Growth and Development in Trees- A Note

The tree starts its life as a small seedling which grows by increase in length and diameter of its shoot and root. As the shoot grows upwards, it develops branches and foliage. The root grows downward and develops lateral roots and its branches. Thus the seedling grows not only by increase in the size of its shoot and root but also by the formation of new organs. The increase in size is commonly referred to as growth or increment' and the formation of new organs is referred to as development. Thus both growth and development are responsible for the change that takes place in a small seedling growing into a tree.

Various stages of growth and development of a plant are designated as follows:

• Seedling—Seedling is a plant grown from seed till it attains a height of about one meter, i.e., before it reaches the sapling stage.

• Sapling—Sapling is defined as a young tree from the time when it reaches about one meter (3 feet) in height till the lower branches begin to fall. A sapling is characterized by the absence of dead bark and its vigorous height growth'.

• Pole—Pole is defined as a young tree from the time when the lower branches begin to fall off to the time when the rate of height growth begins to slow down and crown expansion becomes marked'.
• Tree—Tree is the stage of growth beyond the pole stage when the rate of height growth begins to slow down and crown expansion becomes marked.

As the plant grows, certain changes occur in its Morphology. The plant sheds its leaves and produces new leaves every
year. It produces flowers and seeds after a certain age and sheds or disperses them on ripening. These changes are important events in the life of plant and in order to know the silviculture of a particular species, it is necessary to study such changes in the members of that species. The science that deals with the study of these changes in plants is known as phenology; it is defined as

the science that deals with the time of appearance of characteristic periodic events such as leaf shedding, etc., in the life cycle of organisms in nature especially as those events are influenced by environmental factors.

These events do not occur on the same date every year. The variation in time of these periodic events every year can, in most cases, be correlated with changes in the climatic factors but it also depends upon the species. For example, if the weather has unusually warmed up a just before leaf fall or at the time of fruiting has began, leaf fall is fastened and fruit ripening is quickened. But if that occurs before the normal time of leafing or flowering, these events are delayed. Heavier rainfall and higher humidity also, sometimes, quicken fruit ripening in Albizzia procera and sal, but they appear to delay new leafing; and flowering. Fruit ripening in Mangifera indica and  Dalbergia sissoo does not show affect of the changes in climatic factors.

In addition to these morphological changes, some anatomical changes also take place in the plants annually and this result in the growth. The growth in trees is confined only in certain regions in the plant body, called the growing points. These consist of meristematic cells which have the capacity to divide and give rise to new cells. The apical meristems are those meristematic cells which are found in the apices of shoot and root. They are responsible for the growth and elongation in height of shoot and length of root. Besides the apical meristems, the trees also have primary lateral meristem present in the form of cylindrical sheath. These lateral meristems are responsible for diameter growth of shoot and root, while secondary lateral meristem responsible for the growth  of  bark.

As a plant grows, its physiological activity increase and thus requiring division of labour. To achieve this, a growing plant develops several kinds of tissues that perform diverse functions.
The growths in plants do not remain uniform all round the year. Usually, the periods of rapid growth precede and are followed by periods of slow growth. Hence, creating a difference between the wood formed during the two distinct growth periods. This brings about the formation of visibly distinct annual rings in some species of trees. The number of years the tree took to grow to stump height when added to the number of annual rings counted on stump gives the age of the tree at the time of felling. Also, the width of the rings indicates the rate of growth. A fast growth is being indicated by rings that are wider than 5 mm. Width of growth rings has considerable effect on the strength properties of wood.

A forester is interested in diameter and height growth of trees since both these factors affect the total volume growth. Additionally, a forester is interested in quality of timber. Therefore, they need a more detailed description.

