Plants may stabilize slopes, yet rainfall often intensifies soil erosion. Until now, just how these two things interact to form mountain topography was only clear for a few small regions on Earth. In a new study, Professor Todd Ehlers, Dr. Jessica Starke and Dr. Mirjam Schaller of the Geosciences department at the University of Tübingen, Germany, investigated how plants and climate shape topography. They did this in a large study of the 3,500 kilometer long western edge of the Andes Mountains in Peru and Chile. They found that the question of how plants influence the landscape and erosion can have different answers, depending on what area is investigated. Key factors identified are the climate zone and plant cover. In the dry Atacama Desert, for example, sparse vegetation is sufficient to hold the soil in place; while higher erosion rates can be seen in the wetter and more temperate regions where plant cover is denser. The study has been published in the latest edition of the journal Science.
The area of the Andes investigated extends almost the entire length of South America, from 6 to 36 degrees south latitude. This region covers six climate zones, from very dry to temperate. “Along the western edge of the Andes, many individual studies have determined the rates of soil erosion over the past million years,” says Todd Ehlers. “Yet the results were inconsistent and could not be easily explained.” To evaluate the rate of erosion, the researchers used what’s known as cosmogenic nuclides, which are produced on the Earth’s surface by cosmic rays from space. The nuclides accumulate only when the ground is exposed. Using the concentration of the isotopes from sediments in 86 rivers, the researchers could calculate how fast the mountains eroded. “We supplemented earlier results from 74 study sites in Peru and Chile with 12 new measurements to fill the gaps," says Ehlers. This enabled him and his team to study changes in mountain erosion in different vegetation and climate zones along the Andes. The erosion rates varied between 1.4 meters to 150 meters per million years.
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