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.