants and animals are wiped out, leaving a monoculture.
• Ecosystem: includes the community of organisms in a habitat, and all the other factors, such as air, water, soil and light.
• Competition: All living organisms have evolved to compete for food, light, space to root, and so on. Rhododendron Ponticum is so successful that it wipes out other populations, becoming the dominant species.

The success of Rhododendron Ponticum is due to a number of factors.

Seed propagation

The plant produces vast numbers of seeds that are dispersed by the wind over large distances. The seed pods dry in the sun and shrivel. The outer fibres on both sides of the pods shrink and create a tension. This eventually causes the pod to split open suddenly and eject the seeds. The seeds themselves are small and low-density so that they carry easily in the wind. In physical terms: when their terminal velocity in free-fall is small compared with the wind speed, they get swept along for large distances.

Root domination

It is thought that the roots of Rhododendron Ponticum are closely associated with a fungus that assists the plant by digesting organic matter while itself benefiting by receiving simple carbon compounds from the plant. Such a relationship between two species is known as symbiosis or mutualism. In this particular symbiotic relationship there is an added advantage for the plant. The fungus generates a toxin that poisons any neighbouring plants through their roots, and thus destroys the competition.

Efficient photosynthesis

All plants need energy from the sun to help them create food. Rhododendron Ponticum can photosynthesise at very low light levels, when as little as 4% of the sunlight is reaching the leaves. The broad leaves also create deep shadows and thus prevent enough light reaching smaller plants, which therefore cannot photosynthesise satisfactorily and die out.

Translocation

Food, in the form of sugars made in the leaves, is transported both up and down the plant in the phloem, which comprises vertical columns of living cells with perforations in their end walls. This is called translocation. The phloem can also carry poisons sprayed on to the leaves to kill the plant. In some plants there are cross-linkages between phloem channels so that food (and poisons) can reach the whole plant from one group of leaves. This is known as tangential translocation. But this is not the case in the rhododendron. There are fewer cross-connections, so poison sprayed on only some of the leaves will kill only some of the plant. So to eradicate Rhododendron Ponticum all leaves must be sprayed or the plant must be allowed to re-grow from undamaged roots and then sprayed again.