Redwoods and Sequoias
What is a redwood?

The term 'redwood' is a colloquial term applied to a number of trees around the World (all with redwood) including the
Scot's Pine (
Pinus sylvestris) which is altogether a much smaller tree (but with its own unique character). Commonly the
term is applied to three similar looking conifer trees belonging to the cypress family, these are:
Sequoiodendron
giganteum
, Sequoia sempervirens and Metasequoia glyptostroboides. The first two are unique to California, whilst the
third is found in China.
Metasequoia glyptostroboides is commonly called the dawn redwood and although an
impressive tree in its own right (about the size of an English oak) it is much smaller than
Sequoia and Sequoiadendron.
Sequoia sempervirens is commonly known as the coast redwood or simply redwood, whilst Sequoiadendron giganteum
is commonly known as the giant sequoia or simply sequoia (and was formerly called
Sequoia gigantea, before being
reclassified into its own genus). Some people have tried to avoid confusion by using the term sequoia exclusively for
Sequoiadendron and the term redwood for Sequoia sempervirens. However, this is not strictly botanically correct, since
the redwood's Latin name is
Sequoia and Sequoiadendron is a redwood tree! Sequoiadendron literally means 'sequoia
tree' so is not so helpful.
Sequoiadendron was originally classified as a Sequoia species, but its mode of development
differs and
Sequoia giganteum caused a name clash as it was a name used for a variant of Sequoia sempervirens!

This taxonomical mess is perhaps best resolved as follows: consider
Sequoia, Sequoiadendron and Metasequoia as
redwoods and sequoias (they all have very similar bark and other external features) but use the full names giant
sequoia, coast redwood and dawn redwood. Remember though, that Californians consider the Redwood to be
Sequoia
sempervirens
only and the sequoia to be Sequoiadendron giganteum only; and these are Californian trees so what
Californians prefer to call them is important! We should thus distinguish between redwoods and the Redwood (
Sequoia
sempervirens
). This confusion is a classic example of why botanists prefer to use Latin names! (Although these have
been changed too!). Personally, I would not get too hung up on what humans call them, so long as your audience
knows which you are referring to, for they are what they are.

Both General Sherman and General Grant are giant sequoias (
Sequoiadendron giganteum). It is perhaps confusing to
call them 'sequoias' since they are now classified in a different genus. The giant sequoia has a more voluminous and
more massive trunk than the coast redwood, but the coast redwood is generally the taller tree, with its buttressed trunk
tapering to a graceful point, in classical conifer fashion, whereas the giant sequoia maintains a column-like stem for
most of its height. Below are more pictures of giant sequoias - click each picture to enlarge.

The
Coast Redwoods (Redwoods), Sequoia sempervirens, once occurred as forest in a narrow coastal belt some
300 miles (480 km) long but they did not extend past the Bay of Monterey nor past the line of Oregon. They are
remarkably tenacious, regrowing by sprouting multiple shoots from stumps, something not all conifers can do. They are
taller than sequoiadendron and one of the World's tallest trees at up to 110 m  (some Australian eucalypts can reach
greater heights) and live for about 1000 to 2000 years, with some reaching about 2200.

The
giant sequoia (sequoia, sequoiadendron), Sequoiadendron giganteum, is shorter, though still colossal at 90 m,
but with a thicker trunk (up to about 8 m or 24 feet in diameter) and is overall the larger of the two trees. They occur
only in a few choice spots along the Sierra Nevada mountains, wherever the summer conditions are moist enough and
the average temperatures suitable. They are now in something of a difficulty - unable to extend downwards into drier
spots, or upwards into more exposed conditions. It is a swamp-loving tree and though it grows well on dry soil (like
many swamp-lovers) it's seeds will not germinate in drier conditions. In terms of numbers it loses out in these regions to
pines, firs and incense-cedars, which do not, however, rival its height. The oldest of these trees possibly reach 2000 to
3000 years or more.

