"If you don't have good values, you'll embrace vices," she says. And if we give in to the vices, "We destroy ourselves. We destroy the environment. If we can embrace [good] values, we also heal ourselves. And in the process we heal the environment."
That's the message of her new book, "Replenishing the Earth: Spiritual Values for Healing Ourselves and the World."
People learn these values from their parents, their teachers, and their religious leaders, says Maathai, whose own background is Christian.
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.