North American lumber prices moved lower again in August, with futures falling below $570 per thousand board feet as weaker US housing activity and uncertainty over Canada-US trade relations kept buyers cautious.    The decline follows a much stronger market earlier in the summer. Spot lumber prices reached their highest level in four years in late July, supported by sawmill closures, production cuts and lower imports. Futures then reversed sharply, with September contracts losing more than 12% over roughly two weeks as traders reacted to weaker housing indicators and higher mortgage rates.     ...