We shouldn't be playing THEIR game. Forget who's subsidising whom.

We should arbitrarily and unilaterally  "raise our price 2%." 
Their producers will have nothing to argue about and when their domestic need exceeds their supply (the reason we ship to them in the first place), our producers will get a 2% raise, while our government gets increased taxes.
In other words, raise our price 2% and let the lumber industry take a holiday for a few months and when they return they get a 2% raise, which will more than compensate for any losses, in a very short time! If they want our lumber, they will have to pay the price, it was their idea, so they can have no objections. Of course their construction industry is going to be screaming about the higher cost.
We are not the ones that need their lumber. WE, should be setting our own price and stop just giving away our resources for nothing! NAFTA was a mistake for Canada, you just can't compete on an even playing field when confronted by Goliath.