When planting trees it’s important to remove all the soil that it comes with. Besides making sure you’re planting it at the correct depth (very important!) which is very hard to tell without removing the soil, you’re helping the tree get accustomed to its new home quicker. Often the soil it is grown in at the nursery is heavily amended to make the trees grow quicker. If this is left on the root ball of the tree when you plant it, and your soil is not as appealing to the tree, its roots may just hang out in this little zone for years without much exploratory growth outwards to create a strong and stable root system. It’s also important to look for and remove any girdling or circling roots and trim off broken or damaged ends. Poor planting often doesn’t show up for years, and may not become a problem until there’s a particularly strong windstorm, or a very dry summer, and those circling roots can’t hold the tree up or find sufficient water.
I removed a number of dead Arborvitae shrubs last month in North Seattle that died in the summer 2017 drought. It was very obvious that the only preparation to the tree was the burlap taken off their roots, and they were then just dropped into a hole. You can see the slick edge of the root ball from one of the dead arborvitae in the photo below, hardly any roots grew out of this packed soil into the native soil it was planted it.
The slick edge of the rootball is still visible 3 years after transplanting. New roots had trouble growing through this into the native soil, eventually leading to its demise, as it couldn’t get enough water during a drought.
Carefully washed roots from a Japanese maple, ready to be planted