Plantwatch: the trees that feed on metal

These plants can clean contaminated soils, could they also offer a greener way of collecting much-needed substances?

A magnificent tall tree called Pycnandra acuminata grows on the island of New Caledonia in the South Pacific, and it does something strange - when its bark is cut it bleeds a bright blue-green latex that contains up to 25% nickel, a metal highly poisonous to most plants in more than tiny amounts.

An estimated 700 plant species have unusually high levels of metal, mostly nickel but not in all cases. The macadamia tree has leaves and sap rich in manganese, although fortunately not in the nut. If such metal-containing plants are dried and burned to ash they yield extremely rich, high-grade metal ore, with far less pollution and using far less energy than needed in conventional mining. Perhaps this could offer new sources of much needed metals. It is highly unlikely they could fully replace traditional mining, although they can also help clean up soils contaminated with toxic metals.

Exactly why some plants feed on large amounts of metal is a puzzle, but it could be a way to protect against insect attack or perhaps to resist fungal infections. And how the plants manage to tolerate poisonous metals in huge amounts without killing themselves remains a mystery.