It is suspected that deforestation has contributed to the mysterious collapse of Mayan civilization more than 1,000 years ago. A new study shows that deforestation has also decimated carbon reservoirs in the tropical soil of the Yucatan peninsula long after old cities were abandoned and forests receded.
The findings, published in the journal Nature Geoscience, underline how important soils and our treatment can be in determining future levels of greenhouse gases in the planet's atmosphere.
The Maya started farming about 4,000 years ago, and the spread of agriculture and building cities eventually led to widespread deforestation and soil erosion, has shown previous research. The most surprising thing in the new study is that the soils in the region have not fully recovered as carbon sinks in more than a millennium of reforestation, says McGill University geochemist Peter Douglas, lead author of the new article.
Ecosystem & # 39; fundamentally changed & # 39;
"If you go to this area today, much of it looks like a dense, old-growing rainforest," says Douglas, an assistant professor of Earth and Space Sciences at McGill. "But if you look at carbon storage in the soil, the ecosystem seems fundamentally changed and never returned to its original state."
The soil is one of the largest storage sites of carbon on earth, with at least twice as much carbon as the atmosphere of today. Yet scientists know very little about how carbon reservoirs in the soil change on time scales longer than a decade or so. The new study, along with other recently published research, suggests that these reservoirs can dramatically change over time scales that span centuries or even millennia.
To investigate these long-term effects, Douglas and his co-authors examined sediment cores from the bottom of three lakes in the Mayan lowlands in southern Mexico and Guatemala. The researchers used measurements of radiocarbon, an isotope that decays over time, to determine the age of molecules called plant waxes, which are usually stored long in the soil because they become attached to minerals. They then compared the age of wax molecules with those of plant fossils deposited with the sediments.
The team – including scientists from Yale University, ETH Zurich, the University of Florida and the University of Wisconsin-Superior – discovered that when the ancient Maya began to deforest the landscape, the age difference between the fossils and the wax of the plant of very big to very small. This implies that carbon was stored in the soil for much shorter periods.
The project resulted from research that Douglas had done a few years ago as a PhD student at Yale, using vegetable wax molecules to follow the climate change of the ancient Maya. At the same time, the work of other researchers indicated that these molecules were a good indicator for changes in soil carbon reservoirs. "By bringing these things together, we realized that here was an important data series about old deforestation of changes in carbon reservoirs in the soil," Douglas explains.
Protection of ancient tropical forests
"This provides another reason – which contributes to a long list – to protect the remaining areas of tropical forests with old growth in the world," says Douglas. "It could also have implications for the way we design things such as carbon compensations, which are often accompanied by reforestation, but do not take full account of the long-term storage of carbon." (Carbon compensations allow companies or individuals to offset their greenhouse gas emissions by buying credits from environmental projects, such as planting trees.)
The technique that the researchers used was only recently developed. In the coming years "it would be great to analyze tropical forests in other regions of the world to see if the same patterns are emerging – and to see whether human deforestation and agriculture have affected carbon reservoirs in the soil worldwide ", says Douglas. . "I am also very interested in applying this technique to permafrost regions in Canada to see what has happened to carbon stored in permafrost during earlier periods of climate change."
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