The tariff had a serious impact on Canadian mills for a while, but these producers were able to overcome much of the cost of the tariff by ramping up their production so they could produce more lumber out of the same facilities. As a result, they were able to gain efficiencies of scale that enabled them to sell the lumber in the U.S. competitively. But to meet the same consumer demand while producing more wood, the lumber industry (particular those giant producers like Boise-Cascade and Weyerhauser that own subsidiaries on both sides of the border) had to shut some of their facilities down. And you guessed it -- whenever possible, they've shut the U.S. facilities down.
Ironically, this resulted in other unforeseen circumstances -- To produce more lumber they had to cut more trees, and since they were cutting more trees they also ramped up their production of other wood products such as paper. And so the U.S. paper industry has been devastated by reduced profit margins, too.