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A tree is a woody, upright plant with three main sections:
• Roots
• Trunk (stem, bole)
• Crown (branches)
Pinus radiata or radiata pine – New Zealand’s primary plantation species – is a softwood. Softwood trees mostly have needles and bear cones. Hardwood trees mostly have broad leaves. In spite of the name, hardwood trees do not always have harder wood than softwoods – the softest wood in the world is balsa wood which comes from a ‘hardwood’ tree.
As a timber, radiata pine is a moderately strong softwood. Plantation grown hardwood trees in New Zealand include eucalypts (gum trees), poplars and some high quality timber species such as Tasmanian blackwood.
Structure of a Pine Tree
What follows is a detailed description of the structure of a pine tree. The structure of trees of other groups and species sometimes differs from this quite significantly.
Roots
Tree roots have two key functions – they anchor the tree to the ground and absorb the water, minerals and oxygen that are essential to growth.
Radiata pine has a strong taproot system. The taproot develops from the primary root, which emerges from the germinating seed and grows vertically down into the soil to absorb water. Lateral roots soon branch out from this and at the end of the new roots is a root cap – a layer of cells that protects the root from damage as it pushes through the soil. The cells behind this cap multiply rapidly making the root lengthen.
Immediately behind this area of growth, the root produces many fine root hairs – these absorb minerals and water. When the resources in one area of soil are depleted, the root grows into fresh soil and new root hairs develop behind the root tip. The old root hairs die and a layer of protective bark develops around the older part of the roots.
Roots absorb oxygen from the gaps between soil particles. Oxygen diffuses into the root through the root hairs. If the soil gets too wet and becomes waterlogged, the roots cannot easily get oxygen from the soil. The tree will not grow in such conditions.
The roots of radiata pine have a mutualistic relationship with a fungus, which is termed mycorrhizae. The hyphae or filaments of the fungus grow around the root hairs and help absorb water and minerals from the thin layer of water surrounding the soil particles. In turn, the fungus absorbs sugars from the roots which the tree has produced in photosynthesis.
While these hyphae are below ground, the reproductive part of the fungus grows above the ground as toadstools or puff balls that release the spores from which new fungi grow. Some of these toadstools are very poisonous. One is the red and white Fly Agaric toadstool which grows up to 20 cm in size and can be seen in plantation forests in the autumn. It gets its name because it can be made into a fly killer by crumbling the cap into water and sugar.
Foliage
Radiata pine is an evergreen tree with long needle-like leaves that usually form in clusters of three, are triangular in cross section and about 100mm long. It is a conifer – it produces cones.
Hardwood trees usually have broad leaves and are either evergreen (for example, eucalypts) or deciduous, shedding their leaves in the autumn (for example, oaks and elms).
The foliage of trees is responsible for three natural and essential processes:
• Transpiration
• Photosynthesis
• Respiration
Transpiration
Radiata pine needles are stiff and have small pores on their surface call stomata. Water evaporates from the needle via the stomata – this process is called transpiration. The rate of water loss is greater on a warm, windy day as the water molecules which reached the outside of the stomata are blown away and more are diffused to take their place. On a warm, windy day, up to 100 litres of water can be removed from the soil, transported through the tree’s xylem system and moved into the atmosphere.
Leaves can have adaptations which help to reduce the rate of transpiration. These are called xeromorphic features. On the pine needle, these are the small surface area, the waxy cuticle layer and the sunken position of the stomata on the surface.
Transpiration is important because it maintains a supply of water to the cells – essential for the cell processes to continue. A tree takes up a lot more water than it actually needs and the excess is used to carry the dissolved minerals from the roots to the leaves. This watery solution is called the sap. Once the minerals are removed from the sap, the remaining water is no longer needed and is removed by transpiration. In turn, this results in more water and minerals being taken in through the plant roots and moved into the xylem (the system of tubular cells that links the roots with the top of the tree) and up through the tree and out to the leaves.
Photosynthesis
A pine tree makes its own food by photosynthesis. This process is the most important chemical reaction on Earth as it provides the initial energy source in all food chains.
