Persistent invasions by northern nomads prompted ancient kingdoms in China to build the Great Wall. Today a new wall, a shelterbelt of trees, shrubs and grass, is being assembled to keep out even more threatening invaders - the desert sands.

One of the seven wonders of the world and the nation's number one tourist attraction, the Great Wall failed to repulse the Mongol tribes which descended upon China in the 3rd century B.C. Two thousand years later the "green wall" may be more successful in repelling the threat to the soil posed by some of the world's most hostile natural conditions. Chinese agronomists estimate that it already produces a net economic benefit of $630 million a year.Even the streets of Peking have benefited. Noted Peking's mayor Huang Chao: "The dust storms that used to plague the capital in spring and winter have been virtually brought under control." The storms, originating in Inner Mongolia, have been subdued by shelter forests on the capital's northern outskirts and beyond.

With vast areas of desert, northern China is plagued by sandstorms that eat into eight million hectares (20 million acres) of farm land and pasture. Unchecked soil erosion has dyed China's second longest river, the Huang Ho, the ochre color that gives it its English name "Yellow River."

Some 7,000 kilometers (4,300 miles) in length, the new green wall is an immense project encompassing one quarter of China's land mass. Winding its way across northern China from Heilongjiang province in the northeast to Xinjiang Uygur Autonomous Region in the northwest, the shelterbelt was begun in 1978. During the first phase, completed in 1985, more than six million hectares (15 million acres) of barren land have been planted.

"It is extremely difficult to grow vegetation in most of the project area

because precipitation averages below 400 millimeters (1.5 inches) a year," says Wang Zhibing, forestry engineer at the Shelterbelt Bureau in Yinchuan, in the middle section of the new wall. Mr. Wang points to the success of the first phase, measured by the fact that "we have not only surpassed our plans, but achieved a 51 percent tree survival rate."

In recent years, according to Mr. Wang, local peasant households have played an increasingly important role. A new government policy has encouraged peasants to contract to plant trees - something which many afforestation programs have tried and failed at.

Peasants are often, and understandably, reluctant to engage in labor- intensive treeplanting that takes many years to pay off.

Over the past two years this policy has triggered an unprecedented nationwide afforestation drive. Millions of peasants have surged into treeless areas, planted them, and used considerable ingenuity to make the seedlings survive and grow.

According to Chen Guangwu, deputy director of the Shelterbelt Bureau, ''One-third of the regions covered by the project have found their sub- climate and environment have changed for the better." Harvests have improved as a result; one survey showed that on farm land protected by the shelter system, crop yield is up by one-fifth. This is mainly due to a 30 percent decrease in wind speed leading to a decline in evaporation, and a resultant quadrupling of the soil moisture content.

Seemingly inexorably, the Tengger and Mu Us deserts in north central China used to roll southward and force residents to make way. Local people are beginning to wrestle back farm land and pastures. On the southern periphery of the Mu Us, in Shaanxi Prefecture, people have stabilized two-thirds of the shifting sand dunes by planting grass. Tree coverage has increased from nearly zero in 1950 to 20 percent.

Despite the huge effort already improved in it, China's northern shelter system is far from being completed. It took more than 1,000 years to build the Great Wall, and the green wall is no less arduous a task.

Chen Gengtao is a correspondent with China Features and wrote this article specially for Earthscan, a Washington-based news service dealing with environmental issues.

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