Possibly a mole. They eat mostly earthworms, but they'll chew/dig thru anything to get to them, including roots.

Are/were there tunnels underneath?

How sad. I had a ground squirrel or two take out a few Foster Holly and Oak trees that had managed to survive for me along a fence line on my property which borders a field where the farmer alternated growing crops one year, and using the field to graze cattle on the odd years.

It certainly is distressful to watch trees at times struggle to survive, finally look like they managed to pull through, but only then to find that some critter has found, and devoured, or uprooted the tree's most critical parts.

One good thing is that your tree is still small enough, that it should still be putting forth the extra effort to grow. Only question: is there enough root left to help your little sprout to recover?

other option:

In your area, do you have a nearby growing mature Shagbark Hickory tree that sprouts a lot of seeds? If so it might be better at this time to prepare another nursery bed by digging down a few feet and lining the hole with small enough woven, rust proof wire or fencing, which has been attached all along the seams sufficiently to keep ground pests from tunneling under and into the nursery bed.

You could also extend the sides of the wiring up above ground and provice the same kind of protection that you describe doing earlier.

Then next spring those mature trees which earlier had dropped seed, and those seed that have sprouted and grown about 4 inches tall; you could dig one or two, with keeping a handfull virgin soil around their roots, and immediately transplant them into your nursery bed. This should keep the underground pests from damaging you new sprouts

Sounds like vole activity to me. Adding sharp rocks to the backfill can discourage their digging.

I had squirrels get to a few 4" tall first-year hickories in early June. They removed the nuts and in the process destroyed the shoots to the ground. The roots started resprouting multiple shoots about three weeks later. If the root wasn't damaged I expect your tree will do the same.

Oh what a tragedy. I know it wasn't anything that could have attacked it from above ground b/c I had a chicken wire encircling it. Thanks for the further assurance that I may still have some luck. I wanted to post pics just in case anyone could make an exact diagnosis. FYI the holes in the ground were very small. Thanks!

This is how I found it...
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This is what it looked out when i pulled it out to investigate. It was bitten right off...

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Here is some sort of poop beside the dead tree..

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Here are some nearby holes in the ground next to our green bell peppers...
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yep, voles (and I say that while gritting my teeth, they are so maddening).

I have read discussions about using something called Ropel(SP?) liquid and tree wrap to protect against voils on prized saplings. Seems like it was suggested to wrap the sapling trunk and its still attached nut. To improve the efficiency of that effort, it was also said to be beneficial if the liquid repellant was sprayed into the planting hole and soil back fill while planting. Then after the sapling is planted to also spray the surface of the planting bed.

I have never tried those suggestions, so maybe others who have done such could report on how effect such attempts turned out to be for them.

Guess if you have a barn or tall enough large tree, another idea could be to set up an owl box. then, if an owl is attacted to that box, it would begin to benefit you by feeding on the voles and possible other pests that might also be active around your gardens during the evening hours. Of course that solution could only prove effective as long as there was enough food over the Owl's prefered hunting distance from the box. Otherwise the Owl might just "move on to better hunting grounds."

dern. Thanks everyone!. I'm holding out hope that the root suckers will pop out soon, but dug further down and could find anything catching my finger just yet. Woe is me.

I won't hold out much hope for resprout. The varmint probably won this time. Try the Ro-pel. It sounds like it just might work. I'm wondering about using it as a soil drench around existing small seedlings.