There have been few signs in financial markets that the prospect of an unraveling of the Nafta trade accord is worrying investors. …Now, those jitters are surfacing in a somewhat obscure corner of global markets — the lumber industry. Lumber futures soared to a 13-year high on Friday, getting a fresh jolt from the latest U.S. threats of dropping out of the North American Free Trade Agreement. The commodity’s price can be used as a good indicator of Canadian and U.S. trade relations because the countries have already been at an impasse for more than a year over a deal on softwood, with futures closely tracking the tensions. …“The trade file definitely looks uglier this week,” said Paul Quinn, an analyst at RBC Capital Markets in Vancouver. “If Nafta is off the table, it’s going to be very difficult to do anything on softwood lumber.”