[bdamm]

While some lumber production happens in the United States, most lumber is imported from Canada. That's because while the USA does have good tracts of land on which lumber is grown, Canada has much, much more. This is why you see "Made in Canada" stamped on quite a lot of plywood and plenty of timer used in residential construction.

The part that I don't quite know how to make sense of is why Canadian producers seem to have a near monopoly on sandpaper products.

[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).