The landscape that Paulo Brando encountered last October in the Tapajós National Forest in Belterra, a township in the west of Pará, is very different from the one that delighted him on his first trip to the region six years ago. The tallest and most imposing trees had far fewer leaves than normal and their tops were no longer intertwined as before. Several were dry and dead and between the gaps in the tree tops the sky could be seen. The sun’s rays, which are almost always inaccessible to anyone walking in the forest, were reaching the layer of leaves on the ground, leaving it drier and more susceptible to catching fire. Fortunately the transformation seen by the forestry engineer from São Paulo is restricted – at least for the time being – to a small area of the Amazon which over the last ten years has served as a natural laboratory for Brazilian and North American researchers who are interested in discovering what might happen with the world’s largest tropical forest if, as forecast, the temperature of the planet keeps on rising and there is less rainfall in the region.
At the end of the 1990’s, in the middle of this environmental reserve on the banks of the Tapajós River, 67 km south of Santarém, Daniel Nepstad, an ecologist from the Woods Hole Research Center in the United States and founder of the Environmental Research Institute of the Amazon (Ipam), created an elaborate open-air experiment. He selected a hectare of native vegetation, corresponding to a block the sides of which are a 100 meters long, in which he simulated intense drought conditions similar to those caused form time to time in the east of the Amazon by El Niño, the abnormal warming of the surface waters of the Pacific Ocean.
Over five rainy seasons some 30 researchers and helpers from Nepstad’s team installed 5,660 plastic panels just above ground level. Each panel is 3 meters long and 0.5 meters wide and they were all collected at the end of each rainy period. Like a type of umbrella over the forest the panels diverted the falling rain into a system of gutters that carried it far away from there. The effects of this complex and costly experiment (gases emitted into the atmosphere were measured, as were soil humidity, plant growth and other factors) have begun to become clearer recently, with the publication of scientific articles detailing the damage caused by five years of severe experimental drought that reduced the volume of water reaching the soil by between 35% and 40% (the average rainfall in the Santarém region is 2000 millimeters a year and is concentrated between December and June).
Making the forest floor impermeable to rain may even appear to be an extravagant idea, but there was no lack of reasons for continuing with the project. Climate models developed by the National Space Research Institute (Inpe) estimate that some regions in the Amazon may be 8ºC hotter over the next few decades if the consumption of oil-derived fuels and the cutting down and burning of the world’s forests continue at their current rate. This will raise the atmospheric concentration of carbon dioxide, the main agent associated with heating and transform the world’s climate. A probable consequence of this increase in temperature is the change in rainfall patterns on the planet.
“As yet there’s no consensus as to what might happen with rainfall in the Amazon”, explains Carlos Nobre, a climatologist from Inpe and a member of the Intergovernmental Panel on Climate Change (IPCC), the UN body that is analyzing the evidence of change in the Earth’s climate. “Of the 23 climate models that formed the basis of the 2007 report from the IPCC, most show the trend of a 10% to 30% reduction in rainfall in the Amazon, but the remainder indicate the possibility that it will remain at its present levels or even increase”, says Nobre, coordinator of the FAPESP Research Program on Global Climate Change.
Beyond the forest
Despite the uncertainty, the decrease in rainfall on the forest, the result of phenomena like a more frequent and intense El Niño or the heating of the North Atlantic as a result of global warming, is worrying. With less rain, there is a great risk that the dense and exuberant forest that spreads across almost 7 million square kilometers in South America, will be transformed, especially in the south and east, into stunted, sparse and dry vegetation, the appearance of which is reminiscent of the savannahs. The losses to the structure and physiognomy of the forest by this transformation (it would move from being humid to dry) are unlikely to be limited to the Amazon; the water the Amazonian vegetation takes from the soil and launches into the atmosphere controls the climate and rainfall of a good part of Brazil and South America (Pesquisa FAPESP nº 114).
