How are Redwoods fireproof?
During a walk in the forest, we came across a Redwood. Having been told that Redwoods are actually resistant to fire, I wanted to find out more ...
A bit about redwoods
Redwoods originate from California. Redwoods
are known for their longevity, living for 500-1000 years, and sometimes 2000
years or more. They flower during the later autumn/early winter and produce
male and female flowers on the same tree. The male flowers produce pollen and
appear as small yellow/brown tufts at the end of leaves throughout the tree. The
female flowers are embryonic cones. They appear green and only at the ends of
the branches of the upper part of the tree. (3) There are actually three types
of Redwood: the Giant Redwood (Wellingtonia), the Coast Redwood (Sequoia sempervirens) and the Dawn
Redwood (Metasequoia glyptostroboides).
The Giant and Coast Redwoods both have properties that make their bark fire
resistant (1).
The Giant Redwood: This type of Redwood is the biggest of them all. Giant Redwoods thrive in a moist, humid climate. (2) They are recognised as being the heaviest of all trees. It can live for over 3 thousand years, reaching a height of more than 100 metres. The bark is soft and spongy. It is also very thick, being up to 2 feet thick in mature trees. The trunk has a conical outwards sweep. The Giant Redwood has short, spiky leaves. Its seedlings need the right conditions to grow and develop quite slowly in the first six months (1).
The Giant Redwood: This type of Redwood is the biggest of them all. Giant Redwoods thrive in a moist, humid climate. (2) They are recognised as being the heaviest of all trees. It can live for over 3 thousand years, reaching a height of more than 100 metres. The bark is soft and spongy. It is also very thick, being up to 2 feet thick in mature trees. The trunk has a conical outwards sweep. The Giant Redwood has short, spiky leaves. Its seedlings need the right conditions to grow and develop quite slowly in the first six months (1).
(9) (1)
The Coast Redwood: This Redwood is
recognised as the tallest tree. Their native habitat is California, where they
grow to around 110 metres. The bark is thick and relatively soft. Its trunk has
a more parallel profile at the base. The leaves are flat, soft and shaped (1).
(1)
(10)
The
Dawn Redwood: This Redwood had been thought to have been extinct for many
years until living examples were located in China. They are now spread across
the globe. They are rare but can be found in many parts of the world. The Dawn
Redwood is a conifer, however, it is deciduous. The branches grow in an upwards
direction. It has fine, flattened, delicate looking leaves. Before falling in
the autumn, the leaves become a bright orange. The trunk is quite slender and
the overall profile is relatively straight. It grows where there is plenty of
water, and in moderately swampy conditions (1).
(11)
(1)
So, how are Redwoods fireproof?
It is only the Giant and Coast redwoods that are fireproof.
These Redwoods are fireproof as they have thick bark, containing tannin, which
protects them against fire (along with insects, fungus and diseases). There is
also a lot of water contained in the wood itself (a large Redwood tree holds
around 34000 pounds of water, transpiring about 200-500 gallons of this a day).
Furthermore, the tree does not contain ‘pitch’, which is very flammable (4). Mature
Redwoods are more resistant to fires than young Redwoods due to the fact that
mature Redwoods have a thicker bark. Because the tree has thick bark, fire will
not burn through this bark easily. Even if the fire does burn through some of
this bark, the part of the tree that keeps it alive is beneath this (the
phloem, cambium, sapwood and heartwood), therefore, the thick bark helps
protect the tree against fire (5). Obviously, water puts out fires, so the fact
that Redwood trees contain a lot of water is advantageous in the fire
resistance aspect of the tree. Pitch is a highly flammable hydrocarbon concealed
by softwood conifers. Native Americans valued pitch as a means of starting
fires. They used kindling containing pitch to help them start fires (6). As
these Redwood trees do not contain pitch, it reduces the flammability of the
trees compared to other trees containing pitch.
Fire can still, however, kill
Redwood trees
Despite Redwoods being fire resistant, repeated fires might
reach the heartwood through cracks in the bark. The damaged heartwood will
decay, causing the tree to be ‘hollowed out’. However, the outside layers
remain intact and still grow. These trees can also be killed as the fire
damages the bark.
Fungi can then invade the damaged wood and cause it to rot (4).
(4) (4)
Redwoods are also resistant to
insects and fungi
The Coast Redwood is occasionally infected by the larva of a
small insect which reduces bark under the surface to a fine powder. However,
this does not endanger the life of the tree (7). The tannin in the bark also
provides protection against insects and fungi. Tannins act as a defence mechanism against
pathogens. When consumed, they induce a negative response which may be
instantaneous. The two main categories of tannins that impact an animal’s
nutrition are hydrolyzable tannins (Hts) and condensed tannins (proanthocyanidins)
(8). Many trees are affected by fungi which then cause them to die after being
damaged by a fire. However, as these Redwood trees are fire resistant, fungi do
not tend to grow on the trees, meaning Redwoods are almost never killed by
fungi (7).
By Lauren Watmough
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