Woodland recolonisation following rhododendron clearance studied

Woodland from which rhododendron has been eradicated is not then recolonised by the same plant communities that it previously hosted, according to award-winning research by a student based at the James Hutton Institute and the University of Aberdeen.

Woodlands: plant populations researched by student from James Hutton Institute and the University of Aberdeen - image: Nicholas Tonelli
Woodlands: plant populations researched by student from James Hutton Institute and the University of Aberdeen - image: Nicholas Tonelli

Janet Maclean won the British Ecological Society's Anne Keymer Prize in 2015 for the best lecture by a postgraduate student at its December annual meeting. Her work looks at woodland restoration after the removal of invasive R. ponticum, focusing on the internationally important Atlantic oak woods of the west of Scotland, noted for their rich flora of bryophytes (mosses and liverworts).

Having analysed plant populations in around 100 plots, ranging from uninvaded control plots and those with varying rhododendron density to those where the shrub had been cleared up to 30 years ago, she concluded that bryophyte populations survived rhododendron invasion and recolonised woodlands quickly after its removal.

But the more familiar populations of forbs and grasses such as bluebells and Deschampsia flexuosa (wavy hair grass) do not recover successfully, she found. "While rhododendron invasion is bad for all species, it is particularly bad for the forbs and grasses, which are all but extirpated," she said.

"There is a message of hope for the recovery of bryophytes, so long as sites are effectively maintained free from rhododendron. However, if people want to see a nice bluebell woodland then further management interventions such as reseeding may be required."

She added: "Because it takes decades, if ever, for native communities to recover following the removal of invasive rhododendron, it is highly important to prevent new areas from becoming invaded as far as possible, and perhaps to target funds at prevention as much as at removing established stands where the native community has already been displaced."

The work, funded by Scottish Natural Heritage (SNH), "is a prime example of how the development of closer relationships between scientific and conservation partners can result in research with practical applications for the care and restoration of our woodlands", said Maclean.

As well as smothering native flora, R. ponticum has also been identified as a host of Phytophthora ramorum. It is on SNH's "species action list" and Forestry Commission Scotland (FCS) has committed to its removal from the National Forest Estate, at a cost initially estimated at £15.5m.

An FCS representative said: "We have undertaken clearance work on more than 20,000ha of the estate, or approximately half the total area affected. We are focusing effort on priority areas such as ancient semi-natural woodlands and sites of special scientific interest." But he conceded: "Achieving complete control is likely to prove a mammoth task with regrowth requiring to be treated repeatedly. It's likely that total removal will take longer than initially thought."

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