Chemical 'fingerprint' of trees can help combat illegal Amazon timber trafficking

A map of isotope distribution in the wood of Amazonian trees can be used to help identify timber illegally extracted from the forest. The use of ratios between different forms of elements such as oxygen, carbon, nitrogen, and strontium makes it possible to pinpoint more precisely the region from which the wood was removed and, in doing so, help combat the illicit trade of the material.

Illegal timber extraction is one of the major causes of deforestation in the Amazon. To combat this practice, environmental agencies and police use a range of tools to try to determine the origin of wood, including analyses of plant anatomy, species occurrence maps, and bureaucratic records, such as the Documento de Origem Florestal — known by the acronym DOF, it is the license required for the transport and storage of forest products, issued by the Ministry of the Environment. But this is not always sufficient to guarantee the legal origin of the merchandise.

The research group of Luiz Antonio Martinelli, a full professor at the Center for Nuclear Energy in Agriculture at USP (University of São Paulo) in Piracicaba, is refining a new tool that can assist in this task. It is based on the use of isotopes — atoms of the same chemical element that have the same number of protons but a different number of neutrons — to narrow down the area from which the wood was extracted.

The group is creating a map of the distribution of these different isotopes in the wood of trees in the Amazon rainforest, which police can use to compare with the composition of wood under investigation.

"We want to provide the Federal Police [PF] with an isotopic model they can use. So that, based on the isotopes, they can determine whether the origin of that wood really is where the DOF states," explains Martinelli.

The main advantage of this approach is that there is no way to falsify the composition of the wood. "We thought about providing the PF with a method that is inviolable. There is no way to falsify stable isotopes," explains Martinelli. Furthermore, once the map — known as an isoscape — is complete, the methodology can be easily applied by experts outside research laboratories. Martinelli's group already has partnerships with PF experts to employ the method in investigations.

The basis of the methodology lies in calculating the ratio between two different weights of the same atom, such as oxygen-18 and oxygen-16, in the cellulose that makes up the wood (the numbers refer to the quantity of neutrons in each atom). In an article published in May in the journal Molecules, the team demonstrates that there is a southwest-northwest gradient in the ratio of the two forms — with the lighter form predominating in the west. In other words, wood originating from the western part of the forest contains more oxygen-16 and less oxygen-18 compared to wood from the east.

This difference can be explained by the gradual loss of the heavier form of oxygen in water as moisture from the Atlantic Ocean moves across the continent. Since the water that reaches the western Amazon has lost some of its oxygen-18 along the way, smaller quantities of this form of the atom will be incorporated into the wood of plants in the region when they absorb rainwater falling on the forest.

Limitations and next steps

It so happens that oxygen cannot be used in isolation for this task. "Because the Amazon is very complex and very large, it is not possible to use just one isotope," explains Martinelli. The ongoing doctoral project of Isabela Maria Souza Silva, developed with support from Fapesp, is mapping the isotopes of two other atoms, carbon and nitrogen. The group also intends to add strontium to the model.

At the current stage of the project, the tool still has some limitations. "Our best model can exclude 80% of the Amazon forest area, which is 3.2 million square kilometers. The problem is that 20% is still a lot. To give you an idea, that amounts to roughly 640,000 square kilometers," says Martinelli. The area is equivalent to two and a half times the size of the state of São Paulo.

Another difficulty faced is the fact that there is greater variability in the results from trees originating from the arc of deforestation, the geographic strip that marks the frontier of the advancing destruction of the biome. This occurs because illegal logging ends up reducing the number of samples available for collection by researchers, making it harder to apply the method to timber coming from precisely the region where the most extraction takes place.

Martinelli and his team intend to reduce these limitations by expanding not only the number of isotopes used, but also the number of samples collected. "We have already collected 800 trees from 63 different locations in the Amazon and have already seen that, if we want a more precise model, we will need to collect even more trees from more locations," he details.

The team is also studying the possibility of using new chemical approaches beyond isotope measurement, such as the creation of maps showing differences in the concentration of certain chemical elements in the wood, known as elementalscapes. By combining these different approaches, the group hopes to create an even more precise tool to help combat deforestation.