Tree Architecture

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Tree Architecture

Most people are familiar with the shoot systems of trees but few people know much about their root systems; fewer still know much about how the architecture of trees helps them stand up against the major natural force threatening to topple them: the wind.

Trees typically have a single woody trunk that projects many meters vertically from the ground. Only toward the top of the tree does repeated branching form ever-narrower branches and twigs, which together make up the compact crown where most of the leaves are held.

It is a commonly held fallacy that the root systems of trees belowground are mirror images of the shoot systems aboveground. The roots do branch, and they extend radially about the same distance from the trunk as the crown, but here the resemblance ends. There is no belowground equivalent of the trunk because the central tap roots of most trees grow very slowly as the tree matures. Instead the system is dominated by several woody lateral roots, which grow horizontally away from the tap root, before branching into smaller, more fibrous, distal roots. The vast majority of the root system therefore grows within a meter of the soil surface where the distal roots obtain resources from the nutrient-rich topsoil. Only the tap root and a few sinker roots that grow vertically down from the woody laterals penetrate down to the subsoil.

Mechanics of Wood

The form of the woody parts of trees, both above and belowground, is strongly influenced by their mechanical function of raising the leaves above other plants, and so outcompeting them for the light. The material of which trees are made—wood—is apparently well designed to withstand the overturning forces caused both by the weight of the tree itself and, more importantly, the wind. Whenever the crown of a tree is blown by the wind the branches and trunk are both bent. This results in longitudinal tensile forces being set up along the windward side, and longitudinal compressive forces being set up on the leeward side. Both forces are efficiently resisted by the walls of the wood cells, which are arranged longitudinally like densely packed drinking straws.

Mechanics of the Shoot System

The aboveground architecture also plays a key part in preventing toppling. The single trunk is better at holding up the crown against strong wind forces than many separate trunks with the same total diameter. This is because the rigidity of a beam is proportional to the fourth power of its radius, whereas its weight is only proportional to the square of the radius. Hence a single trunk will be twice as stiff as two trunks with the same combined mass, and will be able to hold up the crown even in high winds. The much thinner branches and twigs from which the crown is formed, meanwhile, are much better able to bend away from the wind. This reconfiguration helps to streamline the crown and reduce the forces it experiences. This streamlining can be improved by two other mechanisms. First, the leaves themselves reconfigure, folding up together with others along the same branches to reduce drag, while the lobed and pinnate leaves of some broad-leaved trees can each individually roll up into a streamlined tube. Second, in exposed areas wind forces reduce growth on the windward side of the crown and bend branches permanently to leeward. This produces a crown with a permanently streamlined "flagged" shape that presents less drag to the prevailing wind.

Mechanics of the Root System

Even with all of these drag-reducing mechanisms, overturning forces are still transmitted to the trunk and hence to the root system. Fortunately this is also well designed to resist failure. The extensive woody laterals prevent the leeward side of the root system from being pushed into the ground. Instead, the likely mode of failure is for a windward root/soil plate to be levered out of the ground. This movement is resisted strongly by the weight of the soil plate and the resistance of the leeward roots to bending. But the greatest component of anchorage is provided by the sinker and tap roots, which must be pulled out of the ground; their vertical orientation, reminiscent of that of tent pegs, is ideal to resist this movement. The result is that a fairly small woody root system can effectively anchor a large tree.

Growth Responses of Trees

The genetically determined architecture of trees is therefore ideally suited to resist mechanical failure. Their mechanical efficiency is further improved by a growth response called thigmomorphogenesis. The higher the mechanical stresses imposed on trees by the wind, the more wood they lay down to strengthen their structure; consequently trees growing in exposed areas develop shorter but thicker trunks, branches, and roots. In contrast, if a tree grows in a sheltered wood it will grow taller and thinner, improving its chances of reaching the light. A further refinement, which was first suggested by Claus Mattheck, is that the growth response is locally controlled. Wood is laid down fastest in the areas subject to the highest stress and these areas are consequently strengthened. This response ensures that there are no weak areas in the tree, and it also improves the mechanical design. It automatically ensures that branches are joined to the trunk with smooth fairings and that the vulnerable sides of wounds heal fastest. Research also suggests that it might be responsible for the growth of one of the most bizarre features of rain forest trees: the platelike buttresses that join the superficial lateral roots to the trunk like angle brackets.

The combination of efficient material design, good above and below-ground architecture, and adaptive growth responses have ensured that trees can survive even in the face of terrible gales. Moreover, they are the largest and most spectacular of all biological structures. Giant redwoods can grow well over 100 meters tall, weigh over 1,000 tons, and live for over 1,000 years.

see also Anatomy of Plants; Roots; Trees.

Roland Ennos