I've just been reading J. Russell Smith's "Tree Crops: A Permanent Agriculture" (first published 1929) and I'm getting all excited. So when I saw your topic title, I thought "Hey! Here's a textbook case for his methods!"
The book is available here:
http://www.soilandhealth.org/01aglibrary/010175.tree%20crops.pdf It is also available under google books but as a limited preview. Once you've looked at 20 pages or so, they hide the rest of the book.
He advocates planting trees to cover our basic dietary needs. Trees are vastly superior for cultivation than grains. You only need to plant them once, you only irrigate them until their root system is well-developed, they hold soil in place with those roots, they produce their own mulch, they provide wildlife habitat, etc.
That's nothing new. What Smith brings to the table is a variety of species that are already good replacement crops, especially for grains. I don't think I could do justice to his ideas so I would encourage you to read his book for specific details.
For myself, I will plant several honey locust trees, a korean nut pine and a few sweet acorns so that my sugar and staples needs will be covered in case it becomes impossible to import them later on. I'll leave the yearly work of growing grains to others and content myself with harvesting
