A New Year riddle: why an oak tree is just an elaborate way to make aco rns

OF ALL the ideas for spending the revenues of the National Lottery, this is the most seductive. A coalition of Scottish ecology groups, from the green radicals of "Reforesting Scotland" to the Worldwide Fund for Nature, is proposing to restore th e ancient Caledonian Forest which once covered much of the Highlands.

Even today, visitors return from the Highlands and its empty vistas of stone, bog and heather, and bray about "our last natural wilderness". Yet it is more than 50 years since pioneer ecologist Sir Frank Fraser Darling showed up that phrase for the sentimental lie it is. Looking at this landscape with the eye of a scientist and a social historian, he saw, not a natural wilderness, but a man-made desert.

As late as the Middle Ages, much of the Highlands was covered with trees. In the southern foothills, there was deciduous forest of oak and alder, while the steep slopes above the western sea-lochs were covered with bog-oak and scrub birch. In the centralglens, and up the slopes as far as the tree-line, there were enormous stands of native pine - the classic Caledonian Forest - which gave shelter not only to many species of deer but to wolves, bears, lynxes and boar. The tree roots held the soil in place, preventing the rainfall from eroding and scouring away the fertile earth lower down the slopes which human beings used for pasture and crops.

Then two kinds of development shaved the trees off the hills. One was early industry, felling the hardwood to build timber ships and make charcoal for iron furnaces. Higher up, some of the pine forests were deliberately burned to smoke out outlaws. But then, above all in the 19th century, came the Clearances which expelled the Gaelic population from the inland glens and replaced them with sheep or with red deer for the great stalking estates. The sheep and deer ate the new pine seedlings; the forest cea sed to regenerate and shrank to a few surviving woods. Without tree cover, the soil was swept downhill or became waterlogged and turned to peat-bog. Three centuries of abuse were enough to turn the Highlands into the desolate place we know today; one of the ecological disaster-areas of Europe.

The Forestry Commission began the counter-attack while Fraser Darling was still alive. But all those blue-green slopes of sitka spruce, grown for the timber market ... that is not what the Millennium Foresters want. When they talk of forest, they use a glittering, New Age word: "Wildwood". They mean something shaggy and trackless. They mean a green universe into which men and women go to become lost children in a fairy story. When the Scots pines have grown tall, the forest guardians will not mind modest cropping here and there, to provide local jobs or pay a forester's salary. But that will be on the margins. Essentially, the Wildwood of Caledonia will grow untamed, inventing its own underbrush, clearings, flowers and fungi. This wi ll be a pre-human world, not a slice of "heritage".

Is it possible? Probably it is. There are patches where enthusiasts are planting already, and the Scots pine seedlings grow thicker and faster than they expected. "Reforesting Scotland" has published telling photographs showing almost identical glacial glens and bays in Norway and in Scotland: the first mantled with forest above flourishing little fields, the other bare and stony. It can be done. But - and here is the test of nerve for the Millennium Commissioners - a Wildwood is also political. It is nobody's property. It would replace private shooting estates with public forests.

But "public" or "national" in this sense mean more than merely some state-owned resource. A Millennium Forest should evoke something far deeper. It should amount to a sort of apology, at the end of a millennium in which human beings became arrogant enough to believe that they owned the natural world.

Altogether, a noble idea. Everywhere, similar projects for restoring landscapes are springing up. A mile from where I am writing, on the Argyll coast, they are blocking up the drains of a huge peat-bog so that it can return to a primeval, cow-swallowing "raised mire". And yet I feel a prickle of anxiety. "Put everything back as you found it!" It's a fine principle, but you generally apply it before you leave. Are we, the human race, subconsciously beginning to think about leaving? Could the Millennium Forest be the first of a series of handover ceremonies, rather like the departure of the British from their colonial empire, at which the flag of homo sapiens is hauled down and one territory after another is returned to beavers, pine martens and otters?

There are several ways to leave. We cannot abandon the planet yet, neither can the human race at its present size squeeze into a smaller perimeter. But note those words "at its present size". There is another way to leave the space we occupy - by becoming fewer.

At first, the thought of becoming fewer suggests an Asian government campaign for contraception. But I am interested by something more strange and fundamental. It begins to look as if human populations have an in-built capacity to diminish their numbers,to shrink in order to adapt to new circumstances. The easy explanations - that societies too large for their own resources are cut back by starvation, plague or war - do not fit the facts. There is another mechanism, which nobody yet understands.

Take the British case. Historians now think that the population of Roman Britain may have been as high as four million. Then, in the third century AD, the Empire went through economic and political crisis. In Britain, cities were almost abandoned, trade and transport broke down and people left outlying fields to the weeds. At about this point, the population began to fall, dwindling so fast that 200 years later there may have been only two million people in Britain south of Hadrian's Wall. It was not until the early Middle Ages that the population again reached, and then passed, the Roman total.

How was this reduction done? Saxon massacre or disease do not account for it, but that is about all we know. Maybe people started marrying later in life, or less often. Or did Christianity's stricter incest rules reduce births? The bones in the graveyards, curiously enough, suggest that women lived longer and became slightly healthier while the population figure was plunging. But the mystery remains. All that is clear is that the human species possesses this ultimate escape mechanism - like a trapped lizard which can shed its own tail.

In 1995, if the Millennium Forest project goes through, a piece of Britain would begin to be put back as we found it. But what about putting back the British as they were found by the Romans? Four million seems about right, as a target for the 23rd century. Some may protest that to dwindle from nearly 60 million to only four would be an admission of failure. But perhaps it would be just the opposite: a triumph of adaptability. My New Year riddle for 1995 is about trees, but also about the beauty of smallness: Isn't an oak tree just an elaborate way of making an acorn?