Haven or trap? Study finds sinkholes protect endangered trees at evolutionary cost
Huge sinkholes in China, known as Tianqing (“Heavenly Craters” in Chinese)both helping and harming one of the region’s most endangered plants at the same time. It protects the rare Magnolia aromatica tree from climate change, but also keeps its population isolated, reducing its ability to adapt in the future.A new study by researchers from the South China Botanical Garden of the Chinese Academy of Sciences and the Guangxi Institute of Botany found that these deep forest-covered gaps protect the tree from rising temperatures and drought. However, gaps also prevent trees from exchanging genes with other populations, making them more vulnerable to loss of genetic diversity.The findings, published in the journal Current Biology, suggest that this isolation is slowly reducing species’ ability to evolve and threatening their long-term survival.To study the genetic effects of living inside these giant sinkholes, scientists generated a high-quality reference genome for the plant Magnolia aromatica, and analyzed the DNA of 112 trees from 26 populations across southwest China, including trees growing inside and outside Tianqing.
A profound genetic paradox in karst
Magnolia aromatica is a rare evergreen tree that grows in the limestone landscapes of Guangxi, Guizhou and Yunnan provinces. Genetic study has identified four different evolutionary lineages across its range, with two of them occurring mainly in the Leye Tiankeng area.
The results revealed a surprising genetic pattern. Populations within Tianqing had moderate overall genetic diversity compared to populations outside the sewers. However, trees growing deep in sinkholes have less genetic diversity and a much higher number of harmful mutations than nearby trees growing on the surface. This is mainly because isolated populations have very little gene flow and are strongly affected by genetic drift.“Tianqing 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 closed geography can isolate populations and slowly reduce the genetic diversity needed for future adaptation.”
It thrives in deep shade
Despite this isolation, vacuoles remain essential for the tree’s survival due to its unique climate.
The lower part of Tianqing is cool, wet and shaded by steep slopes.The researchers found that the tree adapted to low light conditions. Genes under natural selection in stream populations have been linked to photosynthesis and carbon fixation.
China’s huge craters contain “heavenly” forests with plants adapted to the harsh underground life
Shading experiments supported these findings. Magnolia aromatica seedlings die quickly in strong sun but have survived and grown well in 50% to 90% shade, with best growth in the deepest shade.“This helps explain why Magnolia aromatica thrives in the cool, humid forests of the Tianqing Bottom,” said first author Zhu Xianliang. “This species appears to rely strongly on shaded environments, especially during the early stages of growth.”
Connecting refuges to save species
Researchers say the tree’s long-term future remains uncertain. By combining species distribution models with genomic data and mutation predictions, they studied how climate change will affect species.Their models showed that future climate change will make some habitats less suitable, leaving many populations unable to adapt. Even more worrying is that harmful mutations are expected to increase over time. This gradual loss of genetic health could increase the direct impacts of climate change and further threaten species survival.The study shows that protecting isolated refuges alone is not enough to save endangered species facing rapid environmental change.
To prevent Magnolia aromatica from falling into an evolutionary dead end, conservation efforts must go beyond protecting the gaps themselves.While tianqing should remain protected as a safe habitat, scientists say its conservation should also protect nearby outlier populations and natural corridors that allow the plants to exchange genes. Protecting these surrounding karst landscapes is important for maintaining the genetic diversity that species need 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 special habitats in which they live.”
Chinese researchers uncover a sinkhole evolutionary trap
Giant “hidden worlds” in China
There are more than 300 known tianqing in China, more than any other country in the world. They are found mostly in the southwest of the country, where water slowly formed limestone rocks over millions of years.
Some Tianqing are so large that they have their own unique forests, streams and ecosystems deep underground.Scientists have found many rare plants and animals within these giant sinkholes, including species rarely seen anywhere else. In 2022, explorers discovered an ancient forest within the giant Tianqing in Guangxi. The forest had trees about 40 meters high and thick vegetation covering the ground.
The discovery showed that some of the craters 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 isolated places. They also provide clues about how species cope with climate change and help researchers develop better ways to protect nature in the future.
Nutrient rich environment in the stream?
Another reason why plants survive inside Tianqing is the nutrient-rich environment. Previous research has found that the soil at the bottom of these giant sinkholes contains higher levels of important nutrients such as nitrogen, phosphorus, calcium, magnesium and potassium than the surrounding landscape. As a result, plants absorb these nutrients more easily, allowing them to grow faster and taller despite receiving very little sunlight.
Scientists also found that plants inside Tianqing contain less carbon than those growing on the surface, because the cool, moist conditions reduce water loss, meaning they don’t 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 landscape.