Comment by jjk166

The US imports about 30% of its lumber. Canada is the largest source of imported lumber, but it's still less than a quarter of all lumber consumed in the US. Surprisingly, the limit on US production is not trees but sawmill capacity.

Sandpaper requires specific grades of corundum; Ontario happens to have several notable large deposits of extremely fit-for-purpose corundum. The Canadian deposits were also a conveniently close source for what would become America's largest abrasives products producer, 3M, after its attempt to mine corundum in Minnesota failed (3M stands for Minnesota Mining and Manufacturing company).