[Homestead] Wood, the miracle of

Gene GeRue genegerue at ruralize.com
Tue Sep 21 10:29:10 EDT 2004

THE OVERSTORY #144--How Trees Stand Up
By Roland Ennos

: --> Arrangement of cells
: --> Rays
: --> Structure of the cell walls
: --> Pre-stressing of wood
: --> Withstanding the wind
: --> Reconfiguring in the wind
: --> The mechanical design of bark
: --> Shedding snow
: --> Flagging
: --> Thigmomorphogenesis
: --> Reaction wood

There is no doubt that trees are magnificent structures; a mature
coastal redwood tree would overshadow most church steeples. Like all
engineering structures, trees combine two elements to do this: they
use good materials and they arrange the materials so that they are
used to their best advantage. Trees have only one main structural
material - wood - but as we shall see this is superbly engineered.
Trees are also ingeniously designed structures that combine strength
and flexibility. They can even respond to their environment and
change their design accordingly. This allows them to support their
canopy of leaves using a bare minimum of wood.

Wood needs to combine many useful properties to allow it to support
the leaves of trees. It has to be stiff, so that trees do not droop
under their own weight; it has to be strong, so that the sheer force
of the wind does not snap the trunk and branches; it has to be tough,
so that when the tree gets damaged it does not shatter; finally it
has to be light, so that it does not buckle under its own weight. No
manufactured material could do all of these things: plastics are not
stiff enough; bricks are too weak; glass is too brittle; steel is too
heavy. Weight for weight, wood has probably the best engineering
properties of any material, so it is not surprising that we still use
more wood than any other material to make our own structures! Its
superb properties result from the arrangement of the cells and the
microscopic structure of the cell walls.

--> Arrangement of cells
Over 90% of the cells in wood are long, thin tubes that are closely
packed together, pointing along the branches and trunk. This helps
transport water to the leaves, but it is also ideal for providing
support. This is because they point in the direction in which the
wood is stressed.
Trees mainly have to resist bending forces. Their branches have to
resist being bent down under their own weight, and both the trunk and
branches have to resist being bent sideways by the wind. These
bending forces actually subject the wood inside to forces which are
parallel to the trunk or branch; the concave side is compressed,
while the convex side is stretched. Whichever way the tree is bent,
therefore, the internal forces always act parallel to the cells or
'grain' of the wood. The long, thin wood cells are well suited to
resist the forces; the cells on the concave side resist being
compressed, rather like pillars, while those on the convex side
resist being stretched, rather like ropes. As a consequence, wood is
very strong along the grain.
The cellular nature of wood is also advantageous to the tree for
another reason. Because the cells are hollow, the tree's trunk and
branches can be thicker than if all its wood material was laid down
in a solid mass. (In some trees, such as the tropical pioneer
Cecropia, not only the cells but also the trunk and branches are
hollow.) Weight for weight, tubular structures like these are
stronger than solid structures; this is why tubes are so often used
in large engineering structures.

--> Rays
The arrangement of the cells along the trunk does have one potential
disadvantage. It is relatively easy to split wood parallel to the
trunk, what a carpenter would call along the grain. However, this is
not very important to the tree because its wood is hardly ever
subjected to forces in this transverse direction. As an extra
precaution, trees prevent the wood splitting between successive
growth rings by incorporating into it blocks of cells called rays,
which are oriented radially in the trunk. As well as storing sugars,
these rays act rather like bolts, effectively pinning the wood
together. The result is that when you do see trees that have been
split along their length, for instance after they have been struck by
lightning, it breaks radially from the centre of the trunk out,
parallel to the rays. This is also why the easiest way to cut up wood
with an axe is radially, through the centre of the trunk, like
cutting pieces of pie.

