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growth of trees

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growth of trees - Growth of Trees
growth of trees - Trees are a specialised group of plants adapted to a terrestrial existence of long duration, together with the attainment of height and size which give them dominance over other vegetation wherever local conditions allow them to thrive. They resemble shrubs and woody climbers in being land plants with persistent perennial aerial stems, but in shrubs these stems are much branched and diffuse, whilst in woody climbers the stem lacks rigidity and cannot stand alone.

The key characteristic of trees is their tendency, whenever possible, to develop a single central rigid erect woody axis known as a trunk or bole. It is this factor which enables them to gain ascendancy over other plants by outgrowing and over-shading them. The growth of the trunk involves the construction of a mechanically sound pillar, resistant to the pull of the wind and the thrust of the foliage. The strength of this pillar, or of its component fibres, is the source of the value of timber. In general, therefore, the woodman is interested mainly in trees which develop mechanically sound and useful trunks of dimensions sufficient for profitable working.

There are many other trees of interest to the gardener and orchardist for the sake of flowers, foliage, or fruit, but these find no place in plantations, and their study is less important to forestry.

It will be noted that trees have been defined only as having a tendency to develop a trunk. A tree may be pollarded or coppiced - cut back a few feet or inches above the ground, it may be a tiny sapling or seedling, or even a minute ungerminated seed. It is still a tree, a whole and complete plant with a tendency to develop its distinctive and invaluable trunk form.

Tree form is distinct from the main evolutionary features by which plants are classified. Trees occur amongst all the higher groups of plants, but tree-ferns and trees of the Monocotyledon class (such as palms) are absent from the present-day flora of Britain. Our trees are either conifers, with naked seed-ovules, or broadleaves, which have an enclosed seed-ovule with two cotyledons or seed-leaves. The conifers are a close-knit group, composed mainly of trees, but the broadleaves are related to a wide variety of herbaceous plants. Thus the elm is a cousin of the nettle, and the lime is related to the hollyhock. In most cases the herbs have probably been derived from their related trees, but evolution in the reverse direction is quite possible, through the intermediate stage of the shrubs.

Tree growth differs radically from the growth of herbs.

The herbaceous plant puts forth leaves and flowering shoots and, after their course is run, these fade and wither back to the ground. In perennial plants this process recurs indefinitely, but though the roots may spread, the stem never gets materially bigger than it was in its first year. But in trees the stem persists until a second phase of growth sets in, or growth may be continuous throughout. Around a slender core of pith is built up a series of annual rings or layers of conductive tissue, and as the stem elongates, so do its channels of nourishment enlarge to feed it. At the same time strengthening elements increase in number and resistive power, to hold the weight of the foliage erect and firm against the wind; and storage elements are evolved by which the food reserves built up in one season are preserved for use in the next. This process of expansion usually reaches a point where the area of wood tissues conducting sap up the trunk exceeds the requirements of the foliage above, and also exceeds the counter capacity of the bast vessels which carry the reverse flow of nutrients downwards towards the roots. But the tree must still grow; to expand its crown greater mechanical strength may be needed, although conductive capacity may be more than adequate. Therefore if closes up a portion of its wood vessels at the centre of the trunk, converting them into stronger mechanical tissues. They cease to carry sap and are no longer " sapwood instead they become the stronger and more durable " heartwood ", valuable both for the support of the tree and as a source of finer timber.

The persistence of the tree's stem enables it to postpone flowering and fruiting until it has built up adequate reserves of nutriment. It need not flower every year, but when it does so the seeds are often large and form the food of animals which distribute them more or less accidentally; being large, the seedling has abundant food to support its early growth. Smaller seeds are usually wind-distributed. The height of the trees enables them to use the wind extensively both for pollination and the spreading of fairly large winged seeds.

In its early stages the tree faces a host of enemies, both animal and vegetable. Its persistent stem can only be built up slowly, and it may take several years to reach the height which a herbaceous plant, such as bracken, attains in one season. Thus, a high proportion of wild tree seedlings are crowded out by herbaceous plants before they can rise above them. During the years that must pass before the young tree raises its head above the reach of grazing cattle, and develops a hard bark to repel rabbits, it is very liable to animal attack. A bite that would set back a herb by a few months' growth, may put a struggling sapling back by as many years. All this is in addition to the general risks that all plants must face - attack by insects and fungi, drought, frost and the chance of seed falling on stony ground or where the fowls of the air may devour it.

To counter these losses, trees seed in great abundance and over a long period of years. Thus they hold their own in the moister climates, less exposed sites, and on the soils most suited to their growth. In course of time they build up associations of their own kind, the virgin forests, from which their mutual shade excludes the competing herbs. The young trees have then only to compete with one another, and with their parent trees. Whenever an old tree falls, a thousand saplings spring-up to replace it, the fittest surviving. Against grazing animals the tree has little defence; spines or poisons are rare, as they are not needed by the grown trees. In a state of nature, the grazing and browsing animals are kept in check by carnivorous animals such as wolves. Where wolves have been exterminated, as in Britain, and where rabbits, sheep and other domestic animals are protected and preserved, the trees, slowly but surely, go under. The old trees fell, and nothing replaces them. In such cases, protection is the first task of the forester.

The grown tree itself is almost immune from injury. Wind, lightning and fungal decay take a small toll, the axe of the feller a heavier one. Apart from these, a tree may persist to an indefinite age, even after its heartwood has rotted away. Eventually, however, growth ceases; the tree continues to live on for perhaps a hundred years more, and then decay sets in. The causes of this are but little known, but the prudent forester will have felled his trees many years before this stage is reached. A tree ceases to be profitable when it ceases to increase its timber volume, which may be long before actual decay sets in.

A great deal of the woodman's work is with the younger trees, seedlings and transplants a few years old, raised artificially and requiring tending even after planting out in plantations. These saplings have no present utilisation value, but a great value in prospect as sources of poles, props and timber. Their correct tending, planting and treatment are therefore matters of great economic importance. It is a little like sinking a shaft for a new mine; the shaft itself is useless, but until it has been sunk - a task taking time and capital - the seam of coal below it is valueless: So the timber supplies of the future can only be assured by the planting of seedlings that can bring in no return for many years to come.

Trees, therefore, must be studied and understood at every stage of their life history. An oak is not simply a sturdy forest tree. It may be an acorn, a seedling in the nursery, a sapling fighting its way through a tangle of brambles, or a vital timber taking the strain of a battleship thundering its guns into history. It is still a tree, and the study of it must take account of its beginning, its growth as the years lengthen, and the manner of its end.

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