guia do novo coronavirus
Imprimir Republish

ECOLOGY

Circles of time

With chainsaws and drills, Brazilians and Germans collect tree samples to see how humidity and temperature have changed over the centuries

Léo Ramos in Gália, São Paulo State

The pursuit of science also requires muscle and sweat. Donning a pair of green pants over his clothes, thick gloves, a red helmet with a face protector and earmuffs, botanist Gregório Ceccantini once again starts up the chainsaw on the afternoon of September 12th. The smell of the gasoline engine is spread by the dry air of the Caetetus Ecological Station, an Atlantic Forest reserve in the town of Gália, located in central São Paulo State. The blade spreads sawdust as it cuts a dead tree trunk that lies exposed among those few trees that remain standing. After he finishes sawing the trunk, Ceccantini, a professor with the University of São Paulo (USP) Institute of Biosciences, shuts off the chainsaw and places it on the ground covered with dry leaves, takes off his helmet and pulls off the first disk of the Peroba-Rosa wood.

“Look how beautiful it is,” he says, crouching down and placing the disk of the hard reddish wood on one leg to show the concentric rings of varying width, which indicate the annual rate of growth of the trees: the wider the ring, the more the tree grew that year, in response to its supply of water and nutrients. Ceccantini, in collaboration with Dieter Anhuf of southern Germany’s University of Passau, have been examining the rings to see how several tree species of North and Southeast Brazil have responded to humidity and temperature variations through dry and rainy seasons, that is, to climate change in recent centuries. Using trees from the state of Minas Gerais, the USP team was able to detect local variations in climate since 1940 (see graph on page 44), which complement climate reconstruction studies over a longer timescale, on the order of thousands of years, conducted by studying pollen and cave minerals. This approach has shown that in some species of trees, the rise of carbon dioxide (CO2) levels in the atmosphere, such as predicted for the coming decades, may not be enough to accelerate growth, because the temperature increase, also predicted, can promote water loss and curb the growth of plant tissues.

Gregório Ceccantini saws a dead Peroba-Rosa

Léo RamosGregório Ceccantini saws a dead Peroba-RosaLéo Ramos

The innermost rings of the disk of freshly cut Peroba-Rosa are thin. The most likely explanation, says Ceccantini, is that at first the tree was growing slowly in the shade of other trees. Neighboring rings are wider, suggesting that when the Peroba-Rosa reached the canopy, it absorbed more light and grew at an accelerated pace. The rings closest to the bark are also thin, a sign that the rate of growth was declining, or that the tree was decaying before falling or that other trees had reached the top of the forest and competition for light had intensified.

Then Ceccantini places on the cut-off trunk a chunk of wood weighing 20-30 kilos, scrapes the surface of the disk with a knife and counts the rings with a magnifying glass. “This tree is roughly 180-200 years old,” he estimates. Paula Jardim, a biologist, stretching a tape measure along the length of the trunk, concludes that the tree, measuring 23 meters from the base up to the beginning of the canopy, must have died standing up and then fallen down one or two years ago. Ceccantini takes a breath, resets the equipment and starts again to slice the fallen Peroba-Rosa, generating much noise and red sawdust.

The work is more delicate with living trees. A few meters away, hidden by vines that form a web of branches among the trees, the German geographer Stefan Krottenthaler performs a kind of biopsy, boring into a tree trunk with a metal tube attached to a gas powered drill. The drill goes back and forth over an aluminum track that he designed and built, supported by two legs and a belt attached to the trunk. It takes all his effort to push the drill so that the tube penetrates the hardwood. Krottenthaler lived in Brazil for two and a half years, including six months among the Xavante Indians of Mato Grosso, and is a doctoral candidate at Passau University.

Gregório Ceccantini extracts a wooden cylinder from a living fig tree

Léo RamosGregório Ceccantini extracts a wooden cylinder from a living fig treeLéo Ramos

A good while later Krottenthaler removes the drill, releases the tube and, from inside, withdraws a cylinder 20 centimeters in length with successive layers in shades of pink, corresponding to the growth of the tree rings. His colleague Hans Broschek, who is in Brazil for the first time, places the wooden cylinder in a leather case while Krottenthaler makes the equivalent of a bandage for the tree, sprays a fungicide into the hole and plugs it with a piece of cork. They were in another Atlantic Forest reserve, Vassununga State Park in Santa Rita do Passa Quatro, 250 kilometers from the capital, two weeks earlier where they sweated a lot more to get a sample of a Jequitiba-Rosa that was 40 meters high and 3 meters in diameter.

