Nettle’s-eye view (Image: Alex Monro, The Natural History Museum) Elatostema celingense: staminate inflorescence with major receptacle bracts and flowers (Image: Wei Yi-Gang, Guangxi Institute of Botany) Elatostema retrorstrigulosum (Image: Alex Monro, The Natural History Museum) (Image: Alex Monro, The Natural History Museum) Advertisement

Walk into a cave in south-west China and you could be stepping back 30,000 years in time.

So says Alex Monro, a researcher in tropical plant diversity at the Natural History Museum, London, who thinks the caves could be a time capsule preserving rare nettles from the time of the last ice age.

Working with researchers from the Chinese Academy of Sciences, Monro has identified seven species of nettle that grow in isolated, dark corners of the karst landscapes of Guangxi and Yunnan provinces. Some species can survive in conditions in which just 0.02 per cent of sunlight penetrates the cave – that’s less than reaches 100 metres deep into the oceans. “They grow at the backs of the main caverns in near-dark conditions,” says Monro.

“Some of the specimens came from areas with very low light levels indeed, and one can easily interpret the site as being under full cave conditions,” says Frank Howarth of the Hawaii Biological Survey in Honolulu, a speleologist who specialises in karst caves and their ecologies.

“There must be something quite special about their photosynthesis,” says Monro, although the team has not yet investigated the photosynthetic mechanism. “They probably activate the photosynthetic process very quickly, which enables them to take advantage of very short bursts of light, and they might go for slightly different wavelengths,” he says.

The nettle species seem to be unique to the remote caves and gorges, growing in isolated groups. One species, Elatostema retrorstrigulosum, is limited to only 10 adult plants, some growing in a grotto, hidden among stalagmites. The team have identified two of the species as “critically endangered” under the criteria of the International Union for Conservation of Nature; the rest are either “endangered” or “vulnerable”.

Cold blast from the past

Nettles like these are not found in the surrounding tropical forest. To explain the discovery of these pockets of rare plants in an environment that is too tropical to support them, Monro suggests that the rare species could be “relicts of a vegetation from a previous cooler climate that resembled that of the caves”.

Another suggestion is that the nettles could have evolved inside the caves. Monro intends to check this with DNA sequencing: if correct, this would be rapid evolution, according to Rogier de Kok, a botanist at the Royal Botanical Gardens in London, who has visited the caves. “These caves are not very old – less then a million years – which is usually seen as very quick in the evolution of a series of species.”

Journal reference: Phytotaxa, vol 29, p 1