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Tree
and Forest Restoration - Assessing
Trees
Although the
temptation following a wildfire is to remove every blackened tree,
it is important to first assess actual damage. Trees that look burned
and have their leaves or needles scorched are not necessarily dead.
Fire usually kills trees in two ways: by killing the cambium layer
just under the bark of a tree, and by killing all of the leaves
and buds. Often some of the cambium and some of the leaves have
been burned, but not enough to kill the tree. How much damage was
done will depend on how the fire behaved and what tree species burned.
In most cases, if salvage logging is not being considered, it is
best to wait until the following spring to determine if new leaves
and needles reappear before deciding whether or not to cut down
a scorched tree. If new leaves appear, the tree will survive and
often will fill out to its former glory in 1 to 3 years. If no new
needles or leaves appear by June, the tree is most likely dead.
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There are several
ways of assessing trees after a wildfire has damaged them. The first
is to determine the extent of damage to the live needles on the
crown of the tree.
Heat related
injury to the tree’s cambium layer is as important as foliar
damage. This is the part of the tree that adds woody growth to the
stem every year and is found just underneath the tree’s bark.
Some tree species have evolved a thick bark to insulate this layer.
Ponderosa pine and western larch that have stem diam- eters greater
than 9 inches are often characterized by bark that is 1 to 3 inches
thick with a heat resistant plate-like structure. Older Douglas-fir
can also have heat resistant bark, and are distinguishable from
ponderosa pine and larch by bark that does not flake apart as easily.
All of the other native conifers have a much thinner bark that is
not fire resistant.
To
assess a tree shortly after a fire, peel back the bark on twigs
and the stem in a few strategic areas. Make quarter-sized or smaller
cuts in the bark to determine if the cambium area has been killed.
If the cambium under the scorched area is white or green and juicy-slimy
looking, the stem has survived the fire and the tree has a good
chance of re-sprouting leaves or needles. If the inner bark that
lies next to the wood is dark brown, streaked and/or dried out,
the stem area you are examining is probably dead. Fire resistant
trees like ponderosa pine can have a portion of their cambium killed
and still survive a fire. If the fire has killed more than 75 percent
of the stem circumference, the tree will probably not survive to
become a healthy tree.
Although
stem assessments and percentage scorch are the best tools for assessing
tree health, another assessment that can be made is to check for
intact buds at the ends of branches. Buds that are still green and
moist inside are alive, and if the tree stem has survived the fire
the tree has a good chance of recovering. Be sure to check several
branches and the main stem of the tree.
Depending on
the intensity of the fire, small twigs may be killed but larger
branches left alive, or if there was a lot of fuel around the base
of the tree, the stem adjacent to the ground may have been killed
while the branches still look alive. It is important to check the
entire tree (base of the main stem, cambium, larger branches and
smaller twigs) to determine the extent of the damage. Or, wait until
next spring for new leaves to form.
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