In the 1970s, her call for African villagers to plant trees may have seemed like a simplistic response to complex environmental problems. But she realized trees hold both symbolic and practical value.
"People use trees for so many things. It was easy to get into a community to talk about trees," Maathai recalls. They provide immediate practical resources: food, firewood, and shelter. They fight soil erosion.
Today, her Billion Tree Campaign has resulted in more than 11 billion trees being planted worldwide.
Her Green Belt Movement embraces four key values: love for the environment, gratitude and respect for Earth's resources, self-empowerment and self-betterment, and the spirit of service and volunteerism.
The Green Belt followers, many of them women, take simple actions in their communities, such as tending tree nurseries, terracing their fields to curb erosion, collecting rainwater, planting home gardens, and building low-tech sand dams.
Maathai attributes her ideas as having come from "the Source" or, in a Christian context, God. But listening for ideas must also be accompanied by "an attitude that allows you to take advantage of that awakening," she writes in "Replenishing the Earth." "This entails keeping your mind, eyes, and ears open, so that when an idea arrives you'll be ready for it."
Maathai is critical of some aspects of Christianity's influence on Africa – especially theologies that suggest suffering is inevitable and that relief will come only in the next world – a view that can lead to resignation and defeatism. "I don't think God puts us here on this planet to suffer or to do nothing so that we suffer," she says.
But she's heartened that many religious traditions, including Christianity, teach that humans should be good stewards of the environment.
"People of faith ought to be in the forefront protecting this creation," says Maathai, who has also written a book on her home continent ("The Challenge for Africa") and a memoir ("Unbowed"). "I'm glad to see that even the pope has come up front to make statements in favor of the environment," she says.