The summit meeting, which lasted less than 24 hours, dealt mostly with how the three countries could prepare better for natural or manmade disasters and keep commerce flowing as the United States tightens controls on its borders.

Harper said the free flow of trade was no threat to national sovereignty. The United States, Canada and Mexico are "independent and interdependent," he said.

The summit meeting also dealt with global issues, including climate change.

In a meeting with Bush on Monday, Harper informed the president that he would need to win approval in Parliament to keep Canada's 2,500 troops in Afghanistan beyond February 2009, said Dan Fisk, director for Western Hemisphere affairs on Bush's National Security Council.

Harper also pressed Bush on Canadian assertions that the Northwest Passage, the main shipping lane through the Arctic as the ice fields retreat, belongs to Canada.

Bush reaffirmed his view that the Arctic passage should be kept an international waterway, Fisk said.

The talks were divided into a series of bilateral meetings, informal meals and then a trilateral gathering Tuesday morning, followed by a news conference.

Bush and Calderón also discussed the access that Mexican trucks have to U.S. roads, as required by Nafta. The U.S. House of Representatives, reflecting opposition from highway safety and labor groups, voted in July to postpone the change. The Senate has not acted on the measure.

Canadian trucks have full access to U.S. roads.