The understory layer forest design plants is sometimes called the sub-canopy layer. Subcanopy plants are the plants between the canopy layer and the shrub layer. These plants are usually between 10 and 30 feet tall and sometimes will become the canopy layer if given the chance through succession.
According to the Permaculture Research Institute, there are 7-Layers in a food forest. They are; canopy, understory, shrub, herbaceous, ground cover, vine/climbers, and root/tuber yield.
Below are pictures with links to any plants that we sell at Great Escape Nursery that could be categorized in this layer. Note that some plants may be in more than one layer. The paw paw tree, shown below, starts out as and often remains only an understory plant. But if given the right conditions, the paw paw flourishes as a canopy layer forest design plant.
Our goal is to have more understory layer plants for sale in the future. One plant comes to mind is the American persimmon. We plan to offer the American persimmon here in the next year or two. But, if you have any suggestions on understory trees that would be good for the mid-Atlantic area, please suggest it in the comments suggestion below.