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Ecology

Inverted flow

After successive occurrences of El Niño during the 90s, the Tapajós National Forest now liberates more carbon dioxide gas than it absorbs

Data collected from one of the twelve towers of the Large Scale Biosphere/Atmosphere Experiment in the Amazon (LBA) that measures how much carbon dioxide (CO2 ) – the main gas responsible for the increase in the greenhouse effect, which is progressively raising the average temperature of the Earth – comes in and goes out at different points of the ecosystem has shocked researchers. According to the registers of this tower, situated some fifty-five meter high at a point at the Tapajós National Forest close to the small town of Santarém, in Pará, this stretch of the Amazon launches more carbon dioxide into the atmosphere than it absorbs.

This is the first time that an experimental tower, capable of registering ten times per second the concentrations emitted and assimilated of this compound, points to an overall loss of carbon in some parts of the forest. In the long term, if this behavior persist, this would be equivalent to saying that this point of the forest (and only this one, as far as we know) has contributed to increasing the greenhouse effect – and not to reducing it, as would appear to be the case with the Amazon in general.

The realization that this stretch of the National Tapajós Forest releases more carbon dioxide than it absorbs was carried out by the American Steven Wofsy, from Harvard University, one of the participants in the LBA, a mega international project of US$ 80 million that, since 1999, has been bringing together more than three hundred researchers from Latin America, Europe and the United States under the leadership of Brazil. The scientist, nonetheless, avoided making any more general comment about the role of the Amazon basin over the long term in the global balance of carbon dioxide. “Throughout all of the forest we hope to see various standards, with some areas losing and some areas gaining in carbon”, said Wofsy during an interview for the February edition of the magazine Environmental Science&Technology. “In the specific case of the Santarém tower, the annual deficit is around a half a ton of carbon per hectare”, says Paulo Artaxo, from the Physics Institute of the University of São Paulo, one of the LBA coordinators.

The sea and the forest
For Dr. Wofsy, the cause of the unprecedented negative balance in the flow of carbon dioxide measured at the tower in Santarém is related to the effects on the regional climate brought about during the last decade by successive El Niños, the abnormal warming of the superficial waters of the South Pacific that alters the indices of rainfall and temperature in several spots of the globe. Because of the four El Niños registered during the 90’s, there was less rainfall in the region of Tapajós and more trees succumbed to prolonged drought. With the ending of the effects of this climatic abnormality, the levels of humidity in the Amazon, normally high, returned to their usual levels and accelerated the rotting of a large quantity of tree trunks and vegetables that had not resisted the dry spell.

The result: the rate of decomposition of abundant dead wood became so elevated that the levels of the carbon dioxide release overtook the intake rate by the photosynthesis of the forest. In spite of the fact that the live part of the forest has responded positively to the re-establishment of the region’s high humidity, the growth of healthy trees – read assimilation of CO2 – was not as expressive to the point of overcoming the quantity of gas exhaled by the decaying of the leftovers of the vegetation.

The majority of scientists believe that the tropical rainforest is capable of absorbing more CO2 than it emits. In practical terms, this vision implicitly confers upon Amazonia the role of removing a part of the excess of this gas present in the atmosphere and thus diminishing the increase of the greenhouse effect. Recent data from almost all of the LBA towers are compatible with this theory. Nevertheless, the quantity of carbon dioxide naturally assimilated by this ecosystem is, according to the most current measurements, equal to or only marginally greater than that expelled, with an annual positive balance of between 1 and 2 tons of carbon per hectare of the forest – and not at 6 to 8 tons as previous studies has indicated. Or that is to say, the cleansing effect of the Amazon on the atmosphere seems to exist, but may well be a lot more modest than had been thought.

Heat impact
The overall loss of carbon detected by the Santarém tower obviously does not match up to the above premise and may well be considered as an isolated event, an exception to the rule. This may well be true, but the unusual measurements in the Tapajós National Forest become even more interesting when compared with the results of another recent study, made within an environment similar to that of the Amazon. In an article published in the May 13th edition of the scientific magazine Proceedings of the National Academy of Sciences, in the United States, researchers from the University of Missouri suggest that the trees of the tropical forest of La Selva, on the coast of Costa Rica, are growing less and are making their balance of carbon during the hottest periods to be negative, especially during years with a pronounced El Niño as during the period 87-88.

After having analyzed the flow of carbon from 1984 to 2000 in La Selva, the authors of the study realized that, when the temperatures get higher than normal, the tree vegetation of the region begins to emit more CO2 than it is absorbing. As can be seen, if the question is a loss of carbon during an El Niño year, the forests of Santarém and La Selva appear to exhibit similar behavior. “In order to calculate the influence of the tropical ecosystem upon the greenhouse effect, measurements need to be taken in other locations and for periods of a much greater time span”, Artaxo comments.

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