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08 Aug2005

Overstory #144 - How Trees Stand Up

Written by Roland Ennos.

Introduction

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.

The mechanical design 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.

The mechanical design of the shoot system

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.

The mechanical design of the root system

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.

Growth responses of trees

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 subarctic.

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 shape.

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.

 

Selected books

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.

Original source

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.

To order the book:

Order this well-illustrated book from The Natural History Museum at: http://www.nhm.ac.uk/shop/index.html

About the author

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.

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