Plants "need helping hand" to recolonise Rhododendron-cleared sites

Little native flora has returned to woodland sites cleared of invasive Rhododendron 30 years ago, according to a new study.

Image: HW
Image: HW

The findings, published in the Journal of Applied Ecology, suggest that programmes to eradicate Rhododendron ponticum need to be supplemented by reseeding for the original plant community to re-establish.

Researchers from the James Hutton Institute, the University of Aberdeen and Scottish Natural Heritage studied plots in the Atlantic oak woodlands of Argyll, Kintyre and Lochaber on Scotland's west coast  - some that had never been invaded, others which were covered in dense rhododendron thickets, plus a time-series of sites cleared of rhododendron at different periods between 1984 and 2014.

They found that even 30 years after rhododendron removal the native understorey normally found in Atlantic oak woodlands had not recovered. Instead of dramatic displays of primroses, violets, wild garlic, ferns and grasses, only dense mats of mosses and liverworts had returned.

The study's lead author, Dr Janet Maclean from the University of Aberdeen and the James Hutton Institute, said that in contrast to the native Celtic rainforest, "rhododendron-invaded woodlands present a different vista, with vast stands of this single species replacing all the diverse native flora".

Rhododendron is thought to hamper plant recovery by leaving traces in the soil even after its removal. As well as analysing plant communities, the team also tested the soil at different sites but found that rhododendron had not affected its acidity, nutrient levels or carbon:nitrogen ratio.

Instead, they think that the deep shade which the invasive species casts is responsible for its ongoing impact on native plants.

"Rhododendron dramatically reduces the amount of light reaching the woodland floor throughout the year, causing local extinction of native grasses and herbaceous plants," Maclean explained.

"Mosses and liverworts cling on because they can tolerate the darkness, so when rhododendron is removed they quickly recolonise, creating a thick mossy mat prevents other plant seeds from germinating."

Rhododendron is thought to affect around 827,000 hectares in the UK, particularly in western Scotland and Snowdonia. Eradication programmes cost around £8.6 million a year.

"The results of this study show that land managers should also consider clearing mats of common mosses from the ground and reseeding with typical woodland grasses and flowering plants," Maclean added. "To resemble uninvaded sites, removing rhododendron is not enough."

The study was funded by Scottish Natural Heritage.

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