On Sunday night in London, I received a three word text message from a Kenyan friend: "Wangari Maathai amefariki" ("Wangari Maathai has died"). I slumped in pain. For me, her death brought not only the loss of a great environmental campaigner. It was also the loss of someone I had interviewed many times for the BBC World Service in Africa, someone I had come to think of as a friend.

I grew up in the Mau forest, the largest indigenous forest in east Africa. The Molo river ran from my home village of Moto, and its waters feed Lake Victoria. The "shamba" system, a colonial inheritance, allowed poor families like mine to cultivate food crops in forest areas, in return for which they had to plant and care for trees.

In theory, it was a win-win situation, allowing landless communities to grow food while conserving the forests. For my family, it meant ready access to food crops, firewood and clean water.

But Wangari Maathai was bitterly opposed to the shamba system. She argued that allowing food production within the forest was slowly damaging the centuries-old eco-system, no matter how many new trees were planted.

In a 2005 interview, she explained her opposition to me: "We owe it to ourselves and to the next generation to conserve the environment so that we can bequeath our children a sustainable world that benefits all.''

In 1977, when she founded her Green Belt Movement, the concepts of environmental sustainability and eco-system protection were barely recognised around the world, let alone in rural Kenya. But as time went on, her warnings proved accurate. The Mau forest began to shrink and many of the rivers that flowed from it - including the Molo - dried up.

But she persevered. And hers was a campaign always rooted in the real world, recognising the dilemma facing poor communities with their own basic needs. She had an amazing ability to connect complex environmental issues with their impact on ordinary lives, enabling her to persuade rather than force people to join her movement.

She worked with rural communities to increase their access to land, firewood and clean water outside the forest regions, and launched the "Enough is enough" campaign, which showed the Kenyan authorities how agriculture and tourism around Kenya's lakes and valleys would be damaged if the rivers from the Mau forest continued drying up.

When I interviewed her about job losses in the timber industry as the government finally began to introduce tighter controls on logging, she said: "I know there is pain when sawmills close and people lose jobs, but we have to make a choice. We need water and we need these forests''

She was a fearless opponent of corruption, a thorn in the side of the male-dominated Kenyan authorities – "that woman", they used to call her – and was never afraid to speak the truth to the most powerful world leaders when they dragged their feet on climate change.

Participants at the forthcoming Durban climate change conference will miss her contribution. Yet, with her passing, they have an even greater incentive to make progress and build a lasting legacy to her memory.

In my current job at the aid agency Cafod, I already see her true legacy: ordinary men and women in rural communities around the world, from Cambodia to Brazil, who take her as their inspiration when standing up against the logging companies, exposing collusion and corruption by the authorities, and protecting their forest heritage for future generations.

Personally, my lasting memory of Wangari Maathai will be her hearty laughter even as she articulated complex environmental issues, and her fierce determination to save the forest where I grew up. She was determined to save our lives in a way my family did not initially understand, but which she eventually persuaded us to demand.

She was my heroine, a global inspiration, and a true Kenyan warrior.

• Joseph Kabiru is Nairobi communicator at Cafod