When does planting a tree become a revolutionary act – and unleash an army of gunmen who want to shoot you dead? The answer to this question lies in the unlikely story of Wangari Maathai.

She was born on the floor of a mud hut with no water or electricity in the middle of rural Kenya, in the place where human beings took their first steps. There was no money but there was at least lush green rainforest and cool, clear drinking water. But Maathai watched as the life-preserving landscape of her childhood was hacked down. The forests were felled, the soils dried up, and the rivers died, so a corrupt and distant clique could profit. She started a movement to begin to make the land green again – and in the process she went to prison, nearly died, toppled a dictator, transformed how African women saw themselves, and won a Nobel Prize.

Now Maathai is travelling the world with a warning. As she told the United Nations climate summit last Tuesday, it is not just her beloved rainforest that is threatened now, but all rainforests. "As human beings, we are attacking our own life-support system," she says. "And if we carry on like this, we are digging our own grave."

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Her story begins with one particular tree in the heart of Africa. In 1940, Maathai became the third of six children born to illiterate peasant farmers. Her father worked as a "glorified slave" for the British settlers who occupied Kenya. He was forced to do what he was told on their farms, and forbidden – like all black people – from growing his own food and selling it. The nearby town of Nakuru was strictly segregated, with Africans banned from the "European areas." As a child, Maathai escaped into the natural landscape. She studied the forests – how they absorbed water and turned them into streams, and how they were filled with life. She would sit for hours under one particular fig tree, which her mother told her was sacred and life-giving and should never be damaged.

"That tree inspired awe. It was protected. It was the place of God. But in the Sixties, after I had gone far away, I went back to where I grew up," she says, "and I found God had been relocated to a little stone building called a church. The tree was no longer sacred. It had been cut down. I mourned for that tree. And I knew the trees had to live. They have to live so we can live."

I. A Daughter of the Soil

I am meeting Maathai in a busy hotel in London. She approaches me in the lobby – a tall, broad woman with a bright blue headdress and a slight limp – looking frazzled. "I have a flight in a few hours and I have packed nothing! Ah, you know how it is," she says. "Let's have coffee." As soon as we sit, she begins to talk about the trees, and a calm settles over her unlined 69-year-old face. "I am a daughter of the soil, and trees have been my life," she says. She begins to talk reverentially about how trees store carbon, regulate rainfall, hold soil in place, and provide food.

"I can't live without the green trees, and nor can you. I'm humbled by the understanding that they could get along without me though! They sustain us, not the other way round. We don't really know where we came from, where we are going, and what the purpose of all this is. But we can look at the trees and the animals and each other, and realise we are part of a web we can't really control."

She was only able to learn the hard science of the forests because her parents made a bold decision. At a time when girls were not often educated, her mother resolved to send her girl to school, and give her all the opportunities she had never had. The British settler her father worked for was furious: who was going to pick his pyrethrum? But her mother insisted – a rare and risky act of defiance. Maathai soon shot to the top of the class, and was offered a place at a Catholic boarding school run by Irish nuns.

When she was 13, in her first year away at school, the rebellion against the British occupation broke out. The Mau Mau guerrilla fighters took on the British occupiers to drive them away, killing around 100 people. The British fought back with astonishing ferocity, killing around 100,000 Kenyans. "The Home Guards had a reputation for extreme cruelty and all manner of terror," she says. Her mother was forced out of her home at gunpoint and ordered to live in an "emergency village" – a glorified camp surrounded by trenches. Men were not allowed in.

"My mother and father didn't see each other for seven years," she says. "I carried messages between them. That's how I ended up imprisoned for the first time." When she was 16, she was caught by British soldiers, and thrown into a detention camp. "The conditions were horrible – designed to break people's spirits and self-confidence and instil sufficient fear that they would abandon their struggle." It stank. She slept on the floor and wept. After two days, she was released. She adds: "I will never forget the misery in that camp. There is terrible trauma for everyone from those times." What does she think of the British historians who lyrically laud the British Empire, and say what happened in Kenya was merely a blip? "Well, that is the propaganda we all give to our subjects! They have to do that for them to support these terrible crimes."

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It was not only humans who were being cut down. Her forests began to be slashed by the British and replaced with vast commercial plantations growing tea for export. These plantations couldn't absorb and store water in the same way, so the groundwater levels fell to almost nothing, and the local streams dried up. After independence, Kenya's corrupt new ruling class continued the same policies, treating the forests as their private property to be pillaged.

