Trees

Trees are tall Perennial (plants lasting for three seasons or more)Woody Plants having a main Trunk and Branches forming a distinct elevated crown. Trees are long living Plants as compared to most other Plants. Some even reach several thousand years of age and may grow up to 115 m (379 ft) in height. There are an estimated 100,000 number of Tree Species, the number of Tree Species worldwide might total 25% of all living Plant Species. Trees are included in the division Spermatophyta. Spermatophytes include all Plants that have seeds. Spermatophytes are divided into two subdivisions, Angiospermae (encased seeds) and Gymnospermae (naked seeds). Trees are included in both of these subdivisions. A small group of Trees growing together is called a 'Grove' or 'Copse', and a landscape covered by a dense growth of Trees is called a Forest. Several biotopes are defined largely by the Trees that inhabit them, for example Rainforests and Taiga. A landscape of Trees scattered or spaced across grassland (usually grazed or burned over periodically) is called a Savannah. A young Tree is called a Sapling.

General Classification of Trees based on Foliage Type

Evergreen Trees - Evergreen Trees are type of Trees which retain their foliage year-round. By retaining their leaves Evergreen Trees conserve energy and nutrients by slowly growing new foliage year-round. Most Evergreens have needle-shaped leaves, to conserve water, and many Evergreens have slightly waxy foliage, which also helps to prevent evaporation through the leaves. Evergreen Trees are common in temperate climates. Evergreen Trees are often collectively referred to as Softwoods. Evergreen Trees are popular as ornamentals because they retain their foliage year-round. These type of Trees include Pine, Spruce, Cedar, Fir etc.

Deciduous Trees - Deciduous Trees are type of Trees shed their leaves annually. Deciduous Trees lose their leaves to conserve water or to better survive winter weather condition. As such Deciduous Trees require a lot of energy and nutrients in the spring, when they put out new leaves. Most Deciduous Trees have broad leaves to absorb more of sunlight. Depending on the variety, Deciduous Trees have leaves that turn from bright yellow to crimson and many shades of orange in between. Deciduous Trees are common in tropical regions. Deciduous Trees are often collectively referred to as Hardwoods. These type of Trees include Maple, Oaks, Elm, Aspen, Birch etc.

General Classification of Trees based on Wood Type

Hardwood Trees - These type of Trees are included in the subdivision Angiospermae. Angiosperm refers to the presence of an ovary that encloses the ovules or seeds. The ovary is the fruit found on the Tree. These Trees are usually broad-leaved. The wood from these Trees is generally harder than Softwoods. Wood hardness varies among Hardwood Species, and some Hardwoods are actually softer than some Softwoods. Hardwoods have a more complex structure than Softwoods. The dominant feature separating Hardwoods from Softwoods is the presence of pores, or vessels. The vessels may show considerable variation in size, shape of perforation plates (simple, scalariform, reticulate, foraminate), and structure of cell wall. Hardwoods are also known as Broadleaf Trees. Some of the most popular Species of Hardwoods are Oak, Maple, Hickory, Walnut, and Cherry Trees. Hardwoods are used for cabinets, furniture, mouldings and other mill work. Hardwoods are more varied than Softwoods. There are many more Hardwood Species than Softwoods Species. Hardwoods are usually more expensive to purchase compared to Softwoods. Because Hardwoods are generally denser, they contain more calories or volume and thus are popular for cooking, smoking meat, and camp fires because they tend to burn hotter and longer than Softwoods.

Softwood Trees - These type of Trees are included in the subdivision Gymnospermae.Gymnospermae are Plants having naked seeds not enclosed in an ovary. Softwood contains only two types of cells, longitudinal wood fibres and transverse ray cells. Softwoods lack vessel elements for water transport that Hardwoods have; these vessels manifest in Hardwoods as pores. These Trees have needle-like or scale-like leaves. Softwoods are also known as Conifers. These type of Trees include Pine, Spruce, Cedar, Fir, Larch, Douglas-Fir, Hemlock, Cypress, Redwood and Yew Trees. There are approximately 500 Species of Softwood Trees.

