Generally speaking, mushrooms grow better under trees than trees do. When talking about bonsai, there are a number of reasons not to grow them underneath larger trees. To take just one, there is usually not enough light. The following photo illustrates this poignantly…
This is on my morning walk. Three small pines were planted under a large spruce. The one on the left is getting the bare minimum light to be healthy. The one in the middle isn’t getting enough, so it is weak and leggy. The one on the right, in deepest shade and fully under the canopy of the spruce, is dead.
Conifers in particular need a LOT of sun. But even a deciduous tree would be weak if grown under the canopy of the spruce in the photo.
Keep your bonsai in the sun! And then water them.