How Not To Mulch
Let me clear: the image you see here is an abomination. This volcano of mulch heaped against the trunk of the tree will act like a soaked wet bandaid on skin--encouraging rot, insect penetration, fungi, suckering, bark damage by rodents and suffocation. This is NOT how you mulch a tree. Especially trees planted in clay soil. If you have someone tend your yard who recommends this--fire them. They'll kill your trees. Now, since probably none of reading this make this particular mistake, go find the businesses in your community that you support and hand them a copy of this post. I mean--saving them the replacement costs of a tree looks out for local business, right?
Mulch can be a very, very good thing. It is infinitely better for your soil ecology than turf grass. It inhibits weed germination, retains soil moisture, moderates soil temperatures, protects roots from winter cold and summer heat, and provides cover to soil insects and organisms that work on enriching your soil for you. BUT it shouldn't be a foot deep. Even in those areas with sandy soil, where protecting that moisture and adding organic material to your "soil" is essential, three inches (at any one time) is plenty. And even that shouldn't be three inches deep right up against the trunks of your trees and the stems of your shrubs.
Ideally, mulch should be about an inch deep under the leaves of shrubs, increasing in depth the further from the stem you move. The same would be true with trees, with the majority of your mulch application being about two inches deep, with just the outermost area of mulch reaching three inches. This will allow penetration by rain, maintain good drainage and encourage retention of moisture especially out at the ends of those roots.
What type of mulch you use is partly a personal choice and partly based on the resources you have. Fallen leaves or pine needles are fantastic. If you have to "import" mulch to your yard, I recommend a shredded bark mulch.
In the meantime, go out there and kick over some volcanoes. The life you save may be a tree!