Some trees can survive a wildfire and even thrive. But many trees can be severely damaged or killed by wildfire. For property owners, it can be difficult to tell if trees have been killed by a fire or merely singed. The length of exposure and the intensity of the fire are big factors in the amount of damage done to a tree during a fire. Additionally, the more stressed the tree was before the fire, whether from drought, disease, or insects, the more susceptible a tree is during a wildfire.
As a wildfire burns, a tree can be damaged in several ways. The injury can vary from leaves or needles being burned off to root damage. For trees, any fire damage that impacts its ability to pull moisture from the soil is going to cause more severe damage or death.
If 50% or more of the bark has been completely burned off around the circumference of the trunk, then the tree has likely been killed. Some trees with extermely thick bark, like sequoias, may be able to survive even when more than 50% of the bark has been burned.
If a tree has been burned but a substantial portion of its bark remains, then you can check to determine whether the tree is still alive by cutting a small hole about the size of quarter through the bark. If there is a white or green moist cambial layer just beneath the bark, then the tree has a good chance of surviving. However, if the fire burned into part of the trunk, then the tree will likely be unstable and a hazard even if it does survive.
Evergreen trees that have less than 10% green needles or with less than 50% live buds remaining will likely not survive. Non-evergreens may survive even if most of the leaves have been burned off.
Even if a tree does survive the immediate fire, the damage it has suffered will make it more vulnerable to suffer attack from bark beetles and other insects that could ultimately kill it.