Will I kill a big birch tree if I put all my grass clippings around the base? The clippings (from 2 acres of grass) are now 2 feet deep around the tree. I have been told it will heat up and cause the tree to burn. I want to make a raised bed garden around the base of the tree.
Aside from the issues of excessive heat and potential anerobic conditions with a deep pile of grass clippings, trees are pretty sensitive to soil level. So compost elsewhere (and try to mix in some shredded paper, leaves, sawdust or other "brown" materials for a more balanced pile) and if you do want a raised bed there, provide it with an inner wall spaced away from the trunk of the tree, and 8-16 radial perforated drainage pipes at the current soil level running to the outer edge of the bed. Otherwise the "simple" act of piling dirt around a tree can kill it, because the roots become buried too deeply and die before they can grow to the new soil level. If you add an inch or less of soil per year you might get away with it - otherwise the drainage pipe solution attempts to keep adequate aeration at the original soil level.
For using grass clippings as mulch, I recommend no more than 3" depth when fresh. After drying, they can be used somewhat deeper.
A very active compost can exceed 140*F, and some can reach 160*F. This heating can destroy plant seeds, bacteria, and organic matter. The heat is temporary as the available fuel dwindles and the pile becomes more balanced. Spreading out the pile will reduce how hot it will get, as it increases the surface area to the open air.
Other issue with compost piles is if they are too large. They start to compact and lose oxygen. This causes a change in the bacteria to anaerobic, which exhume sulphur/methane gases. Roots on plants need oxygen to breathe, so a large compost pile might compromise roots under it.
I've been putting compost (dirt, leaves, grass clippings, small branches, etc) approximately three feet behind and to the left and right of my lemon tree in the back corner fenced off area. Before I did that, it was small, sickly, leaves drying and falling and poor fruiting. After 3 years the tree became stronger (compost was getting higher and higher). Five years later the fruit is the best I've ever seen from a lemon tree and only this year I pulled out the compost to use in my veggie garden. I think if it's not too close to the trunk of the tree, the tree roots benefit from any water that trickles down nutrition from the compost. At least it did in my case.