During the last glaciation, which ended about 10 000 years Before Present (BP), the Brazilian Atlantic forest extended over all the eastern side of the country, covering more than 1 200 000 km², 15% of Brazil's territory. Now only 95 000 km² of this natural habitat survives, just 8% of its initial extent. It is still a large biodiversity reservoir in Brazil, second only to the Amazonian forest. On one hectare of Atlantic forest the biologists recorded over 450 different tree species. But deforestation and intensive farming methods make this tropical forest one of Earth's most seriously threatened ecosystems. In the states of São Paulo and Minas Gerais, regions where agriculture has developed strongly in recent years, the forest is largely fragmented, represented only as small blocks situated on the abrupt slopes which plunge down towards the Atlantic.
With the objective of analysing the changes that have taken place in this ecosystem over the Quaternary era, IRD researchers and counterparts from the University of São Paulo put together the results from three scientific disciplines (botany, palynology, genetics) applied to three species of the tree genus Podocarpus: P. sellowii, P. lambertii and P. brasiliensis. These tropical trees belong to the conifer family. They are good indicators of geographical evolution of the Atlantic forest with time, seeing that the Brazilian species are endemic to this natural habitat. Moreover, pollen grains from the genus Podocarpus have a typical small bladder-like morphology and stay intact for a long time in sediments. These two characteristics make them good candidates for palynological studies.
The team recorded and then collected available plant material from different sites where Atlantic forest stands are still present. This involved 26 sampling points spread over a rectangle 4000 km long by 500 km wide corresponding to the whole of the area of distribution of this ecosystem. They corresponded to 26 different populations of Podocarpus.
This first investigation stage allowed subsequent accurate genetic characterization of each population. In parallel, six sedimentary cores were taken at different latitudes where Atlantic forest still grows so that analysis could be made of the frequency of pollen grains belonging to the Podocarpus genus contained in the various samples collected. The borehole sunk at Colônia (see Map) in the state of São Paulo yielded a core showing that the frequency of these pollen grains fluctuated with time; the phases of expansion and regression of this taxon (3) succeeded one another for periods of varying length. Evidence for a rise in frequency of Podocarpus pollen grains was found for periods of between 60 000 and 45 000 years BP, then between 29 000 and 21 000 years BP in the south of Brazil and between 16 000 and 15 000 years BP in the Nordeste region. These fluctuating rises which occurred during times of glaciation would correspond to phases of expansion of the Atlantic forest in these regions.
In order to test this hypothesis, the Franco-Brazilian team used techniques from molecular biology. For each of the 26 pre-selected populations, the researchers collected leaves of five individual trees of the Podocarpus genus from which they extracted DNA. Nucleotide sequence amplification was performed, then phylogenetic analysis. Comparison was made between that analysis and the level of genetic differentiation between each population of Podocarpus. The scientists thus succeeded in delimiting three large centres of original colonization distributed according to latitude.
The multidisciplinary approach also showed that the expansion of tropical conifer populations never occurred during interglacial periods, in contrast to what usually happened in our temperate latitudes. In the tropics, the populations of Podocarpus that make up the Atlantic forest in fact gained ground in glacial periods owing to an increase in humidity and a cooling of temperatures. At present, in the Nordeste region where a more arid climate prevails, this humid tropical forest occurs in the form of small isolated populations. Nevertheless, it has not always been like that. In that part of the country, the study confirmed the notion that a dense rainforest developed at 15 000 years BP. In the space of about 10 years, its extremely rapid expansion, over an area twice the size of France, was made possible only by the presence of a mosaic of a multitude of patches of forest, now dispersed sparsely over this arid terrain. Predictions for climate changes for the next few decades envisage an increase in the duration and intensity of periods of drought in the intertropical regions, as in Nordeste. If this trend persists, the protection of such surviving areas of Brazil's Atlantic forest, these refugia, will become essential for the conservation of this ecosystem
Gregory Flechet - IRD