The ecology of Rhododendron ponticum L. with special reference to its competitive and invasive capabilities.

A survey of the current status of Rhododendron ponticum in the Peak
District and Sheffield area was carried out. It was found to be
widespread over much of the area, particularly on free-draining,
nutrient-poor, acidic soils, in sheltered, moist situations. It was
largely absent from the Carboniferous and Magnesian Limestone Series, from
the high altitude Kinder/Bleaklow massif and from areas subject to
intensive agricultural or industrial development. Disturbance of habitats
through forestry, grazing or recreational activity seems to encourage
invasion of suitable areas.
The role of mycorrhizal infection in R. ponticum was investigated.
Mycorrhizal plants had lower Root/Shoot Ratios, higher Relative Growth
Rates and showed increased yield compared to non-mycorrhizal plants. The
benefits of infection were strongest on nutrient-poor soils without added
nutrients. Infection of roots was visible after around six weeks and the
effects of mycorrhizas were increasingly apparent during the following six
weeks. The source of fungal inoculum was investigated and considered.
The 'interference' phenomenon described by earlier workers was
investigated. A mixture of competitive and allelopathic influences upon
test seedlings in bioassays was demonstrated. The toxicity . found, was
closely related to the presence of R. ponticum roots in the soil. Both
living or dead roots produced the effects. These were not removed by
nutrient addition. Interference was not dependent on mycorrhizal
infection of the R. ponticum roots.
To help an understanding of the interference demonstrated, a survey was
undertaken of the 'free' phenolic compounds occurring in R. ponticum
tissues, associated soil and litter, and in canopy throughfall. These
compounds have been implicated in allelopathic interactions involving
other members of the Ericaceae. R. ponticum tissues were found to have
very high concentrations of 'free' phenolic compounds compared to other
plant species examined. Considerable variation in form and amount was
found with tissue type and age. Phenolic compounds were detected in
associated soil and litter, as well as in canopy throughfall from R.
ponticum. It was shown that the interference cannot be fully explained by
competition for water and/or nutrients. In some situations a toxic
influence perhaps due to aromatic and aliphatic acids released from the
roots, has a major effect on the interaction between R. ponticum and
associated vegetation. Competition for nutrients and/or water clearly
occurs in some field situations, particularly when R. ponticum bushes are
encroaching on established vegetation. However, with bare-zones (either in the field or under artificial conditions) competition factors may be
almost totally eliminated by the toxicity which inhibits root formation.
Since the seedlings have very restricted root development, they are barely
able to compete for nutrients or moisture, and the toxic effects dominate
the interaction. The natural situation in the field is complicated by the
acidification of soils associated with R. ponticum, the physical and
chemical effects of its litter, shading and the overall influence of the
plant on soils and nutrient cycling.
The large quantity of 'free' phenolic compounds in R. ponticum tissues
(especially new leaves and new stems) probably have anti-herbivore and/or
anti-pathogen functions. This would explain the observed lack of damage
to the plant by invertebrate herbivores, diseases or parasites. These
compounds would thereby enhance the growth and competitive ability of R.
ponticum.