US lumber futures have fallen below $580 per thousand board feet, giving back most of their July gains after Canada-US trade talks collapsed and prospects of removing a 10% tariff on lumber faded. Weak US housing data added further pressure to prices.    Lumber futures traded at around $572.50/mbf on August 24, down from more than $640/mbf in July. The roughly 11% decline from the summer high has returned prices to the range that dominated the market during much of the spring.    The retreat accelerated after Canada-US negotiations ...