Three thousand years ago, dense jungles covered most of Central Africa. But about 2,600 years ago, an event ecologists call the Late Holocene rainforest crisis occurred, and the forests suddenly gave way to savannas dotted with islands of trees. Six hundred years later, the forests grew back almost as quickly as they had disappeared.
But for the past 20 years, paleoecologists have debated what caused the rainforest crisis. Most thought the climate in the region was changing, with either less annual rainfall or a longer dry season with a short but intense monsoon. That climate change, many paleoecologists claim, devastated the rainforests of Central Africa, but created perfect conditions for savannas. But a new study suggests humans may have been the culprits.
A matter of timing
Around the time of the rainforest crisis, farmers from North Africa began to migrate south, bringing with it an advanced culture of pearl millet cultivation, ironworking and palm oil harvesting, all of which take their toll on the landscape. Those northern farmers spoke Bantu languages, which are still spoken by about 300 million people in Africa today.
The currently accepted version among paleoecologists is that warming surface temperatures in the Gulf of Guinea caused a shift in the region’s monsoon cycle, leading to a longer, drier dry season, so people migrated south to farm millet. on the open grasslands. But paleoecologist Yannick Garcin of the University of Potsdam and his colleagues, who just published a new study on the rainforest crisis, argue it happened the other way around: People moved south into the rainforest, clearing land to plant millet. And when their population collapsed 600 years later, the rainforest recovered.
Most of the debate revolves around whether those millet farmers moved south during the rainforest crisis or before, and whether there is evidence of climate change at the same time. And among paleoecologists and archaeologists, the debate is flaring up. If you’re not a paleoecologist, the whole debate may seem a bit esoteric, but it’s a great example of the scientific process at work, with both sides presenting their own evidence and debating what’s most accurate and whose interpretation best fits the data.
Paleoarchaeologist Jean Maley and his colleagues published a paper in October 2017 arguing for the climate hypothesis, citing sediment layers in lakes in Ghana, Gabon and Congo. These show evidence of increased erosion around 2,650 years ago – presumably due to more intense monsoon rains. That lines up well with the other evidence for climate change.
And a previous study sampled pollen from the sediments at the bottom of Lake Victoria, showing that the water level was much lower 2,200 years ago than it is today and that savanna lands had previously been obscured by rainforest canopies.
On the other hand, Garcin and his colleagues recently took a sediment core from Lake Barombi in Cameroon. The 12-meter-long cylinder of mud contained 10,500 years of accumulated sediment layers, which contain microscopic pieces of material called plant wax. Plants secrete waxy mixtures of lipids to protect their outer cells, and these waxes can persist in the soil for thousands of years. Useful for paleoecologists, they record the ratio of hydrogen isotopes the plant has extracted from the water and how the plant uses carbon.
Woody plants such as trees and shrubs get carbon differently from grasses, so their plant crops get different proportions of the stable isotope carbon-13. Before 2600 years ago, the wax of plants in Lake Barombi seemed to come mainly from trees and shrubs, just what you would expect in a thriving rainforest. But within a century, the carbon-13 ratios in the waxes of plants in Lake Barombi began to resemble grassland much more than forest, which matched pollen data from other studies. However, after about 600 years, the carbon-14 signature of forests replaced the grasslands.
Crisis or cause?
Those findings more or less confirmed what the pollen studies had to say about the timing of the Rainforest Crisis, but it said nothing about the cause of the event. But the ratio of hydrogen isotopes in plant waxes can reveal information about climate, because those ratios generally correspond to average annual rainfall on a scale of decades. In the sediment core of Lake Barombi, that evidence pointed to a long, gradual drying out from 7,000 to 2,000 years ago, but there didn’t seem to be a sudden climate change near the onset of the rainforest crisis. According to Lake Barombi data, the area was even wetter during the rainforest crisis than it is today, and today it is still largely covered with rainforest.
According to Garcin and his colleagues, tiny shells preserved in sediment cores from the Gulf of Guinea contain no evidence of a change in sea surface temperature.
But when they examined a database of 460 archaeological sites from across the region, they found that there were very few sites dated earlier than 4,000 years ago. Human activity seems to have started in the region around that time, exploding about 2,600 years ago. Garcin and his colleagues say this is evidence of a major population increase around the start of the rainforest crisis.
They argue that this study is clear evidence that humans, not climate, caused the late Holocene rainforest crisis. But not everyone is convinced. Maley told Ars Technica that the main argument against the blame-the-man hypothesis and in favor of climate change is the geographic scale of the rainforest crisis — it happened at almost the same time from the equator to southern Sahara. Humans, on the other hand, did not migrate south to the rainforests of central Africa with the same kind of synchronization.
In other words, the debate over what happened to the late Holocene rainforests in Central Africa is far from settled.
Science in progress
Paleoecologists, archaeologists, and even linguists are still weighing in on new lines of evidence, and it will likely take some time for them to reach a consensus. Part of the uncertainty is due to the limited resolution of dating methods, be it radiocarbon dating or counting layers in a sediment core.
For example, some of the evidence for the climate hypothesis comes from pollen and diatoms from the bottom of nearby Lake Ossa. But Garcin and his colleagues argue that because of the stale carbon in the sediment itself, there’s about 400 years of uncertainty about dating those layers. And in Garcin’s data, the rainforest crisis appears to have started at Lake Barombi, about 200 years before the Bantu region’s agricultural areas began to crop up.
“We answer that we are at the limit of the precision of the dating method used to resolve such a small delay,” Garcin said, “and since Lake Barombi is on the northern edge of Central Africa, it is it may have witnessed that human impact first compared to the rest of the entire region.”
Scientists all agree on one thing: Understanding the root cause of these kinds of past events is important, because reconstructing past climate events can help predict how people, the climate and fragile ecosystems will interact in the future. interact with each other. And in some ways, the rainforest crisis is an encouraging story, because it means the rainforest can recover from deforestation.
“Rainforest ecosystems are very sensitive to disturbances, but also resilient,” Garcin says.
PNAS2018. DOI: 10.1073/pnas.1715336115 (About DOIs).