Rural: Metal-guzzling trees for a clean, green future

The green implications of planting willows may run deep: research suggests that these scrub plants can be used as natural decontaminators on polluted land. Daniel Butler reports
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The Independent Online
Housing is the biggest threat to Britain's countryside. Because of the growth in single occupancy and our increased lifespan, we will need up to 4.4 million new homes by 2016. Where will we put them? Thanks to a legacy of industrial pollution, urban sites are in short supply, but new research suggests that help may come from an unlikely source: urban woodland.

According to Dr Nicholas Dickinson of Liverpool John Moores University, fast-growing scrub "weeds" such as willow and birch can act as natural decontaminators. Backed by the National Urban Forestry Unit, local authorities and private industry, he has put together a research proposal to monitor the ability of trees to absorb pollutants on 15 contaminated sites around the Black Country.

If his theories prove justified, the implications are certainly exciting. Our industrial heritage has polluted much of the vacant land in and around our cities with heavy metals such as lead, zinc and cadmium, even low levels of which make a site unsuitable for agriculture or development. Conventional remediation techniques are difficult and unsatisfactory; they are also expensive. As a result many of Britain's cities contain areas that have lain unused for decades, or even centuries.

Planting trees on such sites has long been recognised as a cheap alternative to treatment (the most elaborate scheme rarely exceeds pounds 3,000 per acre). Until 20 years ago this tended to be purely cosmetic, designed to hide slag heaps or factory sites, but then trees began to be valued for stabilising potentially dangerous soil. This is because fallen leaves increase the soil humus and tree roots absorb water, both of which reduce pollutants leaching out into surrounding watercourses.

Now Dr Dickinson's preliminary research suggests that they can do much more. The underlying theory is nothing new - scientists have known for some time about "hyperaccumulators" (plants such as the cabbage family, that can absorb high levels of pollutants), but so far these have had limited practical applications. Even though some can absorb up to 1,000 parts per million (ppm), the problem is that they are usually slow growing, and there is the problem of subsequent disposal. So research has concentrated on translocating genes from hyperaccumulators into more marketable crops. Dr Dickinson believes this may well prove to be impracticable: "If it were as easy to put the gene from one plant into another as some make out, we'd have had wheat with its own nitrogen-fixing roots long ago," he points out.

Instead he has concentrated on willow: "The beauty of this species is that it hybridises really easily," he says. "This means that if one type is very pollution tolerant, while another is high yielding, you can cross them to get the best of both."

The results so far are encouraging. Cadmium, for example, is one of the worst pollutants, rendering land unsuitable for agriculture at just 3 ppm. Taking soil containing cadmium at 6-8 ppm, Dr Harrison's team found a remarkable improvement: "Our trials suggest willows could more than halve the level in 16 years," he says. In contrast, it would take 80 years to achieve a reduction of just 10 per cent, if the land were left to recover on its own. "They are less successful in clearing zinc and lead," Dr Harrison admits. "But that's because zinc is found in much higher concentrations and lead is much more stable, therefore difficult to extract."

Although further research is needed on the risk of creating new problems, so far these look relatively insignificant. The most serious danger to date seems to be that as the soil improves, so worms may eat the heavy metals that are released. These could then move up the food chain into blackbirds, for example. "But in practice we think there's no real danger," says Dr Dickinson. "Blackbirds prefer woodland edges and open ground - and so by planting trees on contaminated ground we're actually reducing their contact with the pollution."

If Dr Harrison's trials go according to plan, urban trees could play a major part in improving our cities and in protecting the Green Belt. Certainly he believes that coppicing has the potential to be used widely in the near future. Not only would it give value to land that is currently worthless; it would also reduce the legal risks for developments in neighbouring areas, and produce a valuable crop. Who knows, it could also add a new - and more welcome - meaning to the phrase "urban jungle".