In the following weeks, Ceccantini’s team prepared and examined all this material in something that looks like a USP carpentry shed, alongside a lake with white flowered plants and greenhouses. Once the disks of wood are planed and sanded until smooth and shiny, they are analyzed with a device that determines the distance between the rings, which in turn indicates the annual rate of growth and age of each tree. After a chronology has been done, 5-10 samples of each species from each place will be sent to Potsdam, where the USP team of Dieter Anhuf and Gerd Helle will determine the levels of two stable isotopes (forms) of oxygen, oxygen 16 and 18, and carbon 12 and carbon 18, from the wood cellulose. “To date, studies assessing the growth rings of trees have been mainly carried out in areas of high altitude and northern forests,” Anhuf says. “Research in tropical and subtropical regions is still rare.”

The proportion of each isotope will indicate the effects of rain and temperature on the growth of the trees and even the origin and direction from which the rain came. Leaving aside the technical details, the reasoning is simple: since oxygen 18 is heavier and precipitates more easily than 16, the water containing it in the highest concentration must have come from a nearby region, such as the South or South Atlantic, while water with more oxygen 16 must have come from more distant regions, such as the Amazon.

Pitsch performs a biopsy on a tree

Léo RamosPitsch performs a biopsy on a treeLéo Ramos

“The interior of São Paulo State is a transitional area,” says Krottenthaler, “because it is subject to the influence of air masses from the north and the south. How do you play that game?” He expects that the proportion of each isotope will reveal which air mass over a period of years dominated the other in both the rainy and dry seasons. Then, “through very sophisticated mathematics,” says Ceccantini, researchers hope to convert the information about ring sequences and the proportion of each isotope into graphs showing how climate variation and the movement of moisture in the atmosphere—the so-called flying rivers—influenced the growth of trees of the same species in different locations or different species in the same place over the last few centuries.

It is laborious, slow and difficult work. The good news is that this approach is already working, that is, indicating reduced tree growth in response to lower levels of precipitation and rising temperatures. In 2010, based on the same type of analysis, Roel Brienen, of the University of Leeds in England, and other experts from Mexico and the Netherlands analyzed the rings of a species of tree in southern Mexico, Mimosa acantholoba, and concluded that growth, at least of this species, could decrease by up to 37% in El Niño years, a warming of the waters in the equatorial Pacific and a possible cause of droughts in South America. Furthermore, using mathematical climate simulation models, they estimated a reduction of up to 20% in the growth rate of this species under a scenario of increased carbon dioxide emissions in the atmosphere, as predicted for the coming decades.

In Minas Gerais, variations in the growth rate of Jatoba trees bear witness to the fluctuations in precipitation and temperature since 1940. For two years, Giuliano Locosselli, a member of Ceccantini’s team, examined the trunk rings of two species of Jatoba—from the forest, Hymenaea courbaril, and from the Cerrado, Hymenaea stigonocarpa—which grow in a wooded area in the town of Matosinhos, 80 km from Belo Horizonte. The choice of place happened by chance: Ceccantini was there for an archaeological survey when he learned that the trees were going to be cut down; he revised his plans to take advantage of the moment, managed to save 20 disks of Jatoba and returned happy with the unexpected find: “we loaded up our truck,” he recalled. Until then their oldest sample was 145 years old—from Morro do Diabo, an Atlantic Forest reserve in the far western part of São Paulo State—but, as they would later discover, they had brought back an even older one, 190 years old.

The two species have responded differently to changes in climate over the past five decades. According to analyses of the widths of the rings, caliber of the sap conducting vessels, and the efficiency of water use, described in a study published in 2012 in the journal Trees, rain has a pronounced effect on the growth of rings and sap conducting vessels of the forest Jatobas, while temperature seems to have a greater influence on the growth of the species in the Cerrado. Both used water more efficiently—since they reduced losses through transpiration, but only those trees with the lowest growth rates—suggesting that these species do not respond proportionally to increased CO2 concentration in the atmosphere.

“We see the gradual increase of CO2 in the rings, but these species of Jatoba do not grow more with more CO2, as we expected,” says Locosselli. He believes the increased CO2 concentration in the atmosphere, predicted for the coming decades, should offset the effect of temperature rise: “Trees grow less if the temperature rises, because they tend to lose more water.” Another work in which he was involved indicated that Podocarpus, a genus of conifer that today is restricted to areas of cold and wet weather in southern Brazil, survives in islands of vegetation in Morro do Chapéu in northern Bahia State, growing less than 1 millimeter per year, seven times less than representatives of the same species in the south.

