You may have heard that mulberries were once called "the king of tree crops." I found the original source for that, and it turns out that the context is a bit less flattering. In honor of the close of mulberry season:
For a large section of the Unites States the mulberry is easily the king of tree crops when considered from the standpoint of this book; namely, the establishment of new crops which are easily and quickly grown and reasonably certain to produce crops for which there is a secure and steady market for a large and increasing output.
The mulberry is an excellent food for pigs.To harvest mulberries costs nothing, because the pigs gladly pick up the fruit themselves. Therefore, mulberries fit especially well into American farm economics because labor cost is high.
The book was originally published in 1929, and the rest is pretty interesting as well, especially for permaculturists. He covers other trees as well, including persimmons ("a pasture tree for the beasts, and a kingly fruit for man"). He too wonders why the persimmon hasn't caught on in America.