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botany

The woods are on the move

Forests and fields have advanced and receded in the country over the last 30 thousand years

LUIS CARLOS PESSENDA/USPGrains of history: pollen reveals what the Earth’s climate and vegetation used to be likeLUIS CARLOS PESSENDA/USP

One can still walk easily across the fields at the top of the Serra do Mar mountain range and see the Atlantic on one side and São Paulo City on the other. In 500 years time, however, walking around there will demand more sweat and more attention, because  thick forest should slowly take over the carpet of grass. This is not the first time, or the first place, in which one type of vegetation has replaced another. By examining soil samples ranging from a reddish brown to pale gray, physicist Luiz Pessenda and his team from the University of São Paulo (USP) have detected such transformation all over the country. Little by little, they discovered how different forms of native vegetation have advanced, receded, disappeared or reappeared, largely in response to natural climate change over the last 30 thousand years.

Dense, impenetrable forests are expected to take over the fields that are some 50 km south of the center of São Paulo City even if the climate continues changing because of the pollution produced by humans. “The trees should benefit from the increased carbon dioxide in the atmosphere, the higher mean annual temperatures, and the humidity brought by the rains that are likely to increase in the Southeast,” states botanist Paulo de Oliveira, a researcher from the University of Guarulhos (UnG), and coordinator of the analyses of the pollen collected for this study. According to him, three types of trees that São Paulo inhabitants can see on their way to the coast – the embauba trumpet tree, whose leafs are in the shape of an open hand; the manaca-da-serra vegetable mercury tree, with its white, lilac and purple flowers; and the acacia tree, which produces yellow flowers at this time of the year – attest to  this progressive colonization. These species grow and multiply fast in free areas, generating the shade that eliminates the preceding vegetation while slower-growing, longer-living species appear, such as the canela, peroba and jequitibá trees.

“Wherever we see today a forest bordering on fields or savannas, a lot of change has already occurred,” says Pessenda. “One or the other was larger in the past.” Besides being smaller, the forests at the top of the mountain range in São Paulo were different from the current ones, according to a study coordinated by Pessenda and published in May in Quaternary Research. Analyses of carbon isotopes (variations) in the soil and  grains of pollen and spores collected from black soil fens indicate that 30 thousand years ago, forests of araucauria pines coexisted with the high altitude fields in a stretch of the Mata Atlantica forest in the extreme south of São Paulo City.

With an area of 10 square kilometers today, these fields used to be more extensive. Analysis of the soil and grains of pollen confirmed, furthermore, that these are natural rather than the leftovers of human activity. Vestiges of charcoal production also suggested that this low vegetation might be the outcome of the production of charcoal from the forest trees, to supply São Paulo and the railroads in the early twentieth century. “The extraction of wood must have expanded the area of fields that already existed,” states botanist Ricardo Garcia, co-author of this study and a researcher at the São Paulo Municipal Herbarium. “We mustn’t disregard the influence of a nutrient-poor soil; however, the fields are not left over from a drier past, as we first thought.”

Another conclusion is that the São Paulo State coast must have been the home to forests during the height of the glacial era, when the ice expanded beyond the poles and influenced the climate of the entire Earth – Brazil’s temperature must have been 5 to 10 degrees lower than at present. Previously regarded as dry and unsuitable for plants, this era now looks as if it was moist and vegetation-friendly. This humidity was unexpected, but USP geologist Francisco Cruz reached the same conclusion, upon examining the ratio of oxygen isotopes from the water conserved within the rocks of a cave in the state of Santa Catarina and another in the state of São Paulo. “Two studies reaching the same conclusion through different techniques is not a coincidence. During the glacial period, which lasted from 14 thousand to 90 thousand years ago, there were already forests in Brazil, even in the south of the state of Amazonas,” says Pessenda. Cruz adds: “There was not generalized drought across the country, as we previously thought, but a strong climatic contrast, with drier areas and more humid areas.”

The landscape at the top of the Serra do Mar mountain range changed slowly, in line with the climate change. From 20 to 30 thousand years ago, the araucaria forests coexisted with the fields. In the 2 thousand years after that, they started advancing, thanks to the lower temperatures and the high humidity. However, 18 thousand years ago, the temperature started rising and the araucaria woods gave way to trees, bushes and creepers that have adapted to the hotter and more humid climate. The almost 1,100 samples of soil analyzed over 16 years by Pessenda’s group reflect a picture of change all over the country. “From 4 thousand to 9 thousand years ago, fields and savannas expanded, thanks to a drier climate, throughout the South and the Southeast, all the way to the North and the Northeast,” states the physicist from USP. “From 4 thousand years ago to date, the climate became more humid, as it is now, and the forests have expanded.”

This push-pull effect between different types of vegetation is also part of the history of the Amazon region. In Humaitá, in the south of the state of Amazonas 9 thousand years ago, natural fields probably covered a larger area than the current one, while 5 thousand years ago they expanded to the point of having twice the area they have today. This expansion, however, did not take place in the entire Amazon region. In the town of Altamira, in the Central Amazon Region, and in the town of Porto Velho, in the South of the Amazon Region, the forest resisted.

This low vegetation started shrinking 4 thousand years ago and continues to lose out to thick woodlands. ‘The fields tend to disappear naturally over some tens of centuries, in response to the current climate,” says Pessenda. This shrinkage, he adds, has been sped up in the last few years by the advance of soybean plantations. “Luckily, we arrived two to three years before the sowing started and we have recovered the isotopic traces of the original vegetation in the region’s soils.”

The project
Vegetation and climate reconstruction since the mid-Holocene in Brazil (nº 07/03615-5); Type Regular Line for Research Project; Coordinator Luiz Carlos Ruiz Pessenda – Cena-USP; Investment R$ 358,356.65 (FAPESP).

Scientific article
PESSENDA, L. C. R. et al. The evolution of a tropical rainforest/grassland mosaic in southeastern Brazil since 28,000 14C yr BP based on carbon isotopes and pollen records. Quaternary Research. v. 71, p. 437-452. 2009.

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