A pall in North America’s housing market is slashing demand for lumber and threatening to shut down sawmills in one of the continent’s biggest timber producing regions.  Lumber futures fell 4% to $433 per 1,000 board feet as of 2:46 p.m. in Chicago, extending a slump to about 62% this year. The price is so cheap that it may result in sawmill closures in the heavily forested western Canadian province of British Columbia as producers try to reduce supplies, according to RBC Capital Markets analyst Paul Quinn.   “We are below ...