by Gene Wengert
email@example.com WOOD DOCTOR’S Rx
We have all heard about he terrible forest fires in the West in the past few years, and
especially the Camp Fire in November
2018 in Paradise, California. Let’s look
at forest fires from a forestry point of
view. But before beginning, we do want
to voice our cares and concerns for
those who lost family and friends and
to those that lost homes, belongings
and a lifetime of irreplaceable items in
the Camp Fire and all wildfires.
Forest fire basics
A forest fire needs a source of ignition.
It needs fuel and needs oxygen. The ignition source is often cloud-to-ground
lightning strikes, but human activities
do cause many fires. The fuel source is
wood, leaves, needles and understory
growth, including brush and grass.
The speed of burning and the
speed of expansion of a forest fire into
new fuels depends to a large extent on
the presence of oxygen. In turn, this
means the wind speed is a huge factor
in the life of a fire, as the wind brings
in oxygen, as well as carries burning
embers into new fuel sources.
Further, as might be expected, the
openness of the fuel (open to blowing
wind, such as the needles on a standing
tree versus the needles that have fallen
to the ground) is important in the
intensity of the fire.
A quick lesson in burning can help
understand why fires move at times
with lightning speeds. When a small
piece of wood is first heated (likely
from adjacent fire), the moisture is first
This moisture presence limits the
wood temperature, in the region where
the moisture is, to the boiling point
of water (212F); of course, the boiling
point of water drops with elevation, so
this temperature drops.
As much as 15 percent of the energy
generated by the fire is used for evapo-
ration; stated another way, wet fuels
initially burn more slowly and cooler.
In fact, that is one reason why water is
dropped on a fire. On the other hand,
spells will dry the fuel and result in
worse fires. The role of moisture in the
air is another factor -- dry air means
Once the water is gone, the wood
heats and decomposes into various
flammable gases. These gases ignite
and might burn (using oxygen) at 700F
and hotter. This is a key to understanding forest fires, so we will come back
to this gasification process. Once the
wood has gasified, carbon is all that
This hot carbon combines with
oxygen to form carbon monoxide if
oxygen is sparse or carbon dioxide
when oxygen is plentiful. This carbon
burning occurs at 1100F or much
hotter when oxygen is abundant. (Do
you recall that a blacksmith would use
a bellows on his fire to increase the
oxygen and heat from the burning
carbon?) All in all, in the fire there is
no surplus oxygen and very hot temperatures.
See more at the Wood
Dr. Knowledge Center
See more columns at
Sponsored by Northwest Hardwoods.
Looking at forest fires from a forestry
point of view
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Wood Doctor’s Rx question and answers, go to
Gene Wengert, “The Wood Doctor,” has been training
people in efficient use of wood for 35 years. He is
extension specialist emeritus at the University of Wisconsin-Madison.