Abandoned industrial sites litter the world, and many are contaminated with pollutants that make it difficult to redevelop the land. Yet some species of trees can tolerate polluted soil and thrive. And, oddly enough, some trees growing in contaminated soils are more resistant to natural predators than trees grown in healthy soils.
That’s the conclusion reached by a team of researchers from the University of Montreal, which grew fast-growing willows in greenhouses for six months using soil from both contaminated and uncontaminated sites. Both groups of trees grew at the same rate, so the researchers examined the genetics of the stems, buds, and leaves to learn what mechanism(s) allowed them to tolerate the stresses imposed by the pollutants.
According to Nicholas Brereton, who led the project, the willows produced a variety of chemical defenses against pollutants. “The trees have evolved advanced mechanisms so when they are challenged with nasty contaminants, they produce defensive tools to combat the stress,” Brereton said. “And these chemicals have a secondary effect of fending off insect pests – they’re presumably nasty tasting. So by being stressed, the trees get a massive advantage in nature; they’re primed to defend against attack because of the contamination.”
He was excited to find large quantities of genetic material from other organisms, including fungi, bacteria, and insects, mixed among the tree genes. Spider mite genes, for instance, were found in much higher abundance in 99 percent of the trees grown in non-contaminated soils, suggesting that the trees from contaminated soils were able to defend against the pest insects.
Brereton believes that chemical-tolerant plants like willows could be used to rehabilitate contaminated land in a process called phytoremediation. In addition to absorbing the pollutants and rejuvenating the land, he said, the use of fast-growing trees that produce high yields of biomass could be beneficial to the bioenergy and biofuels industries while making contaminated properties useful again. He even thinks some of the heavy metals and chemicals absorbed by the trees could be harvested and sold, making the concept appealing to entrepreneurs.
Although his studies have focused on willows, Brereton said many tree species probably produce the same chemical response to growing in contaminated soils, though only a few are likely to be economical for a commercial harvesting effort. What he is uncertain of, however, is whether the biochemical response to contaminated soil changes the wood – for better or worse. That’s one of the next steps in his research.
“By understanding the response to contamination, we’re opening up a lot of doors and practical avenues that can be explored,” Brereton said.