Cylinders of wood are identified and packed before heading to Germany

Léo RamosCylinders of wood are identified and packed before heading to GermanyLéo Ramos

Locosselli wanted to collect Jatoba in October for the second time in Pará State—these samples should aid in tracking changes in the extent of the air masses coming from the Atlantic over the Amazon. Since August, when they started working together, the groups from São Paulo and Germany have collected 120 samples of Jatoba, Jequitibá and other species in São Paulo State, and more than a hundred from trees in the states of Goiás, Bahia, Pará and Rondônia,” including three from Cabreuva trees about 240 years old!” says Ceccantini. “In two years, we want to reach 400 new samples.” The wood collection (xylotheque) for which they are responsible has cataloged 5,000 samples, and 3,000 more are in the process of being registered.

One of the August expeditions, to Carlos Botelho State Park, 200 kilometers from the city of São Paulo, was particularly difficult, because it took place under a steady rain. “It didn’t rain a lot,” recalled Locosselli, “but enough for each of us to fall a few times.” They had to walk for hours on end until they found another tree of the same species to add to the collection, crossing areas that were slippery and steep—the altitude there ranges from 50 to 800 meters—amid a tangle of vines. For the first time in Brazil, the German geographer Phillip Pitsch got caught in the rain, saw bromeliads growing over an immense diversity of trees and with a mixture of awe and fear concluded that it was an authentic jungle. “German forests have only two or three species of trees,” he later recalled, in the early evening at the lodge for the reserve in Gália. For him the weariness of the evening was intense not only because he was not used to walking in tropical forests, but also because, having served two years in the Army and being one of the strongest of the group, he was asked to carry more disks of wood and heavy equipment.

“It was a beautiful place,” recalled Erika Righetto Ifanger about the Carlos Botelho forest, who was also in a forest with a group of researchers for the first time. “Yes, it drizzled every day, but it was good because nobody got bitten by a tick or mosquito.” A scout from ages 6 to 21, and now in the final year of the biology program, Ifanger used a chainsaw in the field for the first time on the morning of September 12, in order to cut into a Peroba-Rosa, like the one Ceccantini cut into the day before. Shortly before, wearing a hat just like Indiana Jones and with various work instruments attached to a belt, Ceccantini had, by hand and with a special probe, perforated the trunk of a fig tree, which has no growth rings, but whose age he plans to reconstruct through the isotopes of neighboring species in the forest.

After polishing, the disk is analyzed by a device that determines the weather conditions under which the tree grew

Léo RamosAfter polishing, the disk is analyzed by a device that determines the weather conditions under which the tree grewLéo Ramos

After cutting the wood, Ifanger loaded the disks into the truck parked on a small road that cuts through the Caetetus forest while Pitsch, wearing a blue headscarf, punctured a tree with one of the drills, Broschek and Jardim helping him because the aluminum track was being used by Krottenthaler. “Here, sweat is guaranteed,” says Ceccantini. “The hardest part is after the third or fourth day, when the body is already hurting and we must keep going.” There is much work at Caetetus. About 20 years ago, from 2,000 to 3,000 trees fell probably because of a very strong storm—and those that did not rot away are still being found throughout the reserve.

There is also a little bit of gratitude. “All these big trees are due to one man, Olavo Amaral Ferraz, who protected this forest on his farm, which was later purchased by the government to create the reserve,” says Ceccantini. Locosselli asks us not to forget the bushmen—Nelson Donizetti Correa and Antônio Crema in Vassununga, Natanael Ozorio da Silva and Pedro Ozorio da Silva in Carlos Botelho, and Sergio Aparecido Esborini in Caetetus—who guided them through the woods, helped them to find and identify trees, chose the paths to follow and warned of the dangers of accidents or poisonous animals that usually they were the first to see. “On one of the days in Carlos Botelho, we did not have 62-year-old Pedro Ozorio da Silva to guide us, a man who walks faster in the woods than any of us, and we spent the entire day looking for just one tree,” says Locosselli.” Without them, we would not have collected even half of what we did.”

Project
Twentieth century changes of tree ring isotopes in southeastern Brazilian forests: how the climate conditions influence growth and water use efficiency and thus enforce tree migration (DFG – FAPESP) (2012/50457-4); Grant Mechanism Regular Line of Research Project Award; Coordinators Gregório Tápias Ceccantini – USP and Dieter Anhuf – University of Passau; Investment R$236,482.86 (FAPESP) and €336,380.00 (DFG)

Scientific articles
BRIENEN, R. J. et al. Climate-growth analysis for a Mexican dry forest tree shows strong impact of sea surface temperatures and predicts future growth declines. Global Change Biology. v. 16, p. 2001-12. 2010.
LOCOSSELLI, G. M. et al. A multi-proxy dendroecological analysis of two tropical species (Hymenaea spp., Leguminosae) growing in a vegetation mosaic. Trees. v. 27, No. 1, p. 25-36. 2012

Republish