"Softwood lumber imports from Canada have been of concern to Congress for many years. Under the Constitution, Congress has the power to regulate interstate commerce and exercises authority over trade relations with foreign nations. Lumber production is a significant industry in many states, and U.S. lumber producers are concerned they are at an unfair competitive disadvantage in the domestic market against Canadian lumber producers because of Canada's timber pricing policies. This has resulted in four major disputes (so-called 'lumber wars') between the United States and Canada since the 1980s. The last major dispute was resolved when the 2006 Softwood Lumber Agreement (SLA) was signed. Under the agreement, Canadian softwood lumber shipped to the United States is subject to export charges and quota limitations when the price of U.S. softwood products falls below a certain level. That agreement is set to expire on October 12, 2015, although both countries are prohibited from filing for trade protections for one year after the expiration."
CRS Report for Congress, R42789
National Agricultural Law Center: http://www.nationalaglawcenter.org/