It's such a different viewpoint than we would see now - what I think of as true environmentalism (health of soil, land, people, animals).
Most of the trees he explores are for animal fodder (e.g. pigs eat the acorns, we eat the pigs), since he points out - rightly - that growing grain for livestock is ecologically ruinous and far less productive in the end.
Also with trees you can explore "2-story" agriculture - something grown on the ground later beneath the trees.
Of course permaculturalists will point out that we can have 7 layers in our forest gardens, but Smith is exploring tree agriculture on a larger scale - acre of chestnuts, oaks, hickories, persimmons, etc. Personally I think that focusing on scale is part of what got us into such a mess in the first place (@`jackspirko` has a great episode on the USDA "go big or go home" campaign) but rows of nut trees and productive ground layer would be a dramatic improvement on the monoculture disaster that we have now.
