Paleoclimate and human activity shaped the Araucaria forest in Brazil

The study used advanced DNA sequencing techniques, machine learning and analysis of fossil pollen records to investigate the history of this forest formation.

By José Tadeu Arantes  |  Agência FAPESP – The Araucaria forest in southern and southeastern Brazil has a complex history, influenced by climatic and human factors. Araucaria angustifolia, the dominant tree in this forest formation, provides food and shelter for various species, including humans. A study conducted by an international collaboration of researchers used advanced DNA sequencing techniques, machine learning and analysis of fossil pollen records to investigate the past expansions and contractions of the Araucaria forest and to identify the impact of past climate and the activity of ancestral human groups on this process. The results were published in the journal Ecography.

“Both climatic and human factors have played an important role in the dynamics of the Araucaria forest. However there are significant differences between the process that took place on the Southern Plateau of the Brazilian territory and the one that took place in the Mantiqueira Mountains, which stretches across the states of São Paulo, Rio de Janeiro and Minas Gerais. And this could be related to the more intense human occupation in southern Brazil,” says Mariana Mira Vasconcellos, postdoctoral student at the Institute of Biosciences of the University of São Paulo (IB-USP) and first author of the study.

The researcher explains: “The expansion of the Araucaria population in the Southern Core began during the last Glacial Period, around 70,000 years ago, long before the presumed date of human arrival in South America, which contradicts the idea of an anthropogenic expansion. Instead, we interpret this initial expansion as a response to the climatic changes of the Upper Pleistocene. The wetter and colder climate of this period would also have caused the expansion of other Araucaria species in Australia and New Caledonia. The isolated Mantiqueira population, on the other hand, only expanded very recently, around 3,000 years ago. Although this may have occurred in response to a brief climate cooling event in the Holocene and increased summer monsoon rainfall in South America between 4,000 and 3,300 years ago, a possible impact of human settlements in southeastern Brazil cannot be ruled out.”

However, Vasconcellos points out that this should not be taken as a proven fact, since archaeological evidence of human settlements in the Mantiqueira is scarce compared to that of the Southern Plateau of Brazil.

In the case of the south, genetic analyses indicate that while the initial spread of the Araucarias was not the result of human activity, their subsequent evolution was significantly influenced by anthropogenic activity. This influence resulted in lower genetic diversity, greater endogamy and increased long-distance gene flow, suggesting that humans facilitated seed dispersal and germination through the consumption of pine nuts.

“If you look at two trees, two individuals of the same species, the further apart they are, the greater the genetic differentiation between them. That’s what you’d expect from a natural process. But that isn’t what we observed in the Araucaria population in southern Brazil. Trees separated by long distances are genetically very similar. This suggests that someone traveled these distances, taking the seeds from one place to another. It could be animals that feed on pine nuts. But also, and more likely, it could be humans,” argues the researcher.

He continues: “There are now several studies that show evidence of human management in the Amazon rainforest. We don’t have that in southern Brazil. There are no signs of management on the ground. But the Araucaria is a plant that doesn’t need management. The Araucaria germinates very easily. The pine nut falls to the ground and all it takes is a little rain for it to germinate. The simple fact that humans eat pine nuts and carry the seeds on their expeditions would’ve been enough to spread the species. The storage and transportation of seeds are still common practices among the indigenous peoples of the region.”

There is also other evidence. In southern Brazil and Uruguay, there are a large number of archaeological sites called “underground houses.” It is believed that these structures, built by populations of the Proto-Jê language family, served as shelters for their inhabitants, like the houses in today’s indigenous villages. The study found that the areas where the underground houses are located coincide with the current distribution of the Araucaria forest, suggesting that humans may have chosen these locations because of the abundance of resources provided by the plant species. “The Araucaria has great cultural importance in southern Brazil. Archaeological studies, including cave paintings and analysis of food remains and tartar found in human skull dentitions, indicate that this tree was an important food source and may have been dispersed by indigenous peoples,” Vasconcellos points out.

The Araucaria is a tree native to South America. But there are only two species on the continent: Araucaria angustifolia, in southern Brazil, and Araucaria araucana, in Chile. Other species are found in Australia, New Zealand, New Caledonia, etc. In Brazil, the two populations of Araucaria angustifolia, the one in the south and the one in the Mantiqueira, diverged around the time of the penultimate Glacial Maximum, around 149,000 years ago, when part of the globe was covered by an extensive layer of ice, with a strong impact on the area that corresponds to today’s southeastern region of Brazil. After that, around 70,000 years ago, the population of the south began to expand, and around 16,000 years ago, the Araucaria forest reached its greatest extent, driven by the so-called Heinrich 1 Event, which caused icebergs to break off in the North Atlantic, resulting in peaks of humidity during cold periods in Brazil.

“The data collected in the study suggest that the remaining populations of Araucaria, especially in the Mantiqueira Mountains, which responded well to the long period of climatic stability in the Holocene, are now facing serious threats due to their reduced population size, forest fragmentation caused by recent anthropogenic activities, and the global climate crisis, which is causing higher temperatures in southeastern Brazil. They therefore require effective conservation policies,” concludes Vasconcellos.

The study was coordinated by Ana Carolina Queiroz Carnaval of the City University of New York (CUNY) and Fabián Michelangeli of the New York Botanical Garden (NYBG), both institutions based in New York, USA. It was supported by FAPESP through the Thematic Project “Dimensions US-BIOTA São Paulo: integrating disciplines to predict the biodiversity of the Atlantic Forest in Brazil”.

The article “Evaluating the impact of historical climate and early human groups in the Araucaria Forest of eastern South America” can be accessed at: nsojournals.onlinelibrary.wiley.com/doi/10.1111/ecog.06756.