Trees as witnesses to environmental pollution

Wouldn't it be great if trees could serve as silent witnesses to environmental pollution caused by, for example, an industrial plant? A WSL study was able to show that trees can store tiny particles from air and water in their wood. These nanoparticles are more than one thousand times thinner than human hair. They can be pollutants if they consist of toxic heavy metals such as aluminium or lead, or industrial agents that transport active substances in sun blockers or modern pesticides, for example.

It is already known that agricultural plants absorb such particles from the environment. Paula Ballikaya, a PhD student at WSL, wanted to know whether this is also the case with trees. “Until now, it was not clear if and how nanoparticles penetrate through the leaves, such as gaseous pollutants do", she says. Now, in a greenhouse experiment, she was able to show for the first time that intact nanoparticles can pass through the leaves into other parts of the tree. The results of the study were published in the journal Tree Physiology.

For their experiment, Ballikaya and her colleagues sprayed nanoparticles made from gold onto young European beech and Scots pine trees in the laboratory. They chose gold particles as a model because they do not harm the trees and are easily detectable in plant tissue. After twenty days, the particles were present not only in the leaves, but also in the trunk and roots. The path into the leaf tissue probably leads through the stomata on the leaf surface, which the tree needs for gas exchange with the air. From there, they spread throughout the tree in a still unknown way. When she added nanoparticles to the roots, they also ended up in the trunk, but more particles were found in the trunk of trees with treated leaves.

Tree ring chemistry for monitoring air pollution

The experiment demonstrates that trees absorb nanoparticles, similar to those found in air and water pollution, into their wood. They can still be detected there years later. This is what tree-ring chemistry, Ballikaya's primary field of research, uses to pinpoint pollution to the year. "For example, you can determine if an industrial plant has contributed to pollution in the environment by analysing the tree-ring chemistry" Ballikaya says. "These kinds of applications were a motivation for our nanoparticle experiment."

Previous work shows that this could indeed work, as Ballikaya and her colleagues write in a review article. Various research groups detected gaseous pollutants from car exhausts, metal refineries and coal combustion in tree trunks. They therefore suggest extending tree-ring chemistry to environmental monitoring programs that address nanoparticle contamination. Paolo Cherubini, senior scientist at WSL and Ballikaya’s supervisor, adds: "Tree rings could tell us not only about historical air pollution rates, but even about past climates or events such as volcanic eruptions. First, however, we must find out more about how nanoparticles move around in trees."

Still a long way off, but not completely far-fetched thanks to the study, is the idea of using trees to clean up polluted soil and air. "Fast-growing trees could store heavy metals from the soil or air in their wood, which can then be disposed of properly," says Ballikaya. But she says she needs to find out more about the interactions between nanoparticles and trees before that can happen.