TREES

Basic Structure of a Tree

A tree consists of a trunk, a crown and roots. The functions of the roots are to implant the tree in the soil, absorb moisture and mineral substances it contains and in turn supply them to the trunk. The trunk supports the crown and serves to provide water and nutrients from the roots to the leaves through branches and from the leaves back to the roots.

Classification of Trees

According to the manner of growth, trees may be broadly classified as Endogenous trees and Exogenous trees.

1. Endogenous Trees

These trees grow by the formation of layers. In such trees new wood crosses and penetrates the fibres of the wood previously formed, e.g., bamboo, palmyrah, coconut, etc. Such trees are not useful for engineering works.

2. Exogenous Trees

These trees grow outwards by the addition of rings of young wood. A cross-section of these trees shows distinct approximate concentric rings called annular rings. Timber from such trees is mainly used in engineering works.

They are further classified into two types, viz., softwood and hardwood.

(i) Softwood

Softwoods have needle-like leaves and are also known as conifers as they bear cone-shaped fruits. Trees under this group are pines, deodar, spruces, etc.

The qualities of softwood are:
Light in weight and colour
Has straight fibres and is resinous
Has distinct annular rings
Is weak and can split easily

(ii) Hardwood

Hardwoods are mostly of broad-leafed trees. They are deciduous trees like teak, sap, etc.

Qualities of hardwood are:
Heavy in weight and dark in colour
Close-grained and non-resinous
No distinct annular rings
Strong and durable

Structure of Exogenous Trees

A cross-section of an exogenous tree is shown in Fig. 10.1. It shows almost distinct concentric rings called annual rings. Pith is the innermost central portion consisting of cellular tissues. The inner annular area surrounding the pith is called heartwood. Heartwood is the one which is used for all engineering works. The portion between the cambium layer and the heartwood is called the sapwood, which can not be of any use for engineering purposes since it contains large quantities of water. A very small amount of cells that grow in the horizontal direction occurs as groups or bundles of cells known as medullary rays.

Figure 10.1 Typical cross-section of wood

Freshly cut trees contain large quantities of moisture. If the percentage of moisture is more than about 20%, then the cut trees are subjected to different types of fungus attacks and attacks by insects. Hence, the amount of moisture has to be reduced.

10.2.4 Growth of a Tree

A tree grows through the tops of branches. Just within the inner bark, the tree becomes thicker by the formation of cells. A young tree grows taller and thicker every year. The growth is rapid during spring and slower during late summer and early fall. In young trees, hollow cells are alive. During the growing season the cells are filled with sap, which circulates through each fibre by a series of openings and connecting channels. With time, the older channels become partly clogged and inactive and more mature tissues combine to form the heavier and stronger heartwood. Active growth takes place only in the cells of the inner bark. As long as the layer of cells just within the outer bark is healthy, growth continues. A few annual rings next to the inner bark are less mature, which is known as sapwood. Heartwood is more resistant to decay than sapwood.

Felling of Trees

The process of a cutting down or knocking down or causing trees to fall to the ground is called the felling of a tree. Trees have to be felled only when they are very close to maturity or have just matured. Generally, trees attain maturity between 50 and 100 years. Trees cut before maturity may not be durable. On the other hand, trees cut after maturity may cause the valuable heartwood to decay. The best time for felling trees is mid-summer or mid-winter when the sap quantity is minimum.