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 the building block, a new study has found.
The sinkholes, called “tiankeng”, are some of the last remaining natural refuges for ancient forests and may contain species unknown to science — but it wasn’t clear exactly how these species could thrive at the bottom of these deep pits.
The laurels, nettles, and ferns that live inside the tiankeng have been shown to thrive on vast supplies of nitrogen, phosphorus, potassium, calcium, and magnesium, all of which limit plant growth in other environments where they are scarce. But since these nutrients are abundant in tiankeng, plants according to a study published online July 20 in Chinese Journal of Plant Ecology.
“Plants can adapt to adverse environments by adjusting nutrient content,” the researchers wrote in the study, which was translated from Mandarin using Google Translate.
Very little light reaches the bottom of the tiankeng, which means “sky pits” in Mandarin. Tiankeng is a 330-foot-deep (100-meter) hole in the southwestern karst landscape of China. These deep pits contain plants that prefer moisture and shade, including species unique to the region, according to the study.
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“Because of the towering cliffs and steep terrain, the tiankeng has been less disturbed by human activity,” the researchers wrote. The Tiankeng are a refuge for modern karst forest plants, including the Nepalese plum tree (Choerospondias axillaris) and the Chinese rain bell (Strobilanthes cusia), they wrote.
For the study, the researchers collected samples of 64 plant species inside and outside the tiankeng in Leye County, China’s Guangxi region. Leye County is home to the Dashiwei Tiankeng Group, a geological wonder containing 30 sinks in a landscape of 7.7 square miles (20 square kilometers). To determine whether the nutrient uptake and growth strategies of these plants differed depending on their environment, the team measured the carbon and nutrient content of each sample.
Plants growing inside the tiankeng had lower carbon content than those growing outside, but had higher levels of every other element the researchers measured, such as calcium and potassium, as well as a faster growth rate.
Carbon is essential in plants, making up a large part of their “skeletons” and structures that improve water retention, the study says. But the humid conditions inside the tiankeng mean the plants can get by just fine with lower levels of carbon in their tissues because they don’t need to store as much water. Plants growing on the surface contained more carbon, likely because “the forest outside the pit has high light intensity, rapid water evaporation, poor soil, greater human interference, and easy soil loss,” the researchers wrote.
Compared to surface plants, plants growing inside the tiankeng had higher levels of nitrogen and phosphorus, both of which plants draw from the soil. The soil at the bottom of the tiankeng contained more of these elements than the surface soil, indicating that plants absorbed them more easily. Karst soil is rich in calcium and magnesium, and tiankeng plants showed much higher levels of these elements than surface plants. They also had higher levels of potassium, even though potassium is relatively rare in karst soils.
Tiankeng plants absorb nutrients more easily than “conservative” surface plants because nutrients are more abundant in the shady depths of sinkholes and because plants need to grow, according to the study.
“The nutrient status of the soil in the tiankeng forest is good,” the researchers wrote, and the plants have evolved to make the best use of available resources to grow quickly and harvest more light.