The hillsides around Beijing are all terraced like this. All the terracing was done by hand to combat erosion caused by massive deforestation over the centuries. The trees you see are an effort to reforest the area, partially for land stability, and also for air quality - to catch the sands as they blow in from the Gobi Desert. It is actually required that each resident in Beijing plant one tree every year. The State provides the tree, but they must take a day to go outside the city and plant. On the way up to the Wall we saw several planting "parties" covering the hillsides with saplings.