These trees are not very well established yet because they are young and are reacting to abnormal and harsh winter conditions. We had a droughty fall with high temperatures long past when plants should have been going dormant. Then we plunged into severe cold. I t's been tough for many plants.
They should recover in spring.
Be sure to keep them watered during droughty periods which can be spring or fall, not just summer.
A liquid fertilizer (such as MiracleGro) this spring or composted organic material spread around the roots (fall is best) would be okay. Fertilizer is not needed or recommended each year as it can stress trees when it's over done. Young trees should grow at a natural rate. Over-fertilization invites insect pests that are attracted by overly tender or lush growth.