Plants growing at the bottom of giant sinkholes in China are so packed with nutrients that they grow faster than their surface counterparts, while using fewer building blocks, a new study has found.
The craters, called Tianqing, are among the last remaining. Natural refuges of ancient forests may harbor species unknown to science. But it was not clear exactly how these species could thrive at the bottom of these deep holes.
It turns out that the laurel, nettle and ferns that live inside Tianqing thrive on huge stores 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 Tianqing, the plants Aquatic plants can grow taller and make the most of the sunlight they get, according to a study published online July 20 in the journal Nature. Chinese Journal of Plant Ecology.
“Plants can adapt to adverse environments by adjusting their nutrient content,” the researchers wrote in the study, which was translated from Chinese using Google Translate.
Very little light reaches the bottom of the Tianqing, which means “heavenly pit” in Chinese. The Tianqing are 330-foot-deep (100-meter-deep) sinkholes in the karst landscape of southwestern China. According to the study, these deep holes harbor plants that prefer moisture and shade, including species unique to the region.
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“Due to its steep slopes and steep terrain, Tianqing has been less disturbed by human activities,” the researchers wrote. Tianqing is a haven for modern karst forest plants, including the Nepalese pig plum (Corospondias axillaris) and the Chinese rain bell (Strobilanthes coscia), they wrote.
As part of the study, researchers collected samples from 64 plant species inside and outside the Tianqing area in Lehe County, Guangxi Zhuang Autonomous Region, China. Lehe County is home to the Dashuyi Tianqing Group, a geological wonder. It consists of 30 holes. In an area spanning 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 Tianqing had less carbon than those growing outside, but they had higher levels of every other element the researchers measured, such as calcium and potassium, as well as higher growth rates.
Carbon is essential to plants, making up a large part of their “skeletons,” the structures that improve their ability to hold water, according to the study. But the humid conditions inside Tianqing mean that plants can do well with low levels of carbon in their tissues because they don’t need to hold as much water. Plants growing on the surface hold more carbon, the researchers wrote, likely because “the forest outside the crater has high light intensity, rapid water evaporation, poor soil, greater human interference, and easy soil loss.”
Compared to surface plants, plants growing within Tianqing had higher levels of nitrogen and phosphorus, both of which plants derive from the soil. The soil at the bottom of Tianqing had higher amounts of these elements than the surface soil, suggesting that plants absorb them more easily. Karst soils are rich in calcium and magnesium, and Tianqing plants showed much higher levels of these elements than surface plants. They also had higher levels of potassium, although potassium is relatively scarce in karst soils.
Tianqing plants absorb nutrients faster than “conservative” surface plants, because nutrients are more abundant in the shaded depths of the earthen pits and because the plants need to grow taller, according to the study.
“The nutritional status of the soil within the Tianqing Forest is good,” the researchers wrote, and plants have evolved to make the most of available resources by growing quickly and harvesting more light.
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