Now, scientists at the Kunming Institute of Botany in Yunnan province say they might have the answer. Led by botanist Liu Jie, the team collected samples from pine-growing regions across China to build the country’s largest database of information on the pine species.

Through genetic analysis, they found the species has three lineages – and these differences in genes mean they are not suited to the same conditions.

That explained why the trees planted during the reforestation campaign did not survive – they were not planted in their natural habitat, the scientists said in a paper published in the peer-reviewed journal Evolutionary Applications last month.

They said it also explained why there had not been a similar die-off of the pines since then.

02:07

As China continues planting trees, 23% of the country is now covered in forest

As China continues planting trees, 23% of the country is now covered in forest

Large-scale industrialisation began in China in the 1950s. Less than two decades later, mountains with once-thick forests had been denuded by logging, as railways, factories and new homes were built.

The deforestation problem was serious, and amid the decade-long upheaval of the Cultural Revolution, the Chinese government launched a tree-planting campaign in the late 1960s.

Although it was driven by communist revolutionary spirit, the campaign was not entirely without planning or guidance. The pines were mainly planted in areas where the same species used to grow, and the saplings were guarded and looked after by local forestry authorities.

They looked like the other Mount Hua pines growing naturally nearby, but genetic sequencing showed that even in the same mountain range, the species could have different lineages, the botanists said.

They estimated that the first lineage branched off about 9 million years ago as a large mountain range formed across central China. Some tree species could not adapt to the dramatic change of environment and perished, but the Mount Hua pine survived and thrived.

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Its three lineages each have a distinct set of genes adapted to different environments, and there have been few genetic exchanges between them, according to the study.

But the people planting the trees 50 years ago were not aware of the genetic differences, and planted the pines suited to lower altitude areas high up in the mountains, or the other way round.

A few years later, many of them suddenly started to die, and the reforestation campaign was a disaster.

01:14

1,900-year-old pine discovered in southwest China

1,900-year-old pine discovered in southwest China

Botanists have over the years offered several theories to explain why it happened, including acid rain and a disease-causing fungus, but they have all been proven wrong.

There was no evidence of damage caused by environmental pollution, disease or climate change.

And none of the theories could explain why the same phenomenon had not happened again. Today, nearly all of the affected mountains have been reforested with Mount Hua pines, and the genetic analysis suggested their distribution follows a highly natural pattern. The scientists said there was very little sign of human influence on the distribution of the different pine lineages, which tended to dominate areas they were suited to.

Liu and the team put this down to “traditional wisdom”. Farmers and forestry authorities in the 1980s did not have access to genetic sequencing technology, but when they replanted the pines they used seeds from the trees that were growing naturally nearby. There has been no mass die-off since.

“They have followed the species-site matching principle [selecting a species suitable to the site] and kept human disturbance of the natural landscape to a minimum,” the researchers said in a statement.

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Xu Bo, a researcher with the Institute of Botany in Beijing who was not involved in the study, said the findings would help improve China’s tree-planting programme, which was the biggest in the world.

He also noted that “key traits of a plant cannot be seen from their appearance”. Xu said the traditional classification by species meant a plant’s adaptation to different environments could be overlooked.

Genetic sequencing has been done on important crops such as rice, wheat and corn, but the genes of most plant species remain unknown.

“We are in an age of discovery,” Xu added.

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