Are there primitive creatures in the tiankeng?

Shan Keqiang

Humans are always curious about unknown things. Tiankeng, especially the world-class super tiankeng, is isolated by towering cliffs and preserves rich plant resources. So what is a tiankeng? How is it formed? Are there primitive creatures in it?

In the southwest of my country, typical karst landforms, also known as karst landforms, have developed. They have not only formed the beautiful Guilin landscape, but also established a new karst landform, namely the tiankeng. In 2000, the world's largest tiankeng group was discovered in Guangxi, the largest of which is the Dashiwei tiankeng. The tiankeng is surrounded by vertical rock walls and is well-shaped. The pit mouth is 600 meters long from east to west and 613 meters deep. The world's largest tiankeng is the Xiaozhai tiankeng in Fengjie, Chongqing. Its pit mouth has a diameter of 622 meters and a depth of 666.2 meters, which is roughly equivalent to the height of two Eiffel Towers stacked together.

Tiankeng, as a karst landform, has actually been recorded for a long time. In Fengjie, Chongqing, the locals call it Tiankeng. There are also other names such as Shiwei, Shiyuan, Longgang, and Yanwan. Due to the small number of discoveries in the past, this special karst landform is often studied as a special case of a large collapse funnel. As more and more tiankengs are discovered in southwest my country, this special karst landform has gradually acquired universal significance. In 2003, Professor Zhu Xuewen, a karst geologist, constructed a complete tiankeng theory system in the book "Discovery, Exploration, Definition and Research of the Dashiwei Tiankeng Group in Leye, Guangxi", and separated tiankengs as a typical karst landform from large collapse funnels.

At present, there are more than 300 confirmed tiankengs in the world, of which 270 are located in China. China is truly a "tiankeng country". Tiankeng is a landform formed by the evolution of carbonate rock cave halls. The length and depth of the pit mouth are more than 100 meters, or the volume is greater than 100 cubic meters. It is surrounded by steep cliffs. Its bottom is now or was once connected to an underground river cave. It is known as the most spectacular karst landform on the surface.

Except for the entrance and the underground river at the bottom of the tiankeng, which are connected to the outside, the tiankeng is a relatively closed natural environment due to its huge height and closed cliffs, which minimizes the interference of natural and human factors from the outside world. The plant community at the bottom of the tiankeng retains a large number of ancient plant species. With the evolution of the tiankeng, some new tree species will continue to spread into the tiankeng. Therefore, the plant community in the tiankeng shows the characteristics of the coexistence of ancient relict tree species and modern tree species.

In the early stage of the formation of the tiankeng, the internal space was in the shape of an inverted funnel. As the skylight appeared in the cave hall, sunlight began to enter the cave hall, and the entire tiankeng world instantly became active. Seeds, powder, and even whole plants of various plants would spread into the tiankeng. However, due to the narrow underground and connecting swallows, the small internal light area, the low internal temperature of the tiankeng, the humid air, the thin soil, and the exposed rocks, it was difficult for plants to grow. At this stage, the tiankeng plants were mainly moss communities. Mosses grew in the moist and dark soil, rock surfaces, and cracks in the rocks, which could form a large amount of humus, increasing the fertility of the soil.

As the sky openings increased, light levels increased, and temperatures rose, ferns began to appear and replace the mosses.

The mosses migrated to the dark places inside the tiankeng. With the emergence of a large number of ferns, the humus content and thickness of the soil at the bottom of the tiankeng increased, and stair grass began to appear in the fungal plant community, and more tongtuo wood also survived. The angiosperms that like hot and humid environments are like trees that can be embraced by two people. The large plants that entered the tiankeng in the early days had difficulty surviving due to the nature of the sunlight and soil. After thousands or even tens of millions of years of evolution, the tiankeng mouth further expanded, and the area of ​​the mouth gradually approached the area of ​​the bottom of the pit. The tiankeng evolved into a bucket-shaped shape, and the tiankeng also entered the middle stage of evolution.

Due to the increase in light and the rise in temperature, the cliffs around the tiankeng make it difficult for the cold air in winter to enter the tiankeng. The underground river at the bottom of the tiankeng provides abundant water, and a large amount of sand and green stone also enters the tiankeng. Under the continuous cultivation of the plants at the bottom of the tiankeng in the early stage, highly fertile soil is formed. This environmental regulation is very suitable for the growth of plants, especially large plants. Shrubs, herbaceous trees such as oak, even Chinese holly, and tongtong wood began to flourish in large numbers, forming a tiankeng forest structure with tall trees on the upper layer, small trees and shrubs in the middle layer, and herbs on the lower layer.

Under the background of modern geological tectonic climate change, fragmentation and karst graphitization, the tiankeng has become a refuge for living organisms. The palm tree, which can only grow a few meters high in the outside world, can grow to 20 centimeters in the tiankeng, and the oak cotton can grow to 50 centimeters.

As the tiankeng's mouth expanded further, the steep cliffs surrounding the tiankeng continued to collapse, and the bottom of the tiankeng was continuously raised by the collapsed rocks, blocking the underground river at the bottom. The area of ​​the tiankeng's mouth exceeded the area of ​​the bottom of the tiankeng, and it evolved into a funnel-shaped shape. The tiankeng began to degenerate and entered the later stage of its evolution. The ability of the surrounding cliffs to block the winter cold wind gradually weakened, and the internal environment became dry and cold, gradually losing the characteristics of a humid and hot microenvironment. Many tropical plants were replaced by some deciduous plant species that adapt to cold environments.

In addition to plants, there are also a large number of insects living in the tiankeng. Although a tiankeng forest has been formed, it is a relatively isolated biological island due to its small environment. Animals are rarely found at the bottom of most tiankengs. The most common animal in the expedition is the red and white flying squirrel, also known as the flying cat. But even this flying animal has difficulty surviving at the bottom of the tiankeng. The steep cliffs above the tiankeng, especially at the top of some majestic tiankengs, are often used as ideal habitats by bats and swallows.

Reviewer: Tao Ning, Associate Researcher, Institute of Biophysics, Chinese Academy of Sciences

This article is produced by Science Popularization China-Starry Sky Project (Creation and Cultivation). Please indicate the source when reprinting.