Bryson's America: Fall. It's enough to turn me into John Denver

EVERY YEAR about this time, for a tantalisingly short while - a week or two at most - an amazing thing happens here. The whole of New England explodes in colour. All those trees that for months have formed a sombre green backdrop suddenly burst into a million glowing tints and the countryside, as Frances Trollope put it, "goes to glory".

Yesterday, under the pretence of doing vital research, I drove to Vermont and treated my startled feet to a hike up Killington Peak, 4,235 feet of sturdy splendour in the heart of the Green Mountains. It was one of those sumptuous days when the world is full of autumn muskiness and rangy, crisp perfection, the air so clean and clear that you feel as if you could reach out and ping it with a finger, as you would a polished wine glass. Even the colours were crisp: vivid blue sky, deep green fields, leaves in a thousand luminous hues. It is a truly astounding sight when every tree in a landscape becomes individual, each winding back highway and plump hillside is suddenly splashed with every sharp shade that nature can bestow - flaming scarlet, lustrous gold, throbbing vermilion, fiery orange. Forgive me if I seem a tad effusive, but it is impossible to describe a spectacle this grand without babbling. Even the great naturalist Donald Culross Peattie, a man whose prose is so dry you could use it to mop spills, totally lost his head when he tried to convey the wonder of a New England autumn.

In his classic Natural History Of Trees Of Eastern And Central North America, Peattie drones on for 434 pages in language that can most generously be called workmanlike (typical passage: "Oaks are usually ponderous and heavy-wooded trees, with scaly or furrowed bark, and more or less five- angled twigs and, consequently, five-ranked leaves..."), but when he turns his attention to the New England sugar maple and its vivid autumnal regalia, it is as if someone has spiked his cocoa. In a tumble of breathless metaphors he describes the maple's colours as "like the shout of a great army ... tongues of flame ... the mighty, marching melody that rides upon the crest of some symphonic weltering sea and, with its crying song, gives meaning to all the calculated dissonance of the orchestra".

"Yes, Donald," you can hear his wife saying, "now take your tablets, dear."

For two fevered paragraphs he goes on like this and then abruptly returns to talking about drooping leaf axils, scaly buds and pendulous branchlets. I understand completely. When I reached the preternaturally clear air of Killington's summit, with views to every horizon soaked in autumn lustre, I found it was all I could do not to fling open my arms and burst forth with a medley of John Denver tunes. (For this reason it is a good idea to hike with an experienced companion and to carry a well-stocked first aid kit.)

Occasionally you read about some academic who has gone out with the scientific equivalent of a paint chart and announced with a grave air of discovery that the maples of Michigan or the oaks of the Ozarks achieve even deeper tints, but this is to miss completely the special qualities that make New England's fall display unique. For one thing, the New England landscape provides a setting that no other area of North America can rival. Its sunny white churches, covered bridges tidy farms and clustered villages are an ideal complement to the rich earthy colours of nature. Moreover, there is a variety in its trees that few other areas achieve: oaks, beeches, aspens, sumacs, four varieties of maples, and others almost beyond counting provide a contrast that dazzles the senses. Finally, and above all, there is the brief, perfect balance of its climate in the fall, with crisp, chilly nights and warm, sunny days, which help to bring all the deciduous trees to a co-ordinated climax. So make no mistake. For a few glorious days each autumn, New England is unquestionably the loveliest place on Earth.

What is all the more remarkable about this is that no one knows quite why it happens.

In autumn, as you will recall from your school biology lessons (or, failing that, from Tomorrow's World), trees prepare for their long winter's slumber by ceasing to manufacture chlorophyll, the chemical that makes their leaves green. The absence of chlorophyll allows other pigments, called carotenoids, which have been present in the leaves all along, to show off a bit.

The carotenoids are what account for the yellow and gold of birches, hickories, beeches and some oaks, among others. Now here is where it gets interesting. To allow these golden colours to thrive, the trees must continue to feed the leaves, even though the leaves are not actually doing anything useful except hanging there looking pretty.

Just at a time when a tree ought to be storing up all its energy for use the following spring, instead it is expending a great deal of effort feeding a pigment that brings joy to the hearts of simple folk like me but doesn't do anything for the tree.

What is even more mysterious is that some species of trees go a step further and, at considerable cost to themselves, manufacture another type of chemical called anthocyanins, which result in the spectacular oranges and scarlets that are so characteristic of New England. It isn't that the trees of New England manufacture more of these anthocyanins, but rather that the New England climate and soil provide exactly the right conditions for these colours to bloom in style.

In climates that are wetter or warmer, the trees still go to all this trouble - have done for years - but it doesn't come to anything. No one knows why the trees make this immense effort when they get nothing evident in return.

But here is the greatest mystery of all. Every year literally millions of people, genially and collectively known to locals as "leaf peepers", get in their cars, drive great distances to New England and spend a succession of weekends shuffling round craft shops and places with names like Norm's Antiques and Collectables. I would estimate that no more than 0.05 per cent of them stray above 150 feet from their cars. What a strange, inexplicable misfortune that is, to come to the edge of perfection and then turn your back on it.

They miss not only the bosky joys of the out of doors - the fresh air, the rich, organic smells, the ineffable delight of strolling through drifts of dry leaves - but the singular pleasure of hearing the hills ringing with Take Me Home, Country Roads sung in a loud voice in a pleasingly distinctive Anglo-Iowa twang.

And that, if I say so myself, is definitely worth getting out of your car for.

`Notes from a Big Country' (Doubleday, pounds 16.99)