Porto Velho, the capital of the Brazilian state of Rondônia, is very hot and sultry today, but 30,000 years ago the region was probably cold, like present-day Porto Alegre, capital of the state of Rio Grande do Sul, which lies 3,500 kilometers (km) to the south. The average annual temperature is thought to have been 18º Celsius (C) at most–six degrees below the current average. There was no ice—although ice did cover extensive areas of the Earth’s northern and southern latitudes—but winter temperatures probably dipped as low as 10ºC, cold enough to cause the present-day inhabitants of the southwestern Brazilian Amazon Region to feel quite chilled. By analyzing pollen and isotopes (variations) of carbon and nitrogen in sediments taken from depths of up to 20 meters, researchers from the states of Pará and São Paulo have concluded that the vegetation also must have been different. In addition to tree species still found in the region today, the forest was home to other species typical of a cold climate, which disappeared as the climate became warmer.
Alnus, a genus of trees that are now extinct, offers a clear indication of the changes in climate and vegetation in the area between northern Rondônia and southern Amazonas State. “Alnus grows only in cold climates,” says Marcelo Cohen, a professor at the Federal University of Pará. In this study, he identified grains of pollen from 65 groups of trees and herbaceous plants drawn from sediment samples, and believes he has found the first record of Alnus trees in the Brazilian Amazon. In South America, trees of this genus are currently found at altitudes above 2,000 meters in the Andes range, at least a thousand km from Porto Velho.
Because they are light and miniscule, with a diameter ranging from 10 to 40 micrometers (one micrometer equals one thousandth of a millimeter), pollen grains can be transported easily by wind, rain or rivers. “In the region studied,” Cohen says, “Alnus pollen accounted for 11% of the total found—much higher than expected for dispersion by rivers or wind.” This, he says, was an indication that the Alnus populations, which probably came from the Andes, are likely to have encountered favorable conditions for growth in the lowlands of western Amazonia between 40,000 and 20,000 years ago, and later became extinct as the climate warmed. Cohen also identified pollen from other genera of cold-climate trees, such as Hedyosmum, Weinmannia, Podocarpus, Ilex and Drimys, already identified in other parts of Amazonia. Podocarpus, for example, is a tree genus of the conifer group that includes araucária (Brazilian pine), which still grows in southern and southeastern Brazil.
On the basis of this work, it becomes possible to imagine a continuous forest connecting the Andes and Amazonia during that period, with the most common cold-climate tree species to the west and the warm-climate species to the east. “There was a mixture of tree species that formed a very unusual glacial forest, unlike anything that exists today,” Cohen says. As the climate warmed, plants that grow only in lower temperatures disappeared, enabling those more adapted to warm climate or resistant to extreme climate variations to spread. The researchers also found sections of abandoned rivers that had formed lakes, which were later filled in by sediments and covered over by herbaceous vegetation, creating savannahs.
The identification of many tree species and of a cold climate is also an indication that the climate between 40,000 and 30,000 years ago was cold and moist, rather than cold and dry, as other experts had indicated, according to Luiz Carlos Pessenda, a researcher at the Center for Nuclear Energy in Agriculture (CENA) at the University of São Paulo (USP). In 2001 he obtained the first indications that the climate in southwestern Amazonia was moist and probably featured regular rainfall patterns. Pessenda and his team collected soil samples along a 250-km line between Humaitá in southern Amazonas State and Porto Velho, and confirmed the predominance of moisture-adapted plants. Recent studies of cave rocks conducted by other groups from the states of São Paulo and Minas Gerais reinforce the hypothesis that the climate of that region was moist, and hence more rainy than initially thought, particularly between 30,000 and 20,000 years ago, when scientists believe the sea level was 100 meters below its current level, the coastline lay 100 meters from its present location, and South America and Antarctica were connected by an isthmus of ice. In addition, icecaps as much as 3.5 km thick covered much of North America, Europe and Oceania.
In Pessenda’s view, these findings reinforce his hypothesis that it was the Amazon forest that supplied the moisture for another hybrid forest in the Serra do Mar mountain range in São Paulo State, some 3,000 km away. He has examined the vegetation of that forest in other studies. The Serra do Mar of 30,000 years ago was covered by tree species typical of two different ecosystems, the Atlantic Forest and the Araucária Forest. There, as in Amazonia, only those trees resistant to higher temperatures subsequently survived, and later disappeared as well, giving way to the present-day grasslands (see Pesquisa FAPESP Issue No. 160).
In the past 15 years, Pessenda has examined pollen and the ratio between the forms (isotopes) of carbon and nitrogen in sediments throughout Brazil. He has also created a collection of about 4,500 samples of pollen grains that serve as a foundation for studies such as the one by Cohen, who did post-doctoral research in Pessenda’s laboratory in 2011. His studies reveal a continuous transformation of the forests and contraction of the grasslands—which had once been more extensive throughout Brazil—since approximately 4,000 years ago. According to Pessenda, most of the areas still occupied today by grasslands in São Paulo and Rondônia, for example, are disappearing, even without the expansion of cities and agriculture, and are on the way to becoming naturally occupied by forests in a few dozen centuries, in response to climate change.
COHEN, M.C.L. et al. Late Pleistocene glacial forest of Humaitá-Western Amazonia.Palaeogeography, Palaeoclimatology, Palaeoecology. dez. 2013.