I was privileged enough to be able to see Mark Shepard at the first Permaculture Voices conference. I will have a write-up of what I learned soon. I want to learn his methods and use those ideas to do some agroforestry in my home state of Mississippi. Permaculture and polyculture food is needed there. I need to read his Book Restoration Agriculture.
The basic idea is to combine trees with annual crops to get a larger yield. The larger yields come from having something like half a yield of each of four crops on a given acre. Individually the yields are lower, but the overall yields are double what can be harvested from planting a single crop. This is also a recognized USDA practice.