Bullied by her sister and overlooked by her parents, teenage misfit Anne finds solace in the woods near her house. Under the canopy of trees her thoughts settle and her large, ungainly frame feels "normal". She decides to make the woods her home until developers disturb her idyll. Beatty's portrait of a gentle and misunderstood outsider is a fascinating glimpse inside the mind of someone labelled "bag lady" by society. Her depiction of Anne's daily routine reveals a Beckettian understanding of the rhythms and habits through which we structure our lives. Pollard is a heartbreaking, triumphant debut that renders in microscopic detail the secret life of the forest and explores the consequences of humankind's desire to tame nature.

Literature's outsiders tend to be male, so it's refreshing to meet Anne, the unlikely heroine of Laura Beatty's offbeat first novel. The odd one out among her ratty little siblings, she was, by the time she was 14, "too high and too wide for most of them to notice". She likes words, but "never achieved full understanding"; she is a lumpen, unwanted presence in the crowded family home, and her future looks like a choice between the abattoir and the poultry plant. And yet, visible from her bedroom window, is the forest: "so many tall things and so happy", pulling her towards it "for a bit of space, a little quiet and to put her size into perspective".

One day, she decides to stay; and after the terror of first nightfall ("Don't attempt the wood on a night as dark as this ... There's too much of your own mind out there"), Pollard becomes her survival story. Beatty's description of the slow process by which Anne goes native - building first a lean-to, then a wooden hut; damming herself a pool; eventually creating a chicken run and vegetable garden - is meticulous and utterly convincing. She steals a dustbin for an oven, digs out a cold store, dries and grinds worms for protein: a 21st-century Robinson Crusoe in the heart of England.

Of course, the modern world is never far away; this is a managed forest, with a café, where they talk to her loudly but tolerate her presence, and a Ranger, who turns a blind eye in return for produce. On her travels, foraging for food and litter, she meets dog walkers and doggers; sees her father cycle past on his way to work ("He didn't cross the road or anything") and even bumps into her unpleasant sister Suzie ("Who the fuck of all the crazy cows ... What do you think you look like?"). Beatty very subtly shows us Anne, who could never define herself, through the eyes of others, including the damaged Falklands veteran who teaches her about survival, and the young boy "Peter Parker", with whom she develops a prickly friendship. Her feelings for these two are as intense and uncontrollable as they are inexpressible. Beatty has a wonderful ear for voice, especially the voices of children, and the characters she constructs through Anne's skewed perception are funny and heartbreaking by turns; but what is really impressive is how she weaves her human comedy with the most powerful nature writing.

Through Anne, Beatty describes with intimate, slow-gained knowledge the flora and fauna and the passage of seasons, Anne struggling through the dead white fogs of winter or strolling summer's "loops of green light". She has noticed how birds always face into the fiercest wind; how "inside a wood, rain is sound first and wet second"; how "Owls can't fly at all. They make a right mess of it, wings everywhere, rocking about. Chaos." Most of all, she describes the trees, from their autumn leaves, bright with "all that stored sunlight", to spring's "secret of transformation . . . turning by multiplication the sealed scales of the bud into fistfuls of leaves".

The trees - "always moving, going nowhere" - act as a chorus, punctuating the novel with their stoical, impassive observations: "Let it go." "Most things have no cure." "We are still working with water and with light." These sections, along with the Prologue, have a rhetorical staginess, as well as a narrative uncertainty, that the rest of the novel - so direct and sure - mercifully lacks. The Prologue shows us Anne as an old woman drifting about the town, "no one at home", before taking us back to her young days in the wood; when she was, indeed, at home. Yet this framing device, with its intimations of disaster, feels unnecessary; as does the rather florid climax in which Beatty gestures at universality, when the central narrative was so precisely about Anne as an individual.

"You do change with exposure." Beatty doesn't labour the point, but over time we see Anne becoming "hard as wood", her fingers "jointed and knobbled like roots". Curling up through winter, hibernating, she adopts animal rhythms. When she bags her first pheasant, she boils and eats the lot, drinks the cooking water, and sleeps it off like a lion; she and the fox are the wood's "great predators". She begins to speak bird. Through Anne - as both individual and symbol of the forest that has accepted her, but is now "growing signs" about cycle tracks and walkways - Beatty explores questions of ownership and access, wilderness and desecration. "I don't need bag ladies in my wood," says Ranger. Once discovered by him, and throughout their uneasy feudal relationship, Anne is never at ease again; fallen from her state of nature, she starts to question her own role in the forest, to feel her alienation from the natural cycle. Peter Parker bursts into her clearing like a child of nature, who "fitted himself like a glove ... made without awkwardness", but Anne's existence baffles and repels him by turns.

