Trees
All trees are useful in their own ways. We use them for housing, fuel, and food. Most trees reside in forests or jungles, but that may be changing within the next couple decades. It is our job to preserve species so that we do not lose them. One way we do this is incorporate them in our gardens, yards, and pastures. A fantastic source for permaculture information is Temperate Climate Permaculture (http://tcpermaculture.com/site/plant-index/). The following is a small list of trees that popular in orchards, food forests, and pastures.
Canopy Layer: tall trees that provide shade and are wind blockers.
Sub-canopy Layer: tree smaller than those in the canopy layer, most of the fruit trees reside in this layer