You wouldn't think that a tree could go more green than it already is. But with environmental concerns gaining traction in the 1960s and 1970s, Rockefeller Center began recycling its tree after Christmas. In 1971, it turned the tree into 30 three-bushel bags of mulch for nature trails in upper Manhattan. In 2005, Habitat for Humanity used wood from that year's tree to make door frames for its homes. By 2007, the tree's organizers had switched to all energy-saving LED lights, some 30,000 of them. The new bulbs used 1,200 kilowatt hours less electricity per day enough to power a 2,000 sq.-ft. home for an entire month.