I have a large (36" dbh plus) laurel oak in my front yard. Two years ago I placed cardboard over the sod (rather than use chemicals under the tree to kill the sod) and covered with mulch. This summer with our rains I have noticed dark brown mushrooms coming up within 6 feet of the trunk - but not on the trunk. When I tap on the soil it sounds hollow. I haven't dug around yet...I know the tree roots are very close to the surface. The tree is showing signs of canopy dieback and does have shelf fungus on one large branch. I am wondering if putting cardboard down could have hastened its decline by limiting water and/or air transfer. I have another equally large laurel oak about 25 feet from this one with no mushrooms, no hollow sounds and I did the cardboard mulch under it. Any thoughts? Is there any treatment I can do to help lessen the stress on the tree?