Hey, as a fellow Norwegian I keep almost all of my trees outside most of the year, as long as they get the right amount of light and shade to the tree species you're good. But when winter hits it's a good idea to take them inside. Due to roots freezing in the pots, I keep mine in a garage until the frost nights stop. Just to be on the safe aide. A good trick to keep them at a good winter temp is to have them in a space that don't get to much sunlight, so they don't bud to early in spring. And when the soil have dried out I take some snow and put it on the trees and over the soil. This keeps the temp down and slowly give the tree water.
A guy I know in upstate New York does the same, but has a problem every year with mice, who think his garage is warm and really appreciate all that nice tree bark to chew. He has lost a few trees. He tried keeping a cat in the garage but didn't get a lot of help, he thinks.