Bloomberg: Wood is becoming a hot commodity in a new low-carbon world. Power companies are burning more trees because the renewable fuel can be cheaper than coal and ignited without needing permits to release carbon dioxide, the main greenhouse gas blamed for global warming. Vattenfall AB of Sweden, Germany’s RWE AG and American Electric Power Inc. of Ohio, the biggest coal-burner in the U.S., have switched a few plants over to wood and more are planned. So far that hasn’t driven up paper …