Blossoming Among Spoil Heaps: How Lead Mining Created Rare Metal-Tolerant Plant Habitats

The Lead

At first, the small purple flowers are hard to spot in the weak May sunshine. Slowly the drifts of delicate mountain pansies, along with the white rosettes of alpine pennycress, begin to jump out, scattered across an area little bigger than a football pitch, on the banks of the River Allen in Northumberland.

The Metal-Tolerant Ecosystem

This is a pocket of calaminarian grassland, an increasingly rare habitat where specialist plants called metallophytes have adapted to live in soils deeply contaminated by heavy metals, the legacy of more than 1,000 years of lead mining. "This is absolutely a case of nature responding to pollution caused by humans," says Geoff Dobbins, estates manager for the Northumberland Wildlife Trust, who is passionate about saving these grasslands.

The Evolution of Metallophytes

The grasslands originally evolved in small patches around rocky upland outcrops, where veins of lead, cadmium and zinc had been exposed by the elements. As these began to be mined, according to Dr Ruth Starr-Keddle, a botanist at the North Pennines National Landscape, a biocrust of lichens and mosses developed that could tolerate toxic wastewater washing over them.

The Natural Cleanup Process

Despite their delicate appearance, these specialist plants can live in soils 30 times more toxic than most other species can tolerate. As they grow, metallophytes act as "hyper-accumulators," cleansing the soils that feed them through a process called phytoremediation. This turns the metals they absorb through their roots into complex organic compounds, which are locked away below the surface once the plants die.

The Mining Legacy

The barren, rocky uplands of the northern Pennines were first mined by the Romans, but the industry reached its peak in the mid-18th century. Today, the landscape is dotted with abandoned workings and spoil heaps; some high up on the moors, others closer to the rivers and the water the industry needed. "If you took samples from most of the rivers in the North Pennines, most have got contamination from lead mining in them," says Dr Starr-Keddle.

The Future of These Unique Habitats

As they become cloaked in more thuggish plants such as gorse and broom, and the zinc and lead brought by mine-wash became slowly buried beneath a blanket of humus, there is a growing debate about whether these human-made meadows should be protected or allowed to gently fade away. About 30% of Europe's calaminarian grasslands are found in the UK, although they are scarce, covering just 450 hectares (1,100 acres), with pockets in northern England, mid-Wales and the Highlands of Scotland.