Massive sinkholes in China are home to ‘paradise’ forests with plants adapted to harsh underground life

 

Plants growing at the bottom of giant sinkholes in China are so flooded with nutrients that they grow faster than their surface-dwelling counterparts while using less of a fundamental building block, a new study finds.

The sinkholes, called “tiankeng”, are among the last natural refuges for ancient forests and can shelter species unknown to  science — but it was unclear exactly how these species could thrive at the bottom of these deep pits.

It turns out that the laurels, nettles, and ferns that live inside the tiankeng thrive on huge reserves of nitrogen, phosphorus, potassium, calcium, and magnesium, all of which limit plant growth in other environments, where they are scarce. But because these nutrients are abundant in the tiankeng, the plants vacuum them up so they can grow and make the most of the sunlight that reaches them, according to a study published online July 20 in the Chinese Journal of Plant Ecology.

“Plants can adapt to adverse environments by adjusting their nutrient content,” the researchers wrote in the study, translated from Mandarin using Google Translate.

Very little light reaches the bottom of tiankeng, which means “sky pits” in Mandarin. Tiankeng are 100-meter-deep holes in the karst landscape of southwest China. These deep pits are home to plants that like moisture and shade, including species unique to the region, the study said.

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“Because of the Tiankeng’s towering cliffs and steep terrain, it has been less disturbed by human activities,” the researchers wrote. The Tiankeng are refuges for modern karst forest plants, including the Nepalese plum (Choerospondias axillaris) and the Chinese rain bell (Strobilanthes cusia), they wrote.

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For the study, the researchers collected samples of 64 plant species from inside and outside tiankeng in Leye County, Guangxi region of China. Leye County is home to the Dashiwei Tiankeng Group, a geological wonder including 30 sinkholes Across a 20-square-kilometer landscape, the team measured the carbon and nutrient content of each sample to determine whether the nutrient uptake and growth strategies of these plants differed depending on their environment.

Plants growing inside the tiankeng had lower carbon content than those growing outside, but they had higher levels of all other elements the researchers measured, such as calcium and potassium, as well as higher growth rates.

The Chinese rain bell (Strobilanthes cusia), also known as Assamese indigo, is a modern karst plant found deep in chasms in China. (Photo credit: dnixdony via Getty Images)

Carbon is essential to plants, making up much of their “skeletons” and structures that enhance water retention, the study said. But the humid conditions inside the tiankeng mean that plants do just fine with lower levels of carbon in their tissues because they don’t need to hold on to as much water. Plants growing on the surface held more carbon, likely because “the forest outside the well has high light intensity, rapid water evaporation, poor soil, greater interference from human activities, and easy soil loss,” the researchers wrote.

Compared with the aboveground plants, the plants growing inside the tiankeng had higher levels of nitrogen and phosphorus, two elements that plants take up from the soil. The soil at the bottom of the tiankeng contained more of these elements than the aboveground soils, indicating that the plants were more likely to absorb them. Karst soil is rich in calcium and magnesium, and the tiankeng plants had much higher levels of these elements than the aboveground plants. They also had higher levels of potassium, although potassium is relatively rare in karst soils.

Tiankeng plants absorb nutrients more easily than “conservative” surface plants because nutrients are more abundant in the shaded depths of sinkholes and because the plants need to grow taller, the study said.

“The nutritional status of the soil inside the Tiankeng Forest is good,” the researchers wrote, and plants have evolved to make the most of available resources to grow quickly and harvest more light.