http://www.waldeneffect.org/blog/Winter_protection_for_a_large_fig_tree/

It is a neat picture.  But I've tried that technique (or something extremely similar), and found it insufficient protection in zone 5.  I did that with 3 trees in zone 5, and all three died in the 1990's.  Too many variables to say exactly how cold/windy that technique will work for.  

Mike   central NY state, zone 5

I wonder if you need to leave some breathing room, or the tree will compost with the matter around it.   I am sorry for your loss Mike.

In my case, I don't think breathing room / composting were the problem.  The leaves I used were dry, and the trees died not because they decayed (they didn't) or got moist (they didn't)... the trees just froze to death.  I also wrapped the whole structure with several layers of burlap, for additional protection from the wind (though that was probably insufficient).  I think the issue was just that they  got too cold, for too long a time.  

No need for sorrow though... in my case it was an experiment, and the trees were specifically targeted for this experiment.  (They were three-year olds, and had been buried the prior two winters, and kept indoors in their fledgling pots the year before that).

I have seen buried trees begin decaying the bark though (composting, as you say).  If they get too wet (e.g. leaves around the stems) or if they lack air space underground, the bark decays and the top of the tree dies back... in those cases the roots may still survive to grow a new top.

Mike   central NY state, zone 5 

very similar method my dad used to do when he lived in massachusetts. typically every thanksgiving weekend he would drag me and my 2 brothers out to the yard to rake leaves. we would pile up the leaves at the base of the fig trees. meanwhile my dad would prep the trees by pruning and tieing up the branches and me and my brothers would stuff them with leaves. we would pack them in tight, real tight. then we would wrap it once or twice w/burlap. but as we would wrap the tree working our way up we would continue to keep stuffing leaves. once that was done we would wrap it with a plastic tarp and then a placing a plastic barrel or bucket on top. even with some harsh winters we never experienced any die back....