In the hills around Olkusz in southern Poland, an unlikely restoration is underway. Old mining sites left behind centuries ago—so polluted with heavy metals that nothing would grow—are slowly being taken over by trees. Now, researchers in Switzerland have figured out which tree species do the best job at keeping those dangerous metals locked in the soil, where they cannot spread to nearby farms, rivers, or people.
The study, published in the journal Plants, was led by scientists at the Swiss Federal Research Institute WSL and ETH Zurich. They grew seven common Central European tree species for five years in soil packed with zinc, cadmium, copper, and lead—metals that wash up naturally from underground deposits and also leak from old mining and factory sites. These metals can harm wildlife and contaminate drinking water, making polluted sites hard to reuse.
The researchers split the seven species into two groups based on how they naturally grow. Four "acquisitive" species—gray alder, aspen, osier willow, and silver birch—act like hungry teenagers: they pull in lots of nutrients from the soil to grow fast. The other three, called "conservative" species—Norway spruce, European beech, and sycamore maple—take a slower approach, using fewer nutrients and storing them carefully.
Here is where it gets interesting. When grown in polluted soil, all seven species kept similar amounts of metals locked in their roots. But the acquisitive trees ferry more zinc and cadmium up into their trunks and leaves. The conservative trees do the opposite: they keep excess metals away from their above-ground parts, sometimes storing extra zinc in specialized root structures instead.
"We were surprised by how clear-cut the distinction was between the two growth strategies," said Madeleine Günthardt-Goerg, the study's lead author. That clarity gives land managers a simple rule: for polluted sites, plant conservative species.
The payoff is practical. Because conservative trees keep metals out of their wood, that timber could eventually be harvested and sold without worrying about contamination. Forests could reclaim abandoned industrial sites and eventually bring them back into productive use.
"Adapted forest management may allow us to bring polluted sites back into the economic cycle in a cost-effective way," said Pierre Vollenweider, a researcher at WSL who co-authored the study. The next step, his team says, is to test this approach in the real world at actual polluted sites like those around Olkusz.