Washington Post: The United Nations is leading an effort to plant 7 billion trees worldwide — to help protect the environment and slow climate change — by the end of next year. The U.N. Environment Programme (UNEP), an organizer of the planting drive, had an initial goal of 1 billion trees by the end of 2007. “In 2006 we wondered if a billion-tree target was too ambitious; it was not,” said Achim Steiner, head of UNEP. “The goal of planting 7 billion trees, equivalent to just …