--> Structure of the cell walls
The structure of the cell walls also improves the mechanical
properties of wood. Cell walls, like fibreglass, are a composite
material. They are made of tiny cellulose microfibrils, which are
embedded in a matrix of hemicellulose and lignin. The cellulose
fibres stiffen the material, like the glass fibres in fibreglass,
while the matrix protects the fibres and prevents them from buckling,
like the resin in fibreglass. This gives the composite a combination
of high stiffness and strength.
Embedding fibres within a matrix also improves the toughness of
composite materials because more energy is needed to break them; it
is used up pulling the fibres out of the matrix. For this reason
fibreglass is around a thousand times tougher than either resin or
fibres on their own. The arrangement of the fibres within the walls
of wood cells helps to make wood even tougher. Wood cells have walls
with several layers, but the thickest layer making up 80% of the
wall, is the so-called S2 layer. Here the microfibrils are arranged
at an angle of around 20 degrees to the long axis of the cell,
winding round the cell in a narrow helix. This is not far off being
parallel to the cell wall, so they stiffen it up along the grain
quite effectively. But the greatest effect is to dramatically
increase the toughness. As the wood is stretched the cells do not
break straight across; instead, the cell walls buckle parallel to the
fibres and the different strips of the cell wall are then unwound
like springs. This process creates very rough fracture surfaces and
absorbs huge amounts of energy, making wood around a hundred times
tougher even than fibreglass. This mechanism only acts when wood is
cut across the grain, but it explains why wooden boats are far
sturdier than fibreglass ones and can absorb the energy in minor
bumps without being damaged.

--> Pre-stressing of wood
Wood has just one problem; because wood cells are long, thin-walled
tubes, they are very prone to buckling, just like drinking straws.
This means that wood is only about half as strong when compressed as
when stretched, as the cells tend to fail along a so-called
compression crease. If you bend a wooden rod the compression crease
will form on the concave side and it subsequently greatly weakens the
rod. Trees prevent this happening to their trunks and branches by
pre-stressing them.
New wood cells are laid down on the outside of the trunk in a fully
hydrated state. As they mature their cell walls dry out and this
tends to make them shorten. However because they are already attached
to the wood inside they cannot shrink and will be held in tension.
Because this happens to each new layer of cells, the result is that
the outer part of the trunk is held in tension, while the inside of
the trunk is held in compression. The advantage of this is that when
the trunk is bent over by the wind, the wood cells on the concave
surface are not actually compressed but some of the pretension is
released. It is true that on the other convex side the cells will be
subjected to even greater tensile forces, but they can cope very
easily with those. The consequence is that tree trunks can bend a
long way without breaking. This fact was exploited for centuries by
shipwrights, who made their masts as far as possible from complete
tree trunks.
Pre-stressing has two unfortunate consequences. Many trees are prone
to a condition known as 'brittleheart'. This occurs because as the
wood in the centre of the tree ages it can be attacked and broken
down by fungi. Eventually it becomes so weak that the precompression
force makes it crumble, and the tree trunk becomes hollow. Another
problem occurs when trees are harvested. Cutting the trunk frees the
cut end and in some species this allows the pre-stress to be
relieved; the centre of the trunk extends and the outside contracts,
bending the two halves of the trunk outwards and causing the trunk to
split along its length. These splits are known to foresters as
'shakes' and render the timber useless. In some fast-growing species
of Eucalyptus the trunk can spring out so violently that it can kill
the lumberjack who is cutting it down.

There are essentially two parts to the shoot systems of trees: a
rigid trunk and a flexible crown of branches, twigs and leaves. This
combination of rigidity and flexibility plays a key part in helping
trees stand up. In actual fact, it is usually the wind which is most
likely to destroy a tree, or in some areas the weight of snow. Trees
do not collapse under their own weight, unlike some of the structures
made by humans!

--> Withstanding the wind
Trees use a single trunk rather than many separate stems for the same
reason that we use single poles to hold up flags; weight for weight
one thick rod is better at resisting bending than several thin ones.
As a result, a single trunk can support a crown of leaves using a
minimum of wood. Like flagpoles, tree trunks are also tapered; they
are thickest at their base where the bending forces are greatest, but
progressively thinner towards the tip. This also helps to minimize
the amount of wood they use.

