Selective logging and burning cause the loss of 54 million tons of carbon a year in the Amazon rainforest

Loss is equivalent to 40% of that caused by overall deforestation. Study conducted by scientists in Brazil and the United Kingdom linked data from satellites and field studies in 225 areas.

By José Tadeu Arantes

Agência FAPESP – A study conducted by scientists in Brazil and the United Kingdom has quantified the impact that selective logging, partial destruction by burning, and fragmentation resulting from the development of pastures and plantations have had on the Amazon rainforest. In combination, these factors could be removing nearly 54 million tons of carbon from the forest each year, introduced into the atmosphere as greenhouse gases. This total represents up to 40% of the carbon loss caused by deforestation in the region.

The study, which was conducted by 10 researchers from 11 institutions in Brazil and the United Kingdom, was published in the May issue of the journal Global Change Biology.

“The impacts of timber extraction, burning and fragmentation have received little notice because all the efforts have been focused on preventing further deforestation. This attitude has resulted in tremendous progress in conserving the Brazilian Amazon, whose deforestation rate fell more than 70% over the past 10 years. However, our study has shown that this other type of degradation is having a severe impact on the forest, with enormous quantities of previously stored carbon being lost into the atmosphere,” said Erika Berenguer, researcher from the Lancaster Environment Centre at Lancaster University, in the United Kingdom, first author on the study.

According to Joice Ferreira, researcher at the Brazilian Agricultural Research Corporation (Embrapa Amazônia Oriental) in Belém, state of Pará, and second author on the study, one of the reasons that this degradation has gone unnoticed is that it is difficult to monitor. “Satellite imagery allows much easier detection of areas that are totally deforested,” she said.

“Our research combined satellite imagery with field study. We conducted a pixel-by-pixel assessment [each pixel in the image corresponds to an area measuring 900 meters squared (m2)] regarding what has happened over the past 20 years. In the field research, we studied 225 plots (each 3,000 m2) in two large regions in an area measuring 3 million hectares [30,000 square meters], which we used as a model to estimate what occurred in the Amazon as a whole,” Ferreira explained.

The satellite images, compared every two years, have enabled researchers to put together an extensive overview of the degradation of the forest along a 20-year timeline. The field research assessed scarring from burning, timber extraction and other disturbances. The combination of the two investigations resulted in the estimate of carbon stock available today.

Two regions were studied in loco: Santarém and Paragominas, in the eastern part of the Amazon region, both under strong degradation pressures. Two hundred twenty-five areas were investigated in these two regions.

“We collected data from more than 70,000 trees and took more than 5,000 samples of soil, dead wood and other components of what is known as carbon stock. It was the largest study conducted to date regarding carbon loss from tropical forests due to selective logging and wildfires,” Ferreira said.

According to her, the research included four of the five functionally distinct carbon pools whose study is recommended by the United Nations (UN) Intergovernmental Panel on Climate Change (IPCC): aboveground biomass (live plants), dead organic matter, leaf litter (layer that contains a combination of fragments of leaves, branches and other decomposing organic matter) and soil (up to 30 centimeters (cm) in depth). “The only thing we didn’t measure was the carbon stock in the roots,” she said.

For comparative purposes, five categories of forest were considered: primary (totally intact) forest; forest affected by logging; forest affected by fires; forest affected by selective logging and fires; and secondary forests (regenerating after complete clearance).

The forests that were disturbed by logging or fire had from 18% to 57% less carbon than primary forests. One area of primary forest ended up having more than 300 tons of carbon per hectare, while areas of forest that had been burned or subjected to timber extraction had, at most, 200 tons per hectare and, on average, less than 100 tons of carbon per hectare.

Traditional selective logging

The roadmap of degradation was well defined by the researchers. The starting point is often the extraction of higher commercial-value timber, such as mahogany or Brazilian walnut. While these trees are cut selectively, their removal impacts dozens of neighboring trees.

Once extraction begins, several openings are made in the plant cover, which causes the forest to become much more exposed to sun and wind and thus much drier and more susceptible to the propagation of wildfires. The effect is greatly accentuated by the forests’ fragmentation as a result of pastures and plantations.

The combined effects can then transform the forest into dense undergrowth, full of trees and small-scale lianas but with a carbon stock 40% less than that found in undisturbed forests.

“Today, there are several systems for selective logging, some of which have fewer impacts than others. The predominant system, which was what was detected in our study, associated with the age and diameter of the trees removed, may remove an enormous amount of carbon from the forest,” said Plínio Barbosa de Camargo, director of the Tropical Ecosystem Function Division of the Center for Nuclear Energy in Agriculture (CENA) at the University of São Paulo (USP) and member of FAPESP’s Biology Coordination Committee, who also authored the article published in the journal Global Change Biology.

“No matter how much we recommend otherwise, when it comes time to actually cut, few trees with very large diameters are being taken. In another study, we measured the age of the trees with carbon 14. One tree whose trunk measured one meter in diameter would certainly have been 300 to 400 years old. It does no good to remove this tree and think it will be replaced in 30, 40 or 50 years,” Camargo commented.

The degradation underway is becoming even more worrisome in the context of global climate change. “The next step is to better understand how these degraded forests will respond to other forms of disturbances caused by humans, such as more severe dry periods and rainy seasons with much higher levels of precipitation due to climate changes,” said British researcher Jos Barlow of Lancaster University, one of the coordinators of this study as well as one of those in charge of the thematic project “ECOFOR: biodiversity and ecosystem functioning in degraded and recovering Amazonian and Atlantic Forests”.

In addition to the researchers already mentioned, the Global Change Biology article was co-authored by Toby Alan Gardner (University of Cambridge and the Stockholm Environment Institute), Carlos Eduardo Cerri and Mariana Durigan (Luis de Queiroz College of Agriculture/USP), Luiz Eduardo Oliveira e Cruz de Aragão (National Institute for Space Research and the University of Exeter), Raimundo Cosme de Oliveira Junior (Embrapa Amazônia Oriental) and Ima Célia Guimarães Vieira (Emílio Goeldi Museum of Pará).

The article “A large-scale field assessment of carbon stocks in human-modified tropical forests” (doi: 10.1111/gcb.12627) by Erika Berenguer and colleagues may be read at http://onlinelibrary.wiley.com/doi/10.1111/gcb.12627/full.