They stabilize slopes, but also break up the rock with their roots – plants form mountains in a surprisingly complex way, researchers report. In the South American Andes, they gained insights into the interesting interactions between vegetation and climate on mountain formation. Depending on the region, the influence of plants on the erosion processes can even be opposite.
Some areas of the earth’s crust are pushed up and unfolded by geological processes – mountain ranges such as the Alps, the Himalayas or the Andes are formed. How the structures of the mountains subsequently change depends on which erosion factors they are exposed to. In some regions, they are rubbed off by glaciers, in others, heavy rains gradually wash away their material. But life also influences the design of the mountains: It seems clear that plant growth can also have a significant impact on erosion processes. Little is known about the details of the interplay between vegetation, erosion and other environmental factors. A team of scientists from the University of Tübingen has now dedicated itself to this research topic.
They chose the Andes as the region of study: the study area stretches almost the entire length of South America from the 6th to the 36th southern latitude. This region has six climates – from very dry to temperate. The researchers recorded climate information, geological data and information about the vegetation there. They recorded the erosion rate by detecting so-called cosmogenic nuclides. Since these atomic variants are created by radiation from space on the earth’s surface, these nuclides only accumulate in exposed soil. This enables conclusions to be drawn about erosion processes.
On the trail of the vegetation factor
Based on measurements of the nuclide concentration in 86 river sediments, the scientists were able to calculate how quickly the respective rock structures were removed. As they report, the erosion rates were between 1.4 and 150 meters in a million years. On this basis, they were also able to estimate the erosion depending on the different vegetation and different climate zones in the examined areas. Their results show how differently vegetation can influence erosion and thus landscape development depending on the region: For example, while in the dry Atacama region the sparse vegetation tends to hold the ground, in contrast, in temperate, humid regions, there is a higher erosion rate with a denser plant cover linked.
The researchers explain this as follows: Plants hold the ground on slopes with their roots and thereby brake running water on the surface, making mountain sides more stable. On the other hand, plants can also increase erosion: with their roots, they break up solid rock and convert it into loose soil, which can then be removed more easily. Which aspect is more important depends on various factors, which can again have an ambiguous effect, the scientists explain. Rain increases the growth of plants, but is also a crucial factor in erosion.
Complex interaction
“You could assume that the denser the plant cover, the less material there is. This simple connection is actually true for some Andean regions, ”says co-author Todd Ehlers. “But other factors such as precipitation rates also play an important role. It is exciting to see how the erosion of mountains reflects the interaction between plants and precipitation, ”says the scientist. For example, there is only a dense plant cover in the climatically temperate Andes because there is also a lot of rain. Despite the growth, the precipitation patterns there intensify soil erosion overall, the researchers found. However, as plant growth continues to increase in other regions, plants reduce erosion, so slopes can stabilize and become steeper.
“Our investigation along this wide climate gradient in the Andes now helps to bring together the observations from many other studies,” says Ehlers. “Most of these studies were carried out in geographical regions with narrow ecological or climatic conditions. Only an overview now gives a broader impression of how plants and the climate interact with the topography, ”said the scientist.
Source: University of Tübingen, Technical article: Science, doi: 10.1126 / science.aaz0840