--> Reconfiguring in the wind
The trunks of mature trees are too rigid to bend far away from the
wind. Fortunately, because the branches and twigs are so much
thinner, the whole crown of the tree can. This bending of the crown
makes it much more streamlined, reducing the aerodynamic drag force
that it transmits to the trunk. Wind-tunnel tests have shown that
this process of 'reconfiguration' can reduce the force on a 5 m (16
ft) pine tree in high winds to under a third of what it would be if
the tree were rigid. Angiosperm trees can perform even better than
conifers in this respect. Palm trees can bend right over in the wind
and so withstand even the strongest hurricanes. Wind-tunnel tests on
deciduous angiosperms have shown that their leaves can reconfigure as
well as their branches; they roll up in the wind to form streamlined
tubes which greatly reduces their drag. The leaves that do this best
are lobed leaves, such as those of maples, and pinnate leaves, like
those of ash. However, even in trees like oaks or hollies that have
stiff leaves, the drag is reduced because the rigid leaves are
flattened against the branches. Unfortunately, work has not been done
to make it clear just how efficient the reconfiguration of full-sized
angiosperm trees is at reducing their drag. Wind-tunnel tests are
difficult and expensive, and winds are too fickle to get reliable
measurements in the field.

--> The mechanical design of bark
Bark acts as a superb shock-absorber, protecting the delicate phloem
tissue from damage. The key to this ability is that bark is mostly
composed of cork, which has a most ingenious structural design. Cork
is made up of large numbers of closely packed cells, each of which is
dead and filled with air. Each cell is a hexagonal prism in shape
with side walls that are corrugated, like the walls of an open
concertina. Because of the corrugation, a small crushing force can
readily cause the cells to flatten out like a closing concertina.
Each cell can collapse to only a quarter of its original thickness,
so this process can absorb a great deal of energy. Impacts are
therefore safely dissipated. This is good for the tree but even
better for us. The properties have proved to be ideal to produce a
stopper that is watertight yet easy to insert and remove. Real corks
are still better in this respect than the artificial corks that have
been recently introduced by winegrowers. Cork is produced sustainably
by harvesting the thick bark of the cork oak, Quercus suber, which
grows in the Iberian peninsula. The cork is cut from the tree every
10 years or so, without apparently damaging the living trees; they
recover and just produce more cork. Cork has also been used to make
flooring, where its shock-absorbing characteristics make it pleasant
to walk on.

--> Shedding snow
The conifers that grow at high latitudes or high altitudes have a
crown design that allows them to shed snow. They are conical in shape
and both the main branches and side branches of firs point downwards
before curving gently upwards like a ski jump ramp. Snow simply
slides off these branches before its weight can damage the tree.

Despite the reconfiguration of their crowns, trees still transmit
large wind forces to their trunks and down to their root system.
Fortunately the root systems of most trees are well designed to
anchor them firmly in the soil.
The root systems of young trees are dominated by their tap roots.
These anchor the trees directly, like the point of a stake. The rest
of the anchorage is provided by the lateral roots, which radiate
sideways out from the top of the tap root; they act like the guy
ropes of a tent, stopping the tap root rotating.
As trees get older, the tap root becomes less important. Instead, the
lateral roots, many of which grow straight out of the trunk start to
dominate the root system; they get much longer and thicker, branching
as they grow. They produce a network of superficial roots that ramify
through the topsoil as far out as the edge of the crown. Lateral
roots are well placed to take up nutrients, but not to take up water
in times of drought; neither are they well orientated to anchor the
tree. Trees overcome these deficiencies by developing sinker roots
that grow vertically downwards from the laterals, usually quite close
to the trunk. If a tree is pushed over, a plate of roots and soil is
levered upwards, about a hinge on the leeward side of the trunk. Some
anchorage is provided by the bending resistance of the lateral roots
on the leeward side; these roots tend to be elliptical or even
figure-of-eight-shaped in cross-section, ideal at resisting this
deformation. However, the vast majority of the anchorage is provided
by the sinker roots on the windward side of the trunk; they strongly
resist being pulled upwards out of the soil. Sinker roots are so
important that when waterlogging stops them developing, trees can be
very unstable.
Perhaps the most extraordinary anchorage systems are possessed by
those tropical rainforest trees that have huge 'buttress roots'. In
these trees the lateral roots are particularly shallow to help them
exploit the nutrients which are concentrated in just the top few
centimetres of soil. Sinker roots are therefore particularly
important to anchor these trees; they are widely placed away from the
trunk to give them longer lever arms. The buttresses act as angle
brackets, transferring forces smoothly down from the trunk to the
sinker roots. Without the buttresses the narrow lateral roots would
just break.