Characteristics / Features of a Tree

• Parts of a Tree - These are as follows:

Roots - Most Trees have one main Root from which secondary Roots emerge. Roots are under the ground and hold the Tree in a stable position. They also absorb water and minerals form the soil.

Stems - The Stem of a Tree is called Trunk. The Trunk separates the Roots from the crown (upper part of a Tree). The Stem provides support, water and food conduction, and storage. The leaves and stem together are called the Shoot.

Leaves - Leaves are the main organ of photosynthesis and transpiration in Higher Plants.

Branches - Branches are the side Shoots that originate form the bud.

Flowers - Flowers are the sexual organs of the Tree.

• Growth from Top - Trees grow from the top, not from the bottom as is commonly believed. A branch's location on a Tree will only move up the trunk a few inches in 1000 years. Each spring new growth (height) is added from terminal buds that are located in the topmost parts of the Tree.

• Bark -  It is a tough protective covering of the woody stems and Roots of Trees. Bark minimizes water loss from the stems, deters insect and fungal attack, and can be a very effective protector against fire damage, example the Redwoods have a high fire resistance massive Bark.

• Vascular Plants - Trees are Vascular Plants. They possess specialized supporting and water-conducting tissue, called Xylem, and food-conducting tissue, called Phloem. Xylem and Phloem are collectively called Vascular Tissue and form a central column (stele) through the Plant axis.

• Woody Plants - Wood is a hard, fibrous tissue found in Trees. Wood is produced as secondary Xylem in the stems of Trees. It transfers water and nutrients to the leaves and other growing tissues, and has a support function, enabling woody plants to reach large sizes. It is composed of cells, and the cell walls are composed of micro-fibrils of Cellulose (40% – 50%) and Hemicellulose (15% – 25%) impregnated with Lignin (15% – 30%). Sometimes the wood may deform. This deformity is known as a 'Knot'. In the Tree, a Knot is either the base of a side branch or a dormant bud.

• Branching - It is a significant characteristic in Trees. Most conifers form a well-defined dominant trunk with smaller lateral branches (excurrent branching). Many Angiosperms show for some part of their development a well-defined central axis, which then divides continually to form a crown of branches of similar dimensions (deliquescent branching). This can be found in many Oaks, the Honey locust (Gleditsia triacanthos), the Silver Linden (Tilia tomentosa), and the American Elm (Ulmus americana). The Palms illustrate the third major Tree form, Columnar, in which the central axis develops without branching until the apex of the bole (stem).

• Tree Rings - A Tree Ring is simply a layer of wood produced during one Tree's growing season. A cross-section of a Tree often shows a distinct pattern of concentric Tree Rings. Each Tree Ring marks a line between the 'Dark Late Wood' that grew at the end of the previous year and the relatively 'Pale Early Wood' that grew at the start of this year. One Annual Ring is composed of a Ring of 'Early Wood' and a Ring of 'Late Wood'. The growth occurs in the Cambium (the thin, continuous sheath of cells between bark and wood). In spring, the Cambium begins dividing.

This creates new tissue and increases the diameter of the Tree at two places:

Outside the Cambium - The outer cells become part of the Phloem. The Phloem carries food produced in the leaves to the branches, trunk, and roots. Some of the Phloem dies each year and becomes part of the outer Bark.

Inside the Cambium - The inner cells become part of the Xylem. These cells contribute most of a Tree's growth in diameter. The Xylem carries water and nutrients from the roots to the leaves.

These cells show the most annual variation:

1.  When a Tree grows quickly, the Xylem cells are large with thin walls. This Early Wood or Spring-Wood is the lighter-coloured part of a Tree Ring.
2.  In late summer, growth slows; the walls of the Xylem cells are thicker. This Late Wood or Summer-Wood is the darker-coloured part of a Tree Ring.

