China's vast "Great Green Wall" reforestation program -- potentially the largest ecological restoration project ever undertaken -- is getting mixed marks from scientists and conservation groups. While the country has been staving off desertification by increasing forest cover annually by 11,500 square miles, the long-term survival of fast-growing tree species might not be sustainable. Plantings have failed because they "are not native to the region, and don't tolerate local conditions," said David Shankman, a geographer at the University of Alabama.

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