Some trees peel as they grow; bark lost from a trunk that once kept it is the sign to read.
Some trees shed bark as they grow: NC State Extension's Plant Toolbox describes sycamore, river birch, crape myrtle and lacebark elm bark as exfoliating or peeling, and shagbark hickory's plates as curving away. Bark that falls away from a trunk that held it is different; the USDA Forest Service guide calls a spot where bark is sunken or missing a canker.
General field guidance, not a diagnosis from a distance. A tree can hide problems from the fence line and hide strengths too.
Trees whose bark is supposed to peel
Peeling is normal on several yard trees. NC State Extension's Plant Toolbox says sycamore's light gray upper bark "exfoliates to expose the inner bark that is white to cream-colored," that river birch bark "peels away year-round in tough, paper-like layers that remain on the tree," and that crape myrtle bark exfoliates to show hues of brown to gray. It calls lacebark elm bark lacy and exfoliating, and says shagbark hickory, found in NC mainly in the Piedmont, grows plates that separate and curve away from the trunk.
On these trees, the sign is bare wood, not peeling.
A canker is the guide's name for a patch of bark sunken or missing
The USDA Forest Service's defect guide defines a canker as "a localized area on the stem or branch of a tree, where the bark is sunken or missing," and says "Cankers are caused by wounding or disease." In NC State Extension's gardener handbook a canker is a sharply defined dead area on a woody plant part, and the defect guide adds that "The presence of a canker increases the chance of the stem breaking near the canker."
Mower wounds, a bark-dropping fungus and rot at the roots
Wounding is one cause of a canker. Union County's Extension center says a wound from a lawn mower or weed eater "can seriously threaten the life of that tree," and that "Microorganisms often attack injured bark and invade the adjacent healthy tissues, greatly enlarging the affected area."
Disease is the other cause. NC State Extension's factsheet on Biscogniauxia canker, formerly called Hypoxylon canker, a fungal disease that most often impacts stressed oaks, says its spore mats "cause the bark in the area to fall from the tree," and lists sycamore and hickory among other trees it may affect, so a tree that sheds normally is not exempt.
Rot at the roots can show in the bark too: Extension's oak disease guide lists bark that is spongy or wet among the symptoms of Armillaria root rot, the fungus behind some of the mushrooms at the base of a tree.
Soft wood under a bare patch is decay at the wound's surface
Soft wood under a bare patch points to decay that got in where the bark gave way. Henderson County's Extension center says that once bark is damaged, "fungi can enter trees and rot inner wood," and Union County's center says decay fungi on a wound's surface can cause structural deterioration of the wood beneath. How decay hides and spreads inside a trunk is traced on dead and rotten trunks, and how rotten wood changes the method, and so the price, is driver five on what drives a tree removal quote.
How far around and how high the bare patch runs, and why the check says Worth a call
The USDA Forest Service guide measures a canker by how much of the tree's circumference it takes in, and says a tree with one past the guide's line may be hazardous "even if exposed wood appears sound." Union County's Extension center describes the far end of that scale, a tree "completely girdled," ringed all the way around, after microbes attack mower wounds.
Height matters because a canker, the guide says, raises the chance of the stem breaking near it, and Union's center says the root buttress is the most common site of mower injury, with wound decay able to break a tree off at the stem or root collar. The guide also lists a canker "physically connected to a crack, weak branch union, a cavity, or other defect" among its reasons to take action.
Peeling bark over spongy wood sits in the middle tier of the home page's fence-line check, "Worth a call," the check's general reading of that sign. A look at the trunk tells a shedding species from a lost patch, reads its width, its height and its reach around the trunk, checks for soft wood, cracks or mushrooms nearby, and notes what stands below, the target that the hazard tree field guide pairs with each defect.
How big is the bare patch, and how far around the trunk does it reach?
Questions people actually ask.
Is it normal for a sycamore to lose its bark?
Yes, for the upper bark. NC State Extension's Plant Toolbox says sycamore's light gray upper bark exfoliates to expose white to cream-colored inner bark. Bare wood showing is a different matter.
Does a tree heal a patch of missing bark?
No. NC State Extension's pruning guide says trees cannot heal their wounds; woody plants wall the wound off from the surrounding tissues instead, which lets trees endure injuries without succumbing to them.
When is bark most easily torn by a lawn mower?
Union County's Extension center says the most severe mower injuries come when bark "slips," in early spring during leaf emergence and in early fall during leaf drop.
Do mower wounds at the base of a tree show right away?
No, not usually. Caldwell County's Extension center says wounds near ground level usually go unnoticed for a few years as the tree grows, and that grass or weeds at the base hide the wound.