It is very possible that your cedar (Juniperus virginiana) has allelopathic properties. However, even if it didn't, that does not sound like a very suitable location for your tree. At least not how I'm picturing it in my mind. Woody plants, by the way, do much better in the long run if the soil is not amended with compost or anything else. Planting into the plain native soil is best.

That is so true. We had a peach that broke off last year that we thought was stout enough to stand alone . Now all of our fruit trees have tposts with large cages around them. we get a lot of high straight line winds through the orchard area. now to decide which one to grow.... eeny meeny miny mo

Actually magnolias are a classic host for Armillaria infestations, if a body is ever going to see a Hellish example of a honey fungus attack to a planted tree it will be on a magnolia. For example the perfectly fine big-leaf magnolia I grew here for years, only to have Armillaria get up onto the stem one year and pinch the top of the tree off in short order. In other words the fungus kills trees and shrubs at ground level, causing the top to topple. And apparently become more accessible to it, because in the case of a tree with a substantial trunk at least it is now a fallen log, lying onto or much closer to the ground than it was before. Armillaria is susceptible to dryness, needs conditions to be moist for it to do well. Hence producing rhizomorphs for coming up out of the soil and onto stems, like subterranean termites with their mud tubes built to provide bridges between the soil and wooden parts of structures. And the fungus proliferating after it gets under the bark of targeted plants, where it is protected from sun and air.

I think it is Clavulina cristata , a white coral mushroom https://www.google.ca/search?q=White+coral+mushroom&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj4g_ni5tvdAhXgCTQIHcOiAmkQ_AUIDigB&biw=1268&bih=560