“Small changes in the forest may affect the hydric and thermal balance of other regions”, says agronomist, Eneas Salati. A former professor at the University of São Paulo (USP) and ex-director of the National Amazon Research Institute (Inpa), Salati has been studying the natural recycling of water and rain formation in the Amazon for nearly 40 years. In an experiment conducted 20 years ago in the catchment area of a tributary of the Negro River in the State of Amazonas, some 800 km west of Santarém, he discovered that forest plants returned to the atmosphere half of the rain water in the form of vapor eliminated by transpiration. This effect was proved by subsequent studies. Although there are internal variations between one region and another in the Amazon, these figures are unlikely to vary much. Because of this it is calculated that a little less than half the water that falls on the forest in the form of rain returns as vapor into the atmosphere. “Part of this vapor rises to the high troposphere and travels as far as Antarctica, where it produces ice deposits”, says Salati, the current technical director at the Brazilian Foundation for Sustainable Development. In this long journey the vapor eliminated by the trees of the Amazon contributes to the intense rainfall in the southeast and south of the country, which is responsible for an important part of Brazil’s agricultural and livestock production.
Faced with the risk of a drier future Nepstad decided to check experimentally to what extent the forest can resist a reduction in rainfall and how it may be transformed if this situation lasts for a long time. In partnership with biologist Eric Davidson, from Woods Hole, and ecologist Paulo Moutinho, from Ipam, Nepstad devised the dry forest experiment in Tapajós, where the structure and physiognomy of the vegetation are similar to almost one third of the Amazon Forest. The project, which involved researchers from 14 institutions, formed part of the Large Scale Biosphere-Atmosphere Experiment in the Amazon (LBA) and was funded by the Brazilian and North American governments. In addition to the transparent plastic panels above the ground (the panels were tipped over a few times a week for the dead leaves and branches to fall to the ground) the researchers erected 4 towers, 30 meters high, linked by wooden walkways from where it was possible to observe the tops of the trees better. They also dug 5 wells 11 meters deep to measure changes in water reserves in the sub-soil. On another hectare in the same forest they built similar devices, but kept the area uncovered to allow for comparisons; the so-called control area. “We had no intention of predicting what the future of the forest will be, because to do so we’d have to repeat the experiment in different regions since the vegetation of the Amazon is not homogenous”, says Davidson, project director for a segment of the LBA. “We’d just like to discover the possible effects of drought on the forest structure.”
Right at the beginning there were surprises. The forest in the Tapajós area resisted the first two years of artificial drought well, which was something to be expected in a region that frequently suffers from a shortage of rain caused by El Niño. Tree mortality in the area covered by panels remained similar to the area which continued to receive rain. The tree tops, however, shrank by almost 20%. This was apparently not because more leaves died but because new ones simply did not sprout, wrote Nepstad in 2002 in the Journal of Geophysical Research. The opening in the forest canopy allowed more light to enter, thus drying out the layer of leaves and branches that fell to the ground (litterfall), and increasing the fire risk. Nepstad calculated that the area deprived of rain became vulnerable to fire for up to ten weeks, compared with ten days in more humid locations.
That was not all. “Right from the very first year the trees practically stopped growing”, says Paulo Brando, from Ipam, one of the team members. There was a drop of 20% in the growth rate of medium size trees, with trunks of at least 10 centimeters in diameter and up to 15 meters tall, while others, like the louro-amarelo (Licaria brasiliensis) and the tachi-vermelho (Sclerobium chrysopillum), reduced their photosynthesis rates, the process by which they convert solar energy into sugar and remove carbon dioxide from the atmosphere. Working on a similar experiment put together in 2002 in the National Forest of Caxiuanã, some 1300 km east of Santarém, Rosie Fisher and Patrik Meir, from Edinburgh University, Scotland, saw that the probable reason for the drop in transpiration and photosynthesis rates in the forest is the greater difficulty the roots have of absorbing water from the soil.
Brando particularly analyzed the case of the most common species in the region – the caferana (Coussarea racemosa), a 20 meter tall tree, with fine bark and a grayish trunk that lives in the shade of taller trees in the forest under-growth. With restricted rainfall the caferana started producing leaves, flowers and fruit later than normal, perhaps as a strategy for economizing water. Its fruit became lighter and had almost no seeds after the fourth year of drought, which might compromise reproduction of the species. “This is an effect of drought we rarely manage to observe”, says Brando.
