When frost and freezes are around in the fall, I make sure the trees are well watered before nightfall, when the soil isn't frozen. That insures roots won't really be harmed by the temporary cold, as the water in the soil freezes first and insulates roots. Things warm up come morning. However, in the spring, with cut roots that are no longer cold hardened, things can be a bit more dicey.
As long as it doesn't get below 25, things should be fine, UNLESS the tree has leafed out. Then all bets are off. The leaves haven't hardened off yet and are vulnerable to frost and especially freezes. Those will have to come inside, I think.
I have a covered cold frame over a nine inch deep pit that has been open for the last week or so. The mulch and surrouding soil has already warmed, so this will act as a heat sump of sorts after I lower the covering on the cold frame. Unfortunately, I can't get nearly all my trees into it and none of my big 'uns will come close to fitting into it, much less trying to lower 200 lbs of pots tree and soil into it without damage to me and the tree
We regularly get such huge temperature expanses between winter and spring. It's 85 degrees today and it was 82 yesterday. It will be only 40 come Friday with nighttime lows in the mid 20's. This happens every year, as in No. Va and the D.C. metro area is right on the borderline between the Northern zones and the Southern zones. This is a blessing and a curse, as we can grow very southern species like Magnolia Grandiflora, live oak and crape myrtle, but we can also manage some Northern species as well. I can't grow larch --a cold climate tree--here in No. Va, but folks 30 miles north of me can.