Executives at two of North America's lumber producers said they are bracing for volatile wood prices this building season before sharply higher U.S. duties on Canadian lumber kick in.      Despite President Trump's threats, his April 2 tariff barrage didn't hit Canadian lumber. Nonetheless, duties related to a long-running trade dispute with the U.S. are set to more than double later this year, to an average of more than 34%. Canfor and Interfor executives told investors Friday that they're not sure there won't also be ...