Answer: Those are the tunnels left under the bark of Ash trees by the larvae of Emerald Ash Borer (Agrilus planipennis). Emerald Ash Borer is a small metallic-green beetle that is native to east Asia. It was introduced to the United States accidentally in the 1990s.
The larvae tunnel through the inner layer of bark and the outer layer of sapwood just under the bark. Their tunnels disrupt the flow of water and nutrients from the roots of the tree to its crown. The upper part of the tree is essentially cut off from its source of water and dies. In Mid-Michigan the majority of Green, White, and Black Ash trees have been infested by this non-native beetle and are in danger of becoming locally extinct.
These pictures were taken of a three foot section of wood that split off from a fallen Green Ash tree. The tree had previously died from an Emerald Ash Borer infestation.