Kenya's most recognizable woman, Maathai won the Nobel in 2004 for combining environmentalism and social activism. She was the founder of the Green Belt Movement, where over 30 years she mobilized poor women to plant 30 million trees.
In recognizing Maathai, the Nobel committee said that she had stood up to a former oppressive regime - a reference to former Kenyan President Daniel arap Moi - and that her "unique forms of action have contributed to drawing attention to political oppression."
Maathai said during her 2004 acceptance speech 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.
Although the Green Belt Movement's tree-planting campaign did not initially address the issues of peace and democracy, Maathai said it became clear over time that responsible governance of the environment was not possible without democracy.
"Therefore, the tree became a symbol for the democratic struggle in Kenya. Citizens were mobilized to challenge widespread abuses of power, corruption and environmental mismanagement," Maathai said.
Kenyan Prime Minister Raila Odinga said Maathai's death "strikes at the core of our nation's heart."
"I join Kenyans and friends of Kenya in mourning the passing of this hero of our national struggles," Odinga said. "Hers has been heroism easily recognized locally and abroad. ... Prof Maathai has passed on just when the causes she long fought for were just beginning to get the attention they deserved as threats to the survival of the human race and that of our planet."
The United Nations Environment Program called Maathai one of Africa's foremost environmental campaigners. The U.N. agency recalled that Maathai was the inspiration behind UNEP's Billion Tree Campaign, which was launched in 2006. To date, more than 11 billion trees have been planted as part of the campaign.
"Wangari Maathai was a force of nature. While others deployed their power and life force to damage, degrade and extract short term profit from the environment, she used hers to stand in their way, mobilize communities and to argue for conservation and sustainable development over destruction," said Achim Steiner, the executive director of UNEP.
Tributes also poured out for Maathai online, including from Kenyans who remember planting trees alongside her as schoolchildren. One popular posting on Twitter noted that Maathai's knees always seemed to be dirty from showing VIPs how to plant trees. Another poster, noting Nairobi's cloudy skies Monday, said: "No wonder the sun is not shining today."
A long time friend and fellow professor at the University of Nairobi, Vertistine Mbaya said that Maathai showed the world how important it is to have and demonstrate courage.
"The values she had for justice and civil liberties and what she believed were the obligations of civil society and government," Mbaya said. "She also demonstrated the importance of recognizing the contributions that women can make and allowing them the open space to do so."
Her quest to see fewer trees felled and more planted saw her face off against Kenya's powerful elite. At least three times during her activist years she was physically attacked, including being clubbed unconscious by police during a hunger strike in 1992. The former president, arap Moi, once called Maathai "a mad woman" who was a threat to the security of Kenya.
By contrast, Archbishop Desmond Tutu on Monday called Maathai a "true African heroine." The Nelson Mandela Foundation also expressed sadness over Maathai's death. The foundation hosted Maathai in 2005, when she headlined the foundation's annual lecture.
"We need people who love Africa so much that they want to protect her from destructive processes," she said in her address. "There are simple actions we can take. Start by planting 10 trees we each need to absorb the carbon dioxide we exhale."
A former member of Kenya's parliament, Maathai was the first woman to earn a doctorate in East Africa - in 1971 from the University of Nairobi, where she later was an associate professor in the department of veterinary anatomy. She previously earned degrees from Mount St. Scholastica College in Atchison, Kansas and the University of Pittsburgh.
Maathai first latched on to the idea of widespread tree planting while serving as the chairwoman of the National Council of Women in Kenya during the 1980s.
The Green Belt Movement, which was founded in 1977, said on its website that Maathai's death was a great loss to those who "admired her determination to make the world a more peaceful, healthier and better place."
Edward Wageni, the group's deputy executive director, said Maathai died in a Nairobi hospital late Sunday. Maathai had been in and out of the hospital since the beginning of the year, he said.
Maathai is survived by three children. Funeral arrangements were to be announced soon, the Green Belt Movement said.