The great green wall

The great green wall – long-term reforestation in China has reduced dust storms

FIERCE DUST STORMS HAVE PLAGUED northern China for centuries. Cold, dry winds from Siberia pick up dust from the Gobi Desert, lifting it as high as four miles into the air, reducing visibility to zero in some areas and turning afternoon to night. A long history of human abuse–deforestation, overgrazing, and mining–of the arid lands south of the Gobi has exacerbated the problem. But now there are hopeful signs of a reversal. A team of American and Chinese researchers reports that a government reforestation campaign begun in the 1950s is finally beginning to pay off: dust storms seem to be declining.

Over the past 40 years the Chinese government has planted some 300 million trees (mostly Chinese elms) in northern China. A 3,000-mile-long swath now stretches across most of the northern part of the country, forming a Great Green Wall that is 500 miles wide at some points. This leafy wall is so far doing a better job of keeping out undesirable elements than its historic namesake (which was only partially successful in discouraging invasions by nomads from the north). Beijing, 300 miles downwind of the Gobi Desert, suffered 10 to 20 storms each spring in the 1950s. Now the storms are rare–maybe one a year–and the odd ones that do hit are much less severe. Storms like the one that killed 40 people in Gansu Province, near Mongolia, in May of last year have become an anomaly.

“The trees act as a windbreak,” says Farn Parungo, an atmospheric chemist at the National Oceanic and Atmospheric Administration. Besides slowing down wind, the trees keep the ground moist, so less soil is swept up by passing gales. But Parungo also cautions that the Great Green Wall could have more far-reaching environmental effects.

Dust from the Chinese storms travels far out over the Pacific–as far as Hawaii and Alaska–where the dust particles provide nuclei on which water vapor condenses, forming clouds. When the dust finally rains out of the atmosphere, it supplies the ocean with minerals and nutrients that support the growth of phytoplankton–the single-celled plants that are the base of the marine food web. Parungo emphasizes the need for further study to get a better picture of the long-term effects of the Great Green Wall on the environment. “If we reduce the storms too much,” he says, “it may reduce precipitation, and that may affect marine biology.”

I Am Not Lunch. I Am Your Mate.

The challenge faced by a male autumn spider, Metellina segmentata, is a difficult but not altogether unfamiliar one: he must court a female of the species without getting eaten by a female of the species. Even when he flashes the courtship signal, plucking or tapping the web with his legs and abdomen, the female tends to devour him as if he were just another insect caught in her sticky silk. “The female is a fairly rapacious beast,” explains behavioral ecologist John Prenter of The Queen’s University of Belfast. “Her whole focus is to build up enough food reserve and maximize her reproductive output.” The unusual thing about a male Metellina, though, is his solution: he begins to court a female only after she has just bitten into something else. Prenter has seen a male spider stand beside the web for hours and even days, waiting for some other bug to get stuck. And he has found that the male is indeed much more likely to survive and be fruitful if he approaches his mate when she has her mouth full. “By using this window of opportunity,” says Prenter, “his risk of being subjected to attack is reduced.”

COPYRIGHT 1994 Discover

COPYRIGHT 2004 Gale Group