Answer: Mushrooms are various types of fungus and will grow happily on composted mulch. Some types grow particularly well during a cool moist season, as you have discovered, first setting down their "roots" called mycelium and then appearing above the surface. Since they grow from spores, it is impossible to tell if they came in with the mulch or if they were there all along. In the wild, mushrooms are an important part of the forest as they help in the decomposition process.
To get rid of them, you might try handpicking the tops and disposing of them, being very careful to wear gloves and not ingest any. Stirring the mulch to expose it to the light and air, and topping up the mulch with a fresh, less decomposed batch should also help. As the season becomes drier, you may also discover that they disappear on their own. Note that mushrooms sprouting from your mulch will not cause harm to your trees; however, if moist mulch is piled against the tree's trunk, this could cause problems with rot, so pull the mulch a few inches away from the trunk.
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