The structure of wood and the architecture of trees are mainly
genetically determined. However, trees can fine-tune their mechanical
design by detecting their mechanical environment and responding to it
with a range of growth responses.

--> Flagging
In areas with extremely strong prevailing winds, such as the tops of
mountains or sea cliffs, trees receive forces predominantly from one
direction. An involuntary growth response called flagging results.
The leaves on the windward side are killed by wind-borne particles
and the windward branches are bent gradually leeward by the constant
force. The result is that the foliage points mostly downwind of the
trunk, which itself leans away from the wind. This makes the tree
much more streamlined, reducing the wind forces to which it is
subjected. In the most exposed areas, the wind also tends to kill off
the leading shoot at the top of the tree, so that the only living
shoots are the ones that point downwind. The tree seems to become
bent sideways. Trees exhibiting the prostrate 'krummholtz' form that
results are common near the tree line up mountains and in the

--> Thigmomorphogenesis
Trees also exhibit adaptive growth responses to the wind in areas
where there is no strong prevailing wind direction. These responses
are called thigmomorphogenesis. The most obvious response is that
trees exposed to strong winds grow shorter than those growing in
sheltered areas. If you look at the edge of a wood you will see that
the outermost trees are shorter than the rest. Tree height increases
further in, so many copses seem to have something of a streamlined
Closer examination reveals that the exposed trees also have thicker
trunks and thicker structural roots than sheltered ones. The
structure of the wood is also altered. Exposed trees have wood in
which the cellulose fibres are wound at a larger angle to the axis of
the cell. The cells themselves tend to wind around the trunk of the
tree rather than running parallel to it, a condition known to
foresters as 'spiral grain'. All these changes help make the tree
more stable. The reduction in height reduces the drag on the tree,
while the thickening of the trunk and roots strengthens them. The
changes in the wood, meanwhile, tend to make it more flexible, so the
tree can reconfigure more efficiently away from the wind. Trees
growing in windy areas even have smaller leaves and this further
reduces drag as well as water loss.
It has been shown that the growth responses of the wood are
controlled locally. If a small length of a trunk is bent it will
thicken up more than unstressed areas of the trunk, and if it is bent
in one plane only it will become elliptical in cross-section. In both
cases the tree lays down wood where the mechanical stresses are
highest. This response is clever as it ensures that trees only lay
down wood where it is actually needed. This facility bas been shown
to be responsible for many aspects of the shape of trees. It ensures
that branches are strongly joined to the trunk by expanding like the
bell of a trumpet at their base; stresses are concentrated where the
branches join the trunk and this causes the branch to grow thicker
there automatically. It also is the reason why tree wounds heal
fastest along their sides - bending stresses along the trunk are
diverted around the sides of the wound, and so growth proceeds
fastest there. The response also causes lateral roots, which are bent
only in the vertical plane, to grow fastest along their tops and
bottoms, and so develop into mechanically efficient I-beam shapes. It
is even responsible for the growth of the bizarre buttress roots of
rainforest trees. When these trees are flexed by the wind, mechanical
stresses are concentrated along the tops of the lateral roots; this
causes them to grow rapidly upwards, especially at the join with the
trunk, and so form buttresses.
The time delay which is inevitable in these growth responses causes
problems for us when we grow trees. Cutting a road through a forest
or thinning a plantation exposes trees to greater wind forces than
they are used to. The result can be catastrophic wind damage before
the trees can grow thicker. In urban areas, young trees have
traditionally been staked to help support them. Unfortunately, this
means that the lower trunk and roots are not mechanically stressed,
so they will remain slender and weak. When the stake is eventually
removed the trees are therefore extremely vulnerable to damage.
Nowadays, arboricultralists advise us to stake trees as near the
ground as possible, or bury a wire mesh around the root system to
help it anchor the tree. These precautions minimize the chances of
weak areas developing.

