Mushrooms are consumable items, available in all three Risen games. How they are obtained, however, as well as the nature of these items changes from one game to another.
Found on the ground (or rarely, looted from a killed enemy), these pink-colored fungi are scattered across the island of Faranga.
Risen 2 and 3Edit
Unlike in Risen, once a mushroom is picked from the ground, it is immediately converted to provisions. Once converted, it works as any provision (see that page for details).
In all three games, wandering around the landscapes will naturally often take the Castaway or the Son of Steelbeard to locations that have mushrooms growing. They are particularly common near caves or temples.