For decades, the direction of the U.S. lumber market was relatively predictable. When housing starts accelerated, lumber prices followed. When residential construction slowed, sawmills competed for shrinking demand and prices weakened. The relationship was rarely perfect, but it remained the market's defining characteristic.    That relationship has become increasingly difficult to recognise. The latest rally in North American lumber prices is taking place while the U.S. housing sector remains under pressure from elevated mortgage rates, subdued home sales and cautious consumer spending. Instead of stronger demand, the market is being driven by something else ...