Answer: The mushrooms are growing because there's some decaying organic matter beneath the soil. In your case, I suspect the roots from the tree are beginning to decompose. Just rake up the mushrooms when they appear and remove them from the area. If you allow them to mature, they'll spread their spores all around and you'll end up battling even more of the fungi. You can completely eliminate the mushrooms if you dig out the old tree roots, but most gardeners settle for raking them up as they appear.
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