How trees work

FORM & FUNCTION

Trees are unique organisms.

They are the tallest free-standing organisms in the world. Trees live longer and become more massive than any other living thing on earth. How they manage this is a fascinating subject. Understanding how trees work needs a grasp of both their biology and of physics and engineering. We can only scratch the surface and whet your appetite on these web pages. More fascinating and easily understood detail can be found by clicking on More at the foot of this page.

Plants do not grow in isolation. They must compete with other plants from their own and different species for space, water, nutrients and light. And only the fittest survive and go on to leave offspring.

The basic body plan, leaf shape and size, type of bark, reproductive strategy and all the other components of their life strategy have all evolved to serve some purpose which makes that particular tree species successful in the struggle for life in its environment.

What the trees that make it excel at is competing for light as they can hold their leaves above other plants and shade them out.

Different types of trees may look similar but their internal structure follows different strategies and master plans.

Trees have a built-in repair system to maintain their physical structure.

Trees are subject to biological attack and have evolved ways to minimise damage caused by both large and small organisms. Some of those defence mechanisms can be seen by the human eye - for example prickles against larger grazing animals or thick bark. But others are invisible to us - for example many leaves contain chemicals which make them less palatable to feeding insects or biological systems to isolate or compartmentalize infections.

TREE PHYSIOLOGY

The internal workings of trees are marvels of biomechanics, hydraulics and engineering.

Many thousands of leaves make up the canopy of each mature tree. A fully grown oak grows - and sheds - roughly 250,000 leaves a year. The way the leaves are carried, their shape and their form have all evolved to serve some purpose and assist the tree in surviving and out-competing its neighbours.

Like most plants, trees trap light energy from the sun by photosynthesis, converting carbon dioxide and water into sugars and releasing oxygen. That is normally in the leaves and is their main function. The water needed is transported up from the roots, and carbon dioxide is absorbed from the atmosphere as the basic building blocks of the sugars.

Microscopic pores called stomata in the leaf can open and close to control the rate of evaporation as well as letting air in and out.

Besides water for photosynthesis, trees need far more water to transpire from the leaf surfaces to keep them from overheating in the summer. A mature tree can soak up over 1,400 litres of water a day.

This water is replenished from the soil and it needs a very sophisticated and efficient plumbing system to get the water up from the take up point in the roots through the water transporting xylem tissue in the stem or trunk to the end-user in the leaves.

Every spring, the roots of deciduous trees grow millions of tiny hairs, each of which is a single cell. These microscopic hairs absorb water and nutrients from the soil. Each root hair only lasts one to two months and every autumn they all die.

So put simply, water, together with dissolved mineral nutrients, is taken up from the soil by the roots. It then passes up the water-transporting cells (xylem vessels or tracheids) within the wood of the tree. Those cells are microscopically narrow pipes which connect the uptake site in the fine roots via the larger roots, stems and branches to the twigs and leaves where it is needed. Much of the water transport occurs in the newer annual rings of the wood and to a lesser extent in the older ones. The water is needed in the leaves to make sugars and starches and to evaporate or transpire.

The sugars manufactured in the leaves up in the sunlit canopies are then transported in dilute solution to the parts of the tree where they are needed for growth or for storage. Most of that flow is downwards and inwards through the stem and roots or upwards into the growing shoots.

This flow is mainly through the narrow layer of tissue called the phloem just beneath the outer protective bark. In the wall of the growing cells, sugar molecules are joined together to form cellulose and lignin, the main ingredients of wood. Each molecule is a chain of hundreds of sugar units. Much of the sugar is stored for future use in the ray tissue of the branches, stems and roots and burnt up in respiration to produce the energy to keep tissues functioning.

The actual secrets of how trees can get water many metres from their root system up into their leaf canopies has to do with the internal structure and hydrodynamic design of wood.

A mature tree can soak up over 1,400 litres of water a day. And the water in a living tree usually weighs more than its timber.

BIOMECHANICS

Trees are impressive pieces of engineering. Wood is an extremely versatile structural material and is ingeniously arranged to provide a living structure that combines both strength and flexibility.

Trees also have plasticity to grow and adapt to changing environmental conditions on both the short and long-term basis.

Trees are subject to huge physical forces. Their branches sway in the wind and bend when loaded with snow or ice. Wind creates tremendous pressures especially in trees in full leaf with a large canopy or "sail".

The stresses, strains and loads on them can sometimes exceed their built in structural strength and something has to give. That may mean breakage, toppling over, or splits and cracks.

The larger the tree gets, the more subject it is to all these loading forces - the twists, turns and bends which enter into the realms of mechanical and structural engineering. These often come to a head in extreme conditions such as high winds, wet snow or drought.

Trees are not rigid structures. The natural design of their tissues allows them to resist many physical forces. Trees have a built-in repair system to maintain their physical structure.

More: The following books provide an excellent overview of how trees work, how they adapt to different climates and environmental pressures, and the pushes, pulls, and strains and stresses they have evolved to cope with so successfully.

  • "Trees" (2001) by Roland Ennos published by the Natural History Museum;
  • "Trees: their Natural History" by Peter Thomas (2000) Cambridge University Press; "Trees: their Mechanical Design" by Claus Mattheck (1991) Springer-Verlag, Berlin.
  • "The Body Language of Trees" by Claus Mattheck & Helge Breloer (1994) London:HMSO

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Any more recent ones are on our Links & Latest pages.