Professor Wangari Maathai: Environmental and political activist who became the first African woman to win the Nobel Peace Prize

Wangari Maathai's devotion to the cause of saving the forests of Kenya led to death threats, whippings and beatings, but in 2004 her work was rewarded when she became the first African woman to win the Nobel Peace Prize. In her acceptance speech Maathai, who has died at the age of 71 after a long struggle with ovarian cancer, said that the inspiration for her life's work came from her childhood experiences in rural Kenya, where she witnessed forests being cleared and replaced by commercial plantations, which destroyed biodiversity and the capacity of forests to conserve water.

Maathai called forest clearance a "suicidal mission," explaining that "to interfere with them is to interfere with the rain system, the water system and therefore agriculture, not to mention the other industries dependent on hydro-electricity."

In 1977 Maathai founded the Green Belt Movement with the aim of planting trees to prevent environmental and social conditions deteriorating and damaging the lives of the impoverished people, especially women, in rural Kenya. Her movement expanded in the 1980s and 1990s to embrace wider campaigns for social, economic and political change, setting her on a collision course with the government of the then-president, Daniel arap Moi.

At least three times during her activist years she was physically attacked, including being clubbed unconscious by police during a hunger strike in 1992. Arap Moi called her "a mad woman" who was a threat to the security of Kenya.

Wangari Muta Maathai was born in 1940 in the village of Ihithe, near Nyeri in the Central Highlands of Kenya. At that time few Kenyan girls were educated, but at the instigation of her elder brother Nderitu, Maathai was sent to school, where she was taught mainly by Catholic nuns. She graduated from Loreto High School in 1959, and the following year she was part of the so-called "Kennedy Airlift", a scholarship scheme run by the US government and the Kennedy family.

She studied at Mount St Scholastica (now Benedictine College) in Atchison, Kansas, doing a degree in biological sciences and gaining a master's degree at the University of Pittsburgh. She returned to a newly independent Kenya, joining the School of Veterinary Medicine at the University of Nairobi and receiving a PhD in 1971.

In the 1970s she became active in environmental and humanitarian groups such as the National Council of Women of Kenya. Under its auspices she spoke to rural women, who told her about the environmental and social conditions afflicting poorer Kenyans. They lacked firewood, clean water was scarce and nutritious food was hard to come by.

Maathai saw that the core of the problem was deforestation; the solution was to plant trees, which apart from providing firewood, fodder and fencing material, protect watersheds and stabilise the soil. In 1977 she formed the Green Belt Movement, which has since planted more than 47 million trees, and as the GBM's work progressed she realised that the problems went deeper: behind the poverty and environmental damage were issues of disempowerment, bad governance and the erosion of the values upon which communities drew. The GBM weren't just planting trees: they were taking on a wider social, political, economic and environmental agenda.

All this inevitably brought her into conflict with the authorities. In the 1980s and '90s the GBM allied with other movements for change to confront the abuses of arap Moi's dictatorial regime: successful campaigns included stopping a skyscraper being built in Uhuru Park, an oasis of green that flanks the main highway running through the centre of Nairobi, and halting the seizure of public land in Karura Forest in the north of the city (she was whipped and beaten by guards during a demonstration).

She was also one of the leaders of a year-long vigil alongside mothers of political prisoners; 51 men were released as a result. All this was achieved in the teeth of bitter official opposition, but Maathai also became one of the most respected women in Kenya. As she once said, "Every person who has ever achieved anything has been knocked down many times. But all of them picked themselves up and kept going, and that is what I have always tried to do."

The retirement of arap Moi led in December 2002 to Kenya's first free and fair elections for a generation, and Maathai was elected as a National Rainbow Coalition MP for Tetu, a constituency close to where she had grown up. The following year the new president, Mwai Kibaki, appointed her Deputy Minister for the Environment, and she was quick to import the GBM's strategies of empowerment into the Ministry, emphasising reforestation, the protection of existing woodland and the restoration of damaged land. She went further, introducing various educational initiatives, scholarships for children orphaned by HIV/Aids and help with nutrition for those living with the disease.

The 2007 elections were afflicted by violence, and Maathai served as a mediator and voice of peace. She and the GBM played a crucial role in making sure the new constitution included the right to a clean and healthy environment.

In 2004 she had been awarded the Nobel Peace Prize; the Nobel Committee hailed her as "a strong voice speaking for the best forces in Africa topromote peace and good living conditions on that continent ... her unique forms of action have contributed to drawing attention to political oppression." Two years later she co-founded the Nobel Women's Initiative with five fellow Laureates with the aim of promoting justice, peace and equality all round the world.

Maathai became increasingly involved with international efforts to tackle climate change, not merely calling for the protection of forests but pushing for popular involvement in policy decisions. In 2005 10 Central African governments appointed her Goodwill Ambassador for the Congo Basin rainforest.

In 2006 she joined with the UN Environment Programme to launch a campaign to plant a billion trees worldwide. Results were rapid: that target was reached within a year – the target is now 14 billion. Last year she founded the Wangari Maathai Institute for Peace and Environmental Studies, which allies academic research with the strategies used so successfully by the Green Belt Movement.

Maathai was unsurprisingly showered with honours, including the Legion d'Honneur in 2006, Japan's Order of the Rising Sun in 2009 and the Nelson Mandela Award for Health and Human Rights in 2007. She wrote four books documenting her life and work: The Green Belt Movement: Sharing the Approach and the Experience (2003);her autobiography, Unbowed (2006); The Challenge for Africa (2008), which examined the factors that have hampered progress and providing a manifesto for change; and, last year, Replenishing the Earth: Spiritual Values for Healing Ourselves and the World, an exposition of the values that have fuelled and underpinned the Green Belt Movement.

She wrote, with her typically irrepressible optimism, "I have always believed that, no matter how dark the cloud, there is always a thin, silver lining, and that is what we must look for us." Though Maathai is longer with us, her spirit and inspiration will live on. "We cannot tire or give up," she once said. "We owe it to the present and future generations of all species to rise up and walk!"

Wangari Muta Maathai, environmental and social activist and politician: born 1 April 1940; Nobel Peace Prize 2004; married 1969 Mwangi Mathai (marriage dissolved; three children); died Nairobi 25 September 2011.