I strolled through a wood last week and didn’t even realise it. According to a UN definition, London can be classified as a forest, its 8.4 million trees – almost one for every person – adorning and detoxifying this great city. I was on my way to a wood-panelled room in the House of Lords for the launch of a study calculating the value of London’s trees using open-source software developed in America.

The i-Tree study, undertaken by volunteers, charities and government agencies including the Forestry Commission and Natural England, shines vivid light into the urban jungle. London may be renowned for the handsome plane trees that dominate its centre, but the capital’s most common tree is actually the sycamore, followed by English oak and silver birch.

Condemned lime trees in Rustling Road, Sheffield Photograph: Christopher Thomond/The Guardian

We appreciate the aesthetic qualities of trees – trees on leafy streets have been shown to boost house prices by as much as 15% – but i-Tree reveals their practical value. Trees remove 2,261 tonnes of pollution from London’s air each year. Nearly 40% of London’s surface area is impermeable concrete, and so canopy cover plays a crucial role in storing and slowing rainfall: trees prevent 10 times the volume of water in the Serpentine from entering – or flooding – drains each year. Trees also store carbon and cool buildings, reducing energy use in summer and winter.

All these “ecosystem services” mean that it would cost £6.1bn to replace London’s trees, according to i-Tree, and that’s not calculating their contribution to public health and general joy. We’re on shaky ground if we reduce our trees to one price tag. Where individual urban trees have been given a specific value, developers have offered to pay local authorities so they can chop them down. But i-Tree could transform all cities by confirming trees’ status as green infrastructure that is as important as buildings and broadband.

The i-Tree study reveals wonder – London has a diverse 126 species – but also danger. The capital’s grand planes are menaced by plane wilt, a destructive disease moving northwards through France at a much faster rate than previous decades. We need a minister for urban trees (Rory Stewart, the smart environment minister who could pass for a forest elf, would be perfect) who can help us plan, and plant, for the future.

Core values

Perfection. Excelsior. Sleeping Beauty. Hunter’s Majestic. Maclean’s Favourite. Horsford Prolific. These fine boasts are old varieties of apple tree, once in danger of disappearing because of our appetite for Braeburn and Granny Smith. Unusual old varieties have not been lost, however, thanks to charities such as the East of England Apples and Orchards Project, from whom I’ve just ordered some trees for my garden. We are a nation of apple lovers – another i-Tree revelation is that apple is inner London’s third most common species – and developers are belatedly cottoning on. I visited an estate in Bicester last week where each new home is offered with a fruit tree of choice. Warm glow all round, and for less than £15. I can’t wait for the arrival of my new friends: Caroline, Captain Palmer and Jordan’s Weeping.

Phoney hog wars

Two cheers for the Times making the British Hedgehog Preservation Society one of its Christmas charity beneficiaries. With hedgehogs’ rural population halving since 2000 and urban population falling by a third, hedgehogs need all the help they can get. What they got from the Times, unfortunately, was an ill-informed editorial declaring “more hedgehogs means fewer badgers”. Hedgehogs and badgers have lived alongside each other for millennia. The real cause of hedgehog decline? Intensive farming – and intensive gardening. Far better to ban toxic slug pellets and make gardens less impenetrably fenced than cull badgers.