The Great Green Wall(2)

Yu Xinxiao, a professor from the Beijing Forestry University who is in charge of the project, was quoted by China Green Times in a 2012 article as saying that the project, which will be completed in 2015, will tell people what type of tree has the best ability to attract PM 2.5 and what forest structure holds small particles better.

Even though not much data is readily available on the effectiveness of trees, Yuan said one thing is certain: it's a gradual process.

"There are two types of trees we plant in Beijing, one with tree crowns already in place, which usually take effect as soon as they are planted and block the wind and small particles from spreading," he said, "Another type has bare branches, no leaves. We would need to wait for them to grow bigger."

"It's a long-term process," he added.

Planting pains

However, Feng believes that even more important than tree-planting is preserving the entire ecosystem.

"If you just randomly plant trees in the city, these trees have a higher chance of dying because they are isolated from the whole ecosystem.

They are usually reproduced in a greenhouse and lack interaction with local trees and the ecosystem," he said.

Also, when planting the trees, grass and bushes are often destroyed in the process. Such trees can't block out PM 2.5 as the government hopes, Feng said.

The trees alone can't block out small particles, as there's still space between the branches and trunks, he said.

It's more important to protect the existing ecosystems in Beijing rather than plant new trees.

"An ecosystem has more than trees, it also has bushes, grass, and so on," he said.

"If we really want to solve Beijing's air problem, we should let the system develop naturally, and it will trap dust, small particles, even noise. That's more effective."