Sadly, humans have decimated these species for economic purposes. Some are now enclosed in conserved
parklands, but their future remains in doubt. These trees, and their now extinct kin, were once global in extent,
occurring also in Europe. The scarcity of the giant sequoia, which is largely confined to groves in the Sierra mountains,
suggests that it's climate is drier than in ancient times, such that it's seeds often do not germinate. However, in 1878 a
70 mile long forest of the trees, with plenty of saplings, was reported (3) and it may be that in historic times the tree
was at its greatest extent since being decimated by glaciation some 10 000 years ago (until logging of the trees
commenced in 1852. (The first National Park to protect these trees was founded in 1864).

Bark

Very thick, spongy and fibrous with a thick cork layer, peels off in vertical strings or strips rather than scales.
Sclerenchyma fibres occur on the periphery of the phloem and within the secondary phloem (secondary phloem
fibres). The bark is up to 0.3 m thick in the Coast redwood and 0.75 m thick in the Giant Sequoia.

Wood

Tracheids may occur within the rays (ray tracheids), presumbaly for rapid transport of materials across the diameter of
the trunk. The rays of
Sequoia sempervirens have large parenchyma cells and are partly biseriate (two cells thick).
Axial parenchyma (parenchyma cells with living protoplasts arranged in vertical columns in the trunk) is present and
these cells may be filled with resin. The wood has a low density (0.436 g/cm3 or 436 kg per cubic metre for
Sequoia
sempervirens -
see external link) which goes some way to explaining how these trees can support their great size
without collapsing. Vertical resin canals arise traumatically (that is they develop in response to damage, whereas in
some conifers, such as pines, resin ducts may be normally present).

Leaves

The leaves are typical coniferous needles, with distinct palisade and spongy mesophylls and, in Sequoia sempervirens,
a parenchymatous sheath surrounds each vascular bundle. Transfusion tissue (parenchyma cells and tracheids that
transfer nutrients to and from the vascular bundle, found in conifers) occurs on the left and right sides of the vascular
bundle (cf. in pine trees it completely encircles the vascular bundle). One resin duct is situated between the leaf vein
and the lower epidermis.
Above: notice the red-brown bark of the giant sequoiadendron (redder in real-life than my (somewhat
old and basic non-digital) camera would suggest!) though colour does vary. Notice the fibrous nature of
the bark - it is very light and spongy to the touch and comes away in vertical strips (don't try this,
especially not on a protected tree in the National parks!). The lower part of the trunk is somewhat
buttressed and often bears nodular growths (resembling crown galls, not sure what they are though)
especially in older trees. The bark at the base of the tree tends to be more scaly.
Above: notice the fluted columnar trunks - a geometry reminiscent of the stone columns in ancient
Greek temples).
Above: looking into the canopy gives you an idea of the colossal height of these trees - these
trunks are largely straight but taper due to the perspective! The vertically elongated scales of the
bark are more recognisable here as the scales tend to be shorter higher up the tree.
Above: another colossal fluted and buttressed trunk.
Above: another view of General Grant.
Above: the bark on some giant sequoias is noticeably paler and whiter higher up the tree, especially
on the branches (though this might be an illusion due to the brighter light?). The canopy is
concentrated toward the top of the tree, with about the first 40% of the trunk bearing no branches and
then branches increasing in length up to half-way up the tree and the crown tapers like a broad cone,
which often has a blunt looking tip. Young trees have a more classical Christmas-tree shape with a
conical crown starting low on the trunk. Fallen branches leave scars on the lower half of the trunk,
which are often blackened by fire. This tree is General Grant again.
Comparison of giant sequoias and coast redwoods

                       Giant Sequoia                                       Coast Redwood

Height                         up to 90 m                                           up to 110 m
Diameter                     up to 8 m +                                          up to 2 m
Lifespan                      up to 3200 years                                 up to 2200 years
Bark                            up to 0.75 m thick                                up to 0.3 m thick

Female cones            about as large as a chicken's egg        about as large as a large olive
Seeds                        about the size of oat flakes                   about the size of tomato seeds
Reproduction             by seed only (sexual)                            By seed (sexual) or sprout (asexual)

