I went out to water the trees. One of them is (I think) hackberry I dug up a month ago, sitting in a large tub, maybe 2 feet, well over half a meter, mostly dirt from where I dug it up. Right at the base of the tree was a round hole looking exactly like a chipmunk hole. I dumped a cup of sand in the hole, then a cup of dirt, then a cup of sand, then a cup of dirt, then a cup of sand, then another, altogether at least a quart/liter of stuff. I don't believe I had an air hole that large, so the chipmunk diagnosis seems to be right.

Question: what else should I do? I am planning on cutting a large piece of metal screen to cover the entire pot and weighing it down with rocks, to keep the little $#@$^ from returning. Anything else? It has not looked happy for the last couple of weeks, so I don't want to do anything to it if not necessary. The leaves have been looking half wilted but have not dried up. I don't know how long the critter might have been there.

Hm.. Any traces of digging though? If not, it probably is a hole in the soil that caved in. You noticing growth was lackluster would confirm that: Airpockets do cause roots to be unhappy. Water well, and watch whether the hole recurs I would say.

I forgot to mention the chipmunk hiding under the stand where the pine tree was, and the dozen or so chipmunk holes in the yard. I could assume not a chipmunk, but I think that would be pretty optimistic.

I slid the huge pot over under the shade of a large azalea, so it is in pretty much shade right now.