Metal in the sap

When the soil gets into the plant

On the ultramafic massifs of southern Grande Terre, some plants do not merely tolerate the toxic, nickel-saturated soils beneath them — they concentrate the metal inside their own tissue. These are hyperaccumulators: species that actively draw heavy metals up through their roots and store them at levels lethal to most organisms. New Caledonia has more confirmed hyperaccumulator species than any comparable land area on earth.

The most striking example is Pycnandra acuminata, a tree in the family Sapotaceae. Cut its bark, and the latex that bleeds out is blue-green — a colour so unexpected that early botanists could not account for it.

The pigment is nickel complexed with organic compounds at concentrations around 26 percent by dry weight: an extraordinary figure when you consider that most plants tolerate only a few parts per million before suffering damage. Pycnandra stores more nickel in its sap than some nickel ores carry in their rock.

Why would a plant do this? The leading hypothesis is elemental defence. Tissues loaded with heavy metals are unpalatable or actively toxic to insects and browsing animals, an advantage sharp enough to have been selected for repeatedly across the island's long isolation.

The maquis minier — the low, metal-adapted scrub that covers the ultramafic terrain — has produced this strategy in dozens of lineages, from small shrubs in the genus Rinorea to ferns and members of the violet family. Each evolved the trick independently, which tells you something about how consistently punishing these soils are, and how reliably the defensive payoff follows.

The ecological implications run in several directions. Hyperaccumulators alter the chemistry of leaf litter as they shed, loading the topsoil with metals released from decomposing tissue and reinforcing the very conditions that make the ground hostile to competitors. They also attract researchers interested in phytoremediation — using plants to extract contaminants from degraded land — though the deep endemism of these species means any application elsewhere faces serious limits. Pycnandra acuminata grows nowhere but New Caledonia.

The IRD (Institut de Recherche pour le Développement) and the Muséum national d'Histoire naturelle have both contributed substantially to cataloguing New Caledonia's hyperaccumulator flora, a list that continues to grow as botanists work the more remote massifs.

The chemistry is now well enough understood that some researchers treat the plants themselves as a kind of biological assay — where blue-green latex runs, the ground beneath is almost certainly peridotite, weathered deep into the laterite that carries the nickel load. The plant is reading the geology, and writing the result in its own sap.