Photosynthesis can only happen in daylight because the sun’s energy is essential to the process. Leaves absorb carbon and oxygen through the stomata in the form of carbon dioxide. Inside the chloroplast in the cells, the chlorophyll molecules (the plant’s green pigment) use energy from the sun to make the hydrogen from the water combine with the carbon and oxygen and from the soluble carbohydrate – sugars. The sugars can then be made into starch for storage.
Carbon atoms are the food of all organisms. Photosynthesis is the only way in nature of trapping the carbon molecules in the air and making them into food, which is then transferred through food chains into other organisms.
As the amount of carbon dioxide in the atmosphere – largely resulting from the burning of fossil fuels – continues to increase, forests are seen as an effective way of capturing and locking up some of the carbon and reducing atmospheric levels.
The food produced by the tree in photosynthesis is transported in the phloem (a separate system of tubular cells) and the sugars are taken away from the leaves to other parts of the tree at a rate of 100 cm an hour. This food is then used in the production of vitamins, resins and hormones which are need for growth, resistance to disease and insect attack.
Respiration
Trees need energy to grow. This energy is released from the food made by photosynthesis in a process of respiration. This process occurs in the mitochondria of the tree’s cells 24 hours a day. Some of the food is combined with oxygen and the reaction releases stored energy for growth. Respiration uses between a quarter and a half of the food produced in photosynthesis. Respiration uses oxygen and releases energy, carbon dioxide and water. This is the reverse of the photosynthesis which collects the sun’s energy, combines it with carbon dioxide and water and releases oxygen.
The combined effect of photosynthesis and respiration is that the tree will actually absorb more carbon dioxide from the atmosphere than it releases.
The Trunk
Plantation trees are grown for the wood in their trunks. Most of this wood is made up of dead cells. The trunk supports the crown and holds the leaves up to the light to allow photosynthesis to occur. The trunk comprises:
• Outer bark
• Inner bark
• Cambium
• Heartwood
• Pith
Outer Bark
The outer bark is a layer of dead cells which protects the layer of living cells of the cambium against insects and disease. It can also give some protection against fire. As the tree grows in diameter, the bark cracks and is generally sloughed off by the weather.
Inner Bark
The inner bark contains the phloem cells. These transport the sugars produced in the needles to the roots. The flow of food is stopped if the tree is ringbarked and the tree will die.
Cambium
The cambium is just a single cell layer thick. It is an actively dividing layer and continually produces new xylem cells to carry water and make new wood and also new phloem cells. A tree grows in diameter by forming new cells from the cambium and by lengthening the existing cells in the trunk. The cells which cause the tree to increase in height are produced just behind the growing tip of the branches.
Heartwood
The heartwood consists of dead xylem cells. They no longer store food or water and act simply as a support for the crown. The rings seen in a cut tree trunk are formed by the new layers of cells that are produced in the cambium each year. The cells produced in the spring, when the tree is growing quickly, are wide to allow them to carry the large amounts of water needed for the tree’s rapid growth. Later in the season, the cells produced are smaller, thicker walled and slower growing. This difference in the cells is seen as bands or rings – the early, fast growing wood is lighter in colour and the slower growing is darker. From this annual pattern, the age of the tree can be calculated.
(In hardwood trees there is another group of cells, the vessels, which may be large pores that are visible to the naked eye. These also conduct water up the stem.)
Pith
The pith is the middle core of the stem. It is the centre around which the initial growth takes place and it does function after the juvenile growth of the tree. In mature trees the pith is very thin.
Branches
Branches radiate out from the trunk and are the structures that hold the leaves up to the sunlight. Branches are small stems. In radiata pine they occur in whorls or circles around the main trunk. Several whorls of branches may form in one year. Once formed, a branch stays at the same height above the ground for the life of the tree because the height of the trunk and the branches increases behind the growing tips of the trunk and the branches.
Knots in sawn timber are the result of branches. To get clearwood – timber without knots – the branches are pruned at an early age and the wood grows over the branch stumps for the rest of the tree’s life.
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