Trees can snitch on illegal gold miners in the Amazon

Researchers believe they have identified a network of spies to help combat illegal gold mining operations in the Amazon rainforest. But unlike other espionage efforts, there is no risk of betrayal from the new assets: Trees aren’t known for spilling secrets.

Humans have coveted the Amazon’s natural resources for generations, particularly its gold. But with most of the region’s easily accessible precious metals long gone, illegal mining operations now focus on extracting the soil’s hidden gold particles. To do this, the miners use a method that requires adding toxic mercury into the ground and allowing it to bind to any surrounding gold. Because the resulting amalgams have a much lower melting point than gold alone, miners then burn the mercury away to collect the residual lucrative metal. Meanwhile, the mercury-laden smoke disperses into the atmosphere where it can harm the surrounding environment and local populations.

The annual rings inside certain trees near these mines don’t just tell their age—they also store important environmental biomarkers over time. Recently, an international team led by researchers at Cornell University wondered if those biomarkers could indicate excess atmospheric mercury levels. To test their theory, experts traveled to the Peruvian Amazon and documented their findings in a study published on April 8 in Frontiers in Environmental Science.

Researchers first took core samples from fig trees at five sites. Two locations were far removed from mining activity, but three were within roughly 3.1 miles of mining towns previously known to rely on amalgam burning. One site was also adjacent to protected forest lands.

The subsequent analysis results were clear: mercury levels were highest in wood sampled from mining-adjacent sites and lower at those further removed from mines. Additionally, higher mercury levels in mining-adjacent fig trees also coincided with the historical rise in amalgam burning that began after the year 2000.

“We show[ed] that Ficus insipda tree cores can be used as a biomonitor for characterizing the spatial and potentially the temporal footprint of mercury emissions from artisanal gold mining in the neotropics,” Jacqueline Gerson, a Cornell University biological and environmental engineering associate professor and study first author, said in a statement.

Although annual tree rings can tell researchers when mercury levels began rising, they can’t necessarily offer precise locational directions to the illegal miners. At the same time, higher concentrations may at least serve as reference points indicating a closer proximity. Regardless, the literal spy rings of fig trees may soon offer a cheap, powerful means for regional monitoring and conservation work in the Amazon.