A critically endangered moss has been rediscovered growing at a reservoir near Edinburgh, raising hopes it can be introduced to other areas to stop it dying out.

Round-leaved bryum was found recently at Threipmuir Reservoir south of Balerno by NatureScot’s Kat O’Brien, and was identified by David Chamberlain of the Royal Botanic Garden Edinburgh.

Their find marked the first time the extremely rare plant has been seen at the designated Site of Special Scientific Interest north of the Pentland Hills in 13 years.

The moss, which has not been observed anywhere else in the UK since 1987, can only be found in mud by reservoirs and lochans when water levels are temporarily low, a habitat that only forms occasionally.

Human stabilisation of water levels and water pollution in Scotland has placed round-leaved bryum and some other specialist species under threat by removing their preferred habitat conditions.

NatureScot is now working with bryologists at the Royal Botanic Garden Edinburgh to transfer some of the moss to its greenhouses to build up a population that can be moved to new sites.

It is hoped their efforts will secure round-leaved bryum’s continued presence in the UK and lift it from the GB Critically Endangered list, although it may not be known for another eight to 10 years whether the project has been successful.

Ms O’Brien, NatureScot’s bryophyte, fungi and lichen adviser, said: “This incredible rediscovery of such a rare and endangered moss in Scotland shows us that there is a whole world of biodiversity that we can only see and appreciate when we take the time to create the best conditions and work to protect nature.

“Despite a number of dedicated bryologists in the UK looking for it, no-one has found any round-leaved bryum in decades other than at this site, leading to its classification as a GB Critically Endangered.

“It is likely that this moss is losing sites faster than it is gaining new ones, and is declining on a European scale.

“The unique habitat it needs only occasionally appears, and it’s a good sign that the conservation work in the area is helping biodiversity.”

Dr Neil Bell, a bryologist at Royal Botanic Garden Edinburgh, added: “Round-leaved bryum is one of our most threatened mosses, and this find confirms its continued presence at what is now its only site in the UK.

“Because it is specifically adapted to the temporary habitat that emerges when water levels are very low, rapidly completing its life cycle and then surviving in a dormant state in the mud for many years, the increasing artificial stabilisation of lakes and reservoirs is rapidly pushing it towards extinction in Britain.

“We are really excited to now have colonies of round-leaved bryum and other rare species from Threipmuir in cultivation at the Royal Botanic Garden Edinburgh.

“As well as hoping that they can be bulked up sufficiently for translocation, the horticulturists here are learning a lot about how to grow bryophytes generally and about the life cycle of this fascinating, critically endangered moss.”