The karst mountains of south-west China are strewn with vast sinkholes known as tiankengs. The largest of these, Xiaozhai Tiankeng, is a whopping 626m deep and 527m wide.
These great, forested depressions shelter a host of rare plants from the high temperatures and drought that fall upon the surrounding land. But new research reveals that while tiankengs provide protection for some species, they may also limit their gene flow.
Published in the journal Current Biology, the study suggests this isolation may be slowly diminishing the evolutionary potential of certain plants – and therefore their chances of long-term survival.
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To find out whether the isolation inside tiankengs poses genetic risks that surpass their protective benefits, researchers from the Chinese Academy of Sciences (CAS) and the Guangxi Institute of Botany examined an endangered tree known as Magnolia aromatica.
Native to China, this rare, evergreen magnolia grows across limestone habitats in Guangxi, Guizhou and Yunnan provinces.
By conducting genomic analyses of 112 Magnolia aromatica trees, growing both inside and outside the sinkholes, the team found that tiankeng-dwelling populations showed lower genomic diversity and higher levels of harmful mutations than nearby non-tiankeng populations.
“Tiankengs are not simply safe havens,” says co-author Kang Ming. “They provide a buffered microenvironment that helps endangered plants persist, but their enclosed geography can also isolate populations and gradually wear away the genetic variation needed for future adaptation.”
The team also discovered that Magnolia aromatica seedlings are happier and healthier in shade than they do in light. Shading experiments revealed that seedlings survived and grew strongly under 50% to 90% shade, performing especially well under deep shade. In contrast, seedlings died quickly under full light.
“This helps explain why Magnolia aromatica can thrive in the dim, humid forests at the bottom of tiankengs,” says first author Zhu Xian-Liang. “The species appears to be strongly dependent on shaded environments, especially during early seedling establishment.”
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Climate change is likely to impact habitat suitability across the species’ range. More critically, the researchers project detrimental mutations to rise over time, exacerbating the effects of climate change.
The study emphasises the need to conserve both tiankengs and the surrounding landscapes where Magnolia aromatica exists. “Our results suggest that conservation should protect both the shelter and the connections around it,” says Kang. “For endangered karst plants, maintaining gene flow among populations may be just as important as preserving the special habitats where they survive.”
Top image: A typical karst tiankeng landscape in Leye, Guangxi, south-west China. Credit: Tang Jianmin
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