Beatty's forest is both a modern, managed location and a magical place encircling its hundreds of years of history. Anne's hut, its walls decorated with scores of abandoned dummies hung on nails, must look like a witch's lair (and on a fairy-tale reading of the novel, Anne is not backward but a moon-faced changeling). In Pollard, Beatty beautifully conveys the loneliness and the ecstasy of an unknowable character, and the charged, complex presence of the natural world around us. Both are too often only in our peripheral vision; she looks at them directly. This novel heralds an exceptional talent.

The desire to slip the noose of civilisation and escape to the wilderness reverberates through literature. It can be glimpsed in Yeats's longing for the bee-loud glade of Innisfree, in the fearful fantasy of Robinson Crusoe and in Walden, Thoreau's passionate experiment in wild living. It is this impulse that is at the heart of Laura Beatty's bewitching first novel, the tale of a young girl who starts a new life in a forest.

Anne's the odd one out in her family. There's something not quite right about her: she's clumsy and slow, barely speaks and has long since outstripped the rest of her petite, sharp-tongued brothers and sisters. She's subjected to a nagging litany of complaint: 'Get out of the way are you listening to me do something useful do the tatties for me get the pizza out of the freezer if I have to tell you one more time.' The hanging wood that sprawls at the bottom of the valley is the only place where she feels at home and so, at the age of 15, she decides to stay there.

At first, Anne has nothing and every possession she does acquire is the result of a painful lesson in its necessity. But wildness is already rubbing off on her. Drenched and starving, she learns to build a shelter, to dam a stream for water and to catch rabbits with handmade traps. Later, she finds friendship and a touching, awkward, almost-love with Slow Steve, an ex-soldier she encounters over the body of a dead cat. Later still, there's an intense relationship with a boy called Peter Parker. But each period of joy is followed by pain, building up in volume to a final, appalling climax.

Like Nicola Barker, who she occasionally recalls, Beatty is drawn to the margins of society and to the misfits who congregate there. From the beginning, the wood is a negotiated zone, shared out uneasily between humans and wildlife. At its borders are the dump where Steve lives, an estate, an abattoir and a battery farm. This is the countryside as it really is: a place of poachers, litter and the occasional dreamlike presence of deer. But despite the robustness of Beatty's vision, there sounds through the book a note of elegy for a wildness that is under threat, for a beauty that can be destroyed in a flash. Watching a road being laid through the forest, Anne feels 'the slow suffocation of the trees'; the chainsaws that follow are purgatorial.

Set against these passages is the voice of the wood itself. It is a brave writer who attempts to give tongue to a forest, but there is nothing whimsical about Beatty's Chorus of Trees. The woods are dispassionate observers; they witness change in the knowledge that nothing will last. Their concern is for life, not the individual. When it comes to the final tragedy, set in motion the day Anne looks up to find a golden-haired boy throwing stones in her pond, this point of view seems callous and it is only at the novel's end that the long-range vision of the forest begins to take on consolatory properties.

What happens to Anne, who so charms the reader that it is a shock when a stranger points out how hideous she is ('So, is your beard real or did you get it from a joke shop?'), is heartbreaking. She is destroyed piecemeal by the relentless incursion of civilisation into every last thicket and copse of her enchanted home. If there is a moral in this fierce and wonderful book, it is that we, the humans, are the ones who will lose out if we continue to desecrate the complex, subtle world we have inherited. As the Chorus of Trees has it: 'The colours are irrelevant. They are for others. They mean nothing to the trees, the reds and golds. They are just the memory of burning days.' It is up to us what we prize.

But the title suggests a rather more hopeful viewpoint. A pollard is not a natural tree, but one that has been worked by humans, altering its shape. As a symbol, the pollard suggests that something positive and unexpected can arise from our interactions with the wild. Perhaps Anne represents a way of being that remains within our grasp. If that is true, then despite its readability, Pollard is the precise opposite of escapist literature, because it gives the reader back the world. This is just the sort of generous, provocative novel the Booker judges should cherish.

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