I doubt your lumber yard employee is trained or has much knowledge of the effects of international trade decisions. If a lumber yard can sell at a 20% higher price they would have already done so. They don't sell at the price they would like but the price they have to for you to buy it.  The effect will be the opposite due to increased supply of softwood in Canada. Prices for domestic softwood will come down. This will force some operators out of business in this low margin commodity business and as supply shrinks prices should rise again. 
On the other hand due to a reduction in supply US consumers of lumber will pay a higher price.
I think both parties should respect the call of the umpire - the WTO.