• Long Life Span - Most Trees have long life span as compared to other Plants. California Bristlecone Pines and Giant Sequoias are regarded as the oldest Trees and have been known to live 4,000 to 5,000 years. Red Maples live 80-250 years, American Chestnuts 100-300, White Oaks 300-600, Bald Cypress 600-1,200.

• Self Defence Mechanism - Various Specie shave evolved a self defence mechanism which helps them to survive the harsh conditions or hostile organisms. When Willows are attacked by Web-worms and Caterpillars, they emit a chemical that alerts nearby Willow of the danger. The neighbouring Trees then respond by pumping more tannin into their leaves making it difficult for the Insects to digest the leaves.

• Shrinkage and Expansion of Trunks - Tree Trunks often shrink in diameter during sunny summer days because of the large amount of water lost from the leaves by transpiration. The Trunks will then swell during the night as water is restored to the Tree from the soil. Most of the shrinking and swelling takes place in the Bark, but some occurs in the wood.

• Inducers of Rain - Trees can induce rainfall by cooling the land and transpiring water into the sky from their leaves. An acre of Maple Trees can put as much as 20,000 gallons of water into the air each day.

Distribution

Trees are found all over the World except in Antarctica.

Origin and Evolution of Trees

The evolution of distinct Plant body parts with separate functions allowed Plants to move onto the land and undergo an incredible adaptive radiation. During the initial development of terrestrial organisms, land Plants had no roots or leaves. Land Plants had originated from aqueous environments so they did not require roots or leaves for carrying out their biological functions. Because organic soils were not widely developed, the earliest Terrestrial Plants probably first colonized bare rock near large water sources, such as oceans and lakes. Eventually the land Plants had to develop various adaptations to survive on land efficiently. As a result, over a period of time, they developed structural adaptations in the form of  new tissues and organs which permitted localization of the functions of the Plant body. Leaves evolved as specialized photosynthetic organs. Stems provided mechanical strength and enabled transportation of water and nutrients from the roots to the leaves. Roots provided anchorage and absorption of sufficient water and nutrients to support the remainder of the Plant.

Importance of Trees

• Trees play an important role in producing oxygen and reducing carbon dioxide in the atmosphere, as well as moderating ground temperatures. They contribute significantly in recycling nutrients, absorbing carbon dioxide and generating oxygen.

• Trees are important for the preservation of our environment. Trees prevent soil erosion through it's widespread roots which bind the soil together. Trees are of immense importance in soil stabilization and erosion control in mountainous and hilly regions.

• Wood from Trees is used as a building material, as an energy source in many developing countries and also for various other purposes like furniture making.

• Various Species of Trees produce edible fruits produced like apples, cherries, peaches, pears, avocados, figs, persimmons etc. Fruits are food sources which also generates money for fruit growers.

• Trees provide substances with medicinal values such as the active ingredients used in asthma medications and cough remedies. Aspirin is derived from the bark of a Willow Tree. The bark of the Pacific yew (Taxus brevifolia), a gymnosperm or softwood, has recently gained importance as a source for Taxol, an anti-cancer chemical.

• Trees also supply wood pulp which is used in paper making.

• Trees also provide food and shelter for wildlife. Some Species of Animals are found only on Tree-tops and never set their foot on the ground.

• Trees provide shade and shelter, reducing yearly heating and cooling costs by 2.1 billion dollars.

• Trees improve water quality by slowing and filtering rain water as well as protecting aquifers and watersheds.

• Trees provide protection from downward fall of rain, sleet, and hail as well as reduce storm run-off and the possibility of flooding.

• Trees cut down noise pollution by acting as sound barriers.

• Various Species of Trees are ornamental i.e. they good flowers or other structures which add to the beauty of a landscape. Architectures often use them to add aesthetic appeal of an area with patterned and creative arrangement of Trees.

• Dead and decaying Trees on the ground replenish soils by returning important nutrients, and provide food resources for many types of wildlife.