According to the researchers the only reason the damage was no greater was because the trees in the Amazon have at least two important strategies for obtaining water during prolonged dry periods. The first strategy is to have deep roots that are capable of searching for water at a depth of 11 meters. The second is water redistribution, a mechanism for extracting water from more humid areas and depositing it in dehydrated areas. This was identified among trees in the National Forest of Tapajós by biologists Rafael Oliveira, from the State University of Campinas, and Todd Dawson, from the University of California in Berkeley, USA (Pesquisa FAPESP nº 151).
When soil humidity is very low, at night the roots of trees like the breu (Protium robustum) and maçaranduba (Manilkara huberi) absorb water that is stored in the deepest layers and distribute it by means of a network of surface roots to the drier forest floor. Water redistribution, which was discovered by Martyn Caldwell and James Richards at the end of the 1980’s in plants in desert regions, allows these trees and neighboring plants that have shorter roots to survive. In the rainy period this flow inverts: at night, surface roots remove water from the sodden ground and take it to the deep roots that store it meters below the surface. By incorporating the data observed in the Tapajós area to a climate model Jung-Eun Lee, Inez Fung, Oliveira and Dawson saw that water redistribution helps explain how the forest maintains its normal levels of photosynthesis and transpiration, which are essential for the planet’s climate balance, for some time during prolonged drought periods. “If most of the forest trees really do use this mechanism, deforestation of the Amazon may have more serious consequence than we had imagined”, says Oliveira, one of the authors of the article that reported these results in 2005 in the Proceedings of the National Academy of Sciences (PNAS).
Strategies for looking for water, however, were not enough to prevent the damage that emerged from the third year of the experiment. A reduction of a little more than one third of the rainfall for five years reduced deep water reserves that are situated between 2 and 11 meters below the surface by almost 90% in the area covered by the plastic panels. In the control area 70% of the water stored in the subsoil was still available during the dry season. “The minimum rainfall limit for vegetation to remain in this region is 1700 millimeters. Below this level the risk of change increases”, says Oliveira.
Without water the trees did not resist and started dying off – especially the bigger, thicker ones, which account for 90% of the forest’s biomass. Twice as many large trees, with trunks between 10 and 30 centimeters in diameter, died in the area deprived of rain than in the control area. Among the most imposing – those with trunks over 30 centimeters in diameter and between 30 and 40 meters tall – this rate was even higher: 4.5 times as many. In a more general assessment one in ten large trees dried out in the area covered by panels, while the rate was 1 in 200 in the control area. Mortality remained higher a year after the group finally removed the plastic panels from the forest in 2005, informed Nepstad and Ingrid Thover, from Ipam, in 2007 in Ecology.
“The component most affected by the reduction in rainfall was the forest’s carbon stock”, says Paulo Brando. In the five years of rainfall reduction the growth rate of plants, which had initially fallen by 20%, reduced even more: Brando, the forestry engineer from Ipam, who is currently doing a PhD at Florida University in the United States, saw that during the experiment it was 41% less than in the control area. This reduced growth is reflected mainly in the production of timber, which is 33 tons less in the area covered with plastic panels. The drier forest also produced 47 tons more dead organic material.
Presented in May 2008 in the Philosophical Transactions of the Royal Society B, these results indicate that the capacity for removing carbon dioxide (CO2) from the atmosphere is reduced a lot. CO2 is a source of carbon that is taken in by plants and transformed into trunks, leaves, flowers and fruit. “Even though the smaller trees started growing more with the death of the bigger ones, the reduction in leaves in the canopy and the entry of more light, this growth has been far from sufficient for restoring the initial absorption levels of CO2”, says Brando. “It would probably take hundreds of years for the forest to recover its current capacity for storing carbon.”
If other conditions (temperature, forest area and CO2 concentrations) remain at current levels, a reduction in rainfall could transform the Amazon from a gutter into a global emitter. Studies of gas emissions carried out by the LBA indicate that today the forest is in an almost evenly balanced situation as far as the emission and absorption of carbon dioxide is concerned: every hectare of forest is capable of removing 0.5 tons more carbon dioxide from the air than it emits.
This is no small amount. It is calculated that the 700 million hectares of forest extract 350 million tons of carbon dioxide from the atmosphere every year, almost a tenth of what is absorbed by all the planet’s tropical forests – and 3.5% of what is launched into the atmosphere by human activities.
