> and the size of the forests are mindbogglingly vast.
The size of the forests isn't really relevant; compared to lumber demand, they're mostly insignificant. Humans have never had a problem wiping out local forests.
What matters is how much wood a forest can produce per year, not how much has accumulated over the course of the past.
There is plenty of timber in northern California and southern Oregon, these regions are actually temperate rain forests, that are harvested sustainably and aren't old growth forests. Every 30-50 years, (depending on species: redwood or doug fir) the same tracts of forest can be logged again and again.
Once you get further north the taigas are colder and slower growing and may take 200 years or longer to grow back.
The best thing for the USA is to use the resources which supports jobs in logging, wood processing, transportation, and have lower costs associated with transportation and fees from importation. I'm not against importing timber into very northern parts of the US but there is no reason to ONLY use canadian timber.
The boreal forest aka Taiga is quite a bit more at risk than the forests of Oregon. There may be an argument to be made that the wrong kind of firs have grown in the south of Oregon (they're much more susceptible to fire from heat) and that logging and replacement with the right type of firs could be a win economically and environmentally. Somebody with specific knowledge would need to fact check that idea though.
You're still going to miss externalities when you limit the factors you consider. Sure, lumber sellers in Canada could do well and the US could avoid cutting downt heir own trees, but can we really assume that deforedtstion in Canada would be without consequence?
Assuming that stripping resources from other parts of the world is how we got into this ecological mess in the first place.
Could this happen in a way that benefits American construction interests but also Canadian lumber exporting interests?
If there's ever some dispute about it could it be mediated somehow?
What would the outcome of that possibly be?