Her face still warm with blood from a deep head wound, she reported the attack to some local police officers and offered to take them to the scene so her assailants could be arrested. They insisted instead that she sign a formal complaint. Bloodied but defiant, Maathai took a finger, dipped it in her blood and signed the complaint with an X. Nobody was arrested, and no wonder: that evening she saw TV footage suggesting that the police might have colluded with her attackers.

This was not the first or last time she would be attacked for planting trees. Maathai, who became the first African woman to win the Nobel peace prize in 2004, has been beaten frequently, often by riot police, and jailed repeatedly. She was described as "that mad woman" by former Kenyan president Daniel arap Moi, who said she had no moral authority to speak on environmental or any other political matters because she was a divorcee (the fact that he too was divorced did not seem to matter). Once a Kenyan MP from the ruling Kanu party even suggested that she should be forcibly circumcised. Yet no one has succeeded in frightening her away from her chosen course: "It is wonderful when you don't have the fear, and a lot of the time I don't," she says. "I focus on what needs to be done instead."

Why was tree planting so dangerous? The main reason was that it flew in the face of then president Moi's policy of development for Kenya. When Maathai received her head wound, she had been leading a protest against the planned development of Nairobi's only remaining forest. "They wanted to take public land that was there for the common good and give it to friends and political supporters to build expensive houses and golf courses," she says. By that time she had become a veteran at successfully opposing such luxury developments. In 1989 she had campaigned against the construction by Moi's business associates of a 60-storey skyscraper in the middle of Uhuru Park, one of Nairobi's few remaining green lungs. "It would have been like building over Hyde Park," Maathai says. One of the investors, she discovered, was Robert Maxwell. That project foundered in the face of the opposition generated by her campaign.

Later, when Moi had to cancel the plans to build on Karura Forest, he said he couldn't understand why people would object to a development that would be an example of Nairobi striding forward into the future.

But Wangari Maathai's life struggle has been against such deforestation in her country. She was born in 1940 in a village called Ihithe in the central highlands, about which she writes nostalgically. "We lived in a land abundant with shrubs, creepers, fer ns and trees ... Because rain fell regularly and reliably, clean drinking water was everywhere. There were large, well-watered fields of maize, beans, wheat and vegetables. Hunger was virtually unknown."

Little of Kenya is like that now. The colonial era and mismanagement since independence have, she says, given rise to poverty, hunger, soil erosion, even political corruption. Today only 2% of the country's indigenous forest remains. In 1977, she founded the Green Belt Movement, a grassroots organisation made up mostly of poor women from rural areas. Her aim was to help these women by paying them (initially from her own funds, but later with grants, the largest from the UN) to plant trees in their villages; for Maathai, conservation and feminism have always been closely allied. Those 30m trees have been planted by an estimated 100,000 women in Kenya, and Maathai has become known as the "tree woman" whose green activism has become a model around the world.

Given her belief that "without the British, we would not have had the corruption and greed that accompanied the first 30 years of independence", it's disarming that Wangari Maathai manages to smile so much during this interview with a British journalist in a smart hotel in the capital of the former colonial power. British rule not only created Kenya from 42 ethnic groups, but initiated the devastation of the landscape and worsened the position of women in Kenya, she says. "When the British arrived [shortly after the 1885 Congress of Berlin that carved up the continent betweeen European powers], they started cutting down indigenous forests and replacing them with monocultural forests, such as pines and eucalyptus trees, which were quick growing and so would supply material for telephone poles and housing." This led to soil erosion and destroyed natural habitats.

Then there was the colonial administration's eroding of the position of Kenyan women. "When the British came they introduced the concept of title deeds for land, which they insisted be in the name of the head of the household. That was always the man," explains Maathai. "That undermined the traditional setting whereby land belongs to the family. This reform stopped women having legal right to the land." British rule also meant that arable land was used increasingly for cash crops (tea, coffee, sugar cane) rather than subsistence farming. "When the cash came in, it went into a bank account held by the man, even though it was women and children who did the work in the fields. Women were completely disenfranchised." That is a prime reason why the struggle of the Green Belt Movement was closely connected with improving the lot of rural women.

Unlike many of the women for whom she campaigned, Maathai was lucky. She was educated at St Cecilia's in the regional centre of Nyeri, a boarding school run by a Catholic mission. In 1960 she was one of hundreds of Kenyans sent to study at colleges in the US as part of the "Kennedy airlift" of Africans to US tertiary education. She earned her first degree in biology from from Mount St Scholastica College in Kansas and a masters in biological sciences from the University of Pittsburgh. "The time I was there coincided with Martin Luther King's campaigning. When it became clear Kenya was going to become independent, King's words were resonant for me: 'Free at last! Thank God Almighty, we are free at last!'"

She returned to Nairobi filled with hopes, but was brought down with a bump. The job she had been offered in the department of zoology at the University of Nairobi was withdrawn. "I began to see that I was being mistreated as a woman," she says. She got a job in a different university department, but soon found herself campaigning against discriminatory terms of service that not only meant women academics received less pay than men, but were denied pensions and medical insurance for children.

Maathai became the first East African woman to hold a doctorate (one section of Unbowed begins, divertingly: "In 1971, I completed my PhD on the development and differentiation of gonads in bovines"). But such was the sexism she encountered at university in Kenya and from Moi's political allies that she was unable to quietly continue her academic work. "That has been the tragedy of my life and that of many other well-trained African women," she says. "We have not been able to do what we trained to do. I had to take part in the struggle rather than do academic work." Yet, she says, she realised her encounters with discrimination "were luxuries by comparison with the immiseration of poor, rural women. And I knew that healing the environment was central to my country's future health."

Maathai got a reputation for being a strong, and for some troublesome, woman. In 1977, her husband, a former politician, instituted divorce proceedings. In court he is reported to have said that he wanted a divorce because she was "too educated, too strong, too successful, too stubborn and too hard to control".

Moi's rule came to an end in 2002 when an election swept a multi-party coalition to power. One of the new MPs was Maathai, who was elected with 98% of the votes cast. "I got into politics because I wanted to show that we don't need to be thieves. There must be another way of doing politics in my country." From 2003 to 2005, she served as assistant environmental minister, but found the budget for environmental action "peanuts". By comparison with the security budget, "it is nothing. But the Kenyan army has not fought for decades! What we have done is hold seminars for the army and demonstrated to them that the land they are supposed to be protecting is disappearing under their feet." They have now started planting trees, she says, starting in their own barracks. And they are not the only recent converts to tree planting. Rarely does a visiting dignitary miss the chance to be photographed with Maathai planting a tree. Gordon Brown and US senator Barack Obama, whose father was Kenyan, are among the latest.