Trees don't worsen smog, officials say

Contrary to rumor, windbreaks have 'no relation' to airflow over northern areas

National forestry and weather authorities dismissed rumors that rows of trees planted as windbreaks and to block blowing sand have worsened air pollution in northern China by slowing air movement.

Since November, severe smog has frequently engulfed the northern region, including Beijing, Tianjin and Hebei province, partly because there have been more windless days, they said.

In recent weeks, the average wind speed in the region was around 6.8 kilometers per hour, 5 percent lower than the annual average, according to the National Meteorological Center.

Calm days over a six-week period - including all of November and the first two weeks of December increased to 33 days, three more than the same period the previous year.

Calm days are defined as those with wind speeds lower than 7.2 kilometers per hour and are a major reason for the frequent smog, the center said.

"But the slower wind has no relation to the windbreaks," Ma Xuekuan, chief forecaster at the Central Meteorological Observatory, said.

The windbreaks can affect the wind speed, but only at the surface, he said, adding that wind is only affected within a short distance of the tree lines.

Trees can slow wind only to a distance about 10 times their height. For example, if a tree is 20 meters high, it could affect wind within 200 meters, he said.

Windbreaks, also known as shelter belts, were planted to guard against desertification - soil loss - and to encourage the growth of more trees. The closest large tree belt to urban Beijing is about 200 kilometers from the capital in Zhangjiakou, Hebei province.

"Slower wind in Beijing is not a result of the tree belts. It's the result of general weather conditions," Ma said.

Wind is weaker this winter because cold air fronts have been below average, he added.

The State Forestry Administration took a clear-cut stand.

"It's impossible for a tree belt to prevent wind coming to disperse smog," said Zhang Jianlong, director of the administration. "If tree belts have prevented wind from dispersing pollution, I would argue that the skyscrapers, way higher than trees, could do the same."

Zhang Yongli, deputy director of the administration, said trees could actually reduce the harm caused by smog.

"An increase in forest cover could reduce soil erosion and surface dust, which in turn could improve the conditions that generate smog."

The National Meteorological Center also noted other major reasons smog has come frequently to northern China, including higher humidity than usual.