It's tough being a street tree in Britain. Cars soak you in exhaust fumes, workmen douse you in de-icing salt, the skies are grey with acid rain. People attack you with Stanley knives and battery acid. The council plants you, incompetently, and forgets you. Then there are the vivisectors, graduates of the Sweeney Todd school of medicine, performing major operations on your vital organs, digging up the pavements and mangling your roots. It's a wonder you survive.

Yesterday's report from the Department of the Environment on the state of our urban trees confirmed what many people have long suspected - that the days of the big street tree are numbered. The planes, limes and chestnuts planted in Georgian and Victorian times have become arboreal dinosaurs - lumbering beasts out of scale with the tiny, tight, hectic environments our cities have become. When the big trees go, the little trees - if we are lucky - come: cherries and rowans, ornamental maples.

For the landscape of our towns and cities, and also for the quality of life we can experience in them, the consequences will be profound: it represents one of the biggest changes in urban form of the past three centuries. But the demise of the big tree conceals a paradox, one that goes to the heart of our responses to the natural world. We love trees more and more - yet appear to want them near us less and less.

If the United Nations can be believed, our trees are the least healthy in Europe. A survey in July based on 33 countries concluded that nearly 60 per cent of British trees showed signs of ill-health such as leaf loss and discoloration. This is worse than in many former Eastern bloc countries - the Czech Republic, for instance - which have become bywords for pollution. The Forestry Commission disputes the findings - many British trees appear to be poorly foliated but may have been no better in the past. Nevertheless its own recent studies, and those of the Countryside Commission and the Government, suggest that the urban tree is coming under unprecedented stress.

The oak is in declining health - in part because of a mysterious but sometimes lethal disease known as oak dieback. This year there have been reports of alder dieback, too. A couple of summers ago the leaves of London planes began to wilt early. One does not have to be a conspiracy theorist to see a pattern to all this, even where the arboriculturalists handily pinpoint a specific disease - in the case of London planes, for example, a condition called anthracnose. But 'dieback' belongs to that sub-species of noun favoured by scientists at a loss for an explanation.

This ought not to be happening. Historically, among the the most lethal destroyers of plant life in cities was acid rain, the result of sulphur dioxide pollution from burning coal and oil. In the 19th century it was impossible to grow conifers in cities - and in the parks of Manchester even rhododendrons, now viewed as little more than a rampant weed by conservationists, survived only briefly. Acid rain is with us still, but the worst days of sulphur dioxide pollution are over. Shouldn't our trees be faring better?

Sulphur dioxide has been replaced by a more pervasive range of pollutants. The horse-drawn hansom cab, which a century ago provided a handy source of manure, has been replaced by the car, which produces less visible but toxic emissions - notably the oxides of nitrogen, which combine with sunlight to produce ozone. Ground-level ozone, the main component of the infamous Los Angeles photochemical smogs, is a regular feature of British urban summers. In winter traffic demands clear roads - which means de-icing salt, poisonous to many species. Global warming, meanwhile, may be responsible for the prolonged drought of the later 1980s, another source of tree damage. And then there is high-level ozone, the loss of which is letting more UV radiation into the atmosphere - and which is thought to impair photosynthesis.

Much of this, given the lack of research and absence of reliable historical knowledge about the condition of our trees, is bound to be speculative. What is not in doubt is the damage potential of the street tree's most populous neighbour - human beings.

Most people proclaim their love of trees. The forest that spread over most of our landscape when the last ice sheets retreated 10,000 years ago is the wildwood of legend and literature. We worshipped there until we were forced into churches - but we brought it with us in the form of fluted columns and Gothic vaulting. When the age of cities dawned, we planted trees to remind ourselves of our old habitat: some argue that the most popular and most widely practised urban form is that directly borrowed from woodland structure, with tree-height buildings, winding streets (woodland paths), squares and courtyards (glades). In the Sixties and Seventies we filled our new towns with trees, in the Eighties we embarked on the creation of a new national forest, and 12 community or 'urban' forests. Some of us hug trees, many more of us help to defend them from the bulldozer or the chain saw, millions of us pay extra for them if we are buying a house.

Much of this is based on the assumption that trees make cities liveable - and thus that they represent, albeit in terms the Government would not recognise, a social and economic asset. Such reputation as British cities retain for liveability rests to a surprisingly large degree on the big trees planted by the Victorians, who faced for the first time the full-frontal awfulness of the industrial city and developed a sense of amenity that puts ours to shame. Yet the late 20th century has a much better scientific awareness of the reasons for planting trees in cities. Research suggests that they increase neighbourliness as well as property values, offer a 'gateway to a better world', act as a form of therapy: hospital patients recover faster when they can gaze at trees instead of blank walls. We also know that they are excellent 'air conditioners' - absorbing pollution, providing shade, moisture, a home for wildlife. And they guzzle up carbon dioxide, the main cause of global warming.

We know all this and we mean well, but reality, in cities, is a different story. Despite half a millennium of trying, we still can't plant them properly. For lack of water, decent soil, regular attention, half die within five years. And we are invading their space.

When our first city trees were planted, the soil was rich, uncluttered. Then came town gas, water, sewerage, electricity. Here, four decades ago, is Martha Quest, heroine of Doris Lessing's novel sequence Children of Violence, not long arrived from an African bush farm, surveying a 'great gaping jagged hole' in a south London street and marvelling at its contents: 'It was a yellowish soil. In it was embedded a system of clay pipes, iron pipes, knotted cables. No roots. No trees in this street, not one tree: heavy damp soil. therefore, no roots How long did roots live under a crust of air-excluding Tarmac?'

Not long, is the answer. The latest insult to tree roots is cable television, being driven under 60,000 miles of urban pavement, severing roots and posing, say experts, the biggest threat to British trees since Dutch elm disease or the great storm of 1987. Yet because of worries about subsidence and property damage - for which trees are often (usually wrongly) blamed - Londoners are already 'in danger of becoming negative' about tree-lined streets, according to the London Tree Officers' Association. Tree root claims are on the increase. In its way, it is the classic dilemma of a society that wants to be green but likes its home comforts. We hate cars unless we're driving them. We love trees - but they get in our way. There is a cold economic logic about it, but it is a recipe for joyless cities.

(Photograph and graphic omitted)