“We need to bear in mind that rainfall reduction is not the only factor that has an influence on the future of the forest”, Carlos Nobre remembers. A climate model that Nobre’s team has been developing at Inpe indicates that, at least initially, the increase in carbon dioxide in the atmosphere may counterbalance the effect of the reduction in rainfall. “The tendency for change in the regions south and east of the Amazon continues, but is lessened”, says Nobre.
Even though it is not assessing the influence of these other factors, the dry forest may contribute towards improving climate change forecasts. Its results may feed into more accurate and realistic climate models, since the current ones do not include changes over the whole area and on the structure of forests as a result of climate change. “This work is quantifying parameters that would be very difficult to calculate”, says Eneas Salati.
Heat and fire
While he was monitoring the transformations in the Tapajós area, where the vegetation that rises up to 30 meters from the ground is dense and closed, Nepstad and his team began to ask themselves: if part of the Amazon Forest really does become drier and more susceptible to fire, what happens next? To find out they planned another huge experiment: to set fire to a dry forest area, similar to how the Amazon may be in the future.
They managed to get authorization to carry out the project, known as the “savannah-ization” experiment, on an André Maggi Group farm in Mato Grosso, which belongs to the family of Blairo Maggi, governor of the state and Brazil’s largest soybean producer. This region receives 1700 millimeters of rain a year and the forest is more open and lower – the canopy is 20 meters above ground, on average. This is transitional vegetation between the Amazon Forest and the Cerrado [savannah/scrubland] (Pesquisa FAPESP nº 103).
For four consecutive years between 2004 and 2007, Nepstad and Davidson’s team set fire to an area of 50 hectares of transitional forest. Now they are beginning to compare what happened there with the changes observed in a 50 hectare area that was burned twice in 2004 and 2007, and another of similar size that was not burned.
Fires below knee-height – fires with higher flames are rare in an area of closed vegetation – consumed mainly smaller trees, the diameters of which were between 10 and 20 centimeters. The mortality rate of these trees more than doubled after the first two fires: every year almost 10% of them died. Another group that suffered were the creepers, climbing plants with woody stems that form impenetrable networks linking the forest floor with the tops of trees. “The damage caused by the fire was complementary to the reduction in rainfall, which mainly affected the tallest trees”, says biologist Jennifer Balch, currently a researcher with the National Center for Ecological Analysis and Synthesis in the United States.
Curiously, successive burning reduced the power of the fires, which every year spread over a smaller area and had lower flames, said the biologist in October, 2008 in Global Change Biology. Jennifer proved that the reason is that, with every fire, there is a reduction in the amount of leaves and dry branches available, the main fuel for forest fires. But this effect seems to be temporary. The death of bigger trees, which takes longer, may once more increase the fuel needed for the fires. Jennifer also noted that burning helps grasses invade the forest edges; this is vegetation that is more likely to burn under drought conditions.
Apparently, repeated burning exhausted the forest’s power to recover. “There were seeds and plantules (young plants) of various species sprouting after the first fire”, says Jennifer. “But after the third time of burning the number of species in regeneration fell by half.”
Oswaldo de Carvalho Junior, a biologist with Ipam, noted that some mammal species initially benefited from the fire, while others reduced in number. “The number of species that visited the area did not decrease but the population of each decreased, with the exception of the tapirs, which prefer the tender sprouting leaves”, says Carvalho.
The researchers from Woods Hole and Ipam intend maintaining the experiment in Mato Grosso for a few more years and returning to the National Forest of the Tapajós to monitor its recovery. While they are looking to discover more about the capacity of the forest to resist and adapt they are collecting indications that the climate is already changing. “Over the last few years”, says Davidson, “farmers in Mato Grosso have been altering their planting patterns because the rains are arriving later. They know that today the fire spreads more rapidly and in a more dangerous way”.
BRANDO, P.M. et al. Drought effects on litterfall, wood production and below-ground carbon cycling in an Amazon forest: results of a throughfall reduction experiment. Philosophical Transactions of the Royal Society B. v. 363, n. 1.498, p. 1.839-1.848, 27 May. 2008.
BALCH, J.K. et al. Negative fireback in a transitional forest in southeastern Amazonia. Global Change Biology. v. 14, n. 10, p. 2.276-2.287, Oct 2008.