Retired US teacher Ronald Sakolsky receives a warm welcome from his longtime friend Yin Yuzhen, who has spent four decades planting trees in North China's Maowusu Desert. Their friendship took root more than two decades ago, when Sakolsky raised $5,000 to support Yin's afforestation efforts. Today, that seed has helped grow more than 50,000 trees. (Video produced by Yu Xingxin, Geng Ziyun, Fan Liuyi)