But Maathai was offered a way out, to a place where she could ignore all this. After scoring extremely highly on the national exams, she was granted a place at an American university. It would be fully paid-for by the US Government, as part of a policy introduced by John Kennedy. She was one of thousands of young Africans – including Barack Obama Sr – who became part of "the Kennedy airlift" to study there. At first, "I felt like I had landed on the moon." She remembers getting into her first elevator: "I thought I was going to be pulled apart!" She was shocked to see men and women dancing pressed up against each other and women with relative freedom. She stayed for four years, majoring in biology in Kansas, and "America changed me in every way. I saw the civil rights movement. It changed what I knew about how to be a citizen, how to be a woman, how to live. But I always knew I would go back." Her forests were calling.

When she arrived back in Kenya, she soon became the first woman ever to get a PhD in East or Central Africa. She was a Professor by her mid-20s. But she was paid far less than men in the same position, and the entirely male student body at first refused to take lessons from her in anatomy. "But I showed them who was boss. A failing grade from me counted as much as from any man! That was a language they understood."

She met a young Kenyan politician called Mwangi Maathai, and adored him. She became swept up in his campaign to gain a seat in parliament, and quickly married him – but it soon started to go bad. "When Mwangi won the election, I was so happy for him. I said – what are we going to do now to get jobs for all the people we promised help for? He just said – oh, that was the campaign." She pauses, disgusted still. "I couldn't believe what I was hearing. He didn't intend to do anything."

She joined a group called the National Council of Women of Kenya, determined to give other people the opportunities she had been given. "Many of the girls I was at school with were back working in the fields and living in huts, and I wanted to help them," she says. When she went out into their areas, she saw the forests had been razed, and malnutrition was rife. She felt helpless and wondered what she could do. "Then it just came to me – why not plant trees? The trees would provide a supply of wood that would enable women to cook nutritious foods. They would also have wood for fencing, and fodder for cattle and goats. The trees would offer shade for humans and animals, protect watersheds and bind the soil, and if they were fruit trees, provide food. They would also heal the land by bringing back birds and small animals and regenerate the vitality of the earth."

She managed to persuade international aid organisations to pay women a very small sum – around 2p – for successfully planting each tree. At first, local men scoffed. What could women do? How could they make trees grow? What did this belong in our traditions? But women were soon organising themselves from village to village into independent committees. "We started by planting trees, but soon we were planting ideas! We were showing women could be an independent force. That they were strong."

But a scandal was waiting that threatened to leave Maathai broken – and broke.

II. Too strong

Mwangi Maathai was jealous of his wife's intellect and expected her to be submissive and obedient. "He wanted me to fake failure and deny my God-given talents. But I wouldn't do it," she says. One morning, he announced he was divorcing her – and it became a national news story. Divorce was, at that time, a huge scandal – and the woman was always blamed. When the case came to open court, it was filled with journalists eager to report on Mwangi's charges that she was an adulterous witch who had caused his high blood pressure and refused to submit to his will. She was, he announced, "too educated, too strong, too successful, too stubborn, and too hard to control." The men in the courtroom cheered.

"With every court proceeding, I felt stripped naked before my children, my family and friends. It was a cruel, cruel punishment," Maathai wrote in her autobiography, Unbowed. She adds: "I was being turned into a sacrificial lamb. Anybody who had a grudge against modern, educated and independent women was being given an opportunity to spit on me. I decided to hold my head high, put my shoulders back, and suffer with dignity: I would give every woman and girl reasons to be proud and never regret being educated, successful, and talented."

The judge found in her husband's favour, saying she had been a disgrace as a wife and deserved nothing. A few days later, she criticised the judge in an interview for his sexism – and he ordered she be tossed into prison for contempt of court. "So not only had I lost my husband but I lost my freedom," she says. The other women prisoners were very kind to her: they let her sleep in the middle of the huddle, so she wouldn't be so cold. "Far from beating me down, I felt stronger. I knew I'd done nothing wrong."

When she was released, she decided to run for parliament herself, to demand rights for women. She resigned from the university and announced her candidacy – only for another judge to declare she was ineligible to stand on the false grounds that she hadn't placed herself on the electoral register. The university – also under political pressure – refused to take her back.

Suddenly, "I was 41 years old and I had no job, no money, and I was about to be evicted from my house. I didn't have enough money to feed my children. I remember them wanting chips, and I just couldn't afford it. You never forget the sound of your child crying with hunger." She rubs her head-dress softly and says: "I thought about my mother. She had survived everything life put at her. She had been kept apart from my father for seven years in an emergency village. She always survived."

It was at this personal midnight that she returned to the small seeds she had begun to plant years before. She decided to urge women to plant whole forests. She wanted to see an entire new green belt across Kenya nurtured by women. A grant by the UN Development Programme and the Norwegian Government spurred it on, and she felt herself awakening again – along with the greening land.

