Known as the giant sinkholes of China Tiankengs (“Heavenly Pit” in Chinese)Both are helping and harming one of the region’s most endangered plants at the same time. They protect the rare Magnolia aromatica tree from climate change, but they also keep its population isolated, reducing its ability to adapt in the future.A new study by researchers at the South China Botanical Garden and the Guangxi Institute of Botany of the Chinese Academy of Sciences found that these deep, forest-covered sinkholes protect trees from rising temperatures and drought. However, sinkholes also prevent trees from exchanging genes with other populations, making them more vulnerable to losing genetic diversity.The findings, published in the journal Current Biology, show that this isolation is gradually reducing the species’ ability to evolve and threatening its long-term survival.To study the genetic effects of living inside these giant sinkholes, scientists created a high-quality reference genome for Magnolia aromatica and analyzed the DNA of 112 trees from 26 populations in southwest China, including trees growing both inside and outside Tiankeng.
A genetic paradox deep in the karst
Magnolia aromatica is a rare evergreen tree that grows in the limestone landscapes of Guangxi, Guizhou and Yunnan provinces. Genetic studies identified four distinct evolutionary lineages across its range, two of which were found primarily in the Ley Tiankeng landscape.The results revealed a surprising genetic pattern. The populations inside the Tiankeng had moderate overall genetic diversity compared to the populations outside the sinkhole. However, trees growing deep inside the sinkhole had less genetic diversity and a much higher number of deleterious mutations than nearby trees growing on the surface. The main reason for this is that isolated populations have very little gene flow and are highly affected by genetic drift.“Tiankengs are not just safe havens,” said co-author Kang Ming of the South China Botanical Garden. “They provide a stable environment that helps endangered plants survive, but their enclosed geography can also isolate populations and gradually reduce the genetic diversity needed for future adaptation.”
Thrives in deep shade
Despite this isolation, sinkholes remain essential to the tree’s survival due to their unique climate. The bottom of Tiankeng is cool, humid, and shaded by steep cliffs.The researchers found that the tree has adapted to these low-light conditions. Genes associated with photosynthesis and carbon fixation were under natural selection in sinkhole populations.
Giant sinkholes in China contain ‘heavenly’ forests containing plants adapted to harsh life underground
Imaging experiments supported these findings. Magnolia aromatica plants died quickly in strong sunlight, but survived and grew well in 50% to 90% shade, with the deepest shade growing best.“This helps explain why Magnolia aromatica can thrive in the cool, humid forests at the foot of the Tiankeng,” said first author Zhu Jian-Liang. “The species is strongly dependent on shady environments, especially during the early stages of development.”
Adding shelters to save species
Researchers say the tree’s long-term future remains uncertain. Combining species distribution models, genomic data and mutation predictions, they investigated how climate change might affect species.Their model showed that future climate change will make some habitats less suitable, leaving many populations poorly adapted. What is even more worrying is that harmful mutations are expected to increase over time. This gradual loss of genetic health could exacerbate the direct effects of climate change and further threaten the survival of the species.The study shows that simply protecting isolated refuges is not enough to save endangered species facing rapid environmental change. To prevent Magnolia aromatica from becoming stuck in an evolutionary impasse, conservation efforts must go beyond protecting sinkholes.While Tiankeng should be protected as a safe habitat, scientists say conservation must also protect nearby outside populations and natural corridors that allow plants to exchange genes. Protecting these surrounding karst landscapes is important to maintain the genetic diversity needed for the species to adapt.“Our results suggest that conservation should protect both the refuge and its surrounding connections,” Kang said. “For endangered karst plants, maintaining gene flow between populations may be as important as protecting the specialized habitats where they survive.”
Chinese researchers reveal sinkhole evolutionary trap
China’s vast ‘hidden world’
There are more than 300 known Tiankeng in China, more than any other country in the world. Most are found in the south-west of the country, where water has slowly shaped limestone rocks over millions of years. Some tiankengs are so large that they contain their own forests, streams, and unique ecosystems below ground.Scientists have found many rare plants and animals inside these giant sinkholes, including species that are rarely seen anywhere else. In 2022, explorers discovered an ancient forest inside a giant Tiankeng in Guangxi. There were trees about 40 meters high in the forest and dense plants on the ground. The discovery revealed that some sinkholes may still be home to plants and animals that scientists have not yet studied or even discovered.Scientists say these hidden ecosystems are valuable because they help us understand how plants and animals survive in different places. They also provide clues about how species might cope with climate change and help researchers develop better ways to protect nature in the future.
Nutrient rich environment in sinkhole?
Another reason for the survival of plants inside Tiankengs is the nutrient-rich environment. Earlier research had found that the soil beneath these giant sinkholes contained higher amounts of vital nutrients such as nitrogen, phosphorus, calcium, magnesium and potassium than the surrounding landscape. As a result, plants absorb these nutrients more easily, causing them to grow faster and taller despite receiving very little sunlight. Scientists also found that plants inside Tiankeng contain less carbon than plants growing on the surface because the cool, humid conditions reduce water loss, meaning they do not need as much carbon-rich tissue to survive. These nutrient-rich conditions help create some of the healthiest and most productive plant communities in China’s karst landscapes.