Habitat                        between 5000 and 9000 feet                on the Californian Pacific coastal planes
                      Mature trees need lots of water            Mature trees need lots of water
                      Only found in California                        Only found in California
                      e.g. the Sierra Nevada                          only in a strip 750 km by 8-75 km
                                                                                  (cool air, heavy rains and fog)
                                                                                  (nutrient poor soils)
                      Mixed conifer forests                             Mixed conifer forests

Crown morphology     Narrow, conical                                     Broad, conical


The habitat of the giant sequoia is the mixed-conifer zone (5000 to 9000 feet, but most between 5000 and 7000
feet) on the Californian mountains. This habitat receives some 110 cm rain per annum (44 inches pa) and has
cool air and heavy snow in winter. Black oak, parasitic snow plants and other conifers also grow here. Animals
found in this habitat include: the black bear, mountain lion, cougar, raccoon, quail, mule deer, frogs, tarantulas,
rattle snakes, peregrine falcons and the yellow-bellied marmot. The grizzly bear used to occur in these areas
but was hunted to extinction in California. The high tannin content in the thick bark and wood deters insects and
other grazers and reduces microbial decay, such that dead trees take ages to decompose. Only stunted trees
grow above 9000 feet and up to the tree line at about 11 000 feet.

Note: if you meet a cougar then make lots of noise and throw stones and fight back if attacked. If you meet a
bear and the bear approaches then make lots of noise to scare it off and throw stones (some have survived
attack by playing dead and allegedly on one occasion by striking the sensitive nose) try to keep a safe distance
from the bear, though you could not outrun it (you may however, as a Florida naturalist pointed out to me, run
faster than the other guy!). Watch where you step to avoid startling rattlesnakes. The tarantula bite is not
usually life threatening but is allegedly very painful! Check your armpits and private areas for deer ticks at the
end of the day and remove them to avoid contracting Lyme's disease which can cause arthritis.

The habitat of the coast redwood is low altitude (30 to 750 metres above sea level) and damp all the year
round, with cool air, fog and heavy rains (250 cm pa). This heavy precipitation leaches nutrients from the soils
which are therefore nutrient poor. The nutrients are all locked up within the ecosystem's biomass, which means
that fallen dead trees must be recycled and many young trees grow on these rotting carcasses of their
predecessors and sometimes still bear hollow cavities at the base of their trunks in old age (where the former
fallen tree once existed as the roots grew around it). These coastal damp forests are mixed conifer forests with
Douglas-fir, western hemlock, tanoak, madrone, ferns, redwood sorrel and of course mosses and fungi.

The role of fire in the Sierra Nevada forest ecosystems

The mountain forests where the giant sequoia lives accumulate the needles from the various conifer trees
which decay only very slowly. These dry leaves are potential fuel for forest fires. Lightning strikes or intense
heat and careless human activity may initiate forest fires quite easily in the dry summer months and such fires
are a regular occurrence. For about 100 years, and until quite recently, these fires were considered a bad
thing and many were put-out in their early stages. However, this caused more pine-needle fuel to accumulate,
as fewer fires kept the fuel levels down, and so fires are no probably more frequent than in the natural state.
The policy now is to allow these wildfires to burn out naturally but protect habited areas. Although harmful to
those animals that do not manage to escape, the thick bark of the sequoias protect them and usually they
suffer mere scarring as a result and a loss of their leaves, but soon recover. Dead vegetation burnt away
opens up clearings for sequoia seedlings, essential since these seedlings are easily out-shaded and
out-competed by faster growing trees and shrubs (though if they manage to survive they will eventually get
their own back!). Thus fire is an essential part of the natural cycle of these forests and essential for the health
and vitality. That said, severe fires (such as happen every 20-40 years or so) may eventually weaken old trees,
contributing to the risk of tree-fall.

