Forests help to keep water from running away,so drought does not often happen.
The trees in theforests can keep rain drops from hiting the soil directly,
so the soil is not easily washed away.
The dead leaves on the ground also keep the water from running away.
The water then stays in one place and is used by the trees in the forests.
In this way,floods are prevented.
Chinese people do not want to see more floods and droughts,
so they have built a new Great Wall across the northern part of the country.
This time,it is a "Great Wall"of trees,millions of trees.
The Great Green Wall is 7,000 kilometres long,
and between 400 and 1,700 kilometres wide.
The Great Green Wall will stop the wind from blowing the earth away.
It will stop the sand from moving towards the rich farmland in the south.
It has already saved a lot of land.
But more"Great Green Walls"are still needed,and not only in China.
They must be built all over the world.
Wang Feng is a worker at Yulin in Shanxi Province.
He works on the Great Green Wall with many other people.
We visited him at his workplace among the young trees
and asked him about his work.
"Many thousands of trees must be planted every year,"he said.
"The more,the better.
This year alone,we've already planted ten thousand trees.
You see all those small trees over there on that hill?
That was sand five years ago.Now it's young forest!
In a few years' time,those mountains will be covered with trees,too!"
He pointed to the high mountains far away.
Was it difficult to work on the Great Green Wall?
"Yes.It's hard work,but it's very important.
The only problem is you can't eat trees!
We have to grow our own food,too.
But thanks to the Great Green Wall,the land produces more crops