Wading into one of the United States' most contentious environmental debates, Vice President Al Gore has pledged to prohibit logging and road building on 43 million acres of undeveloped national forests. Mr. Gore's promise, which applies to 17.4 million hectares, goes beyond a proposal by President Bill Clinton's administration, made three weeks ago, that would bar roadbuilding across a broad swath of roadless land in 155 national forests and grasslands.