HEIGHT GROWTH

Height growth in trees varies with age, species and in the same species with the quality of site on which they grow. Thus the forester has very little control over the height growth of trees. In terms of age, three distinct stages are usually distinguished, viz., the juvenile or the seedling stage, the sapling and pole stages of young ages, and finally the tree stage past the middle age of the tree. During the juvenile stage, the growth varies from very slow to fast according to species. The height growth of Abies pindrow seedlings, for instance, is very slow. Even in nursery condition, they attain a height of only about 30 cm in 4 years. Deodar seedlings grow slightly faster as they attain this height in about 24 years. Amongst the Western Himalayan conifers, Kail seedings are fastest grown. In the tropical zone Sal seedlings are slow grown and continue to die back for several years. Teak on the other hand, is faster grown in juvenile stage. The rate of growth in juvenile stage is a very important factor in the survival of the seedlings. Most of the trees after the juvenile stage, grow fast. By the time they reach the middle age, they attain maximum height for that species and site. After middle age, the rate of growth falls again and reaches a time after which there is no height growth.

DIAMETER GROWTH

Diameter growth of trees affects the volume of wood produced. It has an additional importance for the forester because, while a forester cannot influence the height growth of trees, he can control its diameter growth by various silvicultural treatments. The tree grows in diameter right from the juvenile stage of its life; but the growth in diameter up to pole stage is rather how as the tree concentrates mainly on height growth upto this stage. It is only when the tree has reached the maximum height that it starts increase in diameter with speed. The rate of diameter growth starts to fall, when the tree reaches its maturity.

The diameter growth in trees is affected by a variety of factors such as:

(1) Size of the tree—The smaller the tree, the lesser the diameter increment.

(ii) Climate—The trees that grow on the warmer sites show faster diameter increment than trees of the same species growing on colder sites, given the presence of adequate moisture. The diameter growth is also affected by rainfall. Trees show in higher diameter increment in diameter in warmer season, only when there is adequate moisture.

(iii) Soil—The diameter growth is affected by nutrient content, soil moisture and soil quality.

(iv) Production of seed—In years of heavy seed production, the growth in diameter is retarded as the entire food material is utilized in seed production of seeds

(v) Injuries— Insect attacks, defoliation by insects and forest fires have a severe effect on diameter increment.

(vi) Density- The growth in diameter of individual trees in a dense forests is stunted. Just as when the crop is opened up, the growth in diameter becomes faster.

As already mentioned, the growth in diameter of the tree is not uniform throughout its height; it is maximum near the base and least at the tip. In the crown portion, the diameter growth is maximum at its base and decreases rapidly towards the tip to give it a conical shape. The decrease from the base to the top is very gradual in the stem portion such that the diameter at the top of the stem is usually about half of the diameter at the base. The taper of the tree is characteristic of each species and in the some species it varies with age, density of the stand and site. It is described by the term form. On the best quality sites, the typical form of a tree can be seen in the open.

 Age has a great effect on form; the taper is greatest in young age and reduces gradually towards the old age or maturity. The denser the forest stand, the greater the tree bole tends to the cylindrical form.

 

GROWTH IN VOLUME

The growth in volume is a function of height, diameter and the form of the tree. Therefore, greater the height and diameter and lesser the taper, the greater the volume of the tree.

GROWTH OF TREES IN QUALITY

The object of silviculture is not served just by producing trees of large dimensions; if the timber contained in them is not of good quality, it would not only result in lesser merchantable timber but also lesser economic return. Therefore, the goal of silviculture in regard to producing trees with large volume of timber is changed  to producing large quantity of high quality timber. Thus the growth of tree in quality is as great a concern of the forester as the production of larger volume.

The quality of timbers in trees depends upon size, straightness, taper, knots, other defects and strength. The larger the size of timber obtained, the lesser the wastage and hence, greater the volume. It the tree is straight and not crooked, longer sized timber can be produced with minimum of wastage. The quality of straightness should not be confined to the bole but also to fibres inside, because if the fibres are twisted, the timber would be useless. Taper also affects the quality of timber as it increases wastage. Knots seriously affect the quality of timber. The greater is the number of knots, the lower the quality of timber. In case the knots are loose, they mostly come out on drying, thereby leaving a hole in the wood. In the same way, fungus and insect attacks degrade the quality of timber. Fire also affects the quality of timber. Strength of timber is influenced by the rate and the uniformity of growth. Very fast rate of growth impairs the strength of timber and reduces the quality. If the rate of growth is sometimes fast and sometimes slow, the timber will not be of uniform quality. The better the quality of timber improves with uniformity in growth.