That would be my guess for at least part of the reason, too, but it'd only be a guess. I also wouldn't be surprised to find out that tree species plays a role,

That would be my guess for at least part of the reason, too, but it'd only be a guess. I also wouldn't be surprised to find out that tree species plays a role, and that we plant species for harvest that simply produce worse lumber than what used to be widely available (as old growth).