If you slit somebody's throat, you sever the carotid artery, and blood flow between brain and body ceases. What the carotid is to a human being the cambium layer is to a tree. It connects the leaves above to the roots underground. This vital conductive tissue lies in the thin band just underneath the bark.
Woody garden plants include trees, shrubs and vines. Their stems and branches produce mostly sapwood wrapped in a thin cambium layer covered by bark. Bark can be thin as paper or inches thick as with a redwood.
If you strangle the trunk or branch with guy wire or rope it digs into the cambium and kills the tree. If you stake and tie a young tree for support but forget to remove the ties as the trunk grows in diameter, it will eventually dig trough the bark to strangle the cambium layer. This is sometimes called a "ring barked" tree.
If you buy a container grown tree or shrub, the surface of the soil in the pot marks a vital point on the trunk.
Let's call it the "crown."
With bare root trees it's marked by change in color at the base of the trunk where bark turns into root. It is also the undisputable indicator of exactly how deep to plant it in a hole. The crown must be exactly level with the surrounding soil grade.
If you plant the tree in a too shallow hole, the surface roots will be exposed.
These can eventually harden into trunk and although unattractive, this is rarely a fatal mistake. But if you plant the tree too deep, soil will build up around the base of the trunk. Wherever bark contacts soil it will rot, then the cambium layer beneath dies too. This results in crown rot, and when it rings the tree trunk, death comes quickly.
During my years as a new landscape inspector, if a freshly planted tree was looking poorly, the first thing I checked was the planting depth. Newly planted trees can settle after watering, which drops the crown below grade.
Get down and look closely at the base of the trunk. Push some of the dirt out of the way and nick the exposed surface with your fingernail. If the bark comes off easily and it's not green underneath, the problem is crown rot.
Once this bark dies, there is no way to revive the tree. Strive to get the crown at grade or to be safe at an inch high to allow for settling.
Other situations can cause crown rot, too. Plastic or geotextiles under a layer of bark or gravel mulch can cause crown rot if it's laid against the base of a tree.
Keep at least 6 inches to a foot clear from the new tree trunk. If applying the surfacing around an older tree add more clearance.