MARIE-PIERRE LEDRU/IRDFossil thermometer: pollen carries information on climate and distribution of podocarpuses in the pastMARIE-PIERRE LEDRU/IRD
The three Brazilian species of Podocarpus are among Brazil’s few native conifers – the others are the araucarias. They are better known by landscape gardeners, who generally use them in gardening projects; but not only by gardeners. In the fossilized grains of podocarp pollen, the French biologist Marie-Pierre Ledru sees precious records of climate changes in the Quaternary sub-era, the geological period that began 1.8 million years ago. For the researcher from the Research Institute for Development (IRD, in French), the fossil pollen from the podocarp shows that the Brazil’s northeast, which today is very dry, was humid 15,000 years ago and home to verdant woodlands such as the Atlantic rainforest still found in parts of the Southeast today. It also indicates regions where certain types of vegetation resisted weather fluctuations in the past; if this vegetation is conserved, it just might survive upcoming weather changes.
Marie-Pierre arrived at the Atlantic rainforest via the Amazon. “The conifers of the Podocarpus genus are at the heart of the discussions about the extent, biodiversity and floral composition of the Amazon rainforest during the last glacial period around 20,000 years ago,” she says. When she came to Brazil in 1998, as a visiting professor at the Geosciences Institute of the University of São Paulo (USP), she intended to test the hypothesis that in the planet’s most recent period of intense cold some species of plants from the Andean forests had taken refuge in the Amazon, thereby helping to form its flora. “But during the five years I spent in Brazil I noticed that there were species of Podocarpus to the east of the Amazon that might have ‘entered’ this ecosystem.”
When she analyzed samples of pollen from six locations in the Atlantic rainforest (from Lake Caço, in Maranhão, to Cambará do Sul, in Rio Grande do Sul), Marie-Pierre showed that 15,000 years ago podocarps were already in the Northeast, having spread around there for almost 500 years. She also noticed that these plants were in the Colonia crater, in the State of São Paulo, nearly 130,000 years ago, according to an article published in 2007 in Diversity and Distributions.
Besides reconstituting the Brazilian flora of the time, these discoveries help outline the country’s weather maps at various times. “Pollen grains from Podocarpus are good paleoclimatic markers,” says botanist Gregório Ceccantini, from USP. “They are relatively rare and are not carried over long distances; therefore, both their presence as well as their absence has a precise meaning.” Ceccantini explains that it is common for the pollen from other plants to be carried by the wind and deposited thousands of kilometers away from where the species actually grows. This is not the case with the podocarp. Its fossil pollen is only found where there used to be suitable conditions for it: a permanently humid climate or soil.
Obtaining samples of the pollen, however, is not an easy task. In the area to be analyzed the researchers place in the soil one of the ends of an aluminum tube 10 cm in diameter, suspended from a 5 meter high tripod. Then a motor makes the tube vibrate in order to bury it in the ground. When it is removed, the tube brings up a core of sediment (the ‘witness’) that will be cut into slices 2 – 5 cm thick in which the pollen grains will be looked for. Through carbon-14 dating, these slices enable one to estimate roughly when a certain group of plants lived in that particular site – the deepest sediment layers are the oldest, having been deposited a longer time ago. As it is difficult to obtain ‘witnesses’ more than 7 meters long using this method, researchers’ journey in time is usually limited.
Because it is so difficult to find, the fossilized pollen supplies provide fragmented information giving a broader view of the podocarp’s distribution. It is like looking at points in a forest with a magnifying glass without seeing all the surroundings. Another limitation is that, from the pollen grains, it is impossible to distinguish the three species of podocarp (P. sellowii, P. lambertii and P. brasiliensis). Each has different environmental needs, which help to reconstruct the climate of the past. The first demands a lot of humidity, whereas the second also needs cooler temperatures.
Thus, Marie-Pierre realized that it was necessary to resort to genetics to reveal the history of Brazilian podocarps, so she contacted a married couple of botanists from USP, Maria Luiza and Antonio Salatino. With the help of Fábio Pinheiro, they extracted genetic material (DNA) from plants in 26 locations in Brazil and concluded that, probably before the Andes rose, podocarps were found throughout South America, since the species found today in Chile, Bolivia and Argentina are genetically similar to the ones in Brazil. Afterwards, glaciation cycles alternating with warmer periods fragmented this vast forest of podocarps into smaller populations. In cold and humid periods, distribution expanded. In warmer, drier periods, such as the current times, the podocarps were concentrated in refuges, smaller regions that allowed for the subsistence of organisms with the same ecological needs. Analyses with DNA markers also suggest that Brazil’s current podocarps originated in the Southeast. From there, between 21,000 and 29,000 years ago they spread to the South and between 15,000 and 16,000 years ago to the Northeast.
These results led Maria Luiza and Salatino to question the existence of three Brazilian species. They clearly showed that Podocarpus brasiliensis, restricted to a single narrow belt in Goiás, is not different from P. sellowii. “Molecular markers indicate the existence in South America of two groups of Podocarpus: one formed by P. lambertii and P. parlatorei, the latter found in Argentina and Bolivia; and the other containing P. selowii, P. brasiliensis and P. saligna, from Chile,” says Salatino. He and Maria Luiza intend to extend their studies to include samples from the Amazon and to carry out other molecular analyses. The team hopes that reconstituting the evolutionary history of podocarps may help forecast the effect of climate change on these plants and outline conservation targets that will help other species survive climate fluctuation cycles. “Preserving these refuges is important to bring about an increase in the vegetation ,” concludes Marie-Pierre.
The Project
Distribution of Brazilian conifers and detecting genetic polymorphisms through AFLP (nº 01/07070-7); Modality Regular Line of Research Aid; Coordinator Maria Luiza Faria Salatino – USP; Investment R$ 38,080.35 (FAPESP)
