There's a threshold of temps on roots & moisture issues. If u have "above ground" - your trees are just not going to survive -10 or -20 degrees like we will get sometimes. They get dried out and freeze out. Kinda hard to water above ground trees in winter.
U have many solutions. Not one way to solve this for sure!! I kept mine inside (some may remember pic of filling up 2 garage stalls & even carefully stacking) to pack in around 1,000 or 1,500 trees. What this did..... I was able both: A) keep the temp from getting below about 10-15 degrees. I wanted good freeze but not -20 being above ground in pots. B) Then, at points in winter where we had warm up to say 35-40 degrees or if I took water from house- I could water them so moist. I kept a moisture gauge in garage as well. Zero die off. None.
Outside- u can put them below ground or do a crazy good job of insulating them. With insulating with anything (straw or synthetic means) - u gotta watch for critters as above stated. I have had rabbits eat trees to nothing. Protect in any case some how. Both ways are viable and for sure ups and downs to both and a lot depends on ur space, critter situation, how much work u wanna do, etc. the only times I had die offs was one year I had trees outside above ground in smaller cells.... I was at farms bow hunting & it was -10 for a couple days. Almost every tree died. Lot of ways to get this solved though.