Then one day she read about a threat to some of the country's most precious trees. Daniel Arap Moi, the thuggish dictator of Kenya, decided to build over Uhuru Park, the only green space in the capital of Nairobi. He wanted to replace it with a giant skyscraper, some luxury apartments, and a huge golden statue of himself. So she decided to do something you weren't supposed to do in Moi's Kenya: protest. She led large marches to the park, and wrote to the project's international funders, asking if they would happily pay to concrete over Hyde Park or Central Park.

"People said it would make no difference – that you can't make a dictator hear you, he's too strong," she says. "But I was in Japan a few years ago and I heard a story about a hummingbird. There's a huge fire in the forest and all the animals run out to escape. But the hummingbird stays, flying to and from a nearby river carrying water in its beak to put on the fire. The animals laugh and mock this little hummingbird. They say – the fire is so big, you can't do anything. But the hummingbird replies – I'm doing what I can. There is always something we can do. You can always carry a little water in your beak."

But the initial reaction to her protests was frightening. She began to receive anonymous phone calls telling her should shut up or face death. Moi called her a "madwoman," and announced: "According to African traditions, women should respect their men! She has crossed the line!" When she carried on, she was charged with treason – a crime which carried the death penalty – and was slammed away in prison. She had arthritis, and she says: "In that cold, wet cell my joints ached so much I thought I would die." But she would not apologise, or give in. "What other people see as fearlessness is really persistence. Because I am focused on the solution, I don't see the danger. If you only look at the solution, you can defy anyone and appear strong and fearless."

It was only after international protests began to gather – led by then-Senator Al Gore – that an embarrassed Moi had to let her go. She immediately started protesting again. After three years of campaigning against the developers and relentless death-threats, Moi finally relented. He dropped the project. The park was saved. A dictator defeated by a woman? Nothing like it had happened in Kenya before. It was the moment the Moi regime began to die. She says: "People began to think – if one little woman of no significance except her stubbornness can do this, surely the government can be changed."

A great green wave of trees was starting to grow across the country: some 35 million have been planted by her Green Belt Movement. But her confrontation with Moi was not over. As a symbol of resistance, Maathai was contacted by a group of mothers whose sons had disappeared into the prison system, simply for democratically opposing the regime. They believed their sons were being tortured. They were frantic with fear and grief. Maathai realised she could not refuse them. She told them to gather up blanket and mattresses, because they were going to go to Central Nairobi, plant themselves in Moi's vision, and refuse to leave until he released their sons.

On the first night, the police watched anxiously, unsure what to do. Hundreds of people gathered in solidarity. By the third day, there were thousands – and men started to publicly describe how they had been tortured by the police, and weep. "Nothing like it had happened in our country's history before," she says. But then the police swooped in with tear gas and batons. They beat the women hard, and Maathai hardest of all. She was carried away bleeding.

When she had to sign her name at the police station, she dipped her finger in her own blood, pouring from a crack in her head, and scrawled her name with it. The next morning, all the women went back. Maathai was there too, in a neck brace and bandages, insisting she would not be intimidated.

For a second time, Moi relented. There was a sense of shock in the country – the women had won against a Big Man again, using only peaceful political pressure. But there was a third confrontation coming. Moi announced he was going to raze most of the Karura Forest, one of the most precious green areas in Kenya. Again Maathai was there – this time facing down soldiers with machine guns, armed only with a small tree for her to plant. Again, she was beaten, and she nearly died. Again, she won in the end. Moi's rule was finally broken. Within a year, he was chased from office, and Kenya saw its first democratic elections in a generation. Maathai was elected to parliament in a landslide.

III. Death of the rainforests?

Are we in the rich world rendering these victories meaningless? As a scientist, Maathai is warning now that man-made global warming threatens to make the rainforests dry up and die, whatever she does in Kenya. She could save them from Moi – but can she save them from us?

"People don't realise how much they depend for their own survival on this ecosystem and how fragile it is," she says, almost pleading. "The world's forests are its lungs. Thick, healthy strands of indigenous trees absorb huge amounts of carbon dioxide and keep them out of the atmosphere. If the Congolese rainforests were entirely destroyed, for example, 135 billion tonnes of carbon dioxide would be released – equivalent to more than a decade of man-made emissions. So if we lose these forests, we lose the fight against climate change."

