If by softwood lumber you mean construction lumber - this time I hope the Canadian Feds simply apply an automatic 100% higher tariff on all raw lumber exported to the U.S.; in this case a 40% tariff on raw lumber exports to the U.S. to match the U.S. 20% tariff on softwood lumber imports from Canada.
China is buying raw lumber as fast as they can move it, so if the U.S. stops importing raw lumber, then raw lumber exports continue as before and the U.S. construction industry runs out of softwood lumber.
If we cancel all U.S. softwood lumber imports then our mills should have a good chance of surviving this mad behaviour south of the border.
Talk about the tail wagging the president . . .