A couple of months ago I drove by the north side of Barton State Park (in Barton, Texas) where they lost 90% of their pine forest to wildfires several years ago. The new trees I saw had been planted. You can usually tell when people do it because they always plant them too close together. This technique would work better for replanting there due to the lousy job people have done so far by hand (too close, and sporadically.)
Also, nature is going to be sporadic early on. The ground may be viable all over the St. Helens area, but trees coming up would depend solely on whether there were still seeds in the ground at those points. Plenty of that area was swept not only by that pyroclastic flow but also by mudslides. So, there would be plenty of continued barren parts of that landscape just because nature hasn't yet reached those areas with birds and other means of spreading the seeds.
But, we humans now have at least one good method to spread seeds, seedlings, and saplings much quicker 
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