Abstract
Rhododendron ponticum L. is a damaging invasive alien species in Britain, favouring the moist, temperate climate, and the acidic soils of upland areas. It outshades other species and is thought to create a soil environment of low pH that may be higher in phytotoxic phenolic compounds. We investigated native vegetation restoration and R. ponticum regeneration post-clearance using heathland sites within Snowdonia National Park, Wales; one site had existing R. ponticum stands and three were restoring post-clearance. Each site also had an adjacent, uninvaded control for comparison. We assessed whether native vegetation restoration was influenced post-invasion by soil chemical properties, including pH and phytotoxic compounds, using Lactuca sativa L. (lettuce) bioassays supported by liquid chromatography-mass spectroscopy (LC-MSn). Cleared sites had higher shrub and bare ground cover, and lower grass and herbaceous species cover relative to adjacent uninvaded control sites; regenerating R. ponticum was also observed on all cleared sites. No phenolic compounds associated with R. ponticum were identified in any soil water leachates, and soil leachates from cleared sites had no inhibitory effect in L. sativa germination assays. We therefore conclude that reportedly phytotoxic compounds do not influence restoration post R. ponticum clearance. Soil pH however was lower beneath R. ponticum and on cleared sites, relative to adjacent uninvaded sites. The lower soil pH post-clearance may have favoured shrub species, which are typically tolerant of acidic soils. The higher shrub cover on cleared sites may have greater ecological value than unaffected grass dominated sites, particularly given the recent decline in such valuable heathland habitats. The presence of regenerating R. ponticum on all cleared sites however highlights the critical importance of monitoring and re-treating sites post initial clearance.
Highlights
Invasive alien species are increasingly recognised as a major threat to biodiversity and human welfare, and incur major economic costs[1]
Another factor contributing to the invasiveness of R. ponticum is its unpalatability to herbivores; its foliage contains high concentrations of phenolic compounds and grayanotoxins which protect it from grazing, and as a result it has very few natural enemies in the British Isles giving it a competitive advantage over native species[2,11,12,13]
This study investigated how the cover of different native vegetation classes varied between sites cleared of R. ponticum and adjacent uninvaded control sites
Summary
Invasive alien species are increasingly recognised as a major threat to biodiversity and human welfare, and incur major economic costs[1]. Native to the Caucasus region, the Black Sea Coast and the Iberian Peninsula, it was introduced to the British Isles in 1763 and has since invaded native woodlands and conifer plantations, upland grassland, heaths and bogs[5,6] Once established in these habitats, R. ponticum reduces local biodiversity. R. ponticum has a high seed output with one bush able to produce over a million small, wind dispersed seeds, allowing for rapid spread over large distances[2,6,11] Another factor contributing to the invasiveness of R. ponticum is its unpalatability to herbivores; its foliage contains high concentrations of phenolic compounds and grayanotoxins which protect it from grazing, and as a result it has very few natural enemies in the British Isles giving it a competitive advantage over native species[2,11,12,13]. The above interactions result in the formation of novel, less chemically and biologically diverse soil conditions beneath the invasive plant[25]
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