The annual yield from America's growing stock of forests cannot sustain its growing demand for lumber products.  Whether from public lands or its vast private forest land holdings, the Annual Allowable Cut (AAC) of the USA's softwood growing stock is not sufficient to feed its domestic requirements.  Fully 1/3 of US wood products (lumber, moulding, door frames, forms, plywood, strand board etc) comes from Canada.  Further,  US private forest land owners can, in general, export raw logs without restriction.  This they do in vast amounts, particularly to Asia.

Over time the ~20% tariff will cause  the price of lumber (per thousand board ft) to rise even faster than it has been.  A year ago it was $250, today it's $385.  So, even though a tariff will be imposed, the price for the product will go up, in both Canada and the USA.