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.