The rainforests can be killed from two directions – by the saws of men like Moi, or the warming gases of people like us. That is why she has left the land she loves, armed with the Nobel Peace Prize she won in 2004, and travelled so far: to try to persuade us to let the forests live. "There are moments in history when humans have to raise their consciousness and see the world anew. This is one of those moments. We are being called to assist the earth in healing her wounds, and in the process we can heal our own. We can revive our sense of belonging to a larger community of life. We can see who we really are."

When I tell her some people say environmentalism is a rich person's luxury without any relevance for the poor, she lets out a low scoff. "The exact opposite is the case! The people who are at greatest risk are poor. When the problems hit, it's them who are going to be hit worst. They have no money to adapt or flee. They still get their water from the rivers. Their agriculture is very dependent on rainfall. They will die first."

If man-made global warming continues at the current rate, she says there will be "catastrophe" in Africa. "First of all there will be a fast spread of the Sahara desert. It is spreading now. So events like what is happening in Darfur will get much worse. There will be violent competition over shrinking arable land, grazing land and water points as the desert spreads and dries up the land. Second, there will be crop failure because of changing rainfall patterns, and we will get massive starvation. What happens then? As we all know, people don't sit down and wait to die. They migrate. They will try to come here to Europe."

As she speaks, I remember the last time I saw the Congolese rainforest. Logging on a large scale had just begun, and the stumps of cut trees were visible everywhere, like stubble on a dry, cracking face. Maathai is a UN Goodwill Ambassador for this rainforest – and says saving it is an "urgent priority for us all. It covers 700,000 square miles. It is a quarter of all the tropical rainforest remaining on earth ... It affects weather patterns all over the world. It hasn't been cut a lot until now because the conflict there means it can't be accessed." But even the cutting down of trees in the Congo Basin that has already happened produced a 35 per cent drop in the amount of rain that falls in the Great Lakes Region of the United States in February.

Does she believe there is a choice between encouraging development and saving the environment? "Not at all," she says quickly. "You don't reduce poverty in a vacuum. You reduce it in an environment. In Kenya, one of our biggest exports is coffee. Where do you grow coffee? You grow coffee in the land. To be able to grow coffee you need rain, you need special kinds of soil that are found on the hillsides, and that means you need to protect that land from soil erosion. You need to make sure they can hold the rain so that it flows as rivers and streams. You need forests to regulate the rainfall. You can't grow anything in a wrecked environment." In reality, "poverty is both a cause and a symptom of environmental degradation. They have to be dealt with together. When you're poor, you will take whatever you can to stay alive today, which degrades the environment, and makes you ever poorer. It's a matter of breaking the cycle. Paying people to plant trees was the best idea I had to break the cycle and it is working."

But it is, she adds, not enough. For the forests – and a recognisable climate – to survive, there needs to be dramatic change on both sides. We in the West need to change our carbon emissions – and Africans, she says, need to change their culture, by rediscovering something that was smashed by colonialism. She says there is a "vacuum" at the heart of Africa – one that needs to be met by radically changing how the continent works.

IV. The Taboo

As she oversaw the mass planting of trees, Maathai steadily realised the ripping up and hacking down of the forests is only a symptom of a much-wider problem – one that was crippling Africa. "The disease remained. What made our leaders treat their own country like it was a colony, not something they were part of? It was the same disease that causes corruption and very bad leadership." There was something wrong in how African societies worked – something so deep and puzzling that she has just written a whole book trying to figure it out, called The Challenge For Africa. She touches on many problems, from global warming to unfair trade policies, but the most intriguing section is one that breaches a taboo about Africa – the fact that most Africans identify not with their country, but with their tribe.

"Colonialism destroyed Africans' cultural and spiritual heritage," she says. "Any culture is accumulated knowledge and wisdom, built up over millennia. It tells you how to live in your environment, how to understand life. All our accumulated knowledge was wiped out in just a few years. It wasn't written down, so it died with our elders. Now it is lost forever." This had many effects: "Before, there was something deep in our culture that made us respect the environment. We didn't look at trees and see timber. We didn't look at elephants and see ivory. It was in our culture to let them be. That was wiped out." Part of her work is trying to restore that lost sense of respect for the ecosystem – one that has been proved essential by science.

But this erasure of African culture also left another wound, one harder still to rectify. "It left us with a terrible lack of self-knowledge. Who are we? This is the most natural question for human beings. What group do I belong to? Where did I come from? We no longer had an answer." They were told to forget what came before. In Kenya when the British invaded, there were 42 different tribes – or "micro-nations", as she prefers to call them, because it removes the taint of "primitivism". These old identities were supposed to be abandoned for an identity that consisted of lines arbitrarily draw on a map by their European killers. "The modern African state is a superficial creation: a loose collection of ethnic communities brought together by the colonial powers," she says. "Most Africans didn't understand or relate to the nations created for them. They remained attached to their micronations."