--> Reaction wood
Trees react if their trunks are blown over or deflected away from
vertical, with growth responses that help them grow vertically again
towards the light. The tip of the trunk detects the direction of
gravity and automatically bends upwards. The same is also true all
the way down the trunk; reaction wood is laid down on one side of the
trunk to bend it upwards.
Conifers produce a sort of reaction wood, called compression wood, in
which the cellulose microfibrils are orientated at around 45 degrees
to the long axis of the cells. This stops the cells from shortening
after they are laid down. If a tree is deflected from vertical,
conifers produce compression wood on the underside of the trunk and
it tends to push the trunk upwards.
Angiosperm trees produce a very different sort of reaction wood
called tension wood in which the cellulose microfibrils are almost
parallel to the long axis of the cell. Cells of this wood tend to
shorten even more than normal wood after it is laid down. Angiosperms
produce tension wood on the upper side of leaning trunks and it tends
to pull the trunk upwards.
Both compression wood and tension wood are very useful to the trees,
but their production has disadvantages for foresters. The two types
of wood are both brittle, so planks of wood made from bent trees will
not be very strong. The stresses they set up and differences in the
shrinkage rates will also tend to warp and split the planks. Hence,
misshapen trees have very little commercial value.

The adaptive geometry of trees, Henry Horn. Princeton University
Press, Princeton, 1974.
Ecology of world vegetation, Oliver Archibold. Kluwer Academic
Publishers, Amsterdam, 1995.
The evolution of plants and flowers, Barry Thomas. Peter Lowe,
London, 1981.
A field guide to the trees of Britain and Northern Europe, Alan
Mitchell. Collins, London, 1974.
An introduction to tropical rain forests, Tim Whitmore. Oxford
University Press, Oxford, 1990.
The Oxford encyclopedia of trees of the world, Bayard Hora (ed).
Oxford University Press, Oxford, 1981.
Plant life, Roland Ennos and Elizabeth Sheffield. Blackwell Science,
Oxford, 2000.
Trees and woodland in the British landscape, Oliver Rackham.
Weidenfeld & Nicolson, London, 1990.
Trees in the urban landscape, Anthony Bradshaw, Ben Hunt and Tim
Walmsley. E & FN Spon, London, 1995.
Trees: structure and function, M.H. Zimmermann and C.L. Brown.
Springer-Verlag, Berlin, 1971.
Trees: their mechanical design, Claus Mattheck. Springer-Verlag,
Berlin, 1991.
Trees: their natural history, Peter Thomas. Cambridge University
Press, Cambridge, 2000.

This article was adapted with the kind permission of the author and
publisher from:
Ennos, A.R. 2001. Trees. Smithsonian Institution Press, Washington,
DC, and The Natural History Museum, London.

Order this well-illustrated book from The Natural History Museum at:

Roland Ennos graduated in Natural Sciences from the University of
Cambridge in 1984. Since then he has carried out research in the
science of biomechanics, a field which allows him to combine his
interests in natural history and structural engineering. Among other
things, he has investigated how flies fly, how roots anchor plants in
the ground and why tropical trees develop huge buttresses. Since 1990
he has been a lecturer in the School of Biological Sciences at the
University of Manchester, where his teaching has allowed him to
indulge his fascination with the evolution and diversity of life,
while his research has taken him to forests around the world. His
travels have provided the inspiration for his recent textbook Plant
Life and for this, his first popular science book.

Dr. Alex Shigo's publications on trees:
Botanical Record-Breakers Amazing Trivia About Plants (Wayne's Word):

The Overstory #143--Dendrology
The Overstory #132--How Trees Survive
The Overstory #129--Windbreak Design
The Overstory #126--Trees for Urban Planting
The Overstory #85--Animal Shelter
Overstory #92--Trees and Their Energy Transactions
The Overstory #69--Some Tree Basics
The Overstory #68--Twelve Tree Myths

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