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Home page > Plants by Design > Trees Grow, But Hoe Tell

TREES GROW, BUT HOW TELL?
Plants by Design


In a study on the world's tallest trees, researchers from Northern Arizona University have revealed the factors controlling tree growth. (1,2)

There is evident creation in the tree. The cells that comprise the tree are organised in such a way as to constitute the roots, trunk, bark, water columns, branches and leaves. The cells constitute components that permit the tree to survive by performing their necessary functions, and there is a systematic division of labour among these components.

In addition, a tree resembles a giant chemical factory. Exceedingly complex chemical processes are carried out, in the light of a flawless order. There is evidence that the organs that carry out these processes perform calculations like a computer.

One of the most striking facts about trees is that the information about this organisation and system is loaded into their DNA, when they are still tiny, round seeds. The seed follows the instructions loaded in its DNA and turns into a giant structure with which nothing can compare in terms of appearance and dimensions. The way that a seed puts out roots and turns into a tree after it has fallen to earth and been moistened a little, is a clear sign of God's impeccable creation.

The way that growth in this miraculous living thing comes to a halt after a particular point is part of the equilibrium created on Earth by God. If the cells that comprise a tree were to keep growing in an uncontrolled manner, then consequences might arise that would spell the end of life on Earth.

This picture shows a plan of how water and nutrients are transported in a tree by means of its pipework. No matter how high the tree is, the pipes are strong and resistant enough to carry the water and minerals as far as the furthest leaves.

This system, which scientists have only recently unravelled, has been working in trees since they first emerged.

The scientists researching the factors that determine how much trees can grow carried out a most fascinating study on the world's tallest trees. Climbing up to the top of trees more than 100 metres high, the researchers sought clues regarding these factors by taking measurements.

They studied the world's five tallest trees, including the 112.7 metre tall giant redwood ( Sequia sempervirens ), which holds the title of the world's tallest tree. A tree that high is the same as a 30-storey building.

Previously, scientists thought that the main factor determining the height of a tree lay in the mechanical stresses of height. However, it was realised that trees possessed a rather sound structure of such a nature as to overcome these tensions. This led to research focussing on water raising capacity. In the study in question, performed by a team led by the Northern Arizona University ecologist George Koch, findings along these lines were obtained. The studies carried out by the researchers in a natural environment and under laboratory conditions revealed that the main control of maximum tree height is indeed the water supply to the tree top.

The solar energy trapped by the chlorophyll in the leaf, carbon-dioxide in the air, and water in the plant go through various processes and are used to produce glucose and oxygen. These complex processes do not take place in a factory, but in special structures like those in the leaf in the picture, and which measure only one thousandth of a millimeter across.

Water reaches to treetops by means of transpiration, in other words by vaporising through the pores on the surface of the leaves. Transpiration carries the water into the plant through the roots, and up to the top through the water-conducting cells of the xylem tissue. This movement of the water overcomes the forces of gravity and friction, and water continues to rise upwards in the form of a column. Since the forces of gravity and friction that oppose the movement of water are at their maximum at the top of the tree, the force that propels the water upwards also reaches its maximum level at the top. Water columns are capable of withstanding this tension up to a fragmentation threshold. This expresses the point at which bubbles appear in the water column and interrupt it. This situation is known to botanists as "embolism."

Koch and his colleagues measured the maximum tension of the water column at the tops of the tallest redwoods. This measurement revealed that maximum tension was close to the embolism point. At the same time, this tension level was a factor controlling how tall the tree would grow. Three other factors determining tree height were also revealed in the study.

The water reaching the leaves at the top of the tree would normally have a propulsive effect on cellular growth. However, the increased effect of gravity and friction at the tree top reducing the water flow capacity, thus leading to the cells at the top being small and possessing thick walls. As a result, the leaves at the top are also small and dense. Leaf density reaches its highest level at the top of redwoods. This indicates that tree development is to a large extent prevented. Thus the increasing leaf density at the tree top represents a second factor controlling height.

Cross-Section of a Tree, Spowing the Transport System
One of the most important features of the transport system in trees is the working of the transport tubes, made up of cells appropriate to the materials being carried in this difficult process. As can be seen in this illustration, water and minerals are carried to the leaves by different channels. One important feature of this system in plants is the annual renewal of both the xylem pipes and the phloem pipes. All the elements which bring about the root-leaf connection are renewed perfectly every year.

The small, thick leaves at the top of trees also reduce the photosynthesis carried out in these regions. This effect, which lowers photosynthesis productivity, was identified as the third factor determining tree height.

Koch and his team also determined that the CO 2 level in leaves 100 metres high was at the lowest level observed at ambient atmospheric CO 2 concentrations. This constituted the fourth factor: the limitation on CO 2 diffusion taking place by means of leaf pores.

Based on these four physiological factors, the scientists sought to calculate the maximum height to which tree could grow. As a result, they revealed that trees could achieve a maximum height of between 122 and 130 metres. Observations that trees grow an average of 0.25 metres a year further support this idea.

The restricting factors revealed in this study are of great importance for ecological equilibrium. To summarise in brief, the facts that

" Water that rises in opposition to the forces of gravity and friction cannot progress past a particular level,
" The leaves grower smaller and denser
" There is a reduction in photosynthesis productivity, and,
" The necessary CO 2 diffusion in photosynthesis falls to a minimum,

mean that the tree is prevented from growing past a particular point. In this way, the natural equilibrium brought about by the mutual influence of several living and non-living factors is not endangered by uncontrolled tree growth. Looked at from this point of view, this study comprises the latest example of how biological processes in living things support the wide-ranging balance of nature, and of how perfectly these have all been arranged. There can be no doubt that each one of these factors is a cause that has come into being by the will of God. Every phase, from the sprouting of the seed, to the seed becoming a shrub, and the shrub a tree, and the tree growing until it comes to a halt, takes place under the control of Almighty God. Every stage in the life of the tree, every activity concerning its biology, is a manifestation of the infinite might of God.

References:

(1) Ian Woodward, "Plant science: Tall storeys" Nature 428, 22 April 2004, pp. 807 - 808
(2) George W. Koch, Stephen C. Sillett, Gregory M. Jennings & Stephen D. Davis, "The limits to tree height", Nature 428, 22 April 2004, pp. 851 - 854

 
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