When I was a practicing forester, I was a tree planting administrator and inspector on tree planting projects for the US Forest Service, we found that as little as 20 seconds of root exposure to air or sunlight could cause death of root hairs and either the seedling died or was set back for a year or two. Another VERY important thing to remember is to make sure that ALL of the roots are pointing straight down. If any of the roots are "J'd" or "U'd", then it takes years for those roots to turn and go down, thus you get a stunted tree. In the south, another big issue is making sure the dirt packed around the roots is snug and no gaps for air and mold to grow. There is usually a "root collar" where the seedling was grown in the nursery, andthat's where the ground level should be when you are finished planting. Water at about 1 gallon of water per week if you don't get a good soaking rain, up and until the end of August of the first year. After that, if they start to die, you'll waste your time trying to save them.