The Globe and Mail reports in its Monday edition that hundreds of Canadian sawmills must shoulder the additional burden of paying duties retroactive to late January. The Globe's Brent Jang writes that preliminary countervailing duties for new Canadian exports of lumber, averaging nearly 20 per cent, took effect on Friday. In addition, retroactive duties cover most Canadian producers. A wide range of Canadian forestry firms must pay tariffs backdated 90 days for wood exports previously sent south of the border from Jan. 28 to April 27. British Columbia's trade envoy describes the retroactive tariff as a "diabolical" part of a U.S. "shakedown" of Canada. The duties are being imposed as the United States retaliates for what it calls unfair subsidies in Canada. American producers had urged the department to be on the lookout for "critical circumstances" -- bureaucratic lingo to describe a scenario in which Canadian lumber exports rise at least 15 per cent in the months after an investigation is launched but before duties take effect. The four largest lumber producers, Canfor, West Fraser Timber, Tolko Industries and Resolute Forest Products, escaped the 90-day retroactive penalty for critical circumstances.