At the northern fringe of Saihanba, China's largest man-made forest, just a few hundred meters separate green from yellow.

From a vantage point on a firewatching tower, close to the border of Hebei province and the Inner Mongolia autonomous region, even those few hundred meters are reduced to a thin line. On one side of the line lies an army of trees - tall, vigilant conifers standing side by side - which form a dividing line at which the greenery abruptly gives way to yellow.

Fifty years ago, the divide did not exist, according to Chen Zhiqing, deputy director of the forest's management team. "There used to be just one color - yellow," he said. "Back then, sand ruled this vast land, and was clearly on the move southward, toward Beijing. Today, that move has been halted, and as the forest expands the sand is effectively on the defensive."