So this spurns a thought about winter protection and what we're trying to accomplish in wrapping a tree.
If you wrap a tree for the winter, you aren't really protecting by a certain number of degrees, are you? I'd think you were just mostly protecting it from the elements (wind, ice, etc).
I say that based on this reasoning: If I go outside in a winter coat, I expect it to keep me warm solely based on the fact that I'm generating my own heat that the winter coat keeps close to my body, thus keeping me warm. In the case of wrapping a tree, you aren't really providing a ton of extra "warmth" to the tree because the tree doesn't generate any heat like our bodies do. You may achieve a few degrees of protection, similar to standing in a sheltered location on a cold day. So I'd contend that you're really only protecting the tree from the elements, namely a desiccating wind.
Burying a tree works because the ground takes quite a while to react to changes in outside temperature and will rarely dip below a temperature that would be fatal to figs (at least in my area). Insulating the ground area around a fig tree may prevent the ground from freezing and allow it to continue to pump water up to its wood/buds to keep it from drying out in the winter (this is pure and wild speculation, I'm not a botanist).
From our seasoned fig protectors, does this logic ring true? I'd definitely like to hear other thoughts.