The National Arbor Day Foundation has challenged many cities to earn its "Tree City USA" flag by planting and protecting more trees.
国家植树节基金会向许多城市发起挑战，要求它们通过种植和保护更多的树木来赢得那面“美国树城”的旗帜。
2
Researchers say planting and protecting will be needed to make sure cities can develop healthily, but more needs to be done to have a better understanding of city trees which may be different from countryside forests.
For another, we need to enhance our awareness of trees protection and environment protection, and participate in all kinds of activities of planting trees and protecting trees.