In the race for Mayor, let's all vote for the trees

Nowadays everybody knows trees are the good guys. Trees are becoming symbols of ecological virtue and thus political bargaining chips. In the absence of a really comprehensive green policy for London, Ken and Boris can at least compete in promising to plant more trees than each other. Boris has talked vaguely about such matters as recycling schemes, protecting green spaces and promoting bikes, but by far his most specific green pledge has been to plant an extra 10,000 street trees in London, concentrating on deprived areas - paying for them by abolishing Livingstone's vainglorious freesheet, The Londoner, thus scoring a dendrophile coup on both fronts. It sounds eminently achievable. Or to put it another way, over-modest.

Because Ken thinks big. As well as attacking gas-guzzlers and preventing homeowners turning front gardens into car-parking, he has called for the planting of a million trees by 2012, although he hasn't revealed how the Great Forest of Livingstone would be financed.

He produced this impressive figure while taking his tour group of 80 "ambassadors" and 20 aides around India last year, having been inspired by what has been done in Delhi. "We're heading for some tremendously hot summers," he said. "People will start dying in their thousands if we don't start planting trees to cool the streets."

And he displayed his horticultural expertise by predicting which trees would cope with the heat, observing that many native varieties won't. "The London plane will be fine but a lot of silver birch won't make it," he forecast. "Fig trees are going to be brilliant. In 10 years, we will be growing oranges."

There are few enough native trees in London's streets anyway, Livingstone may or may not know. The horse chestnut came from southern Europe in the 17th century. The London plane is no such thing, a hybrid first recorded around the same time. The gingko came from China in 1689. Robinias, or pseudo-acacias, were introduced from America and spread around London by William Cobbett in the 1800s.

Trees are thus a rich part of London's post-imperial, multicultural heritage, Ken might like to emphasise, as well as each and every one an ecological boon. For his part, Boris needs to raise his bid in the bosky stakes. Trees could yet be great beneficiaries of the candidates' campaign promises. There are said to be five million in London but the more we have the better. Truth be told, some of us prefer them to people.