Trees: Magnificent structures

Roland Ennos

How trees stand up: Shoots, roots and growth responses

Author Roland Ennos

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. Trees never collapse under their own weight--if anything is going to destroy them it is likely to be the wind or, in some areas, the weight of snow.

Withstanding the wind

Trees use a single trunk rather than many separate stems because, 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. Furthermore, tree trunks are tapered; they are thickest at their base where the bending forces are greatest, but progressively thinner towards the top. This also helps to minimize the amount of wood they use.

Reconfiguring in the wind

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 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 bend. 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 five-metre 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. The leaves of deciduous angiosperms can reconfigure as well as their branches; they roll up in the wind to form streamlined tubes which greatly reduces their drag. Lobed leaves, such as those of maples, and pinnate leaves, like those of ash, do this best. However, even in trees such as oaks or hollies that have stiff leaves, the drag is reduced because the rigid leaves are flattened against the branches.

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 out sideways 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 from 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 top soil 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. 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 become very unstable.

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

Pine, Pinus Sylvestri

The Scots (Scotch) pine Pinus sylvestris from the North York Moors, UK, has been flagged by westerly winds blowing from a cliff to the left of the picture. (Image: Roland Ennos)

In areas with extremely strong prevailing winds, such as the tops of mountains or sea cliffs, trees receive forces predominantly from one direction. The result is an involuntary growth response called flagging. 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

Sweet chestnut castanea sativa

The trunk of this sweet chestnut Castanea sativa shows pronounced 'spiral grain', making it more flexible.

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 living in sheltered areas. The trees situated at the outermost part of a wood are always shorter than the rest. Tree height increases further in so many copses seem to have something of a streamlined shape.

The exposed trees also have thicker trunks and structural roots than sheltered ones, and the structure of their wood is 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 to 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. The tree lays down wood where the mechanical stresses are highest, ensuring that wood is laid down only where it is actually needed. This facility has 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 automatically to grow thicker there. 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.

Buttress roots

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 time delay which is inevitable in these growth responses causes problems when trees are cultivated for commercial purposes. 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. Nowadays, arboricultralists advise staking trees as near the ground as possible, or burying a wire mesh around the root system to help anchor the tree. These precautions minimize the chances of weak areas developing.

Reaction wood

Trees react if their trunks are blown over or are 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.

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