Fresh mushrooms are mainly water, but dried ones have nothing left but their strong, earthy, and savory taste. Their concentrated flavor releases into the stock as they rehydrate.
Mushrooms are also excellent sources of umami, a key factor in making a rich stock. Chang uses a little under one cup of mushrooms per quart of water in his broth.
Chang uses the same amount of freeze-dried chicken in his stock. Freeze-drying is one of the most effective ways to retain the meat's flavor close to its original state.