Gruffydd Lloyd Jones1,2, Max Tomlinson1, Rhys Owen2, John Scullion1, Ana Winters1, Tom Jenkins3, John Ratcliffe4 and Dylan Gwynn-Jones1
1IBERS, Aberystwyth University, Penglais Campus, Aberystwyth, Ceredigion. SY23 3DA, UK
2Snowdonia National Park Authority, National Park Office, Penrhyndeudraeth, Gwynedd. LL48 6LF, UK
3Forest Research, Thoday Building, Deiniol Road, Bangor, Gwynedd. LL57 2UW, UK
4Natural Resources Wales, Maes y Ffynnon, Penrhosgarnedd, Bangor, Gwynedd. LL57 4DE, UK
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 on heathland sites within Snowdonia National Park, Wales. One site had existing R. ponticum stands while 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. However, the presence of regenerating R. ponticum on all cleared sites highlights the critical importance of monitoring and re-treating sites post initial clearance.