On a summer day several years ago, Fabrice Lambert, a climatologist at the Pontifical Catholic University of Chile, met one of his colleagues in Quinta Normal, a 35-hectare public park in central Santiago. The researchers weren’t there for a picnic. They carried with them a drill-tipped tool known as an increment borer and a package of beeswax.
Using the borer, the duo extracted pencil-width cores of wood from more than 50 cedar trees, similar to those shown above. It took about 15 minutes to collect each core and seal the resulting hole with beeswax, long enough for curious park goers to wander over. “There were quite a few people looking at what we were doing,” Lambert says.
Back in the lab, the scientists made a surprising discovery: The cores acted as a pollution time machine, revealing the air quality in Santiago decades into the past. The air was especially bad prior to 1990s, when the Chilean government first enacted measures to curb pollution. Yet there was a curious exception—the year in which the country experienced a military coup.
The approach marks the first time trees have been used to reconstruct historic air quality, says Mukund Palat Rao, an ecoclimatologist at the University of California, Davis, who was not involved with the work. Because trees can live centuries and even millennia, he notes, the technique could help researchers track fluctuations in air quality before the advent of modern monitoring stations.
Back in the lab, Lambert and his colleagues scraped bits of wood from each of the trees’ annual growth rings, the oldest of which dated to 1930. They then analyzed the chemical composition of each year’s wood to look for trace amounts of heavy metals—environmental pollutants such as copper, cadmium, and lead that can cause lung disease and impair the nervous system.
The metals, which come from everything from worn vehicle brake pads to fossil fuel emissions, make their way into the atmosphere and groundwater, where they are taken up by plants. “Some come mostly through the leaves, some come through the roots,” Lambert says.
Previous research has focused on tracking heavy metals in trees over time. But Lambert and his colleagues took their investigation one step further: They linked the heavy metal abundances in recent years to measures of air quality recorded at nearby monitoring stations— specifically the concentration of airborne particulate matter. Particulate matter, often measuring just a few thousandths of a millimeter, can lodge itself deep within the lungs and exacerbate conditions like asthma.
“We create this relationship between what we measure in the tree and what is actually measured in the air,” Lambert says, “and we continue this relationship back in time.”
The researchers found “consistently bad” air quality prior to the 1990s, before the Chilean government implemented measures to reduce pollution, Lambert says. The data also revealed a strong downturn in pollution in 1973, the year that Augusto Pinochet, an army general, seized control of the country in a military coup. “That could potentially reflect the pause in economic activities while things calmed down under the new regime,” says Lambert, who reported his team’s results last month at a meeting of the American Geophysical Union.
Rao would like to see similar investigations in other cities to better understand their historic air quality. And because finding older trees in urban centers can be challenging, he says, one solution might be to look in botanical gardens.