|An obvious crack on an aspen.|
The sun and heat may also cause the bark to be stimulated and begin growing new cells on just the one side. A quick return of cold temperatures kills the new cells. This pattern causes the bark on the sun-facing side to become sunken or split. This "sun scald" results in an elongated dried, sunken, or cracked section of bark.
Both of these situations result in a wound that makes the tree susceptible to further damage and even death. The weakened area may not protect the tree from additional cold damage; like when you walk into a storm with your coat unzipped. More importantly, during warmer weather the wound is an invitation for insects, fungus, and tree diseases. If the tree isn't strong enough to persevere, it can die.
|Wrapping my apple tree.|
Only the lower portion of tender trees need to be protected. Branches help shade the trunk, stabilize bark temperature, and reduce sun damage. For this reason, newly-pruned, young or thin-skinned trees may be damaged if not protected. You'll rarely see this kind of bark damage more than a few feet off the ground.
|Apple tree wrapped to the first branch.|
Many trees will survive with this type of damage, but if you have a treasured tree like my Gala Apple tree in the photos it will benefit both you and the tree to take the extra precautions against the damaging effects of winter sun.