Clive Anderson is the president of the Woodland Trust. After practising as a barrister for nearly fifteen years, he began a career on radio and TV, mainly on comedy and chat shows including Whose Line Is it Anyway? (BBC Radio 4 and Channel 4) and Clive Anderson Talks Back (Channel 4), but also on some more serious programmes. He currently presents Loose Ends and Unreliable Evidence (both on BBC Radio 4).

Everyday but extraordinary, trees form the backdrop to London life. But they are also vital to our wellbeing: we neglect them at our peril.

Although we might like to think of Britain as a green and pleasant land, blessed with an abundance of forests, woodlands and hedgerows, tree cover in our countryside is really quite low, considerably less than half of the European average. But, as it happens, London, the most built-up bit of the country, does rather well. There are perhaps 8m trees in the Greater London Area, more or less the same number as there are people.

This is only an estimate. Trees can be tricky to count. Apart from anything else, it can be difficult to decide what a tree is, exactly. A few years ago, for the purposes of a planning case, Mr Justice Cranston had to answer the question ‘what is a tree?’ – which he was only able to answer in a written judgment that ran to 12,000 words (Palm Developments v Secretary of State for Communities & Local Government 2009).

But for the most part trees are obvious enough, your lordship, and a joy to behold. One of the many charming things about trees is that they are at the same time both mundane and magical. Magical in their extraordinary size, shape and form – mere plants which through evolution have become veritable giants, competing to grow upwards, fighting to secure their place in the sun, the biggest and oldest living things most of us will ever meet. They tower above us, overshadow us, and can outlive us by hundreds of years. But they are also mundane: everyday items seen on a regular basis by even the most urban city dweller whose lifestyle more or less ignores the natural world.

Trees have been planted or otherwise found a way to grow in every type of space in London: in the elegant parks and squares of central districts, in recreation grounds, in open spaces and gardens throughout the urban sprawl. They are a byword for comfortable living – leafy suburbs – but they are also found in edgier locations: on the mean streets of the inner city, often locked up in metal cages for their own protection or lined up to camouflage unattractive industrial units. When architects produce a drawing of a new building, they almost always sketch in some semi-mature trees to improve the look of their designs.

We like our trees so much in London that one day there could well be some growing suspended above the Thames, if plans to build the garden bridge designed by Thomas Heatherwick ever come to fruition. There could be more creative use of trees in this sort of way if London were to follow the lead of cities such as Paris and Toronto and strongly encourage commercial buildings to put gardens on their roofs.

Like their country cousins, trees in town do a great deal of good for the natural environment. They provide shelter and sustenance to birds, bees and all sorts of other wildlife; they absorb water in time of flood; and they create shade, which, with the transpiration of water through their leaves, ameliorates the high temperatures generated in built-up areas. They also reduce pollution. A recent study showed that a line of silver birch trees reduced particulate matter in the air by more than 50%.

Life in the city offers trees a different range of challenges compared to the country. There are fewer deer to gnaw at their bark and rabbits to nip them in the bud, but they are more likely to suffer vandalism, broken branches and mindless dog owners getting their pets to practise biting on their lower limbs and trunk. They have to cope with the heat reflected off hard surfaces and damp conditions for their roots underneath the pavements and roadways.

Not all trees can withstand the effects of pollution. Until the clean air legislation of the 1950s London was a city of smoke and smog, caused largely by domestic coal fires, which turned the very bricks and stones of our buildings black and brown. But famously able to cope has always been the plane tree – the London plane as we call it, choosing to ignore its widespread success all around the world. London or not, the plane is a robust hybrid between an oriental plane and the American Sycamore (Platanus × acerifolia). Its naturally peeling bark enables it to shrug off dirty deposits and, as a result, huge numbers of impressive specimens are long established in avenues, lines and squares all over London.

From the coal smoke point of view, London air is much cleaner than it was – but now there are other pollutants belched out by the traffic on our congested roads. As it happens, these may not be too bad for trees. According to a recent study, the levels of nitric oxide generated by traffic fumes reduce the concentration of ozone in the air – and the higher amounts of ozone in country districts apparently stunt tree growth.

Banes and contagions

But not everything in the city garden is rosy: not at all. Trees are facing a succession of diseases, some of which can be devastating. A few decades ago, Dutch elm disease brought down English elms almost everywhere – although, as it happens, a few have survived in London, including one in Marylebone and two in Gibson Square in Islington.

But now ash trees are under attack from a fungal disease, Chalara fraxinea, which has made its way here from the continent and threatens to be almost as bad for the ash as Dutch elm disease was for the elm. Things may not be as bad for London as for the countryside: the news from Denmark, which has already suffered from ash dieback, is that the effects are less marked in towns, due to removal of leaf litter and drier conditions leading to fewer fruiting bodies. Ash trees outside forests can actually live with the disease for years.

Ash dieback is the disease that is occupying centre stage at the moment, but there are plenty more waiting in the wings. Another fungus, Phytophthora ramorum, identified in the United States as the cause of the ominously named ‘sudden oak death’, is not killing oaks in this country but has already caused the destruction of acres of larch trees. Oaks, however, are affected by Acute Oak Decline, which has been observed in British oaks for more than 20 years. It’s not a pretty sight: symptoms include black weeping patches on tree stems. It appears to be caused by two new types of bacteria, possibly spread by the oak jewel beetle.

Then there is the processionary moth, which gets its name from the head-to-tail conga lines formed by its caterpillars. The caterpillars’ food of choice is oak leaves and a long line of them can strip a whole oak tree of its foliage in short order. This may not kill the tree straight away, but it certainly weakens it. The moths have already been found living in several London boroughs. Efforts to eradicate them have included sucking them off the trees with vacuum cleaners and spraying them from the air from helicopters. Mothballs don’t work.

Horse chestnut trees – which have been in Britain for centuries and which, aside from supplying conkers for autumn games, are a highly decorative feature of London streets – may be on their way out. You don’t have to be a committed tree-hugger to have noticed that the leaves on horse chestnut trees no longer wait for the autumn to turn brown. They wither and die when it is still summer. This is a sign of infestation by the horse chestnut leaf miner, a moth recently arrived from Eastern Europe. And if the leaf miners don’t get them, bleeding chestnut canker – conker canker – will. Chestnut trees may not be dying immediately, but they have already lost their good looks and new specimens are unlikely to be planted ever again.

Depressingly, there are also new threats to Scots Pine, juniper and box trees. And should the Asian longhorn beetle or the Emerald Ash Borer establish themselves on these shores, things could get even worse for a whole range of broadleaf trees.

Even the London plane, which has taken everything nature and man has thrown at it for hundreds of years, is suffering from a new disease – massaria. This weakens the branches, which are liable to fall off – bad news for a tree that has people passing underneath it and risk-averse local authorities deciding its fate.

The news from France is even worse: Ceratcystis platani – plane wilt – is killing the avenues of plane trees planted along French roadsides and the shade trees of the Canal du Midi. The disease is travelling north and, if it gets across the Channel, thousands of London planes could be lost: something of an ecological disaster, right in the centre of the city.

So in the future, we may have to pay even greater attention to our trees, otherwise these beautiful and useful giants, both magical and everyday, will become rarer and rather less impressive, a loss to our city streets.