The Leye County in southern China is home to 30 gigantic sinkholes collectively called the Dashiwei Tiankeng Group. The deepest sinkhole in this region is a staggering 613 meters deep, with many others reaching depths of 100 meters and more.

In the local language, these sinkholes are called “tiankengs” which translates to heavenly pits. The geological significance of the Dashiwei Tiankeng Group can be understood from the fact that it hosts 30 percent of all the world’s known tiankengs.

However, in recent years, scientists discovered another fascinating aspect of these heavenly pits. They found that despite receiving very little light, the tiankengs are home to ancient forests with plants and trees found nowhere else on the planet. 

“The scene down there was stunning: an underground pristine forest with no trace of human activities, with 40-meter-high ancient trees and a group of endangered wild plants from the times of dinosaurs,” Tiang Jianmin, a botanist from the Guangxi Institute of Botany, who studies sinkholes and have also been inside one of the tiankengs, told the UNESCO Courier.

Many plant species that have become extinct on the surface and numerous varieties of vegetation unknown to humans are somehow thriving in the depths of these heavenly pits — but what has been driving the birth and growth of plants so deep underground? 

The answer to this question was a mystery until now — until this new study finally revealed what makes life blossom in the heavenly pits of Leye County.

Less carbon but more of everything else

The heavenly pits are surrounded by limestones, soluble rocks that often lead to the formation of underground rivers, tunnels, and caves. When the roof of such underground structures falls, large pits or tiankengs are formed reaching hundreds of meters in depth.

In order to understand the factors supporting vegetation growth in the tiankengs in detail, the study authors collected leaf samples of 64 species of plants from 36 families and 55 genera. They measured the quantities of carbon (C), potassium (K), calcium (Ca), nitrogen (N), magnesium (Mg), phosphorus (P), and various other nutrients in the samples.

The study revealed that the nutrient makeup of species thriving in tiankengs varies drastically from that of terrestrial plants and trees. 

For instance, “The leaf nutrients of plants in the study area have lower C content, higher N, P, K, Ca, and Mg contents, and lower C:N, C:P compared to other karst plants and terrestrial plants in China,” the study authors said.

Carbon plays a crucial role in water retention. Since plants on the surface undergo rapid water evaporation as they receive a good amount of sunlight, they also need high amounts of carbon to prevent excessive water loss. 

However, little light reaches the vegetation in the tiankengs, and the environment underground is mostly humid. Therefore, the low amount of carbon makes sense for them. Surprisingly, other nutrients such as nitrogen, calcium, and phosphorus, the deficiency of which limits the growth of plants on the surface, are abundant in the heavenly pits. 

The plants and trees in the heavenly pits consume these nutrients in large quantities. This enables them to make the most of the little sunlight they receive, causing them to grow fast and become taller than their surface counterparts.

“This indicates that the plants in the study area have the characteristics of low carbon sequestration, high nutrient accumulation, high growth rate, and low nutrient utilization efficiency,” the study authors added.

Tiankengs are nature’s refuge

Tiankengs serve as biological and genetic hotspots as the life inside them remains untouched by humans. This is also evident from the fact that the trees growing in these heavenly pits possess more genetic diversity than their surface relatives. 

For instance, when scientists studied the DNA of Magnolia aromatica trees from a sinkhole and compared it with the DNA of another M. aromatica growing on the surface, they discovered that the tree in the tiankeng has 23 percent more genetic diversity. 

Additionally, while the population of terrestrial M. aromatica is facing a sharp decline, its cousin in the sinkholes is thriving.

This indicates that, “The tiankeng area serves as a refuge for endangered plants and primitive forest ecosystems, and provides a natural germplasm bank of biodiversity,” Jianmin said. He wasn’t involved in the current study.

The heavenly pits are also turning into tourist attractions because of their natural beauty and the diverse environment that surrounds them. This is not always a good development as it can often cause serious damage to these sensitive habitats.

“For example, in 2018, the development of Tiankeng Scenic Spot in Dashiwei resulted in a serious damage to biodiversity in close-by Baidong Tiankeng, including endangered plants being cut down and destroyed,” Jianmin added.   

Therefore, conservationists and local authorities must take proper measures to ensure pristine habitats like the Dashiwei Tiankeng Group stay protected at all costs. After all, these sinkholes are probably some of nature’s last remaining refuges.

The study is published in the Chinese Journal of Plant Ecology.