Revealed: The dirty way in which trees are killing each other

"Air pollution comes from trees," Ronald Reagan declared more than 20 years ago, soon after the start of his presidency. The remark earned him widespread derision as proof of his ignorance of environmental issues. Even his first press secretary, James Brady, teased him about it. Once when Air Force One was flying over a forest he grabbed the President by the elbow and, pointing down out of the window, said in alarm: "Look, Mr President: killer trees!"

But now new scientific research is showing that the former Hollywood B-movie star was at least partially right all along. For studies in both Britain and the United States have shown that some trees do indeed emit pollution and may even be killing forests downwind.

The news comes during National Tree Week when the Government and environmental groups combine to extol the benefits of trees to the environment and health. Even more embarrassing, the British research, at Lancaster University, shows that the English oak, one of the very symbols of nationhood, is among the worst offenders.

Others include the poplar, the red and sessile oaks and the crack, goat and white willows. The scientists say: "Most people assume that trees only benefit air quality. In fact, some tree species can have a negative effect and actually help to form pollutants in the atmosphere.''

Research at the University of California at Berkeley, meanwhile, suggests that pollution from oak trees is destroying the pine forests of the Sierra Nevada Mountains. They have found that the oaks are producing between 40 per cent and 70 per cent of the ozone that is damaging and killing Jeffrey and ponderosa pines that are the dominant species in the forest.

And it's not only plant life that's at risk: other Californian research, as exclusively reported in The Independent on Sunday earlier this year, shows that ozone can cause asthma.

The detective work that led to the incrimination of trees began after American cities found, to their surprise, that ozone levels failed to decline rapidly after the imposition of anti-pollution measures.

Ozone is formed by the effect of sunlight on nitrogen oxides and hydrocarbons, both emitted from car exhausts. It became clear that some trees like oak and poplar were producing huge amounts of a hydrocarbon called isoprene, which dominated in the formation of ozone. Professor Russell Monson of the University of Colorado, who also made a special study of the issue, said: "The problem is that forests around many southern US cities have so many oaks, and isoprene is many times more reactive with the atmosphere than man-made hydrocarbons.

"This meant that, although monitoring programmes showed a significant decline in hydrocarbon emissions from automobiles, ozone did not go away.''

The scientists point out, however, that trees are not the only culprits: isoprene can only form ozone when it combines with nitrogen oxides emitted from cars, industry and power stations. The Lancaster University research also shows that some trees help to clean the air of ozone and nitrogen dioxide.

The best are ash, silver birch, larch, Scots pine, common alder and field and Norway maples. Others – such as apple, holly, sycamore, hawthorn, hazel, lilac and the common elm – also clean the air but less well. The scientists add that cutting down the worst trees will not solve the problem and may even make things worse. For all trees help to cool the air, and rising temperatures cause more ozone to be formed.

What they do recommend is that care should be taken to plant trees that clean the air rather than those that exacerbate the problem.

All trees also help to absorb the carbon dioxide that causes global warming – though again the oak does so less effectively than other species because it grows so slowly – and all also absorb tiny particles emitted by car exhausts that kill thousands of Britons each year through heart disease and cancer. The Lancaster University scientists calculate that doubling the tree cover of the West Midlands, for example, would save 140 lives a year.