The result has been "a kind of political schizophrenia. Africans have been obscured from ourselves. It is like we have looked at ourselves through another person's mirror – and seen only cracked reflections and distorted images." They have been told to adopt identities like "Kenyan" that make no sense to them. "It is impossible to speak meaningfully of a South African, Congolese, Kenyan or Zambian culture," she explains. "There are only micronations. But we are still living in denial. We are denying who we are."

The result is that when a leader comes to power, he doesn't try to govern on behalf of his people – and they don't expect him to. He delivers for his own tribe, at the expense of the others. "What they call 'the nation' is a veneer laid over a cultureless state – without values, identity, or character," she says. The mechanism of democratic accountability breaks down: the leader is not expected to serve his people, but only a small fraction of them. Elections consist of different tribes fighting to hijack the state to use in their own interests. The system of winner-takes-all democracy – where you need 50.5 per cent of the vote and get 100 per cent of the power – encourages this, and will never work in Africa, she says.

Maathai believes there is a way out of this – but it is absolutely not to pretend tribes don't exist, or to urge people to simply overcome these identities. "We have been telling people to transcend their micronations for so long, and it hasn't happened. They are urged to shed the identity of their micronations and become citizens of the new modern state, even though no African really knows what the character of that modern state might be beyond a passport and an identity card. It doesn't work." The tribal violence in Kenya last year after the election was, she says, even more proof.

She wants to find another route. Instead of a melting pot that pretends all identities will merge into one, she wants to create a salad bowl – one where every piece is different, but together they form a perfect whole. "Instead of all attempting the impossible task of being the same, we should learn to embrace our diversity," she writes in the book. "African children should be taught that the peoples of their country are different in some ways, but because of Africa's historical legacy, they need to work together ... The different micro-nations would be much more secure and likely to flourish if they accepted who they are and worked together. In my view, Africans have to re-embrace their micronational cultures, languages and values, and then bring the best of them to the table of the nation state."

In addition to conventional parliaments, she says that in each African nation there should be assemblies bringing together all the different tribal groups, modelled on the United Nations. There, they could find common ground, and negotiate areas of disagreement. "It is the only way to heal a psyche wounded by denial of who they really are," she says. Every African should rediscover and feel comfortable in their tribal identity, and feel it is properly represented in the political structures of the state. Only then can they share and live together, she believes. That way, everyone will feel they have a stake in the state all the time – not just when one of their men has managed to seize the reins.

She discovered this sense of calm when, in her forties, she rediscovered her Kikuyu roots. The Kikuyus had regarded the trees and Mount Kikuyu's glaciers as the closest thing they had to a sacred being, something worthy of respect. She too found value there, rather than in the dessicated texts left behind by the colonialists. "When they erased our culture, we were left with a vacuum, and it was filled with the values of the Bible – but that is not the coded values of our people."

But isn't there a danger that you are romanticising past cultures and the equally-irrational beliefs that went with them? Weren't these cultures also committed to keeping women separate and subordinate? Isn't your great achievement to break with the traditional subordination of women? She nods. "Culture is a double-edged sword," she says. "It can be used to strike a blow for empowerment, or to keep down somebody who wants to be different. There are negative aspects to any culture. We should only retain what is good. We were taught for so long that what came before colonialism was all bad. It wasn't. We were told our attachment to the land was primitive and a block on progress. It wasn't. We had a ritual – ituika – through which leaders were accountable to their people and could be changed. And people took what they needed but didn't accumulate or destroy in the process. Those are values we need to rebuild in Africa."

She suddenly leans forward and says: "That is the way we will save the rainforests, and prevent global warming!" She is going to follow this goal with the same feverish intensity that drove her from a mud-hut to the Nobel Prize, and enabled her to stand firm through beatings and imprisonments so she could knock down a dictator. Can will-power and a relentless focus on the solution pull us through the climate crisis as it pulled her through a tyranny?

Before I can ask this, she stands up. "Now I must finish packing. My flight is so soon, and my clothes are all over the room!" In a whirl of bright green, she laughs and limps off through the lobby. She has a slightly pained gait, the result of too many nights sleeping on the floor of damp jail cells. She turns back and waves with a strange bend in her back – as if she is still weighed down, after all this time, by the ghost of that single felled fig tree she failed to save.

"The Challenge For Africa" by Wangari Maathai is published by William Heinemann Limited. To order a copy at the special price of £18, including p&p, call Independent Books Direct on 08430 600 030

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