The California Department of Forestry and Fire Protection is the lead agency in the state for fire prevention and suppression services on private land. One of their tasks is to learn from previous fires. CDF often keeps detailed records on the fires that burned in an area and is a good place to start. The local library, especially local and regional newspapers, will have accounts of larger local fires, though it helps to have a starting date.
Even if you find records of a fire in the vicinity of your property, the fire may not have affected your land, or may have only affected a portion. A careful visual inspection can be used to evaluate your property's fire history. Walking through the forest on your land, look for evidence of previous fires, primarily charcoal.
Forest fires may affect standing trees to different degrees. Trees may be burned completely, burned enough to die fairly quickly but with most of the dead tree still standing, burned enough to char the outside of the tree but not kill it, or burned with little visually apparent damage.
In a few cases, you may find standing dead trees coated in charcoal. However, since dead trees decay rather quickly, you are more likely to find evidence of past fires on living trees. Trees with the thickest bark are the most likely to withstand fire. Look for charcoal at the base of your largest, most fire resistant trees, such as Douglas firs or ponderosa pines.
If some damage occurred to the tree, you may find a scar caused by the fire. Fire scars are most likely to form in areas where fires burn with low intensity. A fire scar is caused when heat from a fire penetrates the bark and kills the cambium. After the injury, the adjacent live cambium expands slowly over the surface of the scarred area and may eventually enclose it. Once a scar has been created, the next fire is more likely to rescar the same area. This is because the scar is either covered with pitch or resin which ignites easily or if it has healed over completely, still has thinner more susceptible bark than the rest of the tree.
Tree scars may also be caused by many factors
other than fire, including people marking their property boundaries,
animals such as bear and squirrel, mechanical damage from falling
trees or from frost, or by insects or diseases. Care should be taken
not to assume that any scar found on a tree is a result of fire.
Scars are more likely to be a result of fire if they occur on
adjacent trees of the same age and species, date to the same year and
are associated with the presence of charcoal on exposed sapwood or
bark.
Fire scars in which the tree has not completely
healed over will be visible from the outside of the tree. Fire scars
in which the tree healed over may only be identifiable after a tree
has been cut down. A good habit to get into is to examine the stump
when a tree is cut down on your property. Tree rings, which are
visible on stumps, are an excellent source of historical information
since they reflect the conditions under which the trees grew.
Counting the rings between the fire scars will
give you a record of the fire frequency on your property. Fire
frequency is often recorded by the tree through a series of fire
scars. By counting the annual rings, you will find years where fires
occurred over the tree's life span.
A fire scar tells you only that a number of fires
burned next to that tree. The fire may not have burned very far from
the tree. Conversely, a single tree is not likely to have recorded
every fire if these events were frequent, or it may have had some
scars burned out by later fires. The best information is produced
when a number of estimates from different trees can be averaged.
When fires are very severe, trees are killed
outright and may decay rapidly. The only evidence of fire remaining
would be a "missing" age of trees, all of which were presumably
killed by the fire. The oldest trees in the stand would be those
which grew after the fire. In some cases there may be residual trees
from before the fire which survived because they were in riparian
zones or on open slopes.
Dating the fire which reestablished the stand
requires estimating the age of the oldest trees by taking increment
bore samples or by dating the oldest stumps.
Most California forests have been logged. It may be easy to confuse a missing age of trees due to fire with a missing set of trees due to harvesting. Some harvesting techniques include burning the tree material left on the ground. Fire history work is most accurate in areas where little harvesting has been done.
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Publication of this series was in part funded by the
California Department of Forestry and Fire Protection
under Contract numbers 8CA96027 and 8CA96028