Enemies of the redwoods

Until the age of conservation, pioneers deforested vast areas of redwood forests for timber and to access
minerals such as gold. Apart from humans, mature redwoods have few natural enemies, but wind and lightning
are two such enemies. Eventually, wind may topple even a mature redwood, especially if the soil beneath it
becomes unstable or if humans remove the shelter afforded it by its neighbours, or if it is just too old and
rotten. Trees may topple if one side of the tree becomes heavier (such as if one side is badly damaged or dies
in old age). Many old stories abound of the affects of lightning on redwoods, although these stories cannot be
easily verified. More often than not these trees survive lightning strikes, but occasionally lightning has been
reported to do immense damage to these tall trees. Stories tell of trees in which the uppermost or middle-most
third of the trunk was blasted into tiny fragments, in the latter case the unsupported top-most section plummets
and may split the base of the tree asunder. There are even reports of entire great trees being reduced to
matchwood. Apparently redwoods shatter into cubical fragments when the force is great enough (the wood of
many conifers fragments in this way, in contrast to broad-leaved tree wood that tends to splinter into elongated
shards). Why are these catastrophic strikes so rare? Well, they could be caused by positive lightning which is
about a tenth as rare but ten times more powerful than the more common negative lightning. It may also
depend upon the moisture content of the tree or of its bark. Conifer trees with high resin content are more likely
to explode when struck by lightning as the resin canals conduct electricity better. Birch trees have oils that are
said to conduct electricity well and so are less likely to be damaged by lightning. The wetness of the soil is also
an important factor. Trees rooted in wet soil may suffer tremendous root damage when struck by lightning.
Often trees suffer more internal damage from lightning than is apparent from external appearances.

Wind and old age are apparently the main enemies of the redwood trees, but lightning may be a factor limiting
the height of the tallest trees.

Friends of the redwoods

Many of these trees now reside in National Parks and are protected by rangers (where possible) and hopefully
the future of these magnificent giants is assured for a long time to come. We are indebted to the U.S. National
Park Service for protecting these habitats and providing informative and educational materials.

Further Reading

1. A. Gray, 1872. Sequoia and its history. The American Naturalist, 6: 577-596.

2. T. Meehan, 1883. Notes on the
Sequoia gigantea Proceedings of the Academy of Natural Sciences of
Philadelphia, 35: 193-196.  

3. Sequoia, 1878. Bulletin of the Torrey Botanical Club, 6: 268.

4. C. A. Harwell, 1935. The Fall of a Giant Sequoia. The Scientific Monthly, 40: 482-484.          
Above: the whole tree (my camera distorted the perspective too much for these two pictures to
form a montage because I was looking up at quite a steep angle.
This sequoia is next to a fallen dead sequoia. As these dead trees rot very slowly, they form rather
strong hollow shells that you can walk through. Giant sequoias "do not know how to die standing" (with
the possible exception of unusually powerful lightning strikes) but eventually fall when say fire damaged
the base of the tree and rains soften the soil (4). Like most trees, the roots are shallow but spread
widely.
A group of sequoias, quite possibly all descended from the same individual.
Sequoia
Sequoia
Sequoia grove
Sequoia
Sequoia
Pov-ray model of a giant sequoia
A Pov-Ray model of a giant sequoia. Most of the nutrients in  a redwood forest is locked up in the trees and so these
nutrients must be recycled. Thus, new trees often grow from the rotting remains of fallen trees, and may have arched
cavities in their base where the former tree was once embraced. The Sequoia is the blue
whale of the botanical world
and their majesty and vast size can only be appreciated by seeing them 'in the wood' - pictures and graphics can never
capture their true essence, which also defies imagination.

For more nice photos of Sequoiadendron, visit
Wikimedia Commons.
Genral Grant Sequoiadendron
Sequoiadendron
Sequoiadendron from Wikimedia Commons
Left: a wide-field view of a giant sequoia,
probably the second largest tree in the World (at
least it probably has the second largest
voluminous trunk) - second only to the General
Cones

Click this external link for an outline of the life-cycle of a Giant Sequoia.
Sequoiadendron leaves
Above: the tiny leaves of Sequoiadendron