| Summary: | In the United Kingdom, Rhododendron ponticum is one of our most invasive plant species, and yet there have been few published scientific studies compared with many other invasive species. Changes in environmental conditions are often implicated as being responsible for its impact on the native vegetation, and this study demonstrated that light availability, temperature, water availability, organic matter and soil pH were all different beneath stands of R. ponticum, compared to areas of open grassland where growth of the native species was not limited. Studies in the New Forest highlighted that light availability and soil pH were the two environmental conditions most likely to explain the impact of R. ponticum. However, glasshouse experiments testing the effect of these changes on the germination and growth of two native species, Lolium perenne (perennial rye grass) and Trifolium repens (white clover), revealed that the low light conditions only reduced the root elongation and leaf appearance of T. repens, and the soil pH had no inhibitory effect on either species. R. ponticum was also shown to release allelopathic compounds into the soil. However, on their own these compounds had no inhibitory effect on the germination or growth of L. perenne, and germination and leaf appearance of T. repens were reduced by less than 60%, indicating that other factors are involved in the inhibition of growth. Light and nutrient stress were shown to increase the susceptibility of the test species to allelopathic compounds, and the light and pH conditions found in uninvaded woodland in the New Forest increased the synthesis and accumulation of allelopathic compounds in the soil beneath the rhododendron. These findings demonstrate the importance of pre-existing conditions and the presence of other species in the success of invasive species, and that the inhibition of growth of the native species is due to a complex combination of biotic and abiotic factors.
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