There's also a history in cold winter climates of gardeners thickly mulching herbaceous perennial plantings in autumn and then removing it in spring. This annual routine would of course serve to provide winter protection while the plants were without above ground stems while assuring that the 3-4 in. depth would not interfere with this latter feature during the growing season.

2-4 inches of mulch is what is typically suggested. Anything less than 2" will not provide the same degree of soil protection/weed suppression as a thicker layer but more than 4" is unnecessary and provides no additional benefit.

I am getting thrown by the modifier "decomposable". Everything that was once alive is decomposable, so I guess that term eliminates mulches of rocks, plastic and rubber.

If we are talking about decomposed leaf matter - that is leaf mold which is a type of compost. That is fine for mulch, but I would just note that it will be less effective at preventing weed seeds from sprouting since it is enough of a soil for them to take root in.

To that end, undecomposed shredded leaves would be better as mulch since seeds will have a much harder time taking root there. One could also use leaf mold next to the soil to take advantage of its soil conditioning properties and then a layer of shredded leaves on the top to help discourage weeds. It has the added plus of being a good habitat for beneficial insects. This is also pretty much what you find on the forest floor - so mother nature must know something about what she is doing.

My garden management theory is that I apply compost or leaf mold, or something that degrades quickly, to my bare garden surfaces to create an instantly inviting habitat for worms. In addition to all the other reasons I use mulch: for shade, to hold and release moisture, to keep weeds down---I've now come to almost exclusively depend on it for fertilization, in that it attracts pooping worms to my garden beds in not inconsequential numbers. I used chipped leaves, too, but the main reason I use them is because they are part of the fast degrading mulch plan.