Fire created the African savannas

Drill core obliterates typical grasslands around eight million years ago

Savanna landscape © P. Dupont
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Savannahs today shape large parts of Africa, but also of South America and Asia. The triumph of these grasslands began about eight million years ago. What triggered him was unknown for a long time. Now German researchers have found an answer: A combination of a dry climate and recurring conflagrations favored the emergence of savannahs. It was not until she let this typical landscape emerge, according to the researchers in the journal Nature Geoscience.

At first glance, our earth appears as a blue planet. After all, it is more than two-thirds covered by oceans. On the continents, however, the color prevails in many places in brown. Up to 40 percent of the global land surface is covered by savanna. In the vastness of these tropical-subtropical landscapes, plants prevail that are called C4 plants because of their special way of capturing carbon dioxide through photosynthesis.

It was known that this type of landscape had already developed in the period between eight and three million years before today. But why he was so influential was controversial. To find out, Sebastian Hötzel and his colleagues from the MARUM Center for Marine Environmental Sciences at the University of Bremen have studied a well-drilled 452-meter-long sediment core, which was drilled 160 kilometers off Namibia in 1997 in almost 800 meters of water on the whale's back,

Triumph of the grasses

Samples from the seafloor revealed plant pores and pollen that had been blown into the South Atlantic Ocean with the southeastern Kalahari winds prevailing in the region. "We looked in particular at the pollen of grasses and typical steppe and desert plants to trace the landscape development in southwestern Africa, " says Hötzel.

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After determining and counting the remains of the plants and conducting geochemical investigations, the conclusion was conclusive: "According to our analysis, the Kalahari, as we know it today, evolved in time 8.4 and 6.8 million years ago, "explains the researcher. During this time, the proportion of C4 plants, in particular of C4 grasses, which copes well with dry-hot climatic conditions, gradually increased. Six million years ago, they became the dominant plant type. display

Verr terische charcoal particles

The trick: especially in the 7.1 to 5.8 million years old sediments, the researchers found relatively many microscopic particles of charcoal: At that time, there must always have been conflagrations. "In the relatively humid summers, sufficient plant material grew, " says H tzel. If the vegetation dried up at the end of the winter, a few lightning strokes were needed to ignite blanket-sized trees.

As Gr ser and especially C4 growers recover much faster than trees, they gradually gained the upper hand: Computer-supported models in which the effect of flow It has been shown that the coverage of the Kalahari with acacia and other tree species would increase by up to 80 percent

Self-reinforcing cycle

The cycle of drier climatic conditions and increasing surface pressures increases each other: drought promotes grassland; this evaporates less water than more wooded landscapes; the water cycle is slowing, drying times are increasing. This increases the risk of vitrages, with which, in turn, growers are better off than trees. Result: the savannah is spreading and the C4 greats are gaining the upper hand there.

The factors that contributed to the development of large-scale savannahs in southwest Africa apparently also influenced landscape development in other parts of Africa: "We now know from similar investigations that similar mechanisms exist are also occupied in today's savannas of East and West Africa, "says H tzel. (Nature Geoscience, 2013; doi: 10.1038 / ngeo1984)

(MARUM - Center for Marine Environmental Sciences at the University of Bremen, 21.10.2013 - NPO)