Sunday, 21 April 2013


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).



                   (